1.Layout过程

1. View的layout
   view的layout方法用来确定view本身的位置

public void layout(int l, int t, int r, int b) {
	...
}

• 先通过setFrame来设定4个定点位置，view在父容器中位置就确定来
• layout内部调用onLayout, 用于父容器确定子元素位置（View未实现onLayout, 需要自己实现）

/**
     * Called from layout when this view should
     * assign a size and position to each of its children.
     *
     * Derived classes with children should override
     * this method and call layout on each of
     * their children.
     * @param changed This is a new size or position for this view
     * @param left Left position, relative to parent
     * @param top Top position, relative to parent
     * @param right Right position, relative to parent
     * @param bottom Bottom position, relative to parent
     */
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    }

2. ViewGroup的layout
   确定所有子元素的位置

• viewGroup的layout为final方法，不能覆写，其实现为View.layout

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

• ViewGroup.onLayout为抽象方法，需要覆写

    @Override
    protected abstract void onLayout(boolean changed,
            int l, int t, int r, int b);

3.总结

• View与ViewGroup的layout实为一套流程，onLayout都是实现，需要根据具体的布局来实现，可参考TextView， LinearLayout的实现。
• measure过程确定测量宽高，layout过程确定4定点位置即最终宽高，有何不同？
  measure, getMeasureWidth
  layout, getWidth = mRight - mLeft
  二者时序不同，值是相等的, measure先，layout后；
  若覆写View.layout ，修改4点的参数，则会导致二者不同