layout的调用过程

view的三大流程是从performTravsals()开始的，我们从这里看起

ViewRootImpl#performTraversals()

performTraversals()调用performLayout()

final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);
if (didLayout) {
    //lp是窗口的LayoutParams 和 窗口的宽高
    performLayout(lp, mWidth, mHeight);
    ......
}

ViewRootImpl#performLayout()

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
            int desiredWindowHeight) {
    //host就是decroview
    final View host = mView;
    if (host == null) {
        return;
    }
     try {
        //传入decroview的四个顶点
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
        ......
    }
}

ViewGroup#layout()

当我们定位到layout方法时，它进入的是view的layout方法，因为ViewGroup是继承自View的，我们先看它的layout方法，发现layout是用final修饰的，也就是ViewGroup的layout方法是不能被继承的

public final void layout(int l, int t, int r, int b) {
    if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
        if (mTransition != null) {
            mTransition.layoutChange(this);
        }
        //调用view的layout
        super.layout(l, t, r, b);
    } else {
        // record the fact that we noop'd it; request layout when transition finishes
        mLayoutCalledWhileSuppressed = true;
    }
}

View#layout()

调用layout方法，确定view本身的位置

public void layout(int l, int t, int r, int b) {
    ......
    //通过setFrame方法来确定View的四个顶点的位置
    boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
    
    //调用onLayout方法 父容器确定子元素的位置
    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);
        ......
    }
    ......
}

setFrame()

通过确定view的宽高是否发生改变，来修改mLeft mRight mTop mBottom这四个确定view位置的值

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;

            // Remember our drawn bit
            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 our old position
            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);
            }
        ......
    }
}

View#onLayout()

View的onLayout方法是空实现，需要我们人为去实现。ViewGroup的onLayout是一个抽象方法，在我们自定义view的时候需要去重写。

protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}

我们进入FrameLayout的onLayout方法中去看看

FrameLayout#onLayout()

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    //把父容器的参数传进
    layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}

FrameLayout#layoutChildren()

void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
    //获取子view的个数
    final int count = getChildCount();
    
    //下面四个值会影响到子view的布局
    //parentLeft由父容器的padding和foreground决定
    final int parentLeft = getPaddingLeftWithForeground();
    //parentRight由父容器的width、padding、foreground决定
    final int parentRight = right - left - getPaddingRightWithForeground();

    final int parentTop = getPaddingTopWithForeground();
    final int parentBottom = bottom - top - getPaddingBottomWithForeground();
    
    //遍历子元素
    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            
            //获取子view测量的宽高
            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;

            //当子view设置了水平方向上的layout_gravity
            //childLeft是子view的左上角的值
            switch (absoluteGravity &
            Gravity.HORIZONTAL_GRAVITY_MASK) {
                //当子view居中显示，childLeft = （父容器的宽度 - 子view的宽度）/2 + 父view左上角的距离 + 若有margin，左margin - 右margin
                case Gravity.CENTER_HORIZONTAL:
                    childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                    lp.leftMargin - lp.rightMargin;
                    break;
                //水平居右，childLeft = parentRight - 子view宽度-子view的rightMargin
                case Gravity.RIGHT:
                    if (!forceLeftGravity) {
                        childLeft = parentRight - width - lp.rightMargin;
                        break;
                    }
                //水平居左 childLeft = parentLeft+子view的margin
                case Gravity.LEFT:
                default:
                    childLeft = parentLeft + lp.leftMargin;
            }
            
            //当子view设置了水平方向上的layout_gravity,如上所示
            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;
            }
            //调用layout确定子view本身的位置
            child.layout(childLeft, childTop, childLeft + width, childTop + height);
        }
    }
}

总结

view的layout过程

• 从ViewRootImpl的performTraversals()方法开始调用perforLayout()方法
• performLayout方法调用layout方法，通过确定mLeft、mTop、mRight、mBottom四个值确定view的位置
• 再调用onLayout去确定子view的位置