Add shared element transition for camera fab.

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

@@ -0,0 +1,53 @@
+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 com.google.android.material.floatingactionbutton.FloatingActionButton
+
+@RequiresApi(21)
+class FabElevationFadeTransform(context: Context, attrs: AttributeSet?) : Transition(context, attrs) {
+
+  companion object {
+    private const val ELEVATION = "CrossfaderTransition.ELEVATION"
+  }
+
+  override fun captureStartValues(transitionValues: TransitionValues) {
+    if (transitionValues.view is FloatingActionButton) {
+      transitionValues.values[ELEVATION] = transitionValues.view.elevation
+    }
+  }
+
+  override fun captureEndValues(transitionValues: TransitionValues) {
+    if (transitionValues.view is FloatingActionButton) {
+      transitionValues.values[ELEVATION] = transitionValues.view.elevation
+    }
+  }
+
+  override fun createAnimator(sceneRoot: ViewGroup?, startValues: TransitionValues?, endValues: TransitionValues?): Animator? {
+    if (startValues?.view !is FloatingActionButton || endValues?.view !is FloatingActionButton) {
+      return null
+    }
+
+    val startElevation = startValues.view.elevation
+    val endElevation = endValues.view.elevation
+    if (startElevation == endElevation) {
+      return null
+    }
+
+    return ValueAnimator.ofFloat(
+      startValues.values[ELEVATION] as Float,
+      endValues.values[ELEVATION] as Float
+    ).apply {
+      addUpdateListener {
+        val elevation = it.animatedValue as Float
+        endValues.view.elevation = elevation
+      }
+    }
+  }
+}