在Canvas中使控件居中

經常會遇到要在Canvas里將各類控件居中的問題，下面以TextBlock為例說明。

//在將TextBlock添加到Canvas前獲取它的實際尺寸
TextBlock label = new TextBlock;
label.Text = "It is a test";
label.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
Size labelSize = label.DesiredSize;

Canvas canvas = new Canvas();
canvas.Width = 100;
canvas.Height = 40;
canvas.Children.Add(label);
Canvas.SetLeft(label, (canvas.Width - labelSize.Width)/2);
Canvas.SetTop(label, (canvas.Height - labelSize.Height)/2);