Android控件架构与自定义控件（一）

（本文并非原创文章，整理摘抄方便自己查看，原文地址为Android控件架构与自定义控件详解 和讲讲Android事件拦截机制 ）

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1、Andorid控件架构

在Android中，控件大致被分为两类，即ViewGroup控件与View控件。ViewGroup控件作为父控件可以包含多个View控件，并管理其包含的View控件。通过ViewGroup，整个界面上的控件形成了一个树形结构，即控件树，上层控件负责下层子控件的测量与绘制，并传递交互事件。在每棵控件树的顶部，都拥有一个ViewParent对象，这就是整棵树的控制核心，所有的交互管理事件都由它来统一调度和分配，从而可以对整个视图进行整体控制。如下图展示了一个View视图树：

PhoneWindow 将一个 DecorView 设置为整个应用窗口的根 View，DecorView 分为 TitleView 和 ContentView。

1、其中DecorView作为窗口界面的顶层视图，封装了一些窗口操作的通用方法。 可以说，DecorView将要显示的具体内容呈现在了PhoneWindow上，这里面的所有的View的监听事件，都通过WindowManagerService进行接收，并通过Activity对象来回调相应的onClickListener。
2、其中ContentView，是一个ID为content的FrameLayout，activity_main.xml 就是设置在这样一个FrameLayout里，最外层是一个FrameLayout，所以当activity_main.xml最外层是一个FrameLayout会造成层次层叠，用merge来代替FrameLayout进行布局的优化。
3、所以这也就说明了，用户通过设置 requestWindowFeature(Window.FEATURE_NO_TITLE); 来设置全屏显示的时候，它一定要放在 setContentView() 方法之前才能生效。
4、在代码中，当程序在 onCreat() 方法中调用 setContentView（）方法后，ActivityManagerService会回调 onResume（）方法，此时系统才会把整个DecorView添加到 PhoneWindow中，并让其显示出来，从而最终完成界面的绘制。
5、在源码中ViewGroup是继承自View的！！！！！

如果用户通过设置requestWindowFeature(Window.FEATURE_NO_TITLE)来设置全屏显示，那么DecorView中将只有ContentView了，这就解释了为什么调用requestWindowFeature()方法一定要在调用setContentView()方法之前才能生效的原因。在代码中，当程序在onCreate()方法中调用setContentView()方法后，ActivityManagerService会回调onResume()方法，此时系统才会把整个DecorView添加到PhoneWindow中，并让其显示出来，从而完成界面的绘制。

2、ViewRoot和DecorView介绍

2.1、ViewRoot简介

（1）ViewRoot对应于ViewRootImpl 类，它是连接 WindowManager 和 DecorView的纽带，View的三大流程（measure测量、layout布局、draw绘制）都是通过ViewRoot来完成的。
（2）在ActivityThread 中，当Activity对象被创建完毕后，会将 DecorView 添加到Window中，同时会创建 ViewRootImpl对象，并将 ViewRootImpl对象和DecorView建立关联

root = new ViewRootImpl(view.getContext(), display);  
root.setView(view, wparams, panelParentView);  

（3）View的绘制流程是从ViewRoot 的 performTraversals 方法（源码在sources\android\view\ViewRootImpl.java）开始的，它经过 measure、layout和draw三个过程才能最终将一个View绘制出来，其中measure用来测量View的宽高，layout用来确定View在父容器中的放置位置，draw负责将View绘制在屏幕上。
（4）下面是performTraversals 的大致流程： 源码位置：sources\android\view\ViewRootImpl.Java

在onMeasure方法中会对所有的子元素进行measure过程，这个时候measure流程就从父容器传递到子元素中了，这样就完成了一次measure过程。接着子元素又会重复父容器的measure过程，如此反复就完成了整个View树的遍历。performDraw的传递过程是在draw方法中通过dispatchDraw来实现的。
（5）Measure完成后，可以通过 getMeasuredWidth 和getMeasuredHeight 方法来获取到 View 测量后的宽高。Layout完成后，可以通过 getTop、getBottom、getLeft和getRight 来拿到View的四个顶点的位置，并可以通过 getWidth 和getHeight方法来拿到View的最终宽高。

Draw完成后，View显示在屏幕上。

2.2、DecorView简介：

（1）DecorView作为顶级View，它内部是一个竖直的LinearLayout，其中包含TitleBar和Content。
（2）其中Activity中设置 setContentView 就是将布局文件加载到内容栏的。
（3）内容栏是一个FrameLayout，可以布局优化。
（4）如何获得Content？
ViewGroup content = findViewById(R.android.id.content);
（5）如何获得View？
content.getChildAt(0);

3、view的测量

3.1、MeasureSpec简介

源码位置：sources\android\view\View.java
Android系统在绘制View前，必须对View进行测量，这个过程在onMeasure（）方法中进行，借助的是 MeasureSpec 类。MeasureSpec类是一个32位的值，其中高2位为测量的模式SpecMode，低30位为测量的大小SpecSize。

public static class MeasureSpec {
    // 移位用的，后面表示大小的30位
    private static final int MODE_SHIFT = 30;
    private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

    /**
     * Measure specification mode: The parent has not imposed any constraint
     * on the child. It can be whatever size it wants.
     */
    /*
     * dp/px
     * 父容器对子元素没有任何约束，子元素可以是任意大小
     * */
    public static final int UNSPECIFIED = 0 << MODE_SHIFT;

    /**
     * Measure specification mode: The parent has determined an exact size
     * for the child. The child is going to be given those bounds regardless
     * of how big it wants to be.
     */
    /*
     * match_parent
     * 父容器决定了子元素的大小，子元素和父元素一样大
     * */
    public static final int EXACTLY = 1 << MODE_SHIFT;

    /**
     * Measure specification mode: The child can be as large as it wants up
     * to the specified size.
     */
    /*
     * wrap_content
     * 子元素不可以超过父容器的大小。
     * 通常的控件对这个值都会设定一个默认值来表示wrap_content。
     * */
    public static final int AT_MOST = 2 << MODE_SHIFT;

    /**
     * Creates a measure specification based on the supplied size and mode.
     *
     * The mode must always be one of the following:
     * <ul>
     *  <li>{@link android.view.View.MeasureSpec#UNSPECIFIED}</li>
     *  <li>{@link android.view.View.MeasureSpec#EXACTLY}</li>
     *  <li>{@link android.view.View.MeasureSpec#AT_MOST}</li>
     * </ul>
     *
     * <p><strong>Note:</strong> On API level 17 and lower, makeMeasureSpec's
     * implementation was such that the order of arguments did not matter
     * and overflow in either value could impact the resulting MeasureSpec.
     * {@link android.widget.RelativeLayout} was affected by this bug.
     * Apps targeting API levels greater than 17 will get the fixed, more strict
     * behavior.</p>
     *
     * @param size the size of the measure specification
     * @param mode the mode of the measure specification
     * @return the measure specification based on size and mode
     */
    /*
     * 将size和mode打包成一个32位的int值返回：
     * */
    public static int makeMeasureSpec(int size, int mode) {
        if (sUseBrokenMakeMeasureSpec) {
            return size + mode;
        } else {
            return (size & ~MODE_MASK) | (mode & MODE_MASK);
        }
    }

    /**
     * Extracts the mode from the supplied measure specification.
     *
     * @param measureSpec the measure specification to extract the mode from
     * @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
     *         {@link android.view.View.MeasureSpec#AT_MOST} or
     *         {@link android.view.View.MeasureSpec#EXACTLY}
     */
    public static int getMode(int measureSpec) {
        return (measureSpec & MODE_MASK);
    }

    /**
     * Extracts the size from the supplied measure specification.
     *
     * @param measureSpec the measure specification to extract the size from
     * @return the size in pixels defined in the supplied measure specification
     */
    public static int getSize(int measureSpec) {
        return (measureSpec & ~MODE_MASK);
    }

    static int adjust(int measureSpec, int delta) {
        return makeMeasureSpec(getSize(measureSpec + delta), getMode(measureSpec));
    }

    /**
     * Returns a String representation of the specified measure
     * specification.
     *
     * @param measureSpec the measure specification to convert to a String
     * @return a String with the following format: "MeasureSpec: MODE SIZE"
     */
    public static String toString(int measureSpec) {
        int mode = getMode(measureSpec);
        int size = getSize(measureSpec);

        StringBuilder sb = new StringBuilder("MeasureSpec: ");

        if (mode == UNSPECIFIED)
            sb.append("UNSPECIFIED ");
        else if (mode == EXACTLY)
            sb.append("EXACTLY ");
        else if (mode == AT_MOST)
            sb.append("AT_MOST ");
        else
            sb.append(mode).append(" ");

        sb.append(size);
        return sb.toString();
    }
}

MeasureSpec的测量模式有三种：
（1）EXACTLY：具体值或者 match_parent。onMeasure（）方法默认情况下只支持这种模式。
（2）AT_MOST：wrap_content。不可以比父容器大就可以了，不过通常控件都会有一个默认值。
（3）UNSPECIFIED：View想多大就多大，通常自定义View时使用。
注意点：要让自定义View支持 wrap_content 属性，就必须重写onMeasure（）方法来指定wrap_content时的大小。

3.2、MeasureSpec和LayoutParams的对应关系：

（1）在View测量的时候，系统会将LayoutParams在父容器的约束下转换成对应的MeasureSpec，然后再根据这个MeasureSpec来确定View测量后的宽高。
（2）MeasureSpec不仅有LayoutParams决定，还由父容器的大小影响。
（3）DecorView比较特别，由窗口的尺寸和LayoutParams来决定，它没有父容器。
（4）MeasureSpec一旦确定后，onMeasure中就可以确定View的测量宽高。

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DecorView在ViewRootImpl中的源码：

（1）DecorView的MeasureSpec创建过程。在measureHierarchy函数中有如下的语句：

 if (baseSize != 0 && desiredWindowWidth > baseSize) {  
    childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);  
    childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);......  

对于desiredWindowHeight指的是屏幕的高度，那个desiredWindowWidth不能超过baseSize。if下面的两句的作用是获得宽高，第三句就是通过performMeasure来设置宽高了。

（2）getRootMeasureSpec 方法：

/**
 * Figures out the measure spec for the root view in a window based on it's
 * layout params.
 *
 * @param windowSize
 *            The available width or height of the window
 *
 * @param rootDimension
 *            The layout params for one dimension (width or height) of the
 *            window.
 *
 * @return The measure spec to use to measure the root view.
 */
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
    int measureSpec;
    switch (rootDimension) {

    case ViewGroup.LayoutParams.MATCH_PARENT:
        // Window can't resize. Force root view to be windowSize.
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
        break;
    case ViewGroup.LayoutParams.WRAP_CONTENT:
        // Window can resize. Set max size for root view.
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
        break;
    default:
        // Window wants to be an exact size. Force root view to be that size.
        measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
        break;
    }
    return measureSpec;
}

这个方法很明显了，进来以后通过第二个参数来判断是用窗口大小还是用LinearLayout的值。其中的makeMeasureSpec是SpecMode和SpecSize的打包组合。

我们布局中的View：

（1）View的measure过程由ViewGroup传递而来，先看一下ViewGroup的measureChildWithMargins 方法：

/**
 * Ask one of the children of this view to measure itself, taking into
 * account both the MeasureSpec requirements for this view and its padding
 * and margins. The child must have MarginLayoutParams The heavy lifting is
 * done in getChildMeasureSpec.
 *
 * @param child The child to measure
 * @param parentWidthMeasureSpec The width requirements for this view
 * @param widthUsed Extra space that has been used up by the parent
 *        horizontally (possibly by other children of the parent)
 * @param parentHeightMeasureSpec The height requirements for this view
 * @param heightUsed Extra space that has been used up by the parent
 *        vertically (possibly by other children of the parent)
 */
protected void measureChildWithMargins(View child,
        int parentWidthMeasureSpec, int widthUsed,
        int parentHeightMeasureSpec, int heightUsed) {
    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

    /* 也是先获取子元素的MeasureSpec，
     * getChildMeasureSpec这里的参数，第一个就变成了父类的大小，
     * 第二个参数是上下左右的边距
     * 第三个参数是LinearLayout的宽高
     */
    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                    + widthUsed, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                    + heightUsed, lp.height);

    /*
     * 得到子元素的MeasureSpec后，调用子元素的measure来设置宽高。
     * */
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

（2）我们也来看看普通View的getChildMeasureSpec方法：其中的padding指的是父容器中已占用的空间大小。

/**
 * Does the hard part of measureChildren: figuring out the MeasureSpec to
 * pass to a particular child. This method figures out the right MeasureSpec
 * for one dimension (height or width) of one child view.
 *
 * The goal is to combine information from our MeasureSpec with the
 * LayoutParams of the child to get the best possible results. For example,
 * if the this view knows its size (because its MeasureSpec has a mode of
 * EXACTLY), and the child has indicated in its LayoutParams that it wants
 * to be the same size as the parent, the parent should ask the child to
 * layout given an exact size.
 *
 * @param spec The requirements for this view
 * @param padding The padding of this view for the current dimension and
 *        margins, if applicable
 * @param childDimension How big the child wants to be in the current
 *        dimension
 * @return a MeasureSpec integer for the child
 */
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
   
    /*
     * 第一个参数是父类的MeasureSpec，所以获取的模式也就是父容器的：
     * */
    int specMode = MeasureSpec.getMode(spec);
    int specSize = MeasureSpec.getSize(spec);
//子元素可用大小为父容器尺寸减去padding：
    int size = Math.max(0, specSize - padding);

    int resultSize = 0;
    int resultMode = 0;

    /*
     * 这里的这个specMode是父类容器的：
     * */
    switch (specMode) {
    // Parent has imposed an exact size on us
    case MeasureSpec.EXACTLY:
        if (childDimension >= 0) {
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
            /*
             * 这里的LayoutParams.MATCH_PARENT就是子元素它的LinearLayout
             * */
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size. So be it.
            resultSize = size;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size. It can't be
            // bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;

    // Parent has imposed a maximum size on us
    case MeasureSpec.AT_MOST:
        if (childDimension >= 0) {
            // Child wants a specific size... so be it
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size, but our size is not fixed.
            // Constrain child to not be bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size. It can't be
            // bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;

    // Parent asked to see how big we want to be
    case MeasureSpec.UNSPECIFIED:
        if (childDimension >= 0) {
            // Child wants a specific size... let him have it
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size... find out how big it should
            // be
            resultSize = 0;
            resultMode = MeasureSpec.UNSPECIFIED;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size.... find out how
            // big it should be
            resultSize = 0;
            resultMode = MeasureSpec.UNSPECIFIED;
        }
        break;
    }
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

代码的逻辑图：

就是说只要子元素的LinearLayout是精确值，那子元素就是精确值。
子元素如果是match_parent，那子元素就和父容器一样大小。
子元素如果是wrap_content，那子元素就不能超过父容器的剩余空间大小。

3.3、看看具体的onMeasure实现和如何重写这个方法：

（1）原始的onMeasure在源码中是这样的，也就是重写时它自动构成这样：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

（2）然后我们去查看 super.onMeasure（）方法：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

可以发现超类中调用了setMeasuredDimension（）方法，它的两个参数是 MeasureSpec 类型变量，这个方法将测量后的宽高值设置进去，从而完成测量工作。
（3）所以当我们想要重写onMeasure（）方法时，可以直接重写超类中的setMeasuredDimension（）方法，同时自定义两个测量宽高的方法 measureWidth() 和 measureHeight() 来处理 MeasureSpec 类型变量，返回宽高值Size。在超类中是以getDefaultSize()来处理 MeasureSpec 类型变量的，这里我们换成自己写的 measureWidth() 和 measureHeight() 方法：
（注意，这里getDefaultSize返回的是size大小，也就是说将MeasureSpec中的size部分返回。）

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(
            measureWidth(widthMeasureSpec),
            measureHeight(heightMeasureSpec));
}

（4）下面就看看需要编写的measureWidth（）方法如何实现的：

private int measureWidth(int measureSpec) {
    
    int result = 0;
    
    // 首先从MeasureSpec对象中提取出具体的测量模式和大小：
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize = MeasureSpec.getSize(measureSpec);

    // 直接返回精确值
    if (specMode == MeasureSpec.EXACTLY) {
        result = specSize;
    } else {
        // 另外两种模式，200是默认大小
        result = 200;
        // 但如果是AT_MOST即wrap_content时，还需要取两者的最小值。
        // 所以通常情况下，如果我们不重写onMeasure（）方法时，都会给这个控件一个默认的比如说200的大小
        // 但如果重写了，这里就可以为wrap_content设置一个其他的默认大小。
        if (specMode == MeasureSpec.AT_MOST) {
            result = Math.min(result, specSize);
        }
    }
    return result;
}

4、View的measure过程

4.1、View的measure过程

源码位置：sources\android\view\View.java
（1）View的measure过程由measure方法来完成，measure方法是一个final类型的方法，意味着子类不能重写此方法，在View的measure方法中会调用VIew的onMeasure 方法，所以只需要看onMeasure方法就可以了：

/** 
 * <p> 
 * Measure the view and its content to determine the measured width and the 
 * measured height. This method is invoked by {@link #measure(int, int)} and 
 * should be overriden by subclasses to provide accurate and efficient 
 * measurement of their contents. 
 * </p> 
 * 
 * <p> 
 * <strong>CONTRACT:</strong> When overriding this method, you 
 * <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the 
 * measured width and height of this view. Failure to do so will trigger an 
 * <code>IllegalStateException</code>, thrown by 
 * {@link #measure(int, int)}. Calling the superclass' 
 * {@link #onMeasure(int, int)} is a valid use. 
 * </p> 
 * 
 * <p> 
 * The base class implementation of measure defaults to the background size, 
 * unless a larger size is allowed by the MeasureSpec. Subclasses should 
 * override {@link #onMeasure(int, int)} to provide better measurements of 
 * their content. 
 * </p> 
 * 
 * <p> 
 * If this method is overridden, it is the subclass's responsibility to make 
 * sure the measured height and width are at least the view's minimum height 
 * and width ({@link #getSuggestedMinimumHeight()} and 
 * {@link #getSuggestedMinimumWidth()}). 
 * </p> 
 * 
 * @param widthMeasureSpec horizontal space requirements as imposed by the parent. 
 *                         The requirements are encoded with 
 *                         {@link android.view.View.MeasureSpec}. 
 * @param heightMeasureSpec vertical space requirements as imposed by the parent. 
 *                         The requirements are encoded with 
 *                         {@link android.view.View.MeasureSpec}. 
 * 
 * @see #getMeasuredWidth() 
 * @see #getMeasuredHeight() 
 * @see #setMeasuredDimension(int, int) 
 * @see #getSuggestedMinimumHeight() 
 * @see #getSuggestedMinimumWidth() 
 * @see android.view.View.MeasureSpec#getMode(int) 
 * @see android.view.View.MeasureSpec#getSize(int) 
 */  
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {  
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),  
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));  
}  

代码很简单，setMeasuredDimension 方法会设置View宽高的测量值，因此我们主要看看getDefaultSize 这个方法，第一个参数是getSuggestedMinimumWidth返回值，第二个参数是MeasureSpec的测量宽值。
（2）getDefaultSize方法的实现：返回的是size值。

/**
 * Utility to return a default size. Uses the supplied size if the
 * MeasureSpec imposed no constraints. Will get larger if allowed
 * by the MeasureSpec.
 *
 * @param size Default size for this view
 * @param measureSpec Constraints imposed by the parent
 * @return The size this view should be.
 */
public static int getDefaultSize(int size, int measureSpec) {
    int result = size;
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize = MeasureSpec.getSize(measureSpec);

    switch (specMode) {
    case MeasureSpec.UNSPECIFIED:
        result = size;//下面（3）对这个size做了解释
        break;
    case MeasureSpec.AT_MOST:
    case MeasureSpec.EXACTLY:
        result = specSize;//这两种情况下返回的是测量值大小
        break;
    }
    return result;
}.

我们从这个方法可以得出，View的宽高是由specSize决定的，所以我们可以直接继承View的自定义控件需要重写onMeasure方法并设置wrap_content时的自身大小，否则在布局中使用wrap_content就相当于使用match_parent。什么意思呢？就是说默认情况下，上面代码中写的AT_MOST和EXACTLY这两种case的返回值都是specSize，但是我们可以在自定义的View中设置wrap_content的大小，使得它有一个自己默认的大小。所以在大多数的控件中，比如说TextView、ImageView等的源码就可以知道，针对wrap_content情形，它们的 onMeasure 方法均作了特殊的处理。

我们这里给一个自己可以重写onMeasure的代码）：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

    int widthSpaceSize = MeasureSpec.getSize(widthMeasureSpec);
    int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
    int heightSpaceSize = MeasureSpec.getSize(heightMeasureSpec);
    int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);

    if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
        setMeasuredDimension(mWidth, mHight);
    } else if (widthSpecMode == MeasureSpec.AT_MOST) {
        setMeasuredDimension(mWidth, heightSpaceSize);
    } else if (heightSpecMode == MeasureSpec.AT_MOST){
        setMeasuredDimension(widthSpaceSize, mHight);
    }
}

（3）setMeasuredDimension 方法中的第一个参数是getSuggestedMinimumWidth方法返回的，它返回的是View在UNSPECIFIED情况下的测量宽高。

/**
 * Returns the suggested minimum width that the view should use. This
 * returns the maximum of the view's minimum width)
 * and the background's minimum width
 *  ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).
 * <p>
 * When being used in {@link #onMeasure(int, int)}, the caller should still
 * ensure the returned width is within the requirements of the parent.
 *
 * @return The suggested minimum width of the view.
 */
protected int getSuggestedMinimumWidth() {
    return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

如果View没有设置背景，那么View的宽度就是mMinWidh，这个值对应于android:minWidth，这个值默认是为0的。
如果View设置了背景，那么View的宽度就是max(mMinWidth, mBackground.getMinimumWidth())。
（4）mBackground.getMinimumWidth()：

/**
 * Returns the minimum width suggested by this Drawable. If a View uses this
 * Drawable as a background, it is suggested that the View use at least this
 * value for its width. (There will be some scenarios where this will not be
 * possible.) This value should INCLUDE any padding.
 *
 * @return The minimum width suggested by this Drawable. If this Drawable
 *         doesn't have a suggested minimum width, 0 is returned.
 */
public int getMinimumWidth() {
    final int intrinsicWidth = getIntrinsicWidth();
    return intrinsicWidth > 0 ? intrinsicWidth : 0;
}

可以看到他返回的就是Drawable的原始宽度，前提是这个Drawable有原始宽度，否则就返回0.
那么Drawable在什么情况下有原始宽度呢？ShapeDrawable无原始宽高，而BitmapDrawable有原始宽高（图片的尺寸）。

4.2、ViewGroup的measure过程：

源码位置：sources\android\view\ViewGroup.java
（1）对于ViewGroup而言，除了完成自己的measure过程以外，还会去遍历去调用所有子元素的measure方法，各个子元素再递归去执行这个过程。
（2）和View不同的是，ViewGroup是一个抽象类，因此它没有重写View的onMeasure方法，但它提供了一个叫 measureChildren的方法：

/**
 * Ask all of the children of this view to measure themselves, taking into
 * account both the MeasureSpec requirements for this view and its padding.
 * We skip children that are in the GONE state The heavy lifting is done in
 * getChildMeasureSpec.
 *
 * @param widthMeasureSpec The width requirements for this view
 * @param heightMeasureSpec The height requirements for this view
 */
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
    final int size = mChildrenCount;
    final View[] children = mChildren;
    for (int i = 0; i < size; ++i) {
        final View child = children[i];
        if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
            measureChild(child, widthMeasureSpec, heightMeasureSpec);
        }
    }
}

（3）在measureChildren中有一个measureChild：

/**
 * Ask one of the children of this view to measure itself, taking into
 * account both the MeasureSpec requirements for this view and its padding.
 * The heavy lifting is done in getChildMeasureSpec.
 *
 * @param child The child to measure
 * @param parentWidthMeasureSpec The width requirements for this view
 * @param parentHeightMeasureSpec The height requirements for this view
 */
protected void measureChild(View child, int parentWidthMeasureSpec,
        int parentHeightMeasureSpec) {
    final LayoutParams lp = child.getLayoutParams();

    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom, lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

取出子元素的LayoutParams，然后再通过getChildMeasureSpec来创建子元素的MeasureSpec，再然后将它直接传递给View的measure方法来进行测量。
（4）在ViewGroup中并没有定义其测量的具体过程，因为ViewGroup是一个抽象类，它的测量过程的onMeasure方法需要各个子类去实现，比如LinearLayout、RelativeLayout等，因为不同的子类有不同的布局特性，这导致它们的测量细节各不相同。因此ViewGroup无法做统一实现。
（5）我们以LinearLayout为例来讲一下ViewGroup：
源码位置：sources\android\widget\LinearLayout.java
先看看它的onMeasure方法：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    if (mOrientation == VERTICAL) {
        measureVertical(widthMeasureSpec, heightMeasureSpec);
    } else {
        measureHorizontal(widthMeasureSpec, heightMeasureSpec);
    }
}

再去看看measureVertical方法部分代码：

/**
 * Measures the children when the orientation of this LinearLayout is set
 * to {@link #VERTICAL}.
 *
 * @param widthMeasureSpec Horizontal space requirements as imposed by the parent.
 * @param heightMeasureSpec Vertical space requirements as imposed by the parent.
 *
 * @see #getOrientation()
 * @see #setOrientation(int)
 * @see #onMeasure(int, int)
 */
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {

    ...

    // See how tall everyone is. Also remember max width.
    for (int i = 0; i < count; ++i) {
        final View child = getVirtualChildAt(i);

        ...

            // Determine how big this child would like to be. If this or
            // previous children have given a weight, then we allow it to
            // use all available space (and we will shrink things later
            // if needed).
            /*
             * 遍历子元素并对子元素执行这个方法，
             * 这个方法内部会调用子元素的measure方法，
             * 这样各个子元素就开始一次进入measure过程，
             * 并且系统会通过mTotalLength这个变量来存储LinearLayout在竖直方向的初步高度。
             * 没测量一个子元素，mTotalLength都会增加。
             * */
            measureChildBeforeLayout(
                   child, i, widthMeasureSpec, 0, heightMeasureSpec,
                   totalWeight == 0 ? mTotalLength : 0);

            if (oldHeight != Integer.MIN_VALUE) {
               lp.height = oldHeight;
            }

            final int childHeight = child.getMeasuredHeight();
            final int totalLength = mTotalLength;
            mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
                   lp.bottomMargin + getNextLocationOffset(child));

            ...
        }
    
    ...

    // Add in our padding
    mTotalLength += mPaddingTop + mPaddingBottom;

    int heightSize = mTotalLength;

    // Check against our minimum height
    heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
    
    // Reconcile our calculated size with the heightMeasureSpec
    int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
    heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
    
    ...
    
    // 等子元素都测量完毕后，LinearLayout测量自己的大小：
    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),//这个方法在下面有介绍
            heightSizeAndState);

    if (matchWidth) {
        forceUniformWidth(count, heightMeasureSpec);
    }
}

针对竖直的LinearLayout而言，它的水平方向的测量遵循View的测量过程，在竖直方向的测量过程则和View有所不同。
具体来说，如果它的布局中高度采用的是match_parent或者具体数值，那么它的测量过程和View一致，即高度为specSize；如果它的布局中高度采用的是wrap_content，那么它的高度是所有子元素所占用的高度总和，但是仍然不能超过它的父容器的剩余空间，当然它的最终高度还需要考虑其在竖直方向的padding，这个过程可以参考如下的源码：这个方法是在View.java中实现的：

/**
 * Utility to reconcile a desired size and state, with constraints imposed
 * by a MeasureSpec.  Will take the desired size, unless a different size
 * is imposed by the constraints.  The returned value is a compound integer,
 * with the resolved size in the {@link #MEASURED_SIZE_MASK} bits and
 * optionally the bit {@link #MEASURED_STATE_TOO_SMALL} set if the resulting
 * size is smaller than the size the view wants to be.
 *
 * @param size How big the view wants to be
 * @param measureSpec Constraints imposed by the parent
 * @return Size information bit mask as defined by
 * {@link #MEASURED_SIZE_MASK} and {@link #MEASURED_STATE_TOO_SMALL}.
 */
public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
    int result = size;
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize =  MeasureSpec.getSize(measureSpec);
    switch (specMode) {
    case MeasureSpec.UNSPECIFIED:
        result = size;
        break;
    case MeasureSpec.AT_MOST:
        if (specSize < size) {
            result = specSize | MEASURED_STATE_TOO_SMALL;
        } else {
            result = size;
        }
        break;
    case MeasureSpec.EXACTLY:
        result = specSize;
        break;
    }
    return result | (childMeasuredState&MEASURED_STATE_MASK);
}

到此为止，measure完成以后，通过 个图MeasuredWidth、MeasuredHeight方法就可以正确地获取到View的测量宽高。注意点：在某些极端情况下，系统需要多次测量measure才能确定最终的测量宽高，这种情况下，在onMeasure方法中拿到的测量宽高很可能是不准确的。一个比较好的习惯是在onLayout方法中去获取View的测量宽高或者说是最终宽高。

4.3、解决View的measure过程和Activity生命周期不同步的问题

（1）当我们想要在Activity已启动的时候就做一个任务，但是这个任务需要获取某个View 的宽高。但是由于View的measure和Activity的生命周期不同步执行，因此无法保证Activity执行了onCreate、onStart、onResume时某个View已经测量完毕了，如果View 还没有测量完毕，那么获得的宽高就是0。
（2）具体的解决方法有四种。
（3）解决方法一：onWindowFocusChanged (源码位置：sources\android\view\View.java)。onWindowFocusChanged 这个方法的含义是：View已经初始化完毕了，宽高已经准备好了，这个时候获取宽高是没有问题的。onWindowFocusChanged会被调用多次，当Activity的窗口得到焦点或失去焦点的时候都会被调用一次。也就是说，当Activity继续执行或暂停执行的时候，onWindowFocusChanged就会被调用。那如果频繁的onResume或onPause时，onWindowFocusChanged 也会被频繁的调用。
源码如下：

/**
 * Called when the window containing this view gains or loses focus.  Note
 * that this is separate from view focus: to receive key events, both
 * your view and its window must have focus.  If a window is displayed
 * on top of yours that takes input focus, then your own window will lose
 * focus but the view focus will remain unchanged.
 *
 * @param hasWindowFocus True if the window containing this view now has
 *        focus, false otherwise.
 */
public void onWindowFocusChanged(boolean hasWindowFocus) {
    InputMethodManager imm = InputMethodManager.peekInstance();
    if (!hasWindowFocus) {
        if (isPressed()) {
            setPressed(false);
        }
        if (imm != null && (mPrivateFlags & PFLAG_FOCUSED) != 0) {
            imm.focusOut(this);
        }
        removeLongPressCallback();
        removeTapCallback();
        onFocusLost();
    } else if (imm != null && (mPrivateFlags & PFLAG_FOCUSED) != 0) {
        imm.focusIn(this);
    }
    refreshDrawableState();
}

重写onWindowFocusChanged 代码如下：

public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus);
    // 如果重新获得焦点，那就获取宽高值：
    if(hasFocus){
        int width = view.getMeasuredWidth();
        int height = view.getMeasureHeight();
    }
}

（4）解决方法二：view.post(runnable) （源码位置：sources\android\app\Activity.java）。通过post可以将一个runnable投递到消息队列的尾部，然后等待Looper调用此runnable的时候，View 也已经初始化好了。
源码如下：

/**
 * Called after {@link #onCreate} — or after {@link #onRestart} when  
 * the activity had been stopped, but is now again being displayed to the 
 * user.  It will be followed by {@link #onResume}.
 *
 * <p><em>Derived classes must call through to the super class's
 * implementation of this method.  If they do not, an exception will be
 * thrown.</em></p>
 * 
 * @see #onCreate
 * @see #onStop
 * @see #onResume
 */
protected void onStart() {
    if (DEBUG_LIFECYCLE) Slog.v(TAG, "onStart " + this);
    mCalled = true;
    
    if (!mLoadersStarted) {
        mLoadersStarted = true;
        if (mLoaderManager != null) {
            mLoaderManager.doStart();
        } else if (!mCheckedForLoaderManager) {
            mLoaderManager = getLoaderManager("(root)", mLoadersStarted, false);
        }
        mCheckedForLoaderManager = true;
    }

    getApplication().dispatchActivityStarted(this);
}

重写代码如下：

protected void onStart() {
    super.onStart();
    
    view.post(new Runnable){
        @Override
        public void run(){
            int width = view.getMeasuredWidth();
            int height = view.getMeasuredHeight();
        }
    };
}

（5）解决方法三：ViewTreeObserver
（6）解决方法四：view.measure(int widthMeasureSpec， int heightMeasureSpec)。通过手动对View进行measure来得到View的宽高。这种方法比较复杂，这里要分情况处理，根据View的 LayoutParames 来分：
match_parent：
直接放弃，无法measure出具体的宽高。因为我们此时还没有办法知道父容器的剩余空间。
具体的数值（dp/ps）：

// 比如宽高都是100px
int widthMeasureSpec = MeasureSpec.makeMeasureSpec(100, MeasureSpec.EXACTLY);
int heightMeasureSpec = MeasureSpec.makeMeasureSpec(100,MeasureSpec.EXACTLY);

view.measure(widthMeasureSpec, heightMeasureSpec);

wrap_content：

int widthMeasureSpec = MeasureSpec.makeMeasureSpec( (1 << 30) - 1, MeasureSpec.AT_MOST);
int heightMeasureSpec = MeasureSpec.makeMeasureSpec( (1 << 30) - 1,MeasureSpec.AT_MOST);

view.measure(widthMeasureSpec, heightMeasureSpec);

要知道的是(1 << 30) - 1，VIew 的尺寸使用30位二进制表示，也就是说最大是30个1，也就是(1 << 30) - 1。
在最大化模式下，我们使用View理论上能支持的最大值去构造MeasureSpec是合理的。

4.4、常见Measure错误使用方法：

无法通过错误的MeasureSpec去得出合法的SpecMode，从而导致measure过程出错，违背了系统内部的实现规范。
其次不能保证一定能measure出正确结果。
（1）

int widthMeasureSpec = MeasureSpec.makeMeasureSpec( -1, MeasureSpec.UNSPECIFIED);
int heightMeasureSpec = MeasureSpec.makeMeasureSpec( -1, MeasureSpec.UNSPECIFIED);

view.measure(widthMeasureSpec, heightMeasureSpec);

（2）

view.measure(LayoutParames.WRAP_CONTENT, LayoutParames.WRAP_CONTENT);  TENT);  

5、View的layout过程

（1）Layout的作用是ViewGroup用来确定子元素的位置，当ViewGroup的位置被确定后，它在 onLayout 中会遍历所有的子元素并调用其layout方法。在layout方法中onLayout方法又会被调用。
（2）layout方法会确定View本身的位置！！！！而onLayout方法会确定所有子元素的位置！！！
（3）先看View的layout方法：源码位置：sources\android\view\View.java。

/**
 * Assign a size and position to a view and all of its
 * descendants
 *
 * <p>This is the second phase of the layout mechanism.
 * (The first is measuring). In this phase, each parent calls
 * layout on all of its children to position them.
 * This is typically done using the child measurements
 * that were stored in the measure pass().</p>
 *
 * <p>Derived classes should not override this method.
 * Derived classes with children should override
 * onLayout. In that method, they should
 * call layout on each of their children.</p>
 *
 * @param l Left position, relative to parent
 * @param t Top position, relative to parent
 * @param r Right position, relative to parent
 * @param b Bottom position, relative to parent
 */
@SuppressWarnings({"unchecked"})
public void layout(int l, int t, int r, int b) {
    if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
        onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
        mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
    }

    int oldL = mLeft;
    int oldT = mTop;
    int oldB = mBottom;
    int oldR = mRight;

    /*
     * 首先通过setFrame方法来设定View的四个顶点的位置，
     * 即初始化mLeft、mTop、mBottom、mRight这四个值，
     * 这四个顶点一旦被确定，那么View在父容器中的位置也就确定了
     * */
    boolean changed = isLayoutModeOptical(mParent) ?
            setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        /*
         * 接着调用onLayout方法，这个方法的用途是父容器确定子元素的位置，和onMeasure方法类似。
         * onLayout的具体实现同样和具体的布局有关，
         * 所以View和ViewGroup都没有真正实现onLayout方法。
         * */
        onLayout(changed, l, t, r, b);
        mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnLayoutChangeListeners != null) {
            ArrayList<OnLayoutChangeListener> listenersCopy =
                    (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
            int numListeners = listenersCopy.size();
            for (int i = 0; i < numListeners; ++i) {
                listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
            }
        }
    }

    mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
    mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}

其中涉及到onLayout方法，我们看看View源码中是如何写的：

/**
 * Called from layout when this view should
 * assign a size and position to each of its children.
 *
 * Derived classes with children should override
 * this method and call layout on each of
 * their children.
 * @param changed This is a new size or position for this view
 * @param left Left position, relative to parent
 * @param top Top position, relative to parent
 * @param right Right position, relative to parent
 * @param bottom Bottom position, relative to parent
 */
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}

（4）LinearLayout的onLayout方法：源码位置：sources\android\widget\LinearLayout.java。

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    if (mOrientation == VERTICAL) {
        layoutVertical(l, t, r, b);
    } else {
        layoutHorizontal(l, t, r, b);
    }
}

同样的，分为垂直和水平两种情况。下面去看看layoutVertical好了。
（5）layoutVertical方法，只看有注释的地方就可以了

/**
 * Position the children during a layout pass if the orientation of this
 * LinearLayout is set to {@link #VERTICAL}.
 *
 * @see #getOrientation()
 * @see #setOrientation(int)
 * @see #onLayout(boolean, int, int, int, int)
 * @param left
 * @param top
 * @param right
 * @param bottom
 */
void layoutVertical(int left, int top, int right, int bottom) {
    final int paddingLeft = mPaddingLeft;

    int childTop;
    int childLeft;
    
    // Where right end of child should go
    final int width = right - left;
    int childRight = width - mPaddingRight;
    
    // Space available for child
    int childSpace = width - paddingLeft - mPaddingRight;
    
    // 获取子元素个数：
    final int count = getVirtualChildCount();

    final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
    final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;

    switch (majorGravity) {
       case Gravity.BOTTOM:
           // mTotalLength contains the padding already
           childTop = mPaddingTop + bottom - top - mTotalLength;
           break;

           // mTotalLength contains the padding already
       case Gravity.CENTER_VERTICAL:
           childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
           break;

       case Gravity.TOP:
       default:
           childTop = mPaddingTop;
           break;
    }

    /*
     * 遍历所有的子元素，并调用setChildFrame：
     * */
    for (int i = 0; i < count; i++) {
        final View child = getVirtualChildAt(i);
        if (child == null) {
            childTop += measureNullChild(i);
        } else if (child.getVisibility() != GONE) {
            final int childWidth = child.getMeasuredWidth();
            final int childHeight = child.getMeasuredHeight();
            
            final LinearLayout.LayoutParams lp =
                    (LinearLayout.LayoutParams) child.getLayoutParams();
            
            int gravity = lp.gravity;
            if (gravity < 0) {
                gravity = minorGravity;
            }
            final int layoutDirection = getLayoutDirection();
            final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
            switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                case Gravity.CENTER_HORIZONTAL:
                    childLeft = paddingLeft + ((childSpace - childWidth) / 2)
                            + lp.leftMargin - lp.rightMargin;
                    break;

                case Gravity.RIGHT:
                    childLeft = childRight - childWidth - lp.rightMargin;
                    break;

                case Gravity.LEFT:
                default:
                    childLeft = paddingLeft + lp.leftMargin;
                    break;
            }

            if (hasDividerBeforeChildAt(i)) {
                childTop += mDividerHeight;
            }

            childTop += lp.topMargin;
            /*
             * 在这里设置子元素的四个顶点值，
             * 其中的childTop会不断增大，
             * 这就意味着后面的子元素会被放置在靠下的位置，
             * 这刚好符合竖直方向的LinearLayout的特性。
             * 但在setChildFrame中，其实它仅仅是调用了子元素的layout方法而已，
             * */
            setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                    childWidth, childHeight);
            childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

            i += getChildrenSkipCount(child, i);
        }
    }
}

其中的setChildFrame方法是这样写的：

private void setChildFrame(View child, int left, int top, int width, int height) {        
    child.layout(left, top, left + width, top + height);
}

还可以发现在setChildFrame中的width和height实际上就是子元素的测量宽高，就是在setChildFrame中的后两个参数是这样获取的：

final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();

（6）这样父元素在layout方法中完成自己的定位以后，就通过onLayout方法去调用子元素的layout方法，子元素又会通过自己的layout方法来确定自己的位置，这样一层一层地传递下去就完成了整个View树的layout过程。
（7）测量宽高和最终宽高之间的联系：源码位置：sources\android\view\View.java。
getHeight获取的是最终高度：

/**
 * Return the height of your view.
 *
 * @return The height of your view, in pixels.
 */
@ViewDebug.ExportedProperty(category = "layout")
public final int getHeight() {
    return mBottom - mTop;
}

getMeasuredHeight获取的是测量高度：

/**
 * Like {@link #getMeasuredHeightAndState()}, but only returns the
 * raw width component (that is the result is masked by
 * {@link #MEASURED_SIZE_MASK}).
 *
 * @return The raw measured height of this view.
 */
public final int getMeasuredHeight() {
    return mMeasuredHeight & MEASURED_SIZE_MASK;
}

其实本质上来说他们两个是相同的，只是测量宽高形成于View的measure过程，而最终宽高形成于View的layout过程，即两者的赋值时机不同，测量宽高的赋值时机稍微早一些。

ViewGroup需要负责子View显示的大小。当ViewGroup的大小为wrap_content时，ViewGroup就需要遍历子View，以便获得所有子View的大小，从而决定自己的大小。而在其他模式下则会通过具体的指定值来设置自身的大小。ViewGroup在测量时通过遍历所有子View，从而调用子View的Measure方法来获得每一个子View的测量结果。当子View测量完毕后，就需要将子View放到合适的位置，这个过程就是View 的Layout过程。
ViewGroup在执行Layout过程时，同样是使用遍历来调用子View的Layout方法，并指定其具体显示的位置，从而来决定其布局位置。在自定义ViewGroup时，通常会去重写onLayout（）方法来控制其子View显示位置的逻辑。同样，如果需要支持wrap_content属性，那么它还必须重写onMeasure（）方法，这点与View是相同的。