FrameLayout布局绘制流程解析

最近在深入学习Android视图的绘制流程，从相对简单的FramgLayout开始。
onMeasure函数主要是用来计算子视图以及ViewGroup自身的大小，代码如下：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {    int count = getChildCount();    //如果width或者height的mode有一个不为EXACTLY那么使能measureMatchParentChildren    final boolean measureMatchParentChildren =            MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||            MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;    //清除mMatchParentChildren缓存，mMatchParentChildren中保存了前一次onMeasure中宽或高被设置为match_parent的view    mMatchParentChildren.clear();    int maxHeight = 0;    int maxWidth = 0;    int childState = 0;    //step1.遍历计算所有子View大小    for (int i = 0; i < count; i++) {        final View child = getChildAt(i);        //mMeasureAllChildren如果使能，那么无视可见性，强制measure所有视图，默认false        if (mMeasureAllChildren || child.getVisibility() != GONE) {            //measure子View,该函数属于ViewGroup            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);            final LayoutParams lp = (LayoutParams) child.getLayoutParams();            //记录子视图最大宽度            maxWidth = Math.max(maxWidth,                    child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);            //记录子视图最大高度            maxHeight = Math.max(maxHeight,                    child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);            childState = combineMeasuredStates(childState, child.getMeasuredState());            //如果measureMatchParentChildren使能那么            //向mMatchParentChildren中加入所有高或宽为Match_Parent的view            if (measureMatchParentChildren) {                if (lp.width == LayoutParams.MATCH_PARENT ||                        lp.height == LayoutParams.MATCH_PARENT) {                    mMatchParentChildren.add(child);                }            }        }    }    // Account for padding too    // 考虑padding    maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();    maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();    // Check against our minimum height and width    // 与最小宽高比较，取较大值    maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());    maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());    // Check against our foreground's minimum height and width    // 与前景图片比较，获取宽高相对较大的值    final Drawable drawable = getForeground();    if (drawable != null) {        maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());        maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());    }    //step2.设置FrameLaout自身的尺寸    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),            resolveSizeAndState(maxHeight, heightMeasureSpec,                    childState << MEASURED_HEIGHT_STATE_SHIFT));    //step3.绘制宽或高为Match_Parent的view    count = mMatchParentChildren.size();    if (count > 1) {        for (int i = 0; i < count; i++) {            final View child = mMatchParentChildren.get(i);            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();            int childWidthMeasureSpec;            int childHeightMeasureSpec;            if (lp.width == LayoutParams.MATCH_PARENT) {                childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -                        getPaddingLeftWithForeground() - getPaddingRightWithForeground() -                        lp.leftMargin - lp.rightMargin,                        MeasureSpec.EXACTLY);            } else {                childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,                        getPaddingLeftWithForeground() + getPaddingRightWithForeground() +                        lp.leftMargin + lp.rightMargin,                        lp.width);            }            if (lp.height == LayoutParams.MATCH_PARENT) {                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -                        getPaddingTopWithForeground() - getPaddingBottomWithForeground() -                        lp.topMargin - lp.bottomMargin,                        MeasureSpec.EXACTLY);            } else {                childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,                        getPaddingTopWithForeground() + getPaddingBottomWithForeground() +                        lp.topMargin + lp.bottomMargin,                        lp.height);            }            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);        }    }}// 考虑子视图外边距计算子视图大小protected void measureChildWithMargins(View child,            int parentWidthMeasureSpec, int widthUsed,            int parentHeightMeasureSpec, int heightUsed) {    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();    //根据parentWidthMeasureSpec，ViewGroup剩余宽度，child的宽度重新计算child的widthMeasureSpec    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,            mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin                    + widthUsed, lp.width);    //根据parentHeightMeasureSpec，ViewGroup剩余高度度，child的高度重新计算childHeightMeasureSpec    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,            mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin                    + heightUsed, lp.height);    //调用measure计算大小    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}

onLayout计算子视图的位置：

@Overrideprotected void onLayout(boolean changed, int left, int top, int right, int bottom) {    // 直接调用了FrameLayout中的layoutChildren方法    layoutChildren(left, top, right, bottom, false /* no force left gravity */);}//该方法中的参数其实都是相对于父视图的void layoutChildren(int left, int top, int right, int bottom,                                  boolean forceLeftGravity) {    final int count = getChildCount();    // 获取各个方向的padding值    final int parentLeft = getPaddingLeftWithForeground();    final int parentRight = right - left - getPaddingRightWithForeground();    final int parentTop = getPaddingTopWithForeground();    final int parentBottom = bottom - top - getPaddingBottomWithForeground();    mForegroundBoundsChanged = true;    // 循环layout所有子View    for (int i = 0; i < count; i++) {        final View child = getChildAt(i);        // 不绘制可见性为GONE的View        if (child.getVisibility() != GONE) {            final LayoutParams lp = (LayoutParams) child.getLayoutParams();            final int width = child.getMeasuredWidth();            final int height = child.getMeasuredHeight();            int childLeft;            int childTop;            int gravity = lp.gravity;            if (gravity == -1) {                gravity = DEFAULT_CHILD_GRAVITY;            }            // 获取布局方向，可能的值为rtl、ltr,分别为从右至左、从左至右布局            final int layoutDirection = getLayoutDirection();            // 结合layoutDirection和View的layout_gravity计算出真正的gravity            // Gravity.getAbsoluteGravity是一个静态方法            // 源码注释:            // if horizontal direction is LTR, then START will set LEFT and END will set RIGHT.            // if horizontal direction is RTL, then START will set RIGHT and END will set LEFT.            // 水平方向如果为从左到右，那么，Gratity为START会被设置成LEFT，END会被设置成RIGHT            // 水平方向如果为从右到左，那么，Gratity为START会被设置成RIGHT，END会被设置成LEFT            final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);            // 获取垂直方向的Gravity            final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;            // 根据不同的水平比重计算View的水平坐标            switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {                case Gravity.CENTER_HORIZONTAL:                    childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +                    lp.leftMargin - lp.rightMargin;                    break;                case Gravity.RIGHT:                    if (!forceLeftGravity) {                        childLeft = parentRight - width - lp.rightMargin;                        break;                    }                case Gravity.LEFT:                default:                    childLeft = parentLeft + lp.leftMargin;            }            //根据垂直比重计算View的垂直坐标            switch (verticalGravity) {                case Gravity.TOP:                    childTop = parentTop + lp.topMargin;                    break;                case Gravity.CENTER_VERTICAL:                    childTop = parentTop + (parentBottom - parentTop - height) / 2 +                    lp.topMargin - lp.bottomMargin;                    break;                case Gravity.BOTTOM:                    childTop = parentBottom - height - lp.bottomMargin;                    break;                default:                    childTop = parentTop + lp.topMargin;            }            child.layout(childLeft, childTop, childLeft + width, childTop + height);        }    }}

FrameLayout并没有继承onDraw方法，而是继承了draw：

// 仅仅绘制了前景图片public void draw(Canvas canvas) {    super.draw(canvas);    if (mForeground != null) {        final Drawable foreground = mForeground;        // 根据mForegroundBoundsChanged判断是否重新设置界限        // mForegroundBoundsChanged在onLayout以及onSizeChanged方法中会被使能        if (mForegroundBoundsChanged) {            mForegroundBoundsChanged = false;            final Rect selfBounds = mSelfBounds;            final Rect overlayBounds = mOverlayBounds;            final int w = mRight-mLeft;            final int h = mBottom-mTop;            // 根据mForegroundInPadding判断前景是否被padding所束缚，重新设置bounds            if (mForegroundInPadding) {                selfBounds.set(0, 0, w, h);            } else {                selfBounds.set(mPaddingLeft, mPaddingTop, w - mPaddingRight, h - mPaddingBottom);            }            final int layoutDirection = getLayoutDirection();            Gravity.apply(mForegroundGravity, foreground.getIntrinsicWidth(),                    foreground.getIntrinsicHeight(), selfBounds, overlayBounds,                    layoutDirection);            foreground.setBounds(overlayBounds);        }        foreground.draw(canvas);    }}

以上。

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