使FrameLayout的Gravity等于Center
由于无法设置FrameLayout的Gravity，所以只能通过重写onLayout事件实现居中的效果了
@Overrideprotected void onLayout(boolean changed, int left, int top, int right,int bottom) {super.onLayout(changed, left, top, right, bottom);// align child to centerint width = right - left;int height = bottom - top;final int childCount = getChildCount();for (int i = 0; i < childCount; i++) {final View childView = getChildAt(i);if (childView.getVisibility() != View.GONE) {final int childLeft = (width - childView.getWidth()) / 2;final int childTop = (height - childView.getHeight()) / 2;childView.layout(childLeft, childTop,childLeft + childView.getWidth(),childTop + childView.getHeight());}}}