Implement operator() for android.

Summary: Fixes #3

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/D2152

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

@@ -16,6 +16,7 @@
 package com.google.android.material.motion.streams;
 
 import android.support.annotation.IntDef;
+import android.support.annotation.Nullable;
 
 import com.google.android.material.motion.observable.IndefiniteObservable;
 import com.google.android.material.motion.observable.Observer;
@@ -77,4 +78,61 @@ public interface MotionObserver<T> extends Observer<T> {
      */
     void state(@MotionState int state);
   }
+
+  /**
+   * An operation is able to transform incoming values before choosing whether or not to pass them
+   * downstream.
+   *
+   * @param <T> The incoming value type.
+   * @param <U> The downstream value type.
+   */
+  public interface Operation<T, U> {
+
+    /**
+     * Modifies the given value before passing it downstream, or blocks the value.
+     *
+     * @param observer Downstream.
+     * @param value The value from upstream.
+     */
+    void next(MotionObserver<U> observer, T value);
+  }
+
+  /**
+   * A light-weight operator builder.
+   * <p>
+   * This is the preferred method for building new operators. This builder can be used to create
+   * any operator that only needs to modify or block values. All state events are forwarded
+   * along.
+   *
+   * @see <a href="https://material-motion.github.io/material-motion/starmap/specifications/streams/operators/$._operator">The
+   * operator() specification</a>
+   */
+  public <U> MotionObservable<U> operator(final Operation<T, U> operation) {
+    final MotionObservable<T> upstream = MotionObservable.this;
+
+    return new MotionObservable<>(new Subscriber<MotionObserver<U>>() {
+      @Nullable
+      @Override
+      public Unsubscriber subscribe(final MotionObserver<U> observer) {
+        final Subscription subscription = upstream.subscribe(new MotionObserver<T>() {
+          @Override
+          public void next(T value) {
+            operation.next(observer, value);
+          }
+
+          @Override
+          public void state(@MotionState int state) {
+            observer.state(state);
+          }
+        });
+
+        return new Unsubscriber() {
+          @Override
+          public void unsubscribe() {
+            subscription.unsubscribe();
+          }
+        };
+      }
+    });
+  }
 }

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

@@ -16,9 +16,13 @@
 package com.google.android.material.motion.streams.sample;
 
 import android.graphics.Color;
+import android.graphics.Typeface;
 import android.os.Bundle;
 import android.support.annotation.Nullable;
 import android.support.v7.app.AppCompatActivity;
+import android.text.Spannable;
+import android.text.SpannableString;
+import android.text.style.StyleSpan;
 import android.view.View;
 import android.widget.Button;
 import android.widget.TextView;
@@ -29,6 +33,7 @@
 import com.google.android.material.motion.streams.MotionObservable;
 import com.google.android.material.motion.streams.MotionObservable.MotionObserver;
 import com.google.android.material.motion.streams.MotionObservable.MotionState;
+import com.google.android.material.motion.streams.MotionObservable.Operation;
 
 import static com.google.android.material.motion.streams.MotionObservable.ACTIVE;
 import static com.google.android.material.motion.streams.MotionObservable.AT_REST;
@@ -55,13 +60,15 @@ protected void onCreate(Bundle savedInstanceState) {
     Button nextButton = (Button) findViewById(R.id.next_button);
     Button unsubscribeButton = (Button) findViewById(R.id.unsubscribe_button);
 
-    MotionObservable<String> observable = new MotionObservable<>(
+    final MotionObservable<String> observable = new MotionObservable<>(
       new Subscriber<MotionObserver<String>>() {
+
         @Nullable
         @Override
         public Unsubscriber subscribe(MotionObserver<String> observer) {
           registerButtonCallback(observer);
           return new Unsubscriber() {
+
             @Override
             public void unsubscribe() {
               unregisterButtonCallback();
@@ -70,9 +77,17 @@ public void unsubscribe() {
         }
       });
 
-    final Subscription subscription = observable.subscribe(new MotionObserver<String>() {
+    final Subscription subscription = observable.operator(new Operation<String, CharSequence>() {
+
+      @Override
+      public void next(MotionObserver<CharSequence> observer, String value) {
+        CharSequence charSequence = italicizeAndCapitalize(value);
+        observer.next(charSequence);
+      }
+    }).subscribe(new MotionObserver<CharSequence>() {
+
       @Override
-      public void next(String value) {
+      public void next(CharSequence value) {
         text.setText(value);
       }
 
@@ -116,6 +131,12 @@ public void onClick(View v) {
     });
   }
 
+  private CharSequence italicizeAndCapitalize(String value) {
+    Spannable spannable = new SpannableString(value.toUpperCase());
+    spannable.setSpan(new StyleSpan(Typeface.ITALIC), 0, spannable.length(), 0);
+    return spannable;
+  }
+
   private void registerButtonCallback(MotionObserver<String> observer) {
     callback = observer;
   }