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Upgrading pre 1.12 Android projects

Gary Qian edited this page Nov 4, 2021 · 23 revisions

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If you flutter created your project prior to version 1.12, this may apply to your project

Background

In order to better support the execution environments of adding Flutter to an existing project, the old Android platform-side wrappers hosting the Flutter runtime at io.flutter.app.FlutterActivity and their associated classes are now deprecated. New wrappers at io.flutter.embedding.android.FlutterActivity and associated classes now replace them.

Those classes better support real world scenarios where the FlutterActivity isn't the first and only Android Activity in an application.

Motivation

Your existing full-Flutter projects aren't immediately affected and will continue to work as before for the foreseeable future.

However, the new Android wrappers also introduce a new set of Android plugin development APIs. Plugins developed exclusively on the new plugins API will not work on older pre-1.12 Android projects. Building a pre-1.12 Android project that uses plugins created after 1.12 will yield a build-time error unless the plugin developer explicitly opted to create a second backward compatible implementation.

Add-to-app was not officially supported on the old Android APIs. If you followed the experimental instructions in the wiki for add-to-app prior to 1.12, you should follow the migration steps under the add-to-app section below.

Full-Flutter app migration

This guide assumes you haven't manually modified your Android host project for your Flutter project. If you did, consult the add-to-app migration guide below.

If you opt to migrate your standard flutter created project, follow the following steps:

1a. If you don't have any of your own added code to android/app/src/main/java/[your/package/name]/MainActivity.java - remove the body of your MainActivity.java and change the FlutterActivity import. The new FlutterActivity no longer requires manually registering your plugins. It will now perform the registration automatically when the underlaying FlutterEngine is created.

// MainActivity.java
-import android.os.Bundle;
-import io.flutter.app.FlutterActivity;
+import io.flutter.embedding.android.FlutterActivity;
-import io.flutter.plugins.GeneratedPluginRegistrant;

public class MainActivity extends FlutterActivity {
-  @Override
-  protected void onCreate(Bundle savedInstanceState) {
-    super.onCreate(savedInstanceState);
-    GeneratedPluginRegistrant.registerWith(this);
-  }
 }
// MainActivity.kt
-import android.os.Bundle
-import io.flutter.app.FlutterActivity
+import io.flutter.embedding.android.FlutterActivity
-import io.flutter.plugins.GeneratedPluginRegistrant

class MainActivity: FlutterActivity() {
-  override fun onCreate(savedInstanceState: Bundle?) {
-    super.onCreate(savedInstanceState)
-    GeneratedPluginRegistrant.registerWith(this)
-  }
}

Since the body of the MainActivity is now empty, you can also optionally delete the MainActivity.java/kt file if you'd like. If you do, you need to change your AndroidManifest.xml's reference to .MainActivity to io.flutter.embedding.android.FlutterActivity.

1b. If you had existing custom platform channel handling code in your MainActivity.java, below is an example of the change you can make to adopt the new embedding API:

-import io.flutter.app.FlutterActivity;
-import io.flutter.plugin.common.MethodCall;
+import androidx.annotation.NonNull;
+import io.flutter.embedding.android.FlutterActivity;
+import io.flutter.embedding.engine.FlutterEngine;
 import io.flutter.plugin.common.MethodChannel;
-import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
-import io.flutter.plugin.common.MethodChannel.Result;
+import io.flutter.plugins.GeneratedPluginRegistrant;

 public class MainActivity extends FlutterActivity {
     private static final String CHANNEL = "samples.flutter.dev/battery";
-
-    @Override
-    public void onCreate(Bundle savedInstanceState) {
-
-        super.onCreate(savedInstanceState);
-        GeneratedPluginRegistrant.registerWith(this);
-
-        new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(
-                new MethodCallHandler() {
-                    @Override
-                    public void onMethodCall(MethodCall call, Result result) {
-                        // Your existing code
-                    }
-                });
-    }
+
+    @Override
+    public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
+        GeneratedPluginRegistrant.registerWith(flutterEngine);
+        new MethodChannel(flutterEngine.getDartExecutor().getBinaryMessenger(), CHANNEL)
+                .setMethodCallHandler(
+                    (call, result) -> {
+                        // Your existing code
+                }
+        );
+    }
 }

In other words, move the channel registration part of the code in your onCreate into the configureFlutterEngine override of the FlutterActivity subclass and use flutterEngine.getDartExecutor().getBinaryMessenger() as the binary messenger rather than getFlutterView().

2. Open android/app/src/main/AndroidManifest.xml.

3. Replace the reference to FlutterApplication in the application tag with ${applicationName}.

Previous configuration:

<application
  android:name="io.flutter.app.FlutterApplication"
  >
  <!-- code omitted -->
</application>

New configuration:

<application
  android:name="${applicationName}"
  >
  <!-- code omitted -->
</application>

4. Update splash screen behavior (if splash behavior is desired).

Remove all <meta-data> tags with key android:name="io.flutter.app.android.SplashScreenUntilFirstFrame".

Add a launch theme to styles.xml that configures the desired launch screen as a background Drawable:

<!-- You can name this style whatever you'd like -->
<style name="LaunchTheme" parent="@android:style/Theme.Black.NoTitleBar">
    <item name="android:windowBackground">@drawable/[your_launch_drawable_here]</item>
</style>

If you created your Flutter project with flutter create then you likely already have a LaunchTheme defined, along with a drawable called launch_background. You can re-use that configuration and adjust it as desired.

Add a normal theme that to styles.xml that should replace the launch screen when the Android process is fully initialized:

<!-- You can name this style whatever you'd like -->
<style name="NormalTheme" parent="@android:style/Theme.Black.NoTitleBar">
    <item name="android:windowBackground">@drawable/[your_normal_background_drawable]</item>
</style>

The "normal theme" draws the background behind your Flutter experience. That background is typically seen for a brief moment just before the first Flutter frame renders. The "normal theme" also controls Android's status bar and navigation bar visual properties for the duration of your Flutter experience.

Configure MainActivity to start with your launch theme and then shift to your normal theme. Also specify that you want your launch screen to continue being displayed until Flutter renders its first frame:

<activity android:name=".MainActivity"
  android:theme="@style/LaunchTheme"
  // some code omitted
  >
  <!-- Specify that the launch screen should continue being displayed -->
  <!-- until Flutter renders its first frame. -->
  <meta-data
    android:name="io.flutter.embedding.android.SplashScreenDrawable"
    android:resource="@drawable/launch_background" />

  <!-- Theme to apply as soon as Flutter begins rendering frames -->
  <meta-data
    android:name="io.flutter.embedding.android.NormalTheme"
    android:resource="@style/NormalTheme"
    />

  <!-- some code omitted -->
</activity>

5. Add a new <meta-data> tag under <application>.

<meta-data
    android:name="flutterEmbedding"
    android:value="2" />

Once you make this declaration in your AndroidManifest and also use plugins, the new GeneratedPluginRegistrants created by the Flutter tools during build will now be able to also use plugins that uses the new Android embedding (which registers the plugins against a FlutterEngine instead of a PluginRegistry.Registrar).

Your app should still build as normal (such as via flutter build apk) but you're now using the new Android classes.

Add-to-app migration

This section details how to take add-to-app scenarios that were built using Flutter's experimental embedding, and transition that code to Flutter's new stable embedding.

Same steps as full-Flutter

Some instructions from the "Full-Flutter app migration" section above still apply. Follow the above steps for:

  1. Remove the reference to FlutterApplication from the application tag.
  2. Update splash screen behavior (if splash behavior is desired).
  3. Add a new <meta-data> tag under <application>.

Changes specific to add-to-app

If you invoke FlutterMain.startInitialization(...) or FlutterMain.ensureInitializationComplete(...) anywhere in your code, you should remove those calls. Flutter now initializes itself at the appropriate time.

Migrating FlutterActivity Uses

Add-to-app scenarios often involve modifications to subclasses of FlutterActivity. For example, such a scenario might introduce new MethodChannels, a custom FlutterEngine instance, custom splash screen behavior, or other behaviors that require overriding existing methods. Therefore, whereas full-Flutter apps can delete their MainActivity and replace it with a standard FlutterActivity, you will need to retain your subclass so that you can keep your behavior overrides.

If your add-to-app use-cases do not modify behavior within FlutterActivity, you should delete your subclasses and replace them with standard FlutterActivitys as described in the previous section.

If your add-to-app use-cases do require modifying behavior within FlutterActivity, you need to migrate your code from the old io.flutter.app.FlutterActivity to the new io.flutter.embedding.android.FlutterActivity.

From:

package [your.package.name];

import android.os.Bundle;
import io.flutter.app.FlutterActivity;
import io.flutter.plugins.GeneratedPluginRegistrant;

public class MainActivity extends FlutterActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    GeneratedPluginRegistrant.registerWith(this);
  }

  // ...some amount of custom code for your app is here.
}

To:

package [your.package.name];

import io.flutter.embedding.android.FlutterActivity;

public class MainActivity extends FlutterActivity {
  // You do not need to override onCreate() in order to invoke
  // GeneratedPluginRegistrant. Flutter now does that on your behalf.

  // ...retain whatever custom code you had from before (if any).
}

Some apps may have required pre-warming a Flutter experience. It is now recommended that all add-to-app use-cases pre-warm Flutter experiences to achieve optimal visual performance when initially rendering a Flutter UI. Please refer to the Flutter guide for pre-warming a FlutterEngine (TODO: mattcarroll) to update your code for pre-warming Flutter.

Your FlutterActivity subclass is now up to date with the new, stable Android embedding.

Migrating FlutterFragment uses

The experimental embedding provides a class called io.flutter.facade.FlutterFragment, along with other classes in the io.flutter.facade package. The entire io.flutter.facade package is deprecated and you should not use any classes in that package.

The experimental io.flutter.facade.FlutterFragment has been replaced by io.flutter.embedding.android.FlutterFragment, which was designed for a much broader set of use-cases than the original FlutterFragment.

If you are instantiating a io.flutter.facade.FlutterFragment via Flutter.createFragment(...), you should delete any such calls and instantiate the new io.flutter.embedding.android.FlutterFragment via one of the following factory methods:

  • FlutterFragment.createDefault()
  • FlutterFragment.withNewEngine()
  • FlutterFragment.withCachedEngine(...)

The use of these factory methods are discussed in depth in website guides at http://flutter.dev.

Migrating FlutterView uses

The deprecated io.flutter.facade.Flutter class has a factory method called createView(...). This method is deprecated, along with all other code in the io.flutter.facade package.

Flutter does not currently provide convenient APIs for utilizing Flutter at the View level, so the use of a FlutterView should be avoided, if possible. However, it is technically feasible to display a FlutterView, if required. Be sure to use io.flutter.embedding.android.FlutterView instead of io.flutter.view.FlutterView. You can instantiate the new FlutterView just like any other Android View. Then, follow instructions in the associated Javadocs to display Flutter via a FlutterView.