android_dynamic_feature_modules.md

App Bundles and Dynamic Feature Modules (DFMs)

[TOC]

About Bundles

Android App bundles is a Play Store feature that allows packaging an app as multiple .apk files, known as "splits". Bundles are zip files with an .aab extension. See android_build_instructions.md#multiple-chrome-targets for a list of buildable bundle targets.

Bundles provide three main advantages over monolithic .apk files:

1. Language resources are split into language-specific .apk files, known as "resource splits". Delivering only the active languages reduces the overhead of UI strings.
   • Resource splits can also be made on a per-screen-density basis (for drawables), but Chrome has not taken advantage of this (yet).
2. Features can be packaged into lazily loaded .apk files, known as "feature splits". Chrome enables isolated splits, which means feature splits have no performance overhead until used (on Android O+ at least).
3. Feature splits can be downloaded on-demand, saving disk space for users that do not need the functionality they provide. These are known as "Dynamic feature modules", or "DFMs".
   • E.g. Chrome's VR support is packaged in this way, via the vr module.

You can inspect which .apk files are produced by a bundle target via:

out/Default/bin/${target_name} build-bundle-apks --output-apks foo.apks
unzip -l foo.apks

*** note Adding new features vis feature splits is highly encouraged when it makes sense to do so:

• Has a non-trivial amount of Dex (>50kb)
• Not needed on startup
• Has a small integration surface (calls into it must be done with reflection)
• Not used by WebView (WebView does not support DFMs)

---

Declaring App Bundles with GN Templates

Here's an example that shows how to declare a simple bundle that contains a single base module, which enables language-based splits:

  android_app_bundle_module("foo_base_module") {
    # Declaration are similar to android_apk here.
    ...
  }

  android_app_bundle("foo_bundle") {
    base_module_target = ":foo_base_module"

    # The name of our bundle file (without any suffix).
    bundle_name = "FooBundle"

    # Enable language-based splits for this bundle. Which means that
    # resources and assets specific to a given language will be placed
    # into their own split APK in the final .apks archive.
    enable_language_splits = true

    # Proguard settings must be passed at the bundle, not module, target.
    proguard_enabled = !is_java_debug
  }

When generating the foo_bundle target with Ninja, you will end up with the following:

• The bundle file under out/Release/apks/FooBundle.aab

• A helper script called out/Release/bin/foo_bundle, which can be used to install / launch / uninstall the bundle on local devices.

  This works like an APK wrapper script (e.g. foo_apk). Use --help to see all possible commands supported by the script.

The remainder of this doc focuses on DFMs.

Declaring Dynamic Feature Modules (DFMs)

This guide walks you through the steps to create a DFM called Foo and add it to the Chrome bundles.

*** note Note: To make your own module you'll essentially have to replace every instance of foo/Foo/FOO with your_feature_name/YourFeatureName/ YOUR_FEATURE_NAME.

---

Reference DFM

In addition to this guide, the Test Dummy module serves as an actively-maintained reference DFM. Test Dummy is used in automated bundle testing, and covers both Java and native code and resource usage.

Create DFM target

DFMs are APKs. They have a manifest and can contain Java and native code as well as resources. This section walks you through creating the module target in our build system.

First, create the file //chrome/android/modules/foo/internal/java/AndroidManifest.xml and add:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:dist="http://schemas.android.com/apk/distribution"
    featureSplit="foo">

    <!-- dist:onDemand="true" makes this a separately installed module.
         dist:onDemand="false" would always install the module alongside the
         rest of Chrome. -->
    <dist:module
        dist:onDemand="true"
        dist:title="@string/foo_module_title">
        <!-- This will fuse the module into the base APK if a system image
             APK is built from this bundle. -->
        <dist:fusing dist:include="true" />
    </dist:module>

    <!-- Remove android:hasCode="false" when adding Java code. -->
    <application android:hasCode="false" />
</manifest>

Next, create a descriptor configuring the Foo module. To do this, create //chrome/android/modules/foo/foo_module.gni and add the following:

foo_module_desc = {
  name = "foo"
  android_manifest =
      "//chrome/android/modules/foo/internal/java/AndroidManifest.xml"
}

Then, add the module descriptor to the appropriate descriptor list in //chrome/android/modules/chrome_feature_modules.gni, e.g. the Chrome Modern list:

import("//chrome/android/modules/foo/foo_module.gni")
...
chrome_modern_module_descs += [ foo_module_desc ]

The next step is to add Foo to the list of feature modules for UMA recording. For this, add foo to the AndroidFeatureModuleName in //tools/metrics/histograms/histograms.xml:

<histogram_suffixes name="AndroidFeatureModuleName" ...>
  ...
  <suffix name="foo" label="Super Duper Foo Module" />
  ...
</histogram_suffixes>

See below for what metrics will be automatically collected after this step.

Lastly, give your module a title that Chrome and Play can use for the install UI. To do this, add a string to //chrome/browser/ui/android/strings/android_chrome_strings.grd:

...
<message name="IDS_FOO_MODULE_TITLE"
  desc="Text shown when the Foo module is referenced in install start, success,
        failure UI (e.g. in IDS_MODULE_INSTALL_START_TEXT, which will expand to
        'Installing Foo for Chrome…').">
  Foo
</message>
...

*** note Note: This is for module title only. Other strings specific to the module should go in the module, not here (in the base module).

---

Congrats! You added the DFM Foo to Monochrome. That is a big step but not very useful so far. In the next sections you'll learn how to add code and resources to it.

Building and installing modules

Before we are going to jump into adding content to Foo, let's take a look on how to build and deploy the Monochrome bundle with the Foo DFM. The remainder of this guide assumes the environment variable OUTDIR is set to a properly configured GN build directory (e.g. out/Debug).

To build and install the Monochrome bundle to your connected device, run:

$ autoninja -C $OUTDIR monochrome_public_bundle
$ $OUTDIR/bin/monochrome_public_bundle install -m foo

This will install the Foo module, the base module, and all modules with an AndroidManifest.xml that:

• Sets <module dist:onDemand="false">, or
• Has <dist:delivery> conditions that are satisfied by the device being installed to.

*** note Note: The install script may install more modules than you specify, e.g. when there are default or conditionally installed modules (see below for details).

---

You can then check that the install worked with:

$ adb shell dumpsys package org.chromium.chrome | grep splits
>   splits=[base, config.en, foo]

Then try installing the Monochrome bundle without your module and print the installed modules:

$ $OUTDIR/bin/monochrome_public_bundle install
$ adb shell dumpsys package org.chromium.chrome | grep splits
>   splits=[base, config.en]

*** note The wrapper script's install command does approximately:

java -jar third_party/android_build_tools/bundletool/bundletool.jar build-apks --output tmp.apks ...
java -jar third_party/android_build_tools/bundletool/bundletool.jar install-apks --apks tmp.apks

The install-apks command uses adb install-multiple under-the-hood.

---

Adding Java code

To make Foo useful, let's add some Java code to it. This section will walk you through the required steps.

First, define a module interface for Foo. This is accomplished by adding the @ModuleInterface annotation to the Foo interface. This annotation automatically creates a FooModule class that can be used later to install and access the module. To do this, add the following in the new file //chrome/browser/foo/android/java/src/org/chromium/chrome/browser/foo/Foo.java:

package org.chromium.chrome.browser.foo;

import org.chromium.components.module_installer.builder.ModuleInterface;

/** Interface to call into Foo feature. */
@ModuleInterface(module = "foo", impl = "org.chromium.chrome.browser.FooImpl")
public interface Foo {
    /** Magical function. */
    void bar();
}

Next, define an implementation that goes into the module in the new file //chrome/browser/foo/internal/android/java/src/org/chromium/chrome/browser/foo/FooImpl.java:

package org.chromium.chrome.browser.foo;

import org.chromium.base.Log;

public class FooImpl implements Foo {
    @Override
    public void bar() {
        Log.i("FOO", "bar in module");
    }
}

You can then use this provider to access the module if it is installed. To test that, instantiate Foo and call bar() somewhere in Chrome:

if (FooModule.isInstalled()) {
    FooModule.getImpl().bar();
} else {
    Log.i("FOO", "module not installed");
}

The interface has to be available regardless of whether the Foo DFM is present. Therefore, put those classes into the base module, creating a new public build target in: //chrome/browser/foo/BUILD.gn:

import("//build/config/android/rules.gni")

android_library("java") {
  sources = [
    "android/java/src/org/chromium/chrome/browser/foo/Foo.java",
  ]
  deps = [
    "//components/module_installer/android:module_installer_java",
    "//components/module_installer/android:module_interface_java",
  ]
  annotation_processor_deps =
    [ "//components/module_installer/android:module_interface_processor" ]
}

Then, depend on this target from where it is used as usual. For example, if the caller is in chrome_java in //chrome/android/BUILD.gn:

...
android_library("chrome_java") {
  deps =[
    ...
    "//chrome/browser/foo:java",
    ...
  ]
}
...

The actual implementation, however, should go into the Foo DFM. For this purpose, create a new file //chrome/browser/foo/internal/BUILD.gn and make a library with the module Java code in it:

import("//build/config/android/rules.gni")

android_library("java") {
  # Define like ordinary Java Android library.
  sources = [
    "android/java/src/org/chromium/chrome/browser/foo/FooImpl.java",
    # Add other Java classes that should go into the Foo DFM here.
  ]
  deps = [
    "//base:base_java",
    # Put other Chrome libs into the classpath so that you can call into them
    # from the Foo DFM.
    "//chrome/browser/bar:java",
    # The module can depend even on `chrome_java` due to factory magic, but this
    # is discouraged. Consider passing a delegate interface in instead.
    "//chrome/android:chrome_java",
    # Also, you'll need to depend on any //third_party or //components code you
    # are using in the module code.
  ]
}

Then, add this new library as a dependency of the Foo module descriptor in //chrome/android/modules/foo/foo_module.gni:

foo_module_desc = {
  ...
  java_deps = [
    "//chrome/browser/foo/internal:java",
  ]
}

Finally, tell Android that your module is now containing code. Do that by removing the android:hasCode="false" attribute from the <application> tag in //chrome/android/modules/foo/internal/java/AndroidManifest.xml. You should be left with an empty tag like so:

...
    <application />
...

Rebuild and install monochrome_public_bundle. Start Chrome and run through a flow that tries to executes bar(). Depending on whether you installed your module (-m foo) "bar in module" or "module not installed" is printed to logcat. Yay!

Adding pre-built native libraries

You can add a third-party native library (or any standalone library that doesn't depend on Chrome code) by adding it as a loadable module to the module descriptor in //chrome/android/moduiles/foo/foo_module.gni:

foo_module_desc = {
  ...
  loadable_modules_32_bit = [ "//path/to/32/bit/lib.so" ]
  loadable_modules_64_bit = [ "//path/to/64/bit/lib.so" ]
}

Adding Chrome native code

Chrome native code may be placed in a DFM. The easiest way to access native feature code is by calling it from Java via JNI. When a module is first accessed, its native library (or potentially libraries, if using a component build), are automatically opened by the DFM framework, and a feature-specific JNI method (supplied by the feature's implementation) is invoked. Hence, a module's Java code may freely use JNI to call module native code.

Using the module framework and JNI to access the native code eliminates concerns with DFM library file names (which vary across build variants), android_dlopen_ext() (needed to open feature libraries), and use of dlsym().

This mechanism can be extended if necessary by DFM implementers to facilitate subsequent native-native calls, by having a JNI-called initialization method create instance of a object or factory, and register it through a call to the base module's native code (DFM native code can call base module code directly).

JNI

Read the jni_generator docs before reading this section.

There are some subtleties to how JNI registration works with DFMs:

• Generated wrapper ClassNameJni classes are packaged into the DFM's dex file
• The class containing the actual native definitions, GEN_JNI.java, is always stored in the base module
• If the DFM is only included in bundles that use implicit JNI registration (i.e. Monochrome and newer), then no extra consideration is necessary
• Otherwise, the DFM will need to provide a generate_jni_registration target that will generate all of the native registration functions

Calling DFM native code via JNI

A linker-assisted partitioning system automates the placement of code into either the main Chrome library or feature-specific .so libraries. Feature code may continue to make use of core Chrome code (eg. base::) without modification, but Chrome must call feature code through a virtual interface (any "direct" calls to the feature code from the main library will cause the feature code to be pulled back into the main library).

Partitioning is explained in Android Native Libraries.

First, build a module native interface. Supply a JNI method named JNI_OnLoad_foo for the module framework to call, in //chrome/android/modules/foo/internal/entrypoints.cc. This method is invoked on all Chrome build variants, including Monochrome (unlike base module JNI).

#include "base/android/jni_generator/jni_generator_helper.h"
#include "base/android/jni_utils.h"
#include "chrome/android/modules/foo/internal/jni_registration.h"

extern "C" {
// This JNI registration method is found and called by module framework code.
JNI_GENERATOR_EXPORT bool JNI_OnLoad_foo(JNIEnv* env) {
  if (!base::android::IsSelectiveJniRegistrationEnabled(env) &&
      !foo::RegisterNonMainDexNatives(env)) {
    return false;
  }
  if (!foo::RegisterMainDexNatives(env)) {
    return false;
  }
  return true;
}
}  // extern "C"

Next, include the module entrypoint and related pieces in the build config at //chrome/android/modules/foo/internal/BUILD.gn:

import("//build/config/android/rules.gni")
import("//chrome/android/modules/buildflags.gni")
...

# Put the JNI entrypoint in a component, so that the component build has a
# library to include in the foo module. This makes things feel consistent with
# a release build.
component("foo") {
  sources = [
    "entrypoints.cc",
  ]
  deps = [
    ":jni_registration",
    "//base",
    "//chrome/browser/foo/internal:native",
  ]

  # Instruct the compiler to flag exported entrypoint function as belonging in
  # foo's library. The linker will use this information when creating the
  # native libraries. The partition name must be <feature>_partition.
  if (use_native_partitions) {
    cflags = [ "-fsymbol-partition=foo_partition" ]
  }
}

# Generate JNI registration for the methods called by the Java side. Note the
# no_transitive_deps argument, which ensures that JNI is generated for only the
# specified Java target, and not all its transitive deps (which could include
# the base module).
generate_jni_registration("jni_registration") {
  targets = [ "//chrome/browser/foo/internal:java" ]
  header_output = "$target_gen_dir/jni_registration.h"
  namespace = "foo"
  no_transitive_deps = true
}

# This group is a convenience alias representing the module's native code,
# allowing it to be named "native" for clarity in module descriptors.
group("native") {
  deps = [
    ":foo",
  ]
}

Now, over to the implementation of the module. These are the parts that shouldn't know or care whether they're living in a module or not.

Add a stub implementation in //chrome/browser/foo/internal/android/foo_impl.cc:

#include "base/logging.h"
#include "chrome/browser/foo/internal/jni_headers/FooImpl_jni.h"

static int JNI_FooImpl_Execute(JNIEnv* env) {
  LOG(INFO) << "Running foo feature code!";
  return 123;
}

And, the associated build config in //chrome/browser/foo/internal/BUILD.gn:

import("//build/config/android/rules.gni")

...

source_set("native") {
  sources = [
    "android/foo_impl.cc",
  ]

  deps = [
    ":jni_headers",
    "//base",
  ]
}

generate_jni("jni_headers") {
  sources = [
    "android/java/src/org/chromium/chrome/browser/foo/FooImpl.java",
  ]
}

With a declaration of the native method on the Java side:

public class FooImpl implements Foo {
    ...

    @NativeMethods
    interface Natives {
        int execute();
    }
}

Finally, augment the module descriptor in //chrome/android/modules/foo/foo_module.gni with the native dependencies:

foo_module_desc = {
  ...
  native_deps = [
    "//chrome/android/modules/foo/internal:native",
    "//chrome/browser/foo/internal:native",
  ]
  load_native_on_get_impl = true
}

If load_native_on_get_impl is set to true then Chrome automatically loads Foo DFM's native libraries and PAK file resources when FooModule.getImpl() is called for the first time. The loading requires Chrome's main native libraries to be loaded. If you wish to call FooModule.getImpl() earlier than that, then you'd need to set load_native_on_get_impl to false, and manage native libraries / resources loading yourself (potentially, on start-up and on install, or on use).

Calling feature module native code from base the module

If planning to use direct native-native calls into DFM code, then the module should have a purely virtual interface available. The main module can obtain a pointer to a DFM-created object or factory (implemented by the feature), and call its virtual methods.

Ideally, the interface to the feature will avoid feature-specific types. If a feature defines complex data types, and uses them in its own interface, then its likely the main library will utilize the code backing these types. That code, and anything it references, will in turn be pulled back into the main library, negating the intent to house code in the DFM.

Therefore, designing the feature interface to use C types, C++ standard types, or classes that aren't expected to move out of Chrome's main library is ideal. If feature-specific classes are needed, they simply need to avoid referencing feature library internals.

Adding Android resources

In this section we will add the required build targets to add Android resources to the Foo DFM.

First, add a resources target to //chrome/browser/foo/internal/BUILD.gn and add it as a dependency on Foo's java target in the same file:

...
android_resources("java_resources") {
  # Define like ordinary Android resources target.
  ...
  custom_package = "org.chromium.chrome.browser.foo"
}
...
android_library("java") {
  ...
  deps = [
    ":java_resources",
  ]
}

To add strings follow steps here to add new Java GRD file. Then create //chrome/browser/foo/internal/android/resources/strings/android_foo_strings.grd as follows:

<?xml version="1.0" encoding="UTF-8"?>
<grit
    current_release="1"
    latest_public_release="0"
    output_all_resource_defines="false">
  <outputs>
    <output
        filename="values-am/android_foo_strings.xml"
        lang="am"
        type="android" />
    <!-- List output file for all other supported languages. See
         //chrome/browser/ui/android/strings/android_chrome_strings.grd for the
         full list. -->
    ...
  </outputs>
  <translations>
    <file lang="am" path="vr_translations/android_foo_strings_am.xtb" />
    <!-- Here, too, list XTB files for all other supported languages. -->
    ...
  </translations>
  <release seq="1">
    <messages fallback_to_english="true">
      <message name="IDS_BAR_IMPL_TEXT" desc="Magical string.">
        impl
      </message>
    </messages>
  </release>
</grit>

Then, create a new GRD target and add it as a dependency on java_resources in //chrome/browser/foo/internal/BUILD.gn:

...
java_strings_grd("java_strings_grd") {
  defines = chrome_grit_defines
  grd_file = "android/resources/strings/android_foo_strings.grd"
  outputs = [
    "values-am/android_foo_strings.xml",
    # Here, too, list output files for other supported languages.
    ...
  ]
}
...
android_resources("java_resources") {
  ...
  deps = [":java_strings_grd"]
  custom_package = "org.chromium.chrome.browser.foo"
}
...

You can then access Foo's resources using the org.chromium.chrome.browser.foo.R class. To do this change //chrome/browser/foo/internal/android/java/src/org/chromium/chrome/browser/foo/FooImpl.java to:

package org.chromium.chrome.browser.foo;

import org.chromium.base.ContextUtils;
import org.chromium.base.Log;
import org.chromium.chrome.browser.foo.R;

public class FooImpl implements Foo {
    @Override
    public void bar() {
        Log.i("FOO", ContextUtils.getApplicationContext().getString(
                R.string.bar_impl_text));
    }
}

Adding non-string native resources

This section describes how to add non-string native resources to Foo DFM. Key ideas:

• The compiled resource file shipped with the DFM is foo_resourcess.pak.
• At run time, native resources need to be loaded before use. Also, DFM native resources can only be used from the Browser process.

Creating PAK file

Two ways to create foo_resourcess.pak (using GRIT) are:

1. (Preferred) Use foo_resourcess.grd to refer to individual files (e.g., images, HTML, JS, or CSS) and assigns resource text IDs. foo_resourcess.pak must have an entry in /tools/gritsettings/resource_ids.spec.
2. Combine existing .pak files via repack rules in GN build files. This is done by the DevUI DFM, which aggregates resources from many DevUI pages.

Loading PAK file

At runtime, foo_resources.pak needs to be loaded (memory-mapped) before any of its resource gets used. Alternatives to do this are:

1. (Simplest) Specify native resources (with native libraries if any exist) to be automatically loaded on first call to FooModule.getImpl(). This behavior is specified via load_native_on_get_impl = true in foo_module_desc.
2. In Java code, call FooModule.ensureNativeLoaded().
3. In C++ code, use JNI to call FooModule.ensureNativeLoaded(). The code to do this can be placed in a helper class, which can also have JNI calls to FooModule.isInstalled() and FooModule.installModule().

Cautionary notes

Compiling foo_resources.pak auto-generates foo_resources.h, which defines textual resource IDs, e.g., IDR_FOO_HTML. C++ code then uses these IDs to get resource bytes. Unfortunately, this behavior is fragile: If IDR_FOO_HTML is accessed before the Foo DFM is (a) installed, or (b) loaded, then runtime error ensues! Some mitigation strategies are as follows:

• (Ideal) Access Foo DFM's native resources only from code in Foo DFM's native libraries. So by the time that IDR_FOO_HTML is accessed, everything is already in place! This isn't always possible; henceforth we assume that IDR_FOO_HTML is accessed by code in the base DFM.
• Before accessing IDR_FOO_HTML, ensure Foo DFM is installed and loaded. The latter can use FooModule.ensureNativeLoaded() (needs to be called from Browser thread).
• Use inclusion of foo_resources.h to restrict availability of IDR_FOO_HTML. Only C++ files dedicated to "DFM-gated code" (code that runs only when its DFM is installed and loaded) should include foo_resources.h.

Associating native resources with DFM

Here are the main GN changes to specify PAK files and default loading behavior for a DFM's native resources:

foo_module_desc = {
  ...
  paks = [ "$root_gen_dir/chrome/browser/foo/internal/foo_resourcess.pak" ]
  pak_deps = [ "//chrome/browser/foo/internal:foo_paks" ]
  load_native_on_get_impl = true
}

Note that load_native_on_get_impl specifies both native libraries and native resources.

Module install

So far, we have installed the Foo DFM as a true split (-m foo option on the install script). In production, however, we have to explicitly install the Foo DFM for users to get it. There are three install options: on-demand, deferred and conditional.

On-demand install

On-demand requesting a module will try to download and install the module as soon as possible regardless of whether the user is on a metered connection or whether they have turned updates off in the Play Store app.

You can use the autogenerated module class to on-demand install the module like so:

FooModule.install((success) -> {
    if (success) {
        FooModule.getImpl().bar();
    }
});

Optionally, you can show UI telling the user about the install flow. For this, add a function like the one below. Note, it is possible to only show either one of the install, failure and success UI or any combination of the three.

public static void installModuleWithUi(
        Tab tab, OnModuleInstallFinishedListener onFinishedListener) {
    ModuleInstallUi ui =
            new ModuleInstallUi(
                    tab,
                    R.string.foo_module_title,
                    new ModuleInstallUi.FailureUiListener() {
                        @Override
                        public void onFailureUiResponse(retry) {
                            if (retry) {
                                installModuleWithUi(tab, onFinishedListener);
                            } else {
                                onFinishedListener.onFinished(false);
                            }
                        }
                    });
    // At the time of writing, shows toast informing user about install start.
    ui.showInstallStartUi();
    FooModule.install(
            (success) -> {
                if (!success) {
                    // At the time of writing, shows infobar allowing user
                    // to retry install.
                    ui.showInstallFailureUi();
                    return;
                }
                // At the time of writing, shows toast informing user about
                // install success.
                ui.showInstallSuccessUi();
                onFinishedListener.onFinished(true);
            });
}

To test on-demand install, "fake-install" the DFM. It's fake because the DFM is not installed as a true split. Instead it will be emulated by play core's --local-testing mode. Fake-install and launch Chrome with the following command:

$ $OUTDIR/bin/monochrome_public_bundle install -f foo
$ $OUTDIR/bin/monochrome_public_bundle launch

When running the install code, the Foo DFM module will be emulated. This will be the case in production right after installing the module. Emulation will last until Play Store has a chance to install your module as a true split. This usually takes about a day. After it has been installed, it will be updated atomically alongside Chrome. Always check that it is installed and available before invoking code within the DFM.

*** note Warning: There are subtle differences between emulating a module and installing it as a true split. We therefore recommend that you always test both install methods.

---

*** note To simplify development, the DevUI DFM (dev_ui) is installed by default, i.e., -m dev_ui is implied by default. This is overridden by:

• --no-module dev_ui, to test error from missing DevUI,
• -f dev_ui, for fake module install.

---

Deferred install

Deferred install means that the DFM is installed in the background when the device is on an unmetered connection and charging. The DFM will only be available after Chrome restarts. When deferred installing a module it will not be faked installed.

To defer install Foo do the following:

FooModule.installDeferred();

Conditional install

Conditional install means the DFM will be installed automatically upon first installing or updating Chrome if the device supports a particular feature. Conditional install is configured in the module's manifest. To install your module on all Daydream-ready devices for instance, your //chrome/android/modules/foo/internal/java/AndroidManifest.xml should look like this:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:dist="http://schemas.android.com/apk/distribution"
    featureSplit="foo">

    <dist:module
      dist:instant="false"
      dist:title="@string/foo_module_title">
      <dist:fusing dist:include="true" />
      <dist:delivery>
        <dist:install-time>
          <dist:conditions>
            <dist:device-feature
              dist:name="android.hardware.vr.high_performance" />
          </dist:conditions>
        </dist:install-time>
        <!-- Allows on-demand or deferred install on non-Daydream-ready
             devices. -->
        <dist:on-demand />
      </dist:delivery>
    </dist:module>

    <application />
</manifest>

You can also specify no conditions to have your module always installed. You might want to do this in order to delay the performance implications of loading your module until its first use (true only on Android O+ where android:isolatedSplits is supported. See go/isolated-splits-dev-guide (googlers only).

Metrics

After adding your module to AndroidFeatureModuleName (see above) we will collect, among others, the following metrics:

• Android.FeatureModules.AvailabilityStatus.Foo: Measures your module's install penetration. That is, the share of users who eventually installed the module after requesting it (once or multiple times).

• Android.FeatureModules.InstallStatus.Foo: The result of an on-demand install request. Can be success or one of several error conditions.

• Android.FeatureModules.UncachedAwakeInstallDuration.Foo: The duration to install your module successfully after on-demand requesting it.

chrome_public_apk and Integration Tests

To make the Foo feature available in the non-bundle chrome_public_apk target, add the java target to the chrome_public_common_apk_or_module_tmpl in //chrome/android/chrome_public_apk_tmpl.gni like so:

template("chrome_public_common_apk_or_module_tmpl") {
  ...
  target(_target_type, target_name) {
    ...
    if (_target_type != "android_app_bundle_module") {
      deps += [
        "//chrome/browser/foo/internal:java",
      ]
    }
  }
}

You may also have to add java as a dependency of chrome_test_java if you want to call into Foo from test code.