Passing Information to a Trusted Web Activity using Query Parameters

When using Trusted Web Activity in their applications, developers may need to pass information from the native part of the application into the Progressive Web App (PWA).

A common use-case for this is implementing custom analytics segmentations to measure installations and sessions started from the Trusted Web Activity. Query parameters can be added to the launch URL to implement this.

Modifying the start URL

If the parameter being passed to the PWA will remain the same across users and launches, the parameter can be appended directly to the launch URL. An example of this usage is when developers want to measure the number of navigation sessions created from a Trusted Web Activity.

Using llama-pack

Llama Pack 🦙 is a tool created to helps developers to creating a Project for an Android application that launches an existing PWAs using a Trusted Web Activity. It contains both a library and a Command Line Interface (CLI).

Creating a new project:

When using the llama-pack CLI, a project is initialized with the init command, and creates default values from a Web Manifest, provided as a parameter:

llama-pack init --manifest https://material.money/manifest.json

The wizard will use the start_url from the Web Manifest as default and will ask users to confirm the value, giving developers the change to add extra parameters to the url used to start the Progressive Web App.

Modifying an existing project

When llama-pack generates a project, information for that particular project is stored in a file called twa-manifest.json, in the project folder. To modify the start url for existing project, developers need to modify the file:

{
  ...

  "startUrl": "/?utm_source=trusted-web-activity",
  ...

}

Then, re-generate the project files and apply the new start URL

llama-pack update

Using Android Studio

When using Android Studio and the default LauncherActivity, the startUrl is defined as a meta tag inside AndroidManifest.xml, and we can change the URL used to launch the Trusted Web Activity by modifying it:

<activity android:name="com.google.androidbrowserhelper.trusted.LauncherActivity"
    android:label="@string/app_name">
    ...
    <meta-data android:name="android.support.customtabs.trusted.DEFAULT_URL"
        android:value="https://svgomg.firebaseapp.com/?utm_source=trusted-web-activity" />
    ...
</activity>

Note: llama-pack takes care of ensuring the URLs across the application match the same origin and, therefore, uses a relative for start URL. When modifying AndroidManifest.xml, the entire URL, including schema and domain must be used.

Modifying the start URL dynamically

In other cases, developers may want to create parameters that change across users or sessions, for instance. In most cases, this will involve collecting details from the Android side of the application to pass it to the Progressive Web App.

Step 1: Create a custom LauncherActivity:

public class CustomQueryStringLauncherActivity extends LauncherActivity {
    private String getDynamicParameterValue() {
        return String.valueOf((int)(Math.random() * 1000));
    }

    @Override
    protected Uri getLaunchingUrl() {
        // Get the original launch Url.
        Uri uri = super.getLaunchingUrl();

        // Get the value we want to use for the parameter value
        String customParameterValue = getDynamicParameterValue();

        // Append the extra parameter to the launch Url
        return uri
                .buildUpon()
                .appendQueryParameter("my_parameter", customParameterValue)
                .build();
    }
}

Step 2: Modify the AndroidManifest.xml to use the custom LauncherActivity

<activity android:name="com.myapp.CustomQueryStringLauncherActivity"
    android:label="@string/app_name">
    ...
    <meta-data android:name="android.support.customtabs.trusted.DEFAULT_URL"
        android:value="https://squoosh.app/?utm_source=trusted-web-activity" />
    ...
</activity>

Note: llama-pack doesn’t support dynamically generating query parameters at this moment. We’re interested in hearing from developers who have the need for this feature. Check out the llama-pack issue tracker and tell us about your use-case.

Conclusion

Passing information from the native part to the web part of an application can be achieved by using query parameters. When a parameter is added to the query string, it will be accessible to scripts running on the page and may also be part of the referral when users navigate to a different page or the developer implements a share action.

Developers must be aware of those implications, and can mitigate them using link rel=noreferrer or cleaning-up the URL using the page location API.

The Trusted Web Activity protocol doesn’t currently provide a mechanism to exchange messages with the native part of the application after the web part is invoked.

We believe existing or upcoming Web Platform APIs enable most use cases needed by developers. If you are looking for new or upcoming Web APIs, check out the New Capabilities status page.

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