另外的一个高代价的操作是布局过程(layout)。 任何时刻对View调用requestLayout()方法，Android UI 框架
都需要遍历整个View树，确定每个视图它们所占用的大小。如果在measure过程中有任何冲突，可能会多次遍历
View树。UI设计人员有时为了实现某些效果，创建了较深层次的ViewGroup。但这些深层次View树会引发效率
问题。确保你的View树层次尽可能浅。
如果你有的UI设计是复杂地，你应该考虑设计一个自定义ViewGroup来实现layout过程。不同于内置View控件，
自定义View能够假定它的每个子View的大小以及形状，同时能够避免为每个子View进行measure过程。 PieChart
展示了如何继承ViewGroup类。 PieChart带有子View，但它从来没有measure它们。相反，它根据自己的布局算法
去直接设置每个子View的大小。
如下代码所示：
/**
* Custom view that shows a pie chart and, optionally, a label.
*/
public class PieChart extends ViewGroup {
...
//
// Measurement functions. This example uses a simple heuristic: it assumes that
// the pie chart should be at least as wide as its label.
//
@Override
protected int getSuggestedMinimumWidth() {
return (int) mTextWidth * 2;
}
@Override
protected int getSuggestedMinimumHeight() {
return (int) mTextWidth;
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// Try for a width based on our minimum
int minw = getPaddingLeft() + getPaddingRight() + getSuggestedMinimumWidth();
int w = Math.max(minw, MeasureSpec.getSize(widthMeasureSpec));
// Whatever the width ends up being, ask for a height that would let the pie
// get as big as it can
int minh = (w - (int) mTextWidth) + getPaddingBottom() + getPaddingTop();
int h = Math.min(MeasureSpec.getSize(heightMeasureSpec), minh);
setMeasuredDimension(w, h);
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
// Do nothing. Do not call the superclass method--that would start a layout pass
// on this view's children. PieChart lays out its children in onSizeChanged().
}
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
//
// Set dimensions for text, pie chart, etc
//
// Account for padding
...
// Lay out the child view that actually draws the pie.
mPieView.layout((int) mPieBounds.left,
(int) mPieBounds.top,
(int) mPieBounds.right,
(int) mPieBounds.bottom);
mPieView.setPivot(mPieBounds.width() / 2, mPieBounds.height() / 2);
mPointerView.layout(0, 0, w, h);
onDataChanged();
}
}
