我學(xué)習(xí)WPF使用的是《WPF編程寶典》（第四版）肋拔。之前讀到自定義控件，也就是18.2顏色拾取器的時(shí)候，有一句話我深深的記在腦海里胎署。

因?yàn)閃PF不允許重新進(jìn)行屬性變化回調(diào)函數(shù)琳骡。
多么智能东揣，多么貼心循衰。為了避免有的人沒(méi)有看過(guò)這本書继低，我把書里的例子貼在這里。

public class ColorPicker:System.Windows.Controls.UserControl
{
    public static DependencyProperty RedProperty;
    public static DependencyProperty GreenProperty;
    public static DependencyProperty BlueProperty;
    public static DependencyProperty ColorProperty;

    static ColorPicker()
    {
        ColorProperty = DependencyProperty.Register("Color", typeof(Color),
                typeof(ColorPicker), new FramewrokPropertyMetadata(Colors.Black, new PropertyChangedCallback(OnColorChanged));
        RedProperty = DependencyProperty.Register("Red", typeof(byte),
                typeof(ColorPicker), new FramewrokPropertyMetadata(new PropertyChangedCallback(OnColorRGBChanged));
        GreenProperty = DependencyProperty.Register("Green", typeof(byte),
                typeof(ColorPicker), new FramewrokPropertyMetadata(new PropertyChangedCallback(OnColorRGBChanged));
        BlueProperty = DependencyProperty.Register("Blue", typeof(byte),
                typeof(ColorPicker), new FramewrokPropertyMetadata(new PropertyChangedCallback(OnColorRGBChanged));
    } 

    public Color Color
    {
        get { return (Color)GetValue(ColorProperty); }
        set { SetValue(ColorProperty, value); }
    }

    public byte Red
    {
        get { return (byte)GetValue(RedProperty); }
        set { SetValue(RedProperty, value); }
    }

    public byte Green
    {
        get { return (byte)GetValue(GreenProperty); }
        set { SetValue(GreebProperty, value); }
    }

    public byte Blue
    {
        get { return (byte)GetValue(BlueProperty); }
        set { SetValue(BlueProperty, value); }
    }
 
    private static void OnColorChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Color newColor = (Color)e.NewValue;
        Color oldColor = (Color)e.OldValue;

        ColorPicker colorPicker = (ColorPicker)sender;
        colorPicker.Red = newColor.R;
        colorPicker.Green = newColor.G;
        colorPikcer.Blue = newColor.B;
    }

    private static void OnColorRGBChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ColorPicker colorPicker = (ColorPicker)sender;
        Color color = colorPicker.Color;

        if (e.Property == RedProperty)
            color.R = (byte)e.NewValue;
        else if (e.Property == GreenProperty)
            color.G = (byte)e.NewValue;
        else if (e.Property == BlueProperty)
            color.B = (byte)e.NewValue;

        colorPicker.Color = color;
    }
}

很明顯劳澄，這段代碼是希望在改變RGB的值時(shí)，能夠更新Color蜈七。同時(shí)在更新Color時(shí)秒拔，能夠更新RGB。并且飒硅，這樣寫并不會(huì)造成循環(huán)調(diào)用砂缩。
我一直堅(jiān)信這是WPF內(nèi)部進(jìn)行優(yōu)化，能夠保證在任何情況下三娩，callback中對(duì)依賴屬性進(jìn)行設(shè)置庵芭，不會(huì)發(fā)生循環(huán)調(diào)用的情況。
知道今天雀监，我把對(duì)應(yīng)的Color改成了TimeSpan双吆，循環(huán)調(diào)用發(fā)生了。那一刻会前，我是懷疑人生的好乐。

先給出結(jié)論：DependencyPropertyChangedCallback中并不會(huì)進(jìn)行什么神奇的操作來(lái)規(guī)避循環(huán)調(diào)用。能夠規(guī)避部分循環(huán)調(diào)用的原因時(shí)瓦宜，SetValue會(huì)對(duì)值進(jìn)行判定蔚万，如果值沒(méi)有發(fā)生改變，則不會(huì)調(diào)用DependencyPropertyChangedCallback临庇。

為什么改成TimeSpan類型會(huì)發(fā)生循環(huán)調(diào)用呢反璃？因?yàn)槿绻裇econd賦值一個(gè)超過(guò)60的數(shù)昵慌，會(huì)進(jìn)位。感興趣的朋友可以想想為什么淮蜈。歡迎留言討論斋攀。