Use StateFlow to View Controller States

StateFlow is a subclass of SharedFlow and internally uses SharedFlow to manage its emissions. However, it provides a stricter API, ensuring that:

1. It always has an initial value.
2. It emits only the latest state.
3. It cannot configure its replay cache (always 1).

Therefore, StateFlow is a specialized type of SharedFlow that represents a state holder, always maintaining the latest value. It is optimized for use cases where you need to observe and react to state changes.

Expose UI states in StateFlow

1. Expose two StateFlows named faceOk and gestureOk in MainViewModel, indicating whether the subject’s face and gestures are ready for a selfie.

    

        
        
    private val _faceOk = MutableStateFlow(false)
    val faceOk: StateFlow<Boolean> = _faceOk

    private val _gestureOk = MutableStateFlow(false)
    val gestureOk: StateFlow<Boolean> = _gestureOk

    

2. In this demo app, you will focus on smiling faces and thumb-up gestures. Append the following constant values to MainViewModel’s companion object:

    

        
        
        private const val FACE_CATEGORY_MOUTH_SMILE = "mouthSmile"
        private const val GESTURE_CATEGORY_THUMB_UP = "Thumb_Up"

    

3. Update onFaceLandmarkerResults and onGestureResults to check if their corresponding results are meeting the conditions above:

    

        
        
    override fun onFaceLandmarkerResults(resultBundle: FaceResultBundle) {
        val faceOk = resultBundle.result.faceBlendshapes().getOrNull()?.let { faceBlendShapes ->
            faceBlendShapes.take(FACES_COUNT).all { shapes ->
                shapes.filter {
                    it.categoryName().contains(FACE_CATEGORY_MOUTH_SMILE)
                }.all {
                    it.score() > HolisticRecognizerHelper.DEFAULT_FACE_SHAPE_SCORE_THRESHOLD
                }
            }
        } ?: false

        _faceOk.tryEmit(faceOk)
        _uiEvents.tryEmit(UiEvent.Face(resultBundle))
    }

    

    

        
        
    override fun onGestureResults(resultBundle: GestureResultBundle) {
        val gestureOk = resultBundle.results.first().gestures()
            .take(HolisticRecognizerHelper.HANDS_COUNT)
            .let { gestures ->
                gestures.isNotEmpty() && gestures
                    .mapNotNull { it.firstOrNull() }
                    .all { GESTURE_CATEGORY_THUMB_UP == it.categoryName() }
            }

        _gestureOk.tryEmit(gestureOk)
        _uiEvents.tryEmit(UiEvent.Gesture(resultBundle))
    }

    

Update UI in the view controller

1. Navigate to the strings.xml file in your app subproject’s src/main/res/values path, then append the following text resources:

    

        
        
    <string name="condition_indicator_text_face">Face</string>
    <string name="condition_indicator_text_gesture">Gesture</string>

    

2. In the same directory, create a new resource file named dimens.xml if it does not exist already. This file is used to define layout related dimension values:

    

        
        
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <dimen name="condition_indicator_padding">12dp</dimen>
    <dimen name="condition_indicator_margin">36dp</dimen>
</resources>

    

3. Navigate to the activity_main.xml layout file and add the following code to the root ConstraintLayout. Add this code after the two overlay views which you have just added in the previous section:

    

        
        
    <androidx.appcompat.widget.SwitchCompat
        android:text="@string/condition_indicator_text_face"
        android:id="@+id/faceReady"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="@dimen/condition_indicator_margin"
        android:padding="@dimen/condition_indicator_padding"
        android:background="@color/white"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintBottom_toBottomOf="parent" />

    <androidx.appcompat.widget.SwitchCompat
        android:text="@string/condition_indicator_text_gesture"
        android:id="@+id/gestureReady"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="@dimen/condition_indicator_margin"
        android:padding="@dimen/condition_indicator_padding"
        android:background="@color/white"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintBottom_toBottomOf="parent" />

    

4. Finally, navigate to MainActivity.kt and append the following code inside the repeatOnLifecycle(Lifecycle.State.RESUMED) block, after the launch block.

This makes sure each of the three parallel launch code sections run in its own co-routine concurrently without blocking each other.

    

        
        
                launch {
                    viewModel.faceOk.collect {
                        viewBinding.faceReady.isChecked = it
                    }
                }

                launch {
                    viewModel.gestureOk.collect {
                        viewBinding.gestureReady.isChecked = it
                    }
                }

    

5. Build and run the app again. Now you should see two switches on the bottom of the screen as shown below, which turn on and off while you smile and show thumb-up gestures. Good job!

Figure 8: Indicator UI.

Recap on SharedFlow vs StateFlow

This app uses SharedFlow for dispatching overlay views’ UI events without mandating a specific stateful model, which avoids redundant computation. Meanwhile, it uses StateFlow for dispatching condition switches’ UI states, which prevents duplicate emission and consequent UI updates.

Here is an overview of the differences between SharedFlow and StateFlow: