View工作原理五：layout流程

View的在屏幕上显示所需的大小确定后，接下来我们需要知道它放屏幕的什么地方。

对于View，根据自己的parent传递的位置信息，来设置自己的位置。layout(int l, int t, int r, int b)，l，t，r，b就是相对父布局的参数。

对于ViewGroup而言知道自己放哪还不行，还需要确定子元素的位置。同理ViewGroup在layout(int l, int t, int r, int b)方法中确定它相在它父布局中的位置，然后调用onLayout(boolean changed, int left, int top, int right, int bottom)确定其子布局相对于它的位置。

页面layout的入口是在ViewRootImpl#performLayout，在这个方法最终会调用到View#layout。

private void performTraversals() {
...
  performLayout(lp, mWidth, mHeight);
...
}
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
            int desiredWindowHeight) {
...
//mView是DecorView
final View host = mView;
...   
//对于DecorView而言就是全屏幕范围  
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
}

DecorView自己没有实现layout，调用的就是View 的layout()。
View#layout：

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;
//是否使用了视图边界布局，setFrame中会设置left，right，top， bottom
        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
//如果left，right，top，bottom 有变，changed就是true
        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);
                }
            }
        }
        mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}

View#layout中核心的方法是 setFrame(...)和 onLayout(...)。
setFrame(...)传入的参数会分别转化为View的mLeft，mTop，mRight，mBottom这四个值，代表的是View的四个顶点这样可以确定View在父容器的位置。
View#setFrame：

 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;
            int drawn = mPrivateFlags & PFLAG_DRAWN;
            int oldWidth = mRight - mLeft;
            int oldHeight = mBottom - mTop;
            int newWidth = right - left;
            int newHeight = bottom - top;
            boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
            invalidate(sizeChanged);
            mLeft = left;
            mTop = top;
            mRight = right;
            mBottom = bottom;
            mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
            mPrivateFlags |= PFLAG_HAS_BOUNDS;
            if (sizeChanged) {
                sizeChange(newWidth, newHeight, oldWidth, oldHeight);
            }
            if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
                mPrivateFlags |= PFLAG_DRAWN;
                invalidate(sizeChanged);
                invalidateParentCaches();
            }
            mPrivateFlags |= drawn;
            mBackgroundSizeChanged = true;
            if (mForegroundInfo != null) {
                mForegroundInfo.mBoundsChanged = true;
            }
            notifySubtreeAccessibilityStateChangedIfNeeded();
        }
        return changed;
    }

View#onLayout是一个空的方法，一般是ViewGroup实现这个方法遍历所有的子元素并调用其layout方法。按照流程View#layouot中调用的onLayout是DecorView中的onLayout。
DecorView#onLayout:

      @Override
        protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
            super.onLayout(changed, left, top, right, bottom);
            getOutsets(mOutsets);
            if (mOutsets.left > 0) {
                offsetLeftAndRight(-mOutsets.left);
            }
            if (mOutsets.top > 0) {
                offsetTopAndBottom(-mOutsets.top);
            }
        }

DecorView#onLayout他会调用父类FrameLayout的onLayout，如果有OutSets就设置偏移。
我们接着看看FrameLayout#onLayout：

 @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        layoutChildren(left, top, right, bottom, false);
    }

    void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
        final int count = getChildCount();

        //去掉View的pad和前背景的pad，获得实践可用的大小
        final int parentLeft = getPaddingLeftWithForeground();
        final int parentRight = right - left - getPaddingRightWithForeground();

        final int parentTop = getPaddingTopWithForeground();
        final int parentBottom = bottom - top - getPaddingBottomWithForeground();

        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            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;
                }

                final int layoutDirection = getLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                        break;                        lp.leftMargin - lp.rightMargin;

                    case Gravity.RIGHT:
                        if (!forceLeftGravity) {
                            childLeft = parentRight - width - lp.rightMargin;
                            break;
                        }
                    case Gravity.LEFT:
                    default:
                        childLeft = parentLeft + lp.leftMargin;
                }

                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);
            }
        }
    }

方法比较简单，计算出子View在父View中的相对位置，然后调用子View的layout(...)。

所以布局过程是从ViewRootImpl对象调用DecorView的layout()方法开始，接着layout()方法调用根视图的onLayout()方法，onLayout()方法会对所包含的子视图逐一执行layout操作，如果子视图是ViewGroup子类对象，则继续调用子视图的layout()，重复这一过程。如果子视图是View子类对象，其onLayout为空，只需要处理自己的layout。

参考：《Android 开发艺术探索》