Add a reactive button target type and an initial isHighlighted stream.

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

Reviewed By: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D3212

src/reactivetypes/Reactive+UIButton.swift

@@ -1,9 +1,58 @@
-//
-//  Reactive+UIButton.swift
-//  Pods
-//
-//  Created by Jeff Verkoeyen on 5/11/17.
-//
-//
+/*
+ Copyright 2017-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
+import UIKit
+
+/**
+ A reactive button target exposes streams for certain button events.
+ */
+public final class ReactiveButtonTarget: NSObject {
+  public init(_ button: UIButton) {
+    super.init()
+
+    button.addTarget(self, action: #selector(didEnterEvent),
+                     for: [.touchDown, .touchDragInside])
+    button.addTarget(self, action: #selector(didExitEvent),
+                     for: [.touchUpInside, .touchUpOutside, .touchDragOutside])
+  }
+
+  // MARK: Streams
+
+  /**
+   Emits true when the button should be highlighted and false when it should not.
+   */
+  public var didHighlight: MotionObservable<Bool> {
+    return MotionObservable { observer in
+      self.didHighlightObservers.append(observer)
+      return {
+        if let index = self.didHighlightObservers.index(where: { $0 === observer }) {
+          self.didHighlightObservers.remove(at: index)
+        }
+      }
+    }
+  }
+
+  func didEnterEvent(_ button: UIButton) {
+    didHighlightObservers.forEach { $0.next(true) }
+  }
+  func didExitEvent(_ button: UIButton) {
+    didHighlightObservers.forEach { $0.next(false) }
+  }
+  private var didHighlightObservers: [MotionObserver<Bool>] = []
+
+  public let metadata = Metadata("Button target")
+}