Add destinations operator for Transition.Direction.

Summary:
Used to build reversible transitions.

Depends on D2414

Reviewers: O2 Material Motion, #material_motion, O4 Material Apple platform reviewers, chuga

Reviewed By: O4 Material Apple platform reviewers, chuga

Tags: #material_motion

Differential Revision: http://codereview.cc/D2415

src/operators/transitions/destinations.swift

@@ -0,0 +1,25 @@
+/*
+ Copyright 2016-present The Material Motion Authors. All Rights Reserved.
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ */
+
+import Foundation
+
+extension ExtendableMotionObservable where T == Transition.Direction {
+
+  /** Emits either the back or fore value when a new direction is received, . */
+  public func destinations<U>(back: U, fore: U) -> MotionObservable<U> {
+    return _map { direction in direction == .forward ? fore : back }
+  }
+}