Android asset packs

Assets such as video, audio, or graphics can occupy a lot of space inside your app package. For example, consider the following files in an example project:

MyProject.csproj
MainActivity.cs
Assets/
    MyLargeAsset.mp4
    MySmallAsset.json
AndroidManifest.xml

MyLargeAsset.mp4 and MySmallAsset.json will both be placed into the Android App Bundle (AAB) package when the app is published. However, if MyLargeAsset.mp4 is greater than 200Mb it can't be placed into the main app bundle due to the package size restrictions enforced by Google Play.

.NET for Android 9 introduces the ability to place assets into separate packages, known as asset packs. This enables you to upload apps that would normally be larger than the basic package size allowed by Google Play. By putting these assets into a separate package you gain the ability to upload a package which is up to 2Gb in size, rather than the basic package size of 200Mb.

Asset pack metadata

To support asset packages in .NET for Android apps, the AndroidAsset item group supports two new metadata attributes:

• AssetPack. If present, this attribute controls which asset pack an asset is placed into. If not present, the asset will be placed into the main app bundle.

  The name of the asset pack will be $(AndroidPackage).%(AssetPack). Therefore, if your package name is com.mycompany.myproject and the AssetPack value is myassets, the asset pack name would be com.mycompany.myproject.myassets.

  Note

  The AssetPack value can also be base, which indicates that the asset should be placed into the main app bundle rather than an asset pack. For more information, see Force an asset into the main app bundle.

• DeliveryType. If present, this attribute controls the type of asset pack is created. Valid values for this are InstallTime, FastFollow, and OnDemand. If not present, the default value is InstallTime. For more information, see Asset delivery options.

Returning to the previous example, MyLargeAsset.mp4 can be moved into its own asset pack by specifying the following <ItemGroup> element in the project's .csproj file:

<ItemGroup>
    <AndroidAsset Update="Assets/MyLargeAsset.mp4" AssetPack="myassets" />
</ItemGroup>

This item group will cause the .NET for Android build system to create a new asset pack called myassets and include MyLargeAsset.mp4 in the pack. The pack will automatically be included in the AAB file. There's no need to specify the delivery type, provided that the default value of InstallTime is the required delivery option.

In scenarios where you have a large number of assets you can use wildcards to update the auto-imported assets:

<ItemGroup>
    <AndroidAsset Update="Assets/*" AssetPack="myassets" />
</ItemGroup>

In this example, all of the assets in the Assets folder will be placed into an asset pack named myassets.

Force an asset into the main app bundle

In scenarios where you have a large number of assets, but you don't want all of them to be placed into an asset pack, you can specify an AssetPack value of base to force an asset into the main app bundle:

<ItemGroup>
    <AndroidAsset Update="Assets/*" AssetPack="myassets" />
    <AndroidAsset Update="Assets/myimportantfile.json" AssetPack="base" />    
</ItemGroup>

In this example, all of the assets in the Assets folder except myimportantfile.json will be placed into an asset pack named myassets. However, myimportantfile.json is placed into the main app bundle via the AssetPack="base" value, rather than the myassets asset pack.

Asset delivery options

Asset packs can have different delivery options, which control when your assets will install on the device:

• Install time packs are installed at the same time as the app. This pack type can be up to 1Gb in size, but you can only have one of them. This delivery type is specified with InstallTime metadata.
• Fast follow packs will install at some point shortly after the app has finished installing. The app will be able to start while this type of pack is being installed so you should check it has finished installing before trying to use the assets. This pack type can be up to 512Mb in size. This delivery type is specified with FastFollow metadata.
• On demand packs will never be downloaded to the device unless the app specifically requests it. This delivery type is specified with OnDemand metadata.

For more information about asset delivery, see Play Asset Delivery on developer.android.com.

Asset packs in .NET MAUI apps

.NET MAUI apps define assets via the MauiAsset build action. An asset pack can be specified via the AssetPack attribute:

<MauiAsset
    Include="Resources\Raw\**"
    LogicalName="%(RecursiveDir)%(Filename)%(Extension)"
    AssetPack="myassetpack" />

If you have specific items you want to place in an asset pack you can use the Update attribute to define the AssetPack metadata:

<MauiAsset Update="Resources\Raw\myvideo.mp4" AssetPack="myassets" />

In .NET MAUI apps, the delivery type can be specified with the DeliveryType attribute on a MauiAsset:

<MauiAsset Update="Resources\Raw\myvideo.mp4" AssetPack="myassets" DeliveryType="FastFollow" />

Check the status of FastFollow asset packs

If your app uses a FastFollow asset pack, it'll need to check that the pack is installed before trying to access its contents.

To do this, add the Xamarin.Google.Android.Play.Asset.Delivery NuGet package to your project. This NuGet package provides access to the AssetPackManager type, which provides the ability to query the location of the asset pack:

using Xamarin.Google.Android.Play.Core.AssetPacks;

var assetPackManager = AssetPackManagerFactory.GetInstance(this);
AssetPackLocation assetPackPath = assetPackManager.GetPackLocation("myfastfollowpack");
string assetsFolderPath = assetPackPath?.AssetsPath() ?? null;
if (assetsFolderPath is null)
{
    // FastFollow asset pack isn't installed.
}

The location of the FastFollow asset pack is queried with the GetPackLocation method, which returns an AssetPackLocation object. If the AssetsPath method returns null for the AssetPackLocation object this indicates that the pack hasn't yet been installed. If the AssetsPath method returns a value, it represents the install location of the pack.

Download OnDemand asset packs

If you app uses an OnDemand asset pack, it'll need to download it manually. To do this, add the Xamarin.Google.Android.Play.Asset.Delivery NuGet package to your project. This NuGet package provides access to the AssetPackStateUpdateListener type, that enables you to monitor the progress of the download. However, in .NET this type is wrapped by the AssetPackStateUpdateListenerWrapper type, which can be used to register an event handler to monitor the download progress.

To monitor the download progress you'll need to declare some fields:

using Xamarin.Google.Android.Play.Core.AssetPacks;

IAssetPackManager assetPackManager;
AssetPackStateUpdateListenerWrapper listener;

Then, declare an event handler to monitor the download:

using Xamarin.Google.Android.Play.Core.AssetPacks.Model;

void Listener_StateUpdate(object? sender, AssetPackStateUpdateListenerWrapper.AssetPackStateEventArgs e)
{
    var status = e.State.Status();
    switch (status)
    {
        case AssetPackStatus.Downloading:
            long downloaded = e.State.BytesDownloaded();
            long totalSize = e.State.TotalBytesToDownload();
            double percent = 100.0 * downloaded / totalSize;
            Android.Util.Log.Info ("Listener_StateUpdate", $"Downloading {percent}");
            break;
        case AssetPackStatus.Completed:
            break;
        case AssetPackStatus.WaitingForWifi:
                assetPackManager.ShowConfirmationDialog(this);
            break;
    }
}

For information about which AssetPackStatus values to use, see AssetPackStatus on developer.android.com.

Then, create an IAssetPackManager instance via the AssetPackManagerFactory.GetInstance method and register the Listener_StateUpdate event handler against an AssetPackStateUpdateListenerWrapper object:

assetPackManager = AssetPackManagerFactory.GetInstance (this);
listener = new AssetPackStateUpdateListenerWrapper();
listener.StateUpdate += Listener_StateUpdate;

You'll also need to register the listener when the app resumes, and unregister the listener when the app pauses:

protected override void OnResume()
{
    assetPackManager.RegisterListener(listener.Listener);
    base.OnResume();
}

protected override void OnPause()
{
    assetPackManager.UnregisterListener(listener.Listener);
    base.OnPause();
}

Finally, before you download the OnDemand asset pack you'll need to check if it's already been installed. This can be accomplished with the AssetPackManager.GetPackLocation method:

using Android.Gms.Extensions;

var assetPackPath = assetPackManager.GetPackLocation ("myondemandpack");
string assetsFolderPath = assetPackPath?.AssetsPath() ?? null;
if (assetsFolderPath is null)
{
    await assetPackManager.Fetch(new string[] { "myondemandpack" }).AsAsync<AssetPackStates>();
}

The location of the OnDemand asset pack is queried with the GetPackLocation method, which returns an AssetPackLocation object. If the AssetsPath method returns null for the AssetPackLocation object this indicates that the pack hasn't yet been installed. assetPackManager.Fetch can then be called to start the download. If the AssetsPath method returns a value, it represents the install location of the pack.

The status of the download can then be monitored via the AssetPackStateUpdateListenerWrapper.

Test asset packs locally

By default, .NET for Android uses the Android Application Package (APK) format for debugging. However, to test asset packs locally you'll need to ensure you're using the Android App Bundle (AAB) package format. To debug your asset packs update your .csproj file with the following build properties:

<PropertyGroup Condition="'$(Configuration)' == 'Debug'">
    <AndroidPackageFormat>aab</AndroidPackageFormat>
    <EmbedAssembliesIntoApk>true</EmbedAssembliesIntoApk>
    <AndroidBundleToolExtraArgs>--local-testing</AndroidBundleToolExtraArgs>
</PropertyGroup>

The --local-testing flag is required for testing on your local device. It tells the bundletool app that all the asset packs should be installed in a cached location on the device. It also sets up the IAssetPackManager to use a mock downloader that will use the cache. This enables you to test installing OnDemand and FastFollow asset packs in a Debug environment.