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Extending Xamarin.Forms.Nuke on iOS to load a placeholder for images that fail to load

msicc profile image Marco Siccardi Originally published at msicc.net on ・4 min read

What is Xamarin.Forms.Nuke?

Xamarin.Forms.Nuke is a Xamarin.Forms implementation of Nuke, one of the most advanced image libraries on iOS today. The Xamarin.Forms implementation focuses heavily on caching only, while the original library has a bunch of more features. I learned about this library when I started to look for an alternative to cache images via Akavache which I used before (I never blogged about that part because it wasn’t ready for that, tbh).

Why do we need to extend the library?

The library currently does only one thing (pretty well) – handling the caching of web images. It uses the default settings of the native Nuke library. The Xamarin.Forms implementation overwrites the ImageSourceHandler for UriImageSource and FileImageSource (optionally), but the case of placeholder loading is not intended in the original version. As I have multiple scenarios where a placeholder comes in handy, I decided to extend the library – and maintain my own fork of it from now on. (Maybe there will be a pull request to the original source, too).

Show me some code, finally!

For our extension, we modify the FormsHandler class as well as the ImageSourceHandler class. Let’s have a look at the FormsHandler class first. We are adding a new property for the placeholder:

public static ImageSource PlaceholderImageSource { get; private set; }
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With that property in place, let’s add two methods. One method is for loading a placeholder image from an embedded resource file, while the second one is for specifying a FontImageSource.

public static void PlaceholderFromResource(string resourceName, Assembly assembly) =>
    PlaceholderImageSource = ImageSource.FromResource(resourceName, assembly);

public static void PlaceholderFromFontImageSource(FontImageSource fontImageSource) =>
    PlaceholderImageSource = fontImageSource;
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I have chosen the FormsHandler class because setting the placeholder is a global thing (in my scenarios, your mileage may vary). That’s already everything in takes in the FormsHandler class, so let’s have a look at the ImageSourceHandler class.

As we are using the default Xamarin.Forms ImageSourceHandler for the resource image (which is a StreamImageSource) and the FontImageSource, we need to add the static fields for them first:

private static readonly StreamImagesourceHandler DefaultStreamImageSourceHandler = new StreamImagesourceHandler();

private static readonly FontImageSourceHandler DefaultFontImageSourcehandler = new FontImageSourceHandler();
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Now let’s implement the loading of the placeholder in a separate method:

private static Task<UIImage> LoadPlaceholderAsync()
{
    switch (FormsHandler.PlaceholderImageSource)
    {
        case StreamImageSource streamImageSource:
            FormsHandler.Warn($"loading placeholder from resource");
            return DefaultStreamImageSourceHandler.LoadImageAsync(streamImageSource);

        case FontImageSource fontImageSource:
            FormsHandler.Warn($"loading placeholder from Font");
            return DefaultFontImageSourcehandler.LoadImageAsync(fontImageSource);
        default:
            FormsHandler.Warn($"no valid placeholder found");
            return null;
    }
}
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As you can see, there is nothing overly complicated in this method. Based on the type of the placeholder set in the FormsHandler class, we are calling the default Xamarin.Forms implementation for the placeholder image. Let’s take this code into action by changing the LoadImageAsync method of the ImageSourceHandler:

public async Task<UIImage> LoadImageAsync(
    ImageSource imageSource,
    CancellationToken cancellationToken = new CancellationToken(),
    float scale = 1)
{
    var result = await NukeHelper.LoadViaNuke(imageSource, cancellationToken, scale);
    if (result == null)
        result = await LoadPlaceholderAsync();

    return result;
}
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As we need to know if the Nukehelper class is able to load the image, we are already running the code by awaiting it at this level. If the result is null, we are loading the placeholder image via our prior implemented method. That’s all we need to do in our forked Xamarin.Forms.Nuke repository.

How to use it in your Xamarin.Forms – iOS project

First, clone my fork (or fork it if you want) of the Xamarin.Forms.Nuke repository and import it into your Xamarin.Forms solution and reference it in your iOS project. Once that is done, we need to initialize the Nuke library (like in the original source) in the AppDelegate‘s FinishedLaunching method:

Xamarin.Forms.Nuke.FormsHandler.Init(true, false);
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The second step is to define the placeholder image source. The FontImageSource should be defined after the LoadApplication method. This way, you can the Xamarin.Forms way of loading the font as a resource.

//Resource image
Xamarin.Forms.Nuke.FormsHandler.PlaceholderFromResource("CachedImageTest.MSicc_Logo_Base_Blue_1024px_pad25.png", Assembly.GetAssembly(typeof(MainViewModel)));

//FontImageSource
Xamarin.Forms.Nuke.FormsHandler.PlaceholderFromFontImageSource(new FontImageSource
{
    Glyph = CachedImageTest.Resources.MaterialDesignIcons.ImageBroken,
    FontFamily = "MaterialDesignIcons",
    Color = Color.Red
});
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Now use the Xamarin.Forms Image control like you always do. If the image from the web cannot be loaded, you will see the placeholder, like in these two examples:

cached image placeholder sample
Left: Single image failed loading, right image in CollectionView failed loading

With a few additions to the Xamarin.Forms.Nuke library, we have implemented a placeholder mechanism for images that can’t be loaded. As always, I hope this post will be useful for some of you. Now that I have the iOS implementation of a fast cached image with placeholder loading in place, I will turn to Android, where I will try to achieve the same using the Glidex.Forms library and extend it to load a placeholder. There will be a full sample once that is implemented as well. Stay tuned for that one!

Until the next post, happy coding, everyone!

The post Extending Xamarin.Forms.Nuke on iOS to load a placeholder for images that fail to load appeared first on MSicc's Blog.

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