Implementation of write() in android.

Summary: Fixes #5

Reviewers: O6 Material Motion Android platform reviewers, O2 Material Motion, featherless

Reviewed By: O6 Material Motion Android platform reviewers, O2 Material Motion, featherless

Subscribers: featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D2160

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

@@ -17,6 +17,7 @@
 
 import android.support.annotation.IntDef;
 import android.support.annotation.Nullable;
+import android.util.Property;
 
 import com.google.android.material.motion.observable.IndefiniteObservable;
 import com.google.android.material.motion.observable.Observer;
@@ -161,7 +162,7 @@ public void unsubscribe() {
   }
 
   /**
-   * Transform the items emitted by an Observable by applying a function to each item.
+   * Transforms the items emitted by an Observable by applying a function to each item.
    *
    * @see <a href="https://material-motion.github.io/material-motion/starmap/specifications/streams/operators/$._map">The
    * filter() specification</a>
@@ -176,7 +177,7 @@ public void next(MotionObserver<U> observer, T value) {
   }
 
   /**
-   * Only emit those values from an Observable that satisfy a predicate.
+   * Only emits those values from an Observable that satisfy a predicate.
    *
    * @see <a href="https://material-motion.github.io/material-motion/starmap/specifications/streams/operators/$._filter">The
    * filter() specification</a>
@@ -191,4 +192,20 @@ public void next(MotionObserver<T> observer, T value) {
       }
     });
   }
+
+  /**
+   * Writes the values from an Observable onto the given target and property.
+   *
+   * @see <a href="https://material-motion.github.io/material-motion/starmap/specifications/streams/operators/$.write">The
+   * write() specification</a>
+   */
+  public <O> MotionObservable<T> write(final O target, final Property<O, T> property) {
+    return operator(new Operation<T, T>() {
+      @Override
+      public void next(MotionObserver<T> observer, T value) {
+        property.set(target, value);
+        observer.next(value);
+      }
+    });
+  }
 }

sample/src/main/java/com/google/android/material/motion/streams/sample/MainActivity.java

@@ -23,6 +23,7 @@
 import android.text.Spannable;
 import android.text.SpannableString;
 import android.text.style.StyleSpan;
+import android.util.Property;
 import android.view.View;
 import android.widget.Button;
 import android.widget.TextView;
@@ -91,11 +92,12 @@ public CharSequence transform(String value) {
           return italicizeAndCapitalize(value);
         }
       })
+      .write(text, TEXT_PROPERTY)
       .subscribe(new MotionObserver<CharSequence>() {
 
         @Override
         public void next(CharSequence value) {
-          text.setText(value);
+          // No-op. Handled by write() above.
         }
 
         @Override
@@ -151,4 +153,17 @@ private void registerButtonCallback(MotionObserver<String> observer) {
   private void unregisterButtonCallback() {
     callback = null;
   }
+
+  private static Property<TextView, CharSequence> TEXT_PROPERTY = new Property<TextView, CharSequence>(CharSequence.class, "text") {
+
+    @Override
+    public CharSequence get(TextView object) {
+      return object.getText();
+    }
+
+    @Override
+    public void set(TextView object, CharSequence value) {
+      object.setText(value);
+    }
+  };
 }