代码滑动panorama-即程序中设置SelectedIndex

我们都知道panorama的SelectedIndex属性是只读的，所以通过修改它，在程序滑动panorama似乎不可能。那么是不是就没有办法了呢？
其实我们可以通过设置SelectedItemProperty这个依赖属性来改变SelectedItem的值。
设置方法如下：

pan.SetValue(Panorama.SelectedItemProperty, pan.Items[newIndex]);

虽然可以通过程序改变SelectedItem，但是panorama不会立即更新，我们只需要加一个小技巧，代码如下：

(pan.Items[curIndex] as PanoramaItem).Visibility = Visibility.Collapsed;
pan.SetValue(Panorama.SelectedItemProperty, pan.Items[(curIndex + 1) % pan.Items.Count]);
pan.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
(pan.Items[curIndex] as PanoramaItem).Visibility = Visibility.Visible;

这样虽然实现了Item切换，却缺少动画效果，那么我们就需要找到相应的item控件，为其添加RenderTransform动画，即动画的target是RenderTransform。
具体实现如下：

private void slidePanorama(Panorama pan)
{
    FrameworkElement panWrapper = VisualTreeHelper.GetChild(pan, 0) as FrameworkElement;
    FrameworkElement panTitle = VisualTreeHelper.GetChild(panWrapper, 1) as FrameworkElement;
    //Get the panorama layer to calculate all panorama items size
    FrameworkElement panLayer = VisualTreeHelper.GetChild(panWrapper, 2) as FrameworkElement;
    //Get the title presenter to calculate the title size
    FrameworkElement panTitlePresenter = VisualTreeHelper.GetChild(VisualTreeHelper.GetChild(panTitle, 0) as FrameworkElement, 1) as FrameworkElement;

    //Current panorama item index
    int curIndex = pan.SelectedIndex;

    //Get the next of next panorama item
    FrameworkElement third = VisualTreeHelper.GetChild(pan.Items[(curIndex + 2) % pan.Items.Count] as PanoramaItem, 0) as FrameworkElement;

    //Be sure the RenderTransform is TranslateTransform
    if (!(pan.RenderTransform is TranslateTransform)
        || !(panTitle.RenderTransform is TranslateTransform))
    {
        pan.RenderTransform = new TranslateTransform();
        panTitle.RenderTransform = new TranslateTransform();
    }

    //Increase width of panorama to let it render the next slide (if not, default panorama is 480px and the null area appear if we transform it)
    pan.Width = 960;

    //Animate panorama control to the right
    Storyboard sb = new Storyboard();
    DoubleAnimation a = new DoubleAnimation();
    a.From = 0;
    a.To = -(pan.Items[curIndex] as PanoramaItem).ActualWidth; //Animate the x transform to a width of one item
    a.Duration = new Duration(TimeSpan.FromMilliseconds(700));
    a.EasingFunction = new CircleEase(); //This is default panorama easing effect
    sb.Children.Add(a);
    Storyboard.SetTarget(a, pan.RenderTransform);
    Storyboard.SetTargetProperty(a, new PropertyPath(TranslateTransform.XProperty));

    //Animate panorama title separately
    DoubleAnimation aTitle = new DoubleAnimation();
    aTitle.From = 0;
    aTitle.To = (panLayer.ActualWidth - panTitlePresenter.ActualWidth) / (pan.Items.Count - 1) * 1.5; //Calculate where should the title animate to
    aTitle.Duration = a.Duration;
    aTitle.EasingFunction = a.EasingFunction; //This is default panorama easing effect
    sb.Children.Add(aTitle);
    Storyboard.SetTarget(aTitle, panTitle.RenderTransform);
    Storyboard.SetTargetProperty(aTitle, new PropertyPath(TranslateTransform.XProperty));

    //Start the effect
    sb.Begin();

    //After effect completed, we change the selected item
    a.Completed += (obj, args) =>
    {
        //Reset panorama width
        pan.Width = 480;
        //Change the selected item
        (pan.Items[curIndex] as PanoramaItem).Visibility = Visibility.Collapsed;
        pan.SetValue(Panorama.SelectedItemProperty, pan.Items[(curIndex + 1) % pan.Items.Count]);
        pan.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
        (pan.Items[curIndex] as PanoramaItem).Visibility = Visibility.Visible;
        //Reset panorama render transform
        (pan.RenderTransform as TranslateTransform).X = 0;
        //Reset title render transform
        (panTitle.RenderTransform as TranslateTransform).X = 0;

        //Because of the next of next item will be load after we change the selected index to next item
        //I do not want it appear immediately without any effect, so I create a custom effect for it
        if (!(third.RenderTransform is TranslateTransform))
        {
            third.RenderTransform = new TranslateTransform();
        }
        Storyboard sb2 = new Storyboard();
        DoubleAnimation aThird = new DoubleAnimation() { From = 100, To = 0, Duration = new Duration(TimeSpan.FromMilliseconds(300)) };

        sb2.Children.Add(aThird);
        Storyboard.SetTarget(aThird, third.RenderTransform);
        Storyboard.SetTargetProperty(aThird, new PropertyPath(TranslateTransform.XProperty));
        sb2.Begin();
    };
}