Implement inline properties for android.

Summary: Fixes material-motion/material-motion#31

Reviewers: O2 Material Motion!, miguelandres, O6 Material Motion Android platform reviewers, #material_motion

Reviewed By: miguelandres, O6 Material Motion Android platform reviewers

Subscribers: featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D2171

library/src/main/java/com/google/android/material/motion/streams/MotionObservable.java

@@ -122,6 +122,28 @@ public static abstract class Predicate<T> {
     public abstract boolean evaluate(T value);
   }
 
+  /**
+   * An inline property that can be read into a MotionObservable stream.
+   */
+  public static abstract class InlineReadable<T> {
+
+    /**
+     * Reads the property's value.
+     */
+    public abstract T read();
+  }
+
+  /**
+   * An inline property that can be written from a MotionObservable stream.
+   */
+  public static abstract class InlineWritable<T> {
+
+    /**
+     * Writes the property with the given value.
+     */
+    public abstract void write(T value);
+  }
+
   /**
    * A light-weight operator builder.
    * <p>
@@ -194,7 +216,7 @@ public void next(MotionObserver<T> observer, T value) {
   }
 
   /**
-   * Writes the values from an Observable onto the given target and property.
+   * Writes the values from an Observable onto the given unscoped property.
    *
    * @see <a href="https://material-motion.github.io/material-motion/starmap/specifications/streams/operators/$.write">The
    * write() specification</a>
@@ -208,4 +230,20 @@ public void next(MotionObserver<T> observer, T value) {
       }
     });
   }
+
+  /**
+   * Writes the values from an Observable onto the given inline property.
+   *
+   * @see <a href="https://material-motion.github.io/material-motion/starmap/specifications/streams/operators/$.write">The
+   * write() specification</a>
+   */
+  public <O> MotionObservable<T> write(final InlineWritable<T> property) {
+    return operator(new Operation<T, T>() {
+      @Override
+      public void next(MotionObserver<T> observer, T value) {
+        property.write(value);
+        observer.next(value);
+      }
+    });
+  }
 }