Implement filter() for android.

Mark Wei · Mark Wei · commit e614b749c134 · 2016-12-08T02:02:45.000-08:00
Summary: Fixes #4 Reviewers: O6 Material Motion Android platform reviewers, O2 Material Motion, featherless Reviewed By: O6 Material Motion Android platform reviewers, O2 Material Motion, featherless Tags: #material_motion Differential Revision: http://codereview.cc/D2155
diff --git a/library/src/main/java/com/google/android/material/motion/streams/MotionObservable.java b/library/src/main/java/com/google/android/material/motion/streams/MotionObservable.java
@@ -97,6 +97,17 @@ public interface Operation<T, U> {
     void next(MotionObserver<U> observer, T value);
   }
 
+  /**
+   * A predicate evaluates whether to pass a value downstream.
+   */
+  public interface Predicate<T> {
+
+    /**
+     * Evaluates whether to pass the value downstream.
+     */
+    boolean evaluate(T value);
+  }
+
   /**
    * A light-weight operator builder.
    * <p>
@@ -135,4 +146,22 @@ public void unsubscribe() {
       }
     });
   }
+
+  /**
+   * Only emit 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>
+   */
+  public MotionObservable<T> filter(final Predicate<T> predicate) {
+    return operator(new Operation<T, T>() {
+
+      @Override
+      public void next(MotionObserver<T> observer, T value) {
+        if (predicate.evaluate(value)) {
+          observer.next(value);
+        }
+      }
+    });
+  }
 }
diff --git a/sample/src/main/java/com/google/android/material/motion/streams/sample/MainActivity.java b/sample/src/main/java/com/google/android/material/motion/streams/sample/MainActivity.java
@@ -44,7 +44,7 @@
 public class MainActivity extends AppCompatActivity {
 
   private static final String[] values = new String[]{
-    "foo", "bar", "baz", "qux"
+    "foo", "skip", "bar", "baz", "qux"
   };
 
   private MotionObserver<String> callback;
@@ -77,32 +77,40 @@ public void unsubscribe() {
         }
       });
 
-    final Subscription subscription = observable.operator(new Operation<String, CharSequence>() {
+    final Subscription subscription = observable
+      .filter(new MotionObservable.Predicate<String>() {
 
-      @Override
-      public void next(MotionObserver<CharSequence> observer, String value) {
-        CharSequence charSequence = italicizeAndCapitalize(value);
-        observer.next(charSequence);
-      }
-    }).subscribe(new MotionObserver<CharSequence>() {
+        @Override
+        public boolean evaluate(String value) {
+          return value != "skip";
+        }
+      })
+      .operator(new Operation<String, CharSequence>() {
 
-      @Override
-      public void next(CharSequence value) {
-        text.setText(value);
-      }
+        @Override
+        public void next(MotionObserver<CharSequence> observer, String value) {
+          CharSequence charSequence = italicizeAndCapitalize(value);
+          observer.next(charSequence);
+        }
+      }).subscribe(new MotionObserver<CharSequence>() {
 
-      @Override
-      public void state(@MotionState int state) {
-        switch (state) {
-          case AT_REST:
-            text.setTextColor(Color.BLACK);
-            break;
-          case ACTIVE:
-            text.setTextColor(Color.RED);
-            break;
+        @Override
+        public void next(CharSequence value) {
+          text.setText(value);
         }
-      }
-    });
+
+        @Override
+        public void state(@MotionState int state) {
+          switch (state) {
+            case AT_REST:
+              text.setTextColor(Color.BLACK);
+              break;
+            case ACTIVE:
+              text.setTextColor(Color.RED);
+              break;
+          }
+        }
+      });
 
     nextButton.setOnClickListener(new View.OnClickListener() {
       @Override
