Implement cross-fade for story thumb shared element animation.

app/src/main/java/org/thoughtcrime/securesms/animation/transitions/CrossfaderTransition.kt

@@ -0,0 +1,63 @@
+package org.thoughtcrime.securesms.animation.transitions
+
+import android.animation.Animator
+import android.animation.ValueAnimator
+import android.content.Context
+import android.transition.Transition
+import android.transition.TransitionValues
+import android.util.AttributeSet
+import android.view.ViewGroup
+import androidx.annotation.RequiresApi
+import androidx.core.animation.doOnEnd
+import androidx.core.animation.doOnStart
+
+@RequiresApi(21)
+class CrossfaderTransition(context: Context, attrs: AttributeSet?) : Transition(context, attrs) {
+
+  companion object {
+    private const val WIDTH = "CrossfaderTransition.WIDTH"
+  }
+
+  override fun captureStartValues(transitionValues: TransitionValues) {
+    if (transitionValues.view is Crossfadeable) {
+      transitionValues.values[WIDTH] = transitionValues.view.width
+    }
+  }
+
+  override fun captureEndValues(transitionValues: TransitionValues) {
+    if (transitionValues.view is Crossfadeable) {
+      transitionValues.values[WIDTH] = transitionValues.view.width
+    }
+  }
+
+  override fun createAnimator(sceneRoot: ViewGroup?, startValues: TransitionValues?, endValues: TransitionValues?): Animator? {
+    if (startValues == null || endValues == null) {
+      return null
+    }
+
+    val startWidth = (startValues.values[WIDTH] ?: 0) as Int
+    val endWidth = (endValues.values[WIDTH] ?: 0) as Int
+    val view: Crossfadeable = endValues.view as? Crossfadeable ?: return null
+    val reverse = startWidth > endWidth
+
+    return ValueAnimator.ofFloat(0f, 1f).apply {
+      addUpdateListener {
+        view.onCrossfadeAnimationUpdated(it.animatedValue as Float, reverse)
+      }
+
+      doOnStart {
+        view.onCrossfadeStarted(reverse)
+      }
+
+      doOnEnd {
+        view.onCrossfadeFinished(reverse)
+      }
+    }
+  }
+
+  interface Crossfadeable {
+    fun onCrossfadeAnimationUpdated(progress: Float, reverse: Boolean)
+    fun onCrossfadeStarted(reverse: Boolean)
+    fun onCrossfadeFinished(reverse: Boolean)
+  }
+}