Add ability to edit default reactions.

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

@@ -0,0 +1,44 @@
+package org.thoughtcrime.securesms.animation.transitions
+
+import android.animation.Animator
+import android.animation.ObjectAnimator
+import android.view.View
+import android.view.ViewGroup
+import androidx.constraintlayout.widget.ConstraintLayout
+import androidx.transition.Transition
+import androidx.transition.TransitionValues
+
+private const val ALPHA = "signal.alpha_transition.alpha"
+
+/**
+ * Alpha transition that can be used with [ConstraintLayout]
+ */
+class AlphaTransition : Transition() {
+
+  override fun captureStartValues(transitionValues: TransitionValues) {
+    captureValues(transitionValues)
+  }
+
+  override fun captureEndValues(transitionValues: TransitionValues) {
+    captureValues(transitionValues)
+  }
+
+  private fun captureValues(transitionValues: TransitionValues) {
+    val view: View = transitionValues.view
+    if (view !is ConstraintLayout) {
+      transitionValues.values[ALPHA] = view.alpha
+    }
+  }
+
+  override fun createAnimator(sceneRoot: ViewGroup, startValues: TransitionValues?, endValues: TransitionValues?): Animator? {
+    if (startValues == null || endValues == null) {
+      return null
+    }
+
+    val view: View = endValues.view
+    val startAlpha: Float = startValues.values[ALPHA] as? Float ?: view.alpha
+    val endAlpha: Float = endValues.values[ALPHA] as? Float ?: view.alpha
+
+    return ObjectAnimator.ofFloat(view, "alpha", startAlpha, endAlpha)
+  }
+}