4.3 View的工作流程(一)

1. View的measure过程

measure是final的，但它里面调用了onMeasure方法

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    ...       
    onMeasure(widthMeasureSpec, heightMeasureSpec);
}    

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

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:
    // unspecified是getSuggestedMinimumWidth()的size
        result = size;
        break;
    case MeasureSpec.AT_MOST:
    case MeasureSpec.EXACTLY:
        // 精确模式和最大模式的测量宽高都是MeasureSpec中的size
        result = specSize;
        break;
    }
    return result;
}

protected int getSuggestedMinimumWidth() {
    return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

如果有背景则返回背景宽度，否则返回mMinWidth值，mMinWidth是android:minWidth的这个属性值所指定的值，没有指定则为0.而mBackground为drawable类型，获取背景宽度如下

public int getMinimumWidth() {
    final int intrinsicWidth = getIntrinsicWidth();
    return intrinsicWidth > 0 ? intrinsicWidth : 0;
}

有原始宽度，则返回原始宽度，否则返回0。那么Drawable在什么情况下有原始宽度？ShapeDrawable就没有原始宽度，BitmapDrawable就有原始宽度。
再看setMeasuredDimension方法

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
    ...
    setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}

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

    mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}

注意：这里mMeasuredWidth和mMeasuredHeight就是最终的测量宽高值

2. 一个问题

从上面的分析中可以知道，View的宽高由specSize决定，所以我们可以得出如下结论：直接继承View的自定义控件要重写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。
首先写个例子试一下：

public class DemoTextView extends View {
    public DemoTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
        setBackgroundColor(getResources().getColor(android.R.color.holo_blue_bright));
    }
}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="qingfengmy.developmentofart._4view.ViewActivity">

    <qingfengmy.developmentofart._4view.DemoTextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</RelativeLayout>

问题确实存在，如果我们这样

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    setMeasuredDimension(200,200);
}

则View的宽高就是200。但TextView却是正常的，可见这些具体的View都已经重写在onMeasure方法，下面简单看下TextView的onMeasure代码

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    int widthSize = MeasureSpec.getSize(widthMeasureSpec);
    int heightSize = MeasureSpec.getSize(heightMeasureSpec);

    int width;
    int height;

    if (widthMode == MeasureSpec.EXACTLY) {
        width = widthSize;
    } else {
        final Drawables dr = mDrawables;
        if (dr != null) {
            width = Math.max(width, dr.mDrawableWidthTop);
            width = Math.max(width, dr.mDrawableWidthBottom);
        }

        width += getCompoundPaddingLeft() + getCompoundPaddingRight();
    }

    if (heightMode == MeasureSpec.EXACTLY) {
        height = heightSize;
        mDesiredHeightAtMeasure = -1;
    } else {
        if (heightMode == MeasureSpec.AT_MOST) {
            height = Math.min(desired, heightSize);
        }
    }

    int unpaddedHeight = height - getCompoundPaddingTop() - getCompoundPaddingBottom();
 
    setMeasuredDimension(width, height);
}

3. ViewGroup的measure过程

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);
        }
    }
}

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);
}

ViewGroup没有定义其测量的具体过程，因为ViewGroup是一个抽象类，其测量过程的onMeasure方法需要各个子类去具体实现，这也是因为不同的ViewGroup子类有不同的布局特性，因此ViewGroup无法做统一实现。

4. LinearLayout的onMeasure方法

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

void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
    final int count = getVirtualChildCount();

    final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    final int heightMode = MeasureSpec.getMode(heightMeasureSpec);

    for (int i = 0; i < count; ++i) {
        // 遍历每个child
        final View child = getVirtualChildAt(i);

        LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();

        if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
            final int totalLength = mTotalLength;
            mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
        } else {
            int oldHeight = Integer.MIN_VALUE;

            if (lp.height == 0 && lp.weight > 0) {
                oldHeight = 0;
                lp.height = LayoutParams.WRAP_CONTENT;
            }

            measureChildBeforeLayout(
                    child, i, widthMeasureSpec, 0, heightMeasureSpec,
                    totalWeight == 0 ? mTotalLength : 0);

            final int childHeight = child.getMeasuredHeight();
            final int totalLength = mTotalLength;
            // 每次都叠加新的高度
            mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
                    lp.bottomMargin + getNextLocationOffset(child));
        }
    }
    // 算LinearLayout自己的高度
    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
            heightSizeAndState);

}

如果它的布局中的高度采用的match_parent或者具体值，那么高度为specSize；如果它的布局中高度采用的wrap_content，那么它的高度是所有元素的高度总和，但不能超过父容器的剩余空间。

5. 几种获取View宽高的方式

• onWindowFocusChanged方法
  当Activity的窗口在得到焦点和失去焦点时均会被调用一次。

@Override
public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus);
    if (hasFocus){
        // 得到焦点
        
    }
}

• view.post
  延时获取
• ViewTreeObserver
  当View树的状态发生或者改变View树内部的View可见性时，onGlobalLayout将会被调用。随着View树的状态改变，onGlobalLayout会多次调用。

ViewTreeObserver observer = view.getViewTreeObserver();
observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
    @Override
    public void onGlobalLayout() {
        
    }
});

• view.measure
  手动通过measure方法来得到View的宽高，根据LayoutParams来分：
  • match_parent
    直接放弃，因为这种需要知道parentSize，我们无法知道，所以直接放弃。
  • 固定像素值

  int widthMeasureSpec = View.MeasureSpec.makeMeasureSpec(100, View.MeasureSpec.EXACTLY);
  int heightMeasureSpec = View.MeasureSpec.makeMeasureSpec(100, View.MeasureSpec.EXACTLY);
  view.measure(widthMeasureSpec,heightMeasureSpec);
  

  • wrap_content

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

  注意上面的(1<<30)-1，因为是最大化，所以宽高值应该是最大值，而specSize最大是30ge1，也就是(1<<30)-1.