WinRT / Metro中的碰撞检测

Question

I'm porting a Silverlight 4 application to WinRT, and the following collision detection code I was using looks like this (and works just fine in Silverlight 4):

private bool IsCollision(Point p)
{
    var hostPoint = this.canvas.TransformToVisual(this.rootVisual).TransformPoint(p);
    return CheckCollisionPoint(hostPoint, this.canvas);
}

private bool CheckCollisionPoint(Point point, UIElement subTree)
{
    var hits = VisualTreeHelper.FindElementsInHostCoordinates(point, subTree);
    return hits.Count() > 0;
}

However, in my Metro app, it doesn't detect collisions correctly at all. I think it might be related to using the wrong rootVisual. The old code that worked out the rootVisual was like this:

private void FindRootVisual()
{
    this.rootVisual = this.canvas;
    while (this.rootVisual.Parent != null)
    {
        this.rootVisual = (FrameworkElement)this.rootVisual.Parent;
    }
}

But in WinRT, this.canvas has a Parent of null. I've tried setting the rootVisual directly to the MainPage object of my application, and to the Grid that the Canvas is in but it doesn't help.

Should this technique still work in WinRT/Metro, and if so, what rootVisual do I need to use? If not, what would be a better way to do collision detection in WinRT?

Answer 1

I've found out what is going wrong. First, I do need to pass in my MainPage to be the rootVisual for it to transform the point correctly before the hit-test.

Second, FindElementsInHostCoordinates seems to return the subTree element itself as a hit, so this needs to be filtered out. In fact it is probably best to explicitly check that the hits found are the things you are collision testing against as I had another unwanted hit in the list as well. Alternatively, you can set IsHitTestVisible to false on items you don't want to match (although doing this on the canvas itself results in no matches with any of its contents)

private bool CheckCollisionPoint(Point point, UIElement subTree)
{
    var hits = VisualTreeHelper.FindElementsInHostCoordinates(point, subTree);
    return hits.Any(x => x != subTree);
}