2.x: Add lazy loading of default main Scheduler

rxandroid/src/main/java/io/reactivex/android/plugins/RxAndroidPlugins.java

@@ -13,6 +13,8 @@
  */
 package io.reactivex.android.plugins;
 
+import java.util.concurrent.Callable;
+
 import io.reactivex.Scheduler;
 import io.reactivex.exceptions.Exceptions;
 import io.reactivex.functions.Function;
@@ -21,39 +23,38 @@
  * Utility class to inject handlers to certain standard RxAndroid operations.
  */
 public final class RxAndroidPlugins {
-    private static volatile Function<Scheduler, Scheduler> onInitMainThreadHandler;
+
+    private static volatile Function<Callable<Scheduler>, Scheduler> onInitMainThreadHandler;
     private static volatile Function<Scheduler, Scheduler> onMainThreadHandler;
 
-    public static void setInitMainThreadSchedulerHandler(Function<Scheduler, Scheduler> handler) {
+    public static void setInitMainThreadSchedulerHandler(Function<Callable<Scheduler>, Scheduler> handler) {
         onInitMainThreadHandler = handler;
     }
 
-    public static Scheduler initMainThreadScheduler(Scheduler scheduler) {
-        Function<Scheduler, Scheduler> f = onInitMainThreadHandler;
-        if (f == null) {
-            return scheduler;
+    public static Scheduler initMainThreadScheduler(Callable<Scheduler> scheduler) {
+        if (scheduler == null) {
+            throw new NullPointerException("scheduler == null");
         }
-        try {
-            return f.apply(scheduler);
-        } catch (Exception e) {
-            throw Exceptions.propagate(e);
+        Function<Callable<Scheduler>, Scheduler> f = onInitMainThreadHandler;
+        if (f == null) {
+            return callRequireNonNull(scheduler);
         }
+        return applyRequireNonNull(f, scheduler);
     }
 
     public static void setMainThreadSchedulerHandler(Function<Scheduler, Scheduler> handler) {
         onMainThreadHandler = handler;
     }
 
     public static Scheduler onMainThreadScheduler(Scheduler scheduler) {
+        if (scheduler == null) {
+            throw new NullPointerException("scheduler == null");
+        }
         Function<Scheduler, Scheduler> f = onMainThreadHandler;
         if (f == null) {
             return scheduler;
         }
-        try {
-            return f.apply(scheduler);
-        } catch (Exception e) {
-            throw Exceptions.propagate(e);
-        }
+        return apply(f, scheduler);
     }
 
     /**
@@ -64,6 +65,34 @@ public static void reset() {
         setMainThreadSchedulerHandler(null);
     }
 
+    static Scheduler callRequireNonNull(Callable<Scheduler> s) {
+        try {
+            Scheduler scheduler = s.call();
+            if (scheduler == null) {
+                throw new NullPointerException("Scheduler Callable returned null");
+            }
+            return scheduler;
+        } catch (Throwable ex) {
+            throw Exceptions.propagate(ex);
+        }
+    }
+
+    static Scheduler applyRequireNonNull(Function<Callable<Scheduler>, Scheduler> f, Callable<Scheduler> s) {
+        Scheduler scheduler = apply(f,s);
+        if (scheduler == null) {
+            throw new NullPointerException("Scheduler Callable returned null");
+        }
+        return scheduler;
+    }
+
+    static <T, R> R apply(Function<T, R> f, T t) {
+        try {
+            return f.apply(t);
+        } catch (Throwable ex) {
+            throw Exceptions.propagate(ex);
+        }
+    }
+
     private RxAndroidPlugins() {
         throw new AssertionError("No instances.");
     }

rxandroid/src/main/java/io/reactivex/android/schedulers/AndroidSchedulers.java

@@ -15,13 +15,26 @@
 
 import android.os.Handler;
 import android.os.Looper;
+
+import java.util.concurrent.Callable;
+
 import io.reactivex.Scheduler;
 import io.reactivex.android.plugins.RxAndroidPlugins;
 
 /** Android-specific Schedulers. */
 public final class AndroidSchedulers {
+
+    private static final class MainHolder {
+
+        static final Scheduler DEFAULT = new HandlerScheduler(new Handler(Looper.getMainLooper()));
+    }
+
     private static final Scheduler MAIN_THREAD = RxAndroidPlugins.initMainThreadScheduler(
-        new HandlerScheduler(new Handler(Looper.getMainLooper())));
+            new Callable<Scheduler>() {
+                @Override public Scheduler call() throws Exception {
+                    return MainHolder.DEFAULT;
+                }
+            });
 
     /** A {@link Scheduler} which executes actions on the Android main thread. */
     public static Scheduler mainThread() {

rxandroid/src/test/java/io/reactivex/android/plugins/RxAndroidPluginsTest.java

@@ -13,17 +13,22 @@
  */
 package io.reactivex.android.plugins;
 
-import io.reactivex.Scheduler;
-import io.reactivex.android.testutil.EmptyScheduler;
-import io.reactivex.functions.Function;
-import java.util.concurrent.atomic.AtomicReference;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.robolectric.RobolectricTestRunner;
 import org.robolectric.annotation.Config;
 
+import java.util.concurrent.Callable;
+import java.util.concurrent.atomic.AtomicReference;
+
+import io.reactivex.Scheduler;
+import io.reactivex.android.testutil.EmptyScheduler;
+import io.reactivex.functions.Function;
+
+import static junit.framework.TestCase.fail;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertSame;
 
 @RunWith(RobolectricTestRunner.class)
@@ -55,7 +60,7 @@ public void mainThreadHandlerCalled() {
     public void resetClearsMainThreadHandler() {
         RxAndroidPlugins.setMainThreadSchedulerHandler(new Function<Scheduler, Scheduler>() {
             @Override public Scheduler apply(Scheduler scheduler) {
-                throw new RuntimeException();
+                throw new AssertionError();
             }
         });
         RxAndroidPlugins.reset();
@@ -67,32 +72,90 @@ public void resetClearsMainThreadHandler() {
 
     @Test
     public void initMainThreadHandlerCalled() {
-        final AtomicReference<Scheduler> schedulerRef = new AtomicReference<>();
+        final AtomicReference<Callable<Scheduler>> schedulerRef = new AtomicReference<>();
         final Scheduler newScheduler = new EmptyScheduler();
-        RxAndroidPlugins.setInitMainThreadSchedulerHandler(new Function<Scheduler, Scheduler>() {
-            @Override public Scheduler apply(Scheduler scheduler) {
-                schedulerRef.set(scheduler);
-                return newScheduler;
-            }
-        });
+        RxAndroidPlugins
+                .setInitMainThreadSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
+                    @Override public Scheduler apply(Callable<Scheduler> scheduler) {
+                        schedulerRef.set(scheduler);
+                        return newScheduler;
+                    }
+                });
 
-        Scheduler scheduler = new EmptyScheduler();
+        Callable<Scheduler> scheduler = new Callable<Scheduler>() {
+            @Override public Scheduler call() throws Exception {
+                throw new AssertionError();
+            }
+        };
         Scheduler actual = RxAndroidPlugins.initMainThreadScheduler(scheduler);
         assertSame(newScheduler, actual);
         assertSame(scheduler, schedulerRef.get());
     }
 
     @Test
-    public void resetClearsInitMainThreadHandler() {
-        RxAndroidPlugins.setInitMainThreadSchedulerHandler(new Function<Scheduler, Scheduler>() {
-            @Override public Scheduler apply(Scheduler scheduler) {
-                throw new RuntimeException();
+    public void resetClearsInitMainThreadHandler() throws Exception {
+        RxAndroidPlugins
+                .setInitMainThreadSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
+                    @Override public Scheduler apply(Callable<Scheduler> scheduler) {
+                        throw new AssertionError();
+                    }
+                });
+
+        final Scheduler scheduler = new EmptyScheduler();
+        Callable<Scheduler> schedulerCallable = new Callable<Scheduler>() {
+            @Override public Scheduler call() throws Exception {
+                return scheduler;
             }
-        });
+        };
+
         RxAndroidPlugins.reset();
 
-        Scheduler scheduler = new EmptyScheduler();
-        Scheduler actual = RxAndroidPlugins.initMainThreadScheduler(scheduler);
-        assertSame(scheduler, actual);
+        Scheduler actual = RxAndroidPlugins.initMainThreadScheduler(schedulerCallable);
+        assertSame(schedulerCallable.call(), actual);
+    }
+
+    @Test
+    public void defaultMainThreadSchedulerIsInitializedLazily() {
+        Function<Callable<Scheduler>, Scheduler> safeOverride =
+                new Function<Callable<Scheduler>, Scheduler>() {
+            @Override public Scheduler apply(Callable<Scheduler> scheduler) {
+                return new EmptyScheduler();
+            }
+        };
+        Callable<Scheduler> unsafeDefault = new Callable<Scheduler>() {
+            @Override public Scheduler call() throws Exception {
+                throw new AssertionError();
+            }
+        };
+
+       RxAndroidPlugins.setInitMainThreadSchedulerHandler(safeOverride);
+       RxAndroidPlugins.initMainThreadScheduler(unsafeDefault);
+    }
+
+    @Test
+    public void overrideInitMainSchedulerThrowsWhenSchedulerCallableIsNull() {
+        try {
+            RxAndroidPlugins.initMainThreadScheduler(null);
+            fail();
+        } catch (NullPointerException e) {
+            assertEquals("scheduler == null", e.getMessage());
+        }
+    }
+
+    @Test
+    public void overrideInitMainSchedulerThrowsWhenSchedulerCallableReturnsNull() {
+        Callable<Scheduler> nullResultCallable = new Callable<Scheduler>() {
+            @Override public Scheduler call() throws Exception {
+                return null;
+            }
+        };
+
+        try {
+            RxAndroidPlugins.initMainThreadScheduler(nullResultCallable);
+            fail();
+        } catch (NullPointerException e) {
+            assertEquals("Scheduler Callable returned null", e.getMessage());
+        }
     }
+
 }