Add flag to indicate whether the platform supports state restoration (#19717)

shell/platform/android/io/flutter/embedding/engine/systemchannels/RestorationChannel.java

@@ -10,6 +10,8 @@ import io.flutter.embedding.engine.dart.DartExecutor;
 import io.flutter.plugin.common.MethodCall;
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.StandardMethodCodec;
+import java.util.HashMap;
+import java.util.Map;
 
 /**
  * System channel to exchange restoration data between framework and engine.
@@ -79,7 +81,7 @@ public class RestorationChannel {
     engineHasProvidedData = true;
     if (pendingFrameworkRestorationChannelRequest != null) {
       // If their is a pending request from the framework, answer it.
-      pendingFrameworkRestorationChannelRequest.success(data);
+      pendingFrameworkRestorationChannelRequest.success(packageData(data));
       pendingFrameworkRestorationChannelRequest = null;
       restorationData = data;
     } else if (frameworkHasRequestedData) {
@@ -90,7 +92,7 @@ public class RestorationChannel {
       // e.g. when the engine is attached to a new activity.
       channel.invokeMethod(
           "push",
-          data,
+          packageData(data),
           new MethodChannel.Result() {
             @Override
             public void success(Object result) {
@@ -142,7 +144,7 @@ public class RestorationChannel {
             case "get":
               frameworkHasRequestedData = true;
               if (engineHasProvidedData || !waitForRestorationData) {
-                result.success(restorationData);
+                result.success(packageData(restorationData));
                 // Do not delete the restoration data on the engine side after sending it to the
                 // framework. We may need to hand this data back to the operating system if the
                 // framework never modifies the data (and thus doesn't send us any
@@ -157,4 +159,11 @@ public class RestorationChannel {
           }
         }
       };
+
+  private Map<String, Object> packageData(byte[] data) {
+    final Map<String, Object> packaged = new HashMap<String, Object>();
+    packaged.put("enabled", true); // Android supports state restoration.
+    packaged.put("data", data);
+    return packaged;
+  }
 }

shell/platform/android/test/io/flutter/embedding/engine/systemchannels/RestorationChannelTest.java

@@ -11,6 +11,8 @@ import static org.mockito.Mockito.verifyZeroInteractions;
 import android.annotation.TargetApi;
 import io.flutter.plugin.common.MethodCall;
 import io.flutter.plugin.common.MethodChannel;
+import java.util.HashMap;
+import java.util.Map;
 import org.json.JSONException;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -51,12 +53,15 @@ public class RestorationChannelTest {
 
     restorationChannel.setRestorationData(data);
     verify(rawChannel, times(0)).invokeMethod(any(), any());
-    verify(result).success(data);
+    Map<String, Object> expected = new HashMap<>();
+    expected.put("enabled", true);
+    expected.put("data", data);
+    verify(result).success(expected);
 
     // Next get request is answered right away.
     MethodChannel.Result result2 = mock(MethodChannel.Result.class);
     argumentCaptor.getValue().onMethodCall(new MethodCall("get", null), result2);
-    verify(result2).success(data);
+    verify(result2).success(expected);
   }
 
   @Test
@@ -72,14 +77,20 @@ public class RestorationChannelTest {
 
     MethodChannel.Result result = mock(MethodChannel.Result.class);
     argumentCaptor.getValue().onMethodCall(new MethodCall("get", null), result);
-    verify(result).success(null);
+    Map<String, Object> expected = new HashMap<>();
+    expected.put("enabled", true);
+    expected.put("data", null);
+    verify(result).success(expected);
 
     restorationChannel.setRestorationData(data);
     assertEquals(restorationChannel.getRestorationData(), null);
 
     ArgumentCaptor<MethodChannel.Result> resultCapture =
         ArgumentCaptor.forClass(MethodChannel.Result.class);
-    verify(rawChannel).invokeMethod(eq("push"), eq(data), resultCapture.capture());
+    Map<String, Object> expected2 = new HashMap<>();
+    expected2.put("enabled", true);
+    expected2.put("data", data);
+    verify(rawChannel).invokeMethod(eq("push"), eq(expected2), resultCapture.capture());
     resultCapture.getValue().success(null);
     assertEquals(restorationChannel.getRestorationData(), data);
   }