Pulling synchronous data from Flutter on the platform vs platform cache for Dart feature developers

gistfile1.txt

// With a platform-thread Dart isolate/cache

pageA.java - our feature team just owns this one file on the Java side
--------------------------
class FeatureAFlutterActivity extends Activity implements SomeLibraryCallback {
  // Long lived engine, owned by someone else, given to me by 
  // an injector.
  @Inject FlutterEngine flutterEngine;
  @Inject SomeLibraryHandle someLibraryHandle;
  
  public void onCreate() {
    someLibraryHandle.setListener(this);
  }
  
  // SomeLibraryCallback
  public X someLibraryCallbackWantingXSynchronously() {
     // I, the Dart dev, have to add this one line to this Java file.
    return SynchronousPigeonApi(flutterEngine).getAnX();
  }
}
--------------------------

// With a platform-side cache

pageA.java - our feature team just owns this one file on the Java side
--------------------------
class FeatureAFlutterActivity extends Activity 
    implements SomeLibraryCallback, PigeonFeatureAHostApi {
  // Long lived engine, owned by someone else, given to me by 
  // an injector.
  @Inject FlutterEngine flutterEngine;
  @Inject SomeLibraryHandle someLibraryHandle;
  
  public void onCreate() {
    someLibraryHandle.setListener(this);
    // This doesn't exist today since there's one big setup, but we can 
    // definitely split it in the schema for handlers that don't have
    // the same lifecycle.
    Pigeon.setupApiForFeatureA(flutterEngine.getDartExecutor(), this);
  }
  
  // PigeonFeatureAHostApi
  public void pigeonCallbackToCacheX(X anX) {
    // Where do I put anX? The FlutterEngine is longer lived than me
    // (Activity). It seems like I need to @Inject some other 
    // container that has the same lifecyle as the FlutterEngine instance
    // I got to store it. I have to ask the infra team where is the 
    // FlutterEngine made and find a nice sibling or parent where 
    // it's semantically clean to store this X thing. Then, as a Dart
    // dev, I have to go figure out how this whole @Inject codegen 
    // thing works and how to get gradle to build it again so this 
    // IDE I don't like to use can autocomplete again. 
  }
  
  // SomeLibraryCallback
  public X someLibraryCallbackWantingXSynchronously() {
    // Where do I get this X instance? I can't keep it just here in the
    // activity instance that our team owns since that value may have never
    // changed or been given to us since our Activity was created. How do I get
    // the previous value the engine emitted?
  }
}
--------------------------

Then rinse and repeat for objective-c, learn how GIMMify works. Learn about 
c pre-processor macros used for dependency injection.