Android @ View 的绘制流程

1. measure、layout、draw
2. 绘制的入口是由 ViewRootImpl 的 performTraversals 方法来发起measure，layout，draw等流程的
3. 父 View 的 measure 的过程会先测量子 View，等子 View 测量结果出来后，再来测量自己
4. MeasureSpec(AT_MOST、EXACTLY、UNSPECIFIED)由父 View 的
   MeasureSpec 和子 View 的 LayoutParams 通过简单的计算得出一个针对子 View 的测量要求
5. measure 是 final 的，所以自定义 View 重写 onMeasure

ViewRootImpl 中的相关源码：

private void performTraversals() {
    ......
    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
    ......
    mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    ......
    mView.layout(0, 0, mView.getMeasuredWidth(), mView.getMeasuredHeight());
    ......
    mView.draw(canvas);
    ......
}

private static int getRootMeasureSpec(int windowSize, int rootDimension) {
    int measureSpec;
    switch (rootDimension) {
    case ViewGroup.LayoutParams.MATCH_PARENT:
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
        break;
    case ViewGroup.LayoutParams.WRAP_CONTENT:
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
        break;
    default:
        measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
        break;
    }
    return measureSpec;
}

FrameLayout 中的相关源码：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    ......
    int maxHeight = 0;
    int maxWidth = 0;
    int childState = 0;
    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (mMeasureAllChildren || child.getVisibility() != GONE) {
            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            maxWidth = Math.max(maxWidth, child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
            maxHeight = Math.max(maxHeight, child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
            ......
        }
    }
    ......
    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
            resolveSizeAndState(maxHeight, heightMeasureSpec, childState << MEASURED_HEIGHT_STATE_SHIFT));
    ......
}

ViewGroup 中的相关源码：

protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) {
    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
    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);
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
    int specMode = MeasureSpec.getMode(spec);
    int specSize = MeasureSpec.getSize(spec);
    int size = Math.max(0, specSize - padding);
    int resultSize = 0;
    int resultMode = 0;
    switch (specMode) {
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
    }
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

View 中的相关源码：

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

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    // 给 mMeasuredWidth 和 mMeasuredHeight 设值，如果这两个值一旦设置了，那么意味着对于这个View的测量结束了，这个 View 的宽高已经有测量的结果出来了。
    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:
        result = size;
        break;
    case MeasureSpec.AT_MOST:
    case MeasureSpec.EXACTLY:
        result = specSize;
        break;
    }
    return result;
}

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

参考：

1. Android View的绘制流程

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