学步园

 2.9讲的是，如果内部设定超过容器大小，怎么办？
StackPanel会裁剪越界部分
DockPanel和Grid会智能判断，从而决定换行。

2.10 自定义布局容器
自定义容器要实现两个方法MeasureOverride和ArrangeOverride，并保证遍历其下的所有子控件，使他们都执行Measure和Arrange方法。

using System;
using System.Windows.Controls;
using System.Windows;

namespace CustomPanel {
    public class DiagonalPanel : Panel {

        protected override Size MeasureOverride( Size availableSize ) {
            double totalWidth = 0;
            double totalHeight = 0;

            foreach( UIElement child in Children ) {
                child.Measure( new Size( double.PositiveInfinity,
                                         double.PositiveInfinity ) );
                Size childSize = child.DesiredSize;
                totalWidth += childSize.Width;
                totalHeight += childSize.Height;
            }

            return new Size( totalWidth, totalHeight );
        }

        protected override Size ArrangeOverride( Size finalSize ) {
            Point currentPosition = new Point( );

            foreach( UIElement child in Children ) {
                Rect childRect = new Rect( currentPosition, child.DesiredSize );
                child.Arrange( childRect );
                currentPosition.Offset( childRect.Width, childRect.Height );
            }

            return new Size( currentPosition.X, currentPosition.Y );
        }
    }
}