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Wednesday, May 13, 2015

Blurred Image Renderer for Xamarin.Forms

In response to a recent Xamarin forum post I spent some time building a custom Xamarin.Forms renderer for a blurred image on iOS and Android. Read on to learn how I did it.

The Problem

The request was to have a blurred image view on Android. Since this is Xamarin.Forms we may as well do it for iOS as well1. The ideal is to have a drop-in replacement for the Image view.

The end results look like this:

Android Screenshot iOS Screenshot

Custom Image View

The first step is to create a custom subclass of the Image view class. We don't need any new properties so it's this easy:

public class BlurredImage : Image
{
}

iOS Renderer Implementation

I started out by trying a UIVisualEffectView. This is very easy to use, fast, and supports blurring any views (even dynamic ones), but it turned out not to match the desired results. It was too blurred, and also lightened or darkened the results. Not quite what we were after.

After some googling I came across this StackOverflow post that explains how to blur an image using CoreImage. That was exactly what I wanted. I just had to translate it to C#. Fortunately that's pretty easy. Here's the result:

private class BlurredImageView : UIImageView
{
    public override UIImage Image
    {
        get { return base.Image; }
        set
        {
            // This may take up to a second so don't block the UI thread.
            Task.Run(() =>
                {
                    using (var context = CIContext.Create())
                    using (var inputImage = CIImage.FromCGImage(value.CGImage))
                    using (var filter = new CIGaussianBlur() { Image = inputImage, Radius = 5 })
                    using (var resultImage = context.CreateCGImage(filter.OutputImage, inputImage.Extent))
                    {
                        InvokeOnMainThread(() => base.Image = new UIImage(resultImage));
                    }
                });
        }
    }
}

I use this simple subclass of UIImageView in a simple subclass of the default ImageRenderer like so:

public class BlurredImageRenderer : ImageRenderer
{
    protected override void OnElementChanged(ElementChangedEventArgs<Image> e)
    {
        if (Control == null)
        {
            SetNativeControl(new BlurredImageView
                {
                    ContentMode = UIViewContentMode.ScaleAspectFit,
                    ClipsToBounds = true
                });
        }

        base.OnElementChanged(e);
    }

    // ... BlurredImageView class goes here
}

Android Renderer Implementation

For Android the core blurring routine came from this Xamarin recipe, which was found by the forum member making the request. That's easy enough to use, but unfortunately I hit a snag. The default ImageRenderer on Android is really poorly suited for subclassing. It uses an internal custom ImageView subclass, which means the trick I used in iOS for blurring the image as it's applied to the view won't work. Instead I had to basically copy all of the default code.

This is a good time to mention a trick that I've been using to learn how Xamarin.Forms works: The Xamarin Studio Assembly Browser. You can get to it by either using "Go to Declaration" on a class that's in Xamarin.Forms (or some other external assembly) or by finding an assembly in your project's list of references and double-clicking it. Once in the Assembly Browser I always immediately change the "Visibility" dropdown to "All Members" and the "Language" dropdown to "C#". Then I can basically see all of the source code (decompiled) of Xamarin.Forms. Using this technique you can learn a huge amount about how things work under the hood, diagnose bugs, find workarounds, etc.

In this case I needed to know how to reimplement my own version of the ImageRenderer class and insert my own step to blur the image. For that I just basically copied the whole thing.

Of course it's not quite that easy. The base implementation used some internal code that I don't have access to. First, the FormsImageView, which does some kind of optimization for skipping invalidation in some situations. For that I just had to make my own BlurredImageView that does the same thing and update code as necessary to use my view.

Next I had to deal with loading images. As I discovered in this post the actual classes involved in loading platform-specific image from an ImageSource are all public, but the nice convenience method for finding the right implementation and doing the load is internal. Therefore I had to write a quick implementation myself, much like the iOS implementation I had written:

IImageSourceHandler handler;

if (imageSource is FileImageSource)
{
    handler = new FileImageSourceHandler();
}
else if (imageSource is StreamImageSource)
{
    handler = new StreamImagesourceHandler(); // sic
}
else if (imageSource is UriImageSource)
{
    handler = new ImageLoaderSourceHandler(); // sic
}
else
{
    throw new NotImplementedException();
}

var originalBitmap = await handler.LoadImageAsync(imageSource, context);

Now I can load an image, but I still want to blur it. For that I just added to the above method:

var blurredBitmap = await Task.Run(() => CreateBlurredImage(originalBitmap, 25));
return blurredBitmap;

And the CreateBlurredImage method comes from the Xamarin recipe:

private Bitmap CreateBlurredImage(Bitmap originalBitmap, int radius)
{
    // Create another bitmap that will hold the results of the filter.
    Bitmap blurredBitmap;
    blurredBitmap = Bitmap.CreateBitmap(originalBitmap);

    // Create the Renderscript instance that will do the work.
    RenderScript rs = RenderScript.Create(Context);

    // Allocate memory for Renderscript to work with
    Allocation input = Allocation.CreateFromBitmap(rs, originalBitmap, Allocation.MipmapControl.MipmapFull, AllocationUsage.Script);
    Allocation output = Allocation.CreateTyped(rs, input.Type);

    // Load up an instance of the specific script that we want to use.
    ScriptIntrinsicBlur script = ScriptIntrinsicBlur.Create(rs, Android.Renderscripts.Element.U8_4(rs));
    script.SetInput(input);

    // Set the blur radius
    script.SetRadius(radius);

    // Start Renderscript working.
    script.ForEach(output);

    // Copy the output to the blurred bitmap
    output.CopyTo(blurredBitmap);

    return blurredBitmap;
}

The next problem was the use of an internal field of the Image class for setting an otherwise read-only property: the IsLoading property. The only workaround for this, unfortunately, is reflection. For that I added this code:

private static FieldInfo _isLoadingPropertyKeyFieldInfo;

private static FieldInfo IsLoadingPropertyKeyFieldInfo
{
    get
    {
        if (_isLoadingPropertyKeyFieldInfo == null)
        {
            _isLoadingPropertyKeyFieldInfo = typeof(Image).GetField("IsLoadingPropertyKey", BindingFlags.Static | BindingFlags.NonPublic);
        }
        return _isLoadingPropertyKeyFieldInfo;
    }
}

private void SetIsLoading(bool value)
{
    var fieldInfo = IsLoadingPropertyKeyFieldInfo;
    ((IElementController)base.Element).SetValueFromRenderer((BindablePropertyKey)fieldInfo.GetValue(null), value);
}

With that code in place I just had to do some simple substitutions to make the rest of the code compile and run. The rest of the code isn't all that interested so I've left it out, but for the complete source see the GitHub project.

Caveats

The Android implementation is brittle because it duplicates code from the base implementation, and it uses reflection.

That's it! Check out the GitHub project. Enjoy!


  1. I don't know how to do it for Windows Phone or WinRT, but pull requests are welcome! 

2 comments:

  1. Good post! Thanks for sharing! If you want to, I can try to make it work for WP. I think it's important for a control to support all three platforms!

    ReplyDelete
  2. Go ahead! Pull requests are welcome. Note that I can't actually test on Windows Phone, but I will at least try to review your code for consistency with the style and correctness of the Xamarin API usage. I look forward to adding that!

    ReplyDelete