Avoid usage of nulls in navi

navi/src/main/java/com/trello/navi2/Event.java

@@ -7,6 +7,7 @@
 import android.content.res.Configuration;
 import android.os.Bundle;
 import android.os.PersistableBundle;
+import android.support.annotation.NonNull;
 import android.view.LayoutInflater;
 import android.view.View;
 import android.view.ViewGroup;
@@ -26,7 +27,8 @@
  * are called *before* their super calls are made. Events that are neither are called *after*
  * their super calls.
  *
- * @param <T> the callback type for the event
+ * @param <T> the callback type for the event. If Object, then the value is just a signal and has
+ * no contents.
  */
 public final class Event<T> {
 
@@ -50,7 +52,7 @@ public final class Event<T> {
   /**
    * Emits {@link Activity#onStart()} and {@link Fragment#onStart()}. Emitted after super().
    */
-  public static final Event<Void> START = new Event<>(Type.START, Void.class);
+  public static final Event<Object> START = new Event<>(Type.START, Object.class);
 
   /**
    * Emits {@link Activity#onPostCreate(Bundle)}. Emitted after super().
@@ -66,22 +68,22 @@ public final class Event<T> {
   /**
    * Emits {@link Activity#onResume()} and {@link Fragment#onResume()}. Emitted after super().
    */
-  public static final Event<Void> RESUME = new Event<>(Type.RESUME, Void.class);
+  public static final Event<Object> RESUME = new Event<>(Type.RESUME, Object.class);
 
   /**
    * Emits {@link Activity#onPause()} and {@link Fragment#onPause()}. Emitted before super().
    */
-  public static final Event<Void> PAUSE = new Event<>(Type.PAUSE, Void.class);
+  public static final Event<Object> PAUSE = new Event<>(Type.PAUSE, Object.class);
 
   /**
    * Emits {@link Activity#onStop()} and {@link Fragment#onStop()}. Emitted before super().
    */
-  public static final Event<Void> STOP = new Event<>(Type.STOP, Void.class);
+  public static final Event<Object> STOP = new Event<>(Type.STOP, Object.class);
 
   /**
    * Emits {@link Activity#onDestroy()} and {@link Fragment#onDestroy()}. Emitted before super().
    */
-  public static final Event<Void> DESTROY = new Event<>(Type.DESTROY, Void.class);
+  public static final Event<Object> DESTROY = new Event<>(Type.DESTROY, Object.class);
 
   /**
    * Emits {@link Activity#onSaveInstanceState(Bundle)} and
@@ -120,7 +122,7 @@ public final class Event<T> {
   /**
    * Emits {@link Activity#onRestart()}. Emitted after super().
    */
-  public static final Event<Void> RESTART = new Event<>(Type.RESTART, Void.class);
+  public static final Event<Object> RESTART = new Event<>(Type.RESTART, Object.class);
 
   /**
    * Emits {@link Activity#onRestoreInstanceState(Bundle)}. Emitted after super().
@@ -143,19 +145,19 @@ public final class Event<T> {
   /**
    * Emits {@link Activity#onBackPressed()}. Emitted after super().
    */
-  public static final Event<Void> BACK_PRESSED = new Event<>(Type.BACK_PRESSED, Void.class);
+  public static final Event<Object> BACK_PRESSED = new Event<>(Type.BACK_PRESSED, Object.class);
 
   /**
    * Emits {@link Activity#onAttachedToWindow()}. Emitted after super().
    */
-  public static final Event<Void> ATTACHED_TO_WINDOW =
-      new Event<>(Type.ATTACHED_TO_WINDOW, Void.class);
+  public static final Event<Object> ATTACHED_TO_WINDOW =
+      new Event<>(Type.ATTACHED_TO_WINDOW, Object.class);
 
   /**
    * Emits {@link Activity#onDetachedFromWindow()}. Emitted after super().
    */
-  public static final Event<Void> DETACHED_FROM_WINDOW =
-      new Event<>(Type.DETACHED_FROM_WINDOW, Void.class);
+  public static final Event<Object> DETACHED_FROM_WINDOW =
+      new Event<>(Type.DETACHED_FROM_WINDOW, Object.class);
 
   /**
    * Emits {@link Fragment#onAttach(Context)}. Emitted after super().
@@ -187,19 +189,19 @@ public final class Event<T> {
   /**
    * Emits {@link Fragment#onDestroyView()}. Emitted before super().
    */
-  public static final Event<Void> DESTROY_VIEW = new Event<>(Type.DESTROY_VIEW, Void.class);
+  public static final Event<Object> DESTROY_VIEW = new Event<>(Type.DESTROY_VIEW, Object.class);
 
   /**
    * Emits {@link Fragment#onDetach()}. Emitted before super().
    */
-  public static final Event<Void> DETACH = new Event<>(Type.DETACH, Void.class);
+  public static final Event<Object> DETACH = new Event<>(Type.DETACH, Object.class);
 
   private final Type eventType;
 
   private final Class<T> callbackType;
 
   // This is purposefully hidden so that we can control available events
-  private Event(Type eventType, Class<T> callbackType) {
+  private Event(@NonNull Type eventType, @NonNull Class<T> callbackType) {
     this.eventType = eventType;
     this.callbackType = callbackType;
   }

navi/src/main/java/com/trello/navi2/Listener.java

@@ -1,10 +1,12 @@
 package com.trello.navi2;
 
+import android.support.annotation.NonNull;
+
 /**
  * Listener callback.
  *
  * @param <T> the callback type; can be Void in cases where event has no metadata
  */
 public interface Listener<T> {
-  void call(T t);
+  void call(@NonNull T t);
 }

navi/src/main/java/com/trello/navi2/NaviComponent.java

@@ -1,5 +1,7 @@
 package com.trello.navi2;
 
+import android.support.annotation.NonNull;
+
 /**
  * Represents an Android component (Activity, Fragment) that can have listeners.
  */
@@ -23,14 +25,13 @@ public interface NaviComponent {
    * @param <T> the callback type for the event
    * @throws IllegalArgumentException if this component cannot handle the event
    */
-  <T> void addListener(Event<T> event, Listener<T> listener);
+  <T> void addListener(@NonNull Event<T> event, @NonNull Listener<T> listener);
 
   /**
    * Removes a listener from this component.
    *
    * @param <T> the callback type for the event
    * @param listener the listener for that event
-   * @throws IllegalArgumentException if this component cannot handle the event
    */
-  <T> void removeListener(Listener<T> listener);
+  <T> void removeListener(@NonNull Listener<T> listener);
 }

navi/src/main/java/com/trello/navi2/component/NaviActivity.java

@@ -20,11 +20,11 @@ public class NaviActivity extends Activity implements NaviComponent {
     return base.handlesEvents(events);
   }
 
-  @Override public final <T> void addListener(Event<T> event, Listener<T> listener) {
+  @Override public final <T> void addListener(@NonNull Event<T> event, @NonNull Listener<T> listener) {
     base.addListener(event, listener);
   }
 
-  @Override public final <T> void removeListener(Listener<T> listener) {
+  @Override public final <T> void removeListener(@NonNull Listener<T> listener) {
     base.removeListener(listener);
   }
 

navi/src/main/java/com/trello/navi2/component/NaviDialogFragment.java

@@ -25,11 +25,11 @@ public class NaviDialogFragment extends DialogFragment implements NaviComponent
     return base.handlesEvents(events);
   }
 
-  @Override public final <T> void addListener(Event<T> event, Listener<T> listener) {
+  @Override public final <T> void addListener(@NonNull Event<T> event, @NonNull Listener<T> listener) {
     base.addListener(event, listener);
   }
 
-  @Override public final <T> void removeListener(Listener<T> listener) {
+  @Override public final <T> void removeListener(@NonNull Listener<T> listener) {
     base.removeListener(listener);
   }
 

navi/src/main/java/com/trello/navi2/component/NaviFragment.java

@@ -25,11 +25,11 @@ public class NaviFragment extends Fragment implements NaviComponent {
     return base.handlesEvents(events);
   }
 
-  @Override public final <T> void addListener(Event<T> event, Listener<T> listener) {
+  @Override public final <T> void addListener(@NonNull Event<T> event, @NonNull Listener<T> listener) {
     base.addListener(event, listener);
   }
 
-  @Override public final <T> void removeListener(Listener<T> listener) {
+  @Override public final <T> void removeListener(@NonNull Listener<T> listener) {
     base.removeListener(listener);
   }
 

navi/src/main/java/com/trello/navi2/component/support/NaviAppCompatActivity.java

@@ -21,11 +21,11 @@ public class NaviAppCompatActivity extends AppCompatActivity implements NaviComp
     return base.handlesEvents(events);
   }
 
-  @Override public final <T> void addListener(Event<T> event, Listener<T> listener) {
+  @Override public final <T> void addListener(@NonNull Event<T> event, @NonNull Listener<T> listener) {
     base.addListener(event, listener);
   }
 
-  @Override public final <T> void removeListener(Listener<T> listener) {
+  @Override public final <T> void removeListener(@NonNull Listener<T> listener) {
     base.removeListener(listener);
   }
 

navi/src/main/java/com/trello/navi2/component/support/NaviAppCompatDialogFragment.java

@@ -25,11 +25,11 @@ public class NaviAppCompatDialogFragment extends DialogFragment implements NaviC
     return base.handlesEvents(events);
   }
 
-  @Override public final <T> void addListener(Event<T> event, Listener<T> listener) {
+  @Override public final <T> void addListener(@NonNull Event<T> event, @NonNull Listener<T> listener) {
     base.addListener(event, listener);
   }
 
-  @Override public final <T> void removeListener(Listener<T> listener) {
+  @Override public final <T> void removeListener(@NonNull Listener<T> listener) {
     base.removeListener(listener);
   }
 

navi/src/main/java/com/trello/navi2/component/support/NaviDialogFragment.java

@@ -25,11 +25,11 @@ public class NaviDialogFragment extends DialogFragment implements NaviComponent
     return base.handlesEvents(events);
   }
 
-  @Override public final <T> void addListener(Event<T> event, Listener<T> listener) {
+  @Override public final <T> void addListener(@NonNull Event<T> event, @NonNull Listener<T> listener) {
     base.addListener(event, listener);
   }
 
-  @Override public final <T> void removeListener(Listener<T> listener) {
+  @Override public final <T> void removeListener(@NonNull Listener<T> listener) {
     base.removeListener(listener);
   }
 

navi/src/main/java/com/trello/navi2/component/support/NaviFragment.java

@@ -25,11 +25,11 @@ public class NaviFragment extends Fragment implements NaviComponent {
     return base.handlesEvents(events);
   }
 
-  @Override public final <T> void addListener(Event<T> event, Listener<T> listener) {
+  @Override public final <T> void addListener(@NonNull Event<T> event, @NonNull Listener<T> listener) {
     base.addListener(event, listener);
   }
 
-  @Override public final <T> void removeListener(Listener<T> listener) {
+  @Override public final <T> void removeListener(@NonNull Listener<T> listener) {
     base.removeListener(listener);
   }
 

navi/src/main/java/com/trello/navi2/internal/Constants.java

@@ -0,0 +1,15 @@
+package com.trello.navi2.internal;
+
+final class Constants {
+
+  /**
+   * Acts as a signal for events that have no data associated with them.
+   *
+   * We reuse a single Object to avoid any extra allocations.
+   */
+  static final Object SIGNAL = new Object();
+
+  private Constants() {
+    throw new AssertionError("No instances!");
+  }
+}