View 绘制体系知识梳理(7) - getMeasuredWi

2017-12-04 本文已影响213人泽毛

前言

前几天被问到了getMeasuredWidth和getWidth之间的区别，因此回来看了一下源码，又顺便复习了一遍measure/layout/draw的过程，有兴趣的同学可以看前面的几篇文章

一、getMeasuredWidth 和 getWidth 的定义

1.1 getMeasuredWidth

我们来看一下getMeasuredWidth方法的内部实现：

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

这里可以看到，该方法返回的是setMeasuredDimensionRaw中设置的mMeasuredWidth的size部分，也就是说，该方法返回的是在 测量阶段中计算出的宽度。

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

    protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int opticalWidth  = insets.left + insets.right;
            int opticalHeight = insets.top  + insets.bottom;

            measuredWidth  += optical ? opticalWidth  : -opticalWidth;
            measuredHeight += optical ? opticalHeight : -opticalHeight;
        }
        setMeasuredDimensionRaw(measuredWidth, measuredHeight);
    }

    private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;
    }

1.2 getWidth

    /**
     * Return the width of the your view.
     *
     * @return The width of your view, in pixels.
     */
    @ViewDebug.ExportedProperty(category = "layout")
    public final int getWidth() {
        return mRight - mLeft;
    }

getWidth的值是根据mRight和mLeft之间的差值计算出来的，在setFrame方法中，会对View的四个点坐标mLeft/mRigth/mTop/mBottom进行赋值，它决定了View在其父容器中所处的位置：

    protected boolean setFrame(int left, int top, int right, int bottom) {
        boolean changed = false;

        if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
            changed = true;
            
            mLeft = left;
            mTop = top;
            mRight = right;
            mBottom = bottom;

            //....
        }
        return changed;
    }

而setFrame就是在layout过程中调用的：

    public void layout(int l, int t, int r, int b) {

        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        //通过之前介绍的 setFrame 方法进行赋值。
        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);

            if (shouldDrawRoundScrollbar()) {
                if(mRoundScrollbarRenderer == null) {
                    mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
                }
            } else {
                mRoundScrollbarRenderer = null;
            }

            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);
                }
            }
        }

    }

1.3 小结

在之前的两篇文章 View 绘制体系知识梳理(3) - 绘制流程之 Measure 详解和 View 绘制体系知识梳理(4) - 绘制过程之 Layout 详解，我们介绍了measure和layout的内部实现，而getMeasuredWidth和getWidth就分别对应于上面这两个阶段获得的宽度，也就是说：

getMeasuredWidth是measure阶段获得的View的原始宽度。
getWidth是layout阶段完成后，其在父容器中所占的最终宽度

1.4 注释说明

下面是Google文档中对于上面这两个方法的注释说明：

getMeasuredWidth：

The width of this view as measured in the most recent call to measure(). 
This should be used during measurement and layout calculations only. 
Use getWidth() to see how wide a view is after layout.

Returns: the measured width of this view

getWidth

Return the width of the your view.

Returns: the width of your view, in pixels

二、示例

我们用一个简单的示例，来演示getMeasuredWidth和getWidth的区别：

2.1 布局定义

首先定义一个自定义的LinearLayout，它包含两个子View，在xml中它们的宽度都被指定为200dp。

<?xml version="1.0" encoding="utf-8"?>
<com.demo.lizejun.repoopt.WidthDemoLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <View
        android:background="@android:color/holo_blue_bright"
        android:layout_width="200dp"
        android:layout_height="100dp"/>
    <View
        android:background="@android:color/holo_orange_light"
        android:layout_width="200dp"
        android:layout_height="100dp"/>
</com.demo.lizejun.repoopt.WidthDemoLayout>

2.2 重写 onLayout 方法

在WidthDemoLayout中，我们重写它的onLayout方法，对它的子View重新摆放，并打印出getMeasuredWidth和getWidth方法的值：

public class WidthDemoLayout extends LinearLayout {

    public WidthDemoLayout(Context context) {
        super(context);
    }

    public WidthDemoLayout(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public WidthDemoLayout(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

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

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
        int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            View child = getChildAt(i);
            if (i == childCount - 1) {
                child.layout(child.getLeft() ,child.getTop(), child.getRight() + 400, child.getBottom());
            }
            Log.d("WidthDemoLayout", "measuredWidth=" + child.getMeasuredWidth() + ",width=" + child.getWidth());
        }
    }
}

输出的结果为：

>> 12-04 12:48:58.788 24935-24935/com.demo.lizejun.repoopt D/WidthDemoLayout: measuredWidth=800,width=800
>> 12-04 12:48:58.788 24935-24935/com.demo.lizejun.repoopt D/WidthDemoLayout: measuredWidth=800,width=1200

最终展示的时候，虽然我们在xml中指定了相同的宽度，但是最终显示是以getWidth为准：