Android app meta tags

Integration Guide

Published on Tuesday, February 4, 2020 • Updated on Tuesday, February 4, 2020

André is a Developer Relations Engineer

Setting up a Trusted Web Activity doesn’t require developers to author Java code, but Android Studio is required. This guide was created using Android Studio 3.3. Check the docs on how to install it.

# Create a Trusted Web Activity Project

When using Trusted Web Activities, the project must target API 16 or higher.

Note: This section will guide you on setting up a new project on Android Studio. If you are already familiar with the tool feel free to skip to the Getting the Trusted Web Activity Library section.

Open Android Studio and click on Start a new Android Studio project.

Android Studio will prompt to choose an Activity type. Since Trusted Web Activities use an Activity provided by support library, choose Add No Activity and click Next.

Next step, the wizard will prompt for configurations for the project. Here’s a short description of each field:

• Name: The name that will be used for your application on the Android Launcher.
• Package Name: An unique identifier for Android Applications on the Play Store and on Android devices. Check the documentation for more information on requirements and best practices for creating package names for Android apps.
• Save location: Where Android Studio will create the project in the file system.
• Language: The project doesn’t require writing any Java or Kotlin code. Select Java, as the default.
• Minimum API Level: The Support Library requires at least API Level 16. Select API 16 any version above.

Leave the remaining checkboxes unchecked, as we will not be using Instant Apps or AndroidX artifacts, and click Finish.

# Get the Trusted Web Activity Support Library

To setup the Trusted Web Activity library in the project you will need to edit the Application build file. Look for the Gradle Scripts section in the Project Navigator. There are two files called build.gradle , which may be a bit confusing and the descriptions in parenthesis help identifying the correct one.

The file we are are looking for is the one with module Module next to its name.

The Trusted Web Activities library uses Java 8 features and the first change enables Java 8. Add a compileOptions section to the bottom of the android section, as below:

The next step will add the Trusted Web Activity Support Library to the project. Add a new dependency to the dependencies section:

Android Studio will show prompt asking to synchronize the project once more. Click on the Sync Now link and synchronize it.

# Launch the Trusted Web Activity

Setting up the Trusted Web Activity is achieved by editing the Android App Manifest.

On the Project Navigator, expand the app section, followed by the manifests and double click on AndroidManifest.xml to open the file.

Since we asked Android Studio not to add any Activity to our project when creating it, the manifest is empty and contains only the application tag.

Add the Trusted Web Activity by inserting an activity tag into the application tag:

The tags added to the XML are standard Android App Manifest. There are two relevant pieces of information for the context of Trusted Web Activities:

1. The meta-data tag tells the Trusted Web Activity which URL it should open. Change the android:value attribute with the URL of the PWA you want to open. In this example, it is https://airhorner.com .
2. The second intent-filter tag allows the Trusted Web Activity to intercept Android Intents that open https://airhorner.com . The android:host attribute inside the data tag must point to the domain being opened by the Trusted Web Activity.

Note: When running the project at this stage, the URL Bar from Custom Tabs will still show on the top of the screen. This is not a bug.

The next section will show how to setup Digital AssetLinks to verify relationship between the website and the app, and remove the URL bar.

# Remove the URL bar

Trusted Web Activities require an association between the Android application and the website to be established to remove the URL bar.

This association is created via Digital Asset Links and the association must be established in both ways, linking from the app to the website and from the website to the app.

It is possible to setup the app to website validation and setup Chrome to skip the website to app validation, for debugging purposes.

# Establish an association from app to the website

Open the string resources file app > res > values > strings.xml and add the Digital AssetLinks statement below:

Change the contents for the site attribute to match the schema and domain opened by the Trusted Web Activity.

Back in the Android App Manifest file, AndroidManifest.xml , link to the statement by adding a new meta-data tag, but this time as a child of the application tag:

We have now established a relationship from the Android application to the website. It is helpful to debug this part of the relationship without creating the website to application validation.

Here’s how to test this on a development device:

# Enable debug mode

1. Open Chrome on the development device, navigate to chrome://flags , search for an item called Enable command line on non-rooted devices and change it to ENABLED and then restart the browser.
2. Next, on the Terminal application of your operating system, use the Android Debug Bridge (installed with Android Studio), and run the following command:

Close Chrome and re-launch your application from Android Studio. The application should now be shown in full-screen.

Note: It may needed to force close Chrome so it restarts with the correct command line. Go to Android Settings > Apps & notifications > Chrome, and click on Force stop.

# Establish an association from the website to the app

There are 2 pieces of information that the developer needs to collect from the app in order to create the association:

• Package Name: The first information is the package name for the app. This is the same package name generated when creating the app. It can also be found inside the Module build.gradle , under Gradle Scripts > build.gradle (Module: app), and is the value of the applicationId attribute.
• SHA-256 Fingerprint: Android applications must be signed in order to be uploaded to the Play Store. The same signature is used to establish the connection between the website and the app through the SHA-256 fingerprint of the upload key.

The Android documentation explains in detail how to generate a key using Android Studio. Make sure to take note the path, alias and passwords for the key store, as you will need it for the next step.

Extract the SHA-256 fingerprint using the keytool, with the following command:

The value for the SHA-256 fingerprint is printed under the Certificate fingerprints section. Here’s an example output:

With both pieces of information at hand, head over to the assetlinks generator, fill-in the fields and hit Generate Statement. Copy the generated statement and serve it from your domain, from the URL /.well-known/assetlinks.json .

Note: The AssetLinks file must be under /.well-known/assetlinks.json , at the root of the domain, as that’s only the place Chrome will look for it.

# Creating an Icon

When Android Studio creates a new project, it will come with a default Icon. As a developer, you will want to create your own icon and differentiate your application from others on the Android Launcher.

Android Studio contains the Image Asset Studio, which provides the tools necessary to create the correct icons, for every resolution and shape your application needs.

Inside Android Studio, navigate to File > New > Image Asset , select Launcher Icons (Adaptative and Legacy) and follow the steps from the Wizard. to create a custom icon for the application.

# Generating a signed APK

With the assetlinks file in place in your domain and the asset_statements tag configured in the Android application, the next step is generating a signed app. Again, the steps for this are widely documented.

The output APK can be installed into a test device, using adb:

If the verification step fails it is possible to check for error messages using the Android Debug Bridge, from your OS’s terminal and with the test device connected.

With the upload APK generated, you can now upload the app to the Play Store.

# Adding a Splash Screen

Starting on Chrome 75, Trusted Web Activities have support for Splash Screens. The Splash Screen can be set up by adding a few new image files and configurations to the project.

Make sure to update to Chrome 75 or above and use the latest version of Trusted Web Activity Support Library.

# Generating the images for the Splash Screen

Android devices can have different screen sizes and pixel densities. To ensure the Splash Screen looks good on all devices, you will need to generate the image for each pixel density.

A full explanation of display-independent pixels (dp or dip) is beyond the scope of this article, but one example would be to create an image that is 320x320dp, which represents a square of 2×2 inches on a device screen of any density and is equivalent to 320×320 pixels at the mdpi density.

From there we can derive the sizes needed for other pixel densities. Below is a list with the pixel densities, the multiplier applied to the base size (320x320dp), the resulting size in pixels and the location where the image should be added in the Android Studio project.

Density	Multiplier	Size	Project Location
mdpi (baseline)	1.0x	320×320 px	/res/drawable-mdpi/
ldpi	0.75x	240×240 px	/res/drawable-ldpi/
hdpi	1.5x	480×480 px	/res/drawable-hdpi/
xhdpi	2.0x	640×640 px	/res/drawable-xhdpi/
xxhdpi	3.0x	960×960 px	/res/drawable-xxhdpi/
xxxhdpi	4.0x	1280×1280 px	/res/drawable-xxxhdpi/

Note: An alternative to creating all the images sizes is to use one Vector Drawable. The Vector Asset Studio offers tools to help developers to transform SVGs into Android Vector Drawables.

# Updating the application

With the images for the splash screen generated, it’s time to add the necessary configurations to the project.

First, add a content-provider to the Android Manifest ( AndroidManifest.xml ).

Note: Make sure to change the android:authorities attribute when creating the provider, as two applications cannot have the same authority on a device.

Then, add res/xml/filepaths.xml resource, and specify the path to the twa splash screen:

Finally, add meta-tags to the Android Manifest to customize the LauncherActivity:

Ensure that the value of the android.support.customtabs.trusted.FILE_PROVIDER_AUTHORITY tag matches the value defined of the android:authorities attribute inside the provider tag.

# Making the LauncherActivity transparent

Additionally, make sure the LauncherActivity is transparent to avoid a white screen showing before the splash by setting a translucent theme for the LauncherActivity:

We are looking forward to see what developers build with Trusted Web Activities. To drop any feedback, reach out to us at @ChromiumDev.

Last updated: Tuesday, February 4, 2020 Improve article

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Android app meta tags

A utility library for Android designed to simplify reading meta-data values from AndroidManifest.xml.

• Quickly access a meta-data values from anywhere with few lines of code.
• Read individual meta-data values into type-specific variables.
• Instantiate objects from meta-data values.
• Read one or more meta-data values into annotated Java classes.
• Use meta-data values to pass configuration parameters to third-party libraries.

Here is the quickest and easiest way to load the metadata from AndroidManifest.xml and get a value. The value, by default, is a String value type.

If the value is not a String type, then you can provide a hint:

You can even directly load a resource from the metadata:

In some cases, you may need to provide additional information about the resource type since different resources types can have the same Java type:

Using Annotations to Load Metadata

Here is the simplest example of using an annotation to define what meta-data value in AndroidManifest.xml it should be initialized with:

In the example above, the field message will be initialized with the value of meta-data tag named my.message. You initialize all values with the @Metadata annotation using a single line of code:

This method will auto-detect the target type, and then assign the value. If the field is not assignable using the meta-data’s value, then an exception will be thrown.

Reading from a Resource

In some cases, you will want to read the value from a resource (i.e., you use android:resource in the meta-data tag). You can use the @Metadata annotation to read resource values as well:

Giving Resource Type Hints

There are some resources that have the same field type, such as integer and color. This makes it hard to auto-detect the resources type. We can therefore provide a hint as follows:

In the example above, the value for backgroundColor will be loaded from resources and interpreted as a color.

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