Add first-class support to ReactInstanceManager for Activity-less usage

Summary:
I'm not sure if we'll ever need to do more than this (probably!) but I want to have a specific option for Activity-less usage, so
that our logic in ReactInstanceManager and elsewhere is more clear. I want to be able to confidently assert that the Activity is present
and correct, or never present, depending on if this `requireActivity` flag is set; instead of adding else statements that imply "well, the Activity should be here, hope everything is okay" which feels hacky.

Doesn't change much for now, but it's an additional constraint in the RN Android codebase that we need to explicitly embed in code so that
we can point to it, and so that the logic is more clear.

Changelog: [Internal]

Reviewed By: javache, motiz88

Differential Revision: D30504616

fbshipit-source-id: d2abdb7c4765a16113c9517406cdbb8cf42822ff

ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java

@@ -156,12 +156,14 @@ public interface ReactInstanceEventListener {
   /* accessed from any thread */
   private final JavaScriptExecutorFactory mJavaScriptExecutorFactory;
 
+  // See {@code ReactInstanceManagerBuilder} for description of all flags here.
   private @Nullable List<String> mViewManagerNames = null;
   private final @Nullable JSBundleLoader mBundleLoader;
   private final @Nullable String mJSMainModulePath; /* path to JS bundle root on Metro */
   private final List<ReactPackage> mPackages;
   private final DevSupportManager mDevSupportManager;
   private final boolean mUseDeveloperSupport;
+  private final boolean mRequireActivity;
   private @Nullable ComponentNameResolverManager mComponentNameResolverManager;
   private @Nullable RuntimeSchedulerManager mRuntimeSchedulerManager;
   private final @Nullable NotThreadSafeBridgeIdleDebugListener mBridgeIdleDebugListener;
@@ -216,6 +218,7 @@ public static ReactInstanceManagerBuilder builder() {
       @Nullable String jsMainModulePath,
       List<ReactPackage> packages,
       boolean useDeveloperSupport,
+      boolean requireActivity,
       @Nullable NotThreadSafeBridgeIdleDebugListener bridgeIdleDebugListener,
       LifecycleState initialLifecycleState,
       @Nullable UIImplementationProvider mUIImplementationProvider,
@@ -233,6 +236,7 @@ public static ReactInstanceManagerBuilder builder() {
 
     DisplayMetricsHolder.initDisplayMetricsIfNotInitialized(applicationContext);
 
+    // See {@code ReactInstanceManagerBuilder} for description of all flags here.
     mApplicationContext = applicationContext;
     mCurrentActivity = currentActivity;
     mDefaultBackButtonImpl = defaultHardwareBackBtnHandler;
@@ -241,6 +245,7 @@ public static ReactInstanceManagerBuilder builder() {
     mJSMainModulePath = jsMainModulePath;
     mPackages = new ArrayList<>();
     mUseDeveloperSupport = useDeveloperSupport;
+    mRequireActivity = requireActivity;
     Systrace.beginSection(
         Systrace.TRACE_TAG_REACT_JAVA_BRIDGE, "ReactInstanceManager.initDevSupportManager");
     mDevSupportManager =
@@ -558,16 +563,20 @@ public void onHostPause() {
    * @param activity the activity being paused
    */
   @ThreadConfined(UI)
-  public void onHostPause(Activity activity) {
-    Assertions.assertNotNull(mCurrentActivity);
-    Assertions.assertCondition(
-        activity == mCurrentActivity,
-        "Pausing an activity that is not the current activity, this is incorrect! "
-            + "Current activity: "
-            + mCurrentActivity.getClass().getSimpleName()
-            + " "
-            + "Paused activity: "
-            + activity.getClass().getSimpleName());
+  public void onHostPause(@Nullable Activity activity) {
+    if (mRequireActivity) {
+      Assertions.assertCondition(mCurrentActivity != null);
+    }
+    if (mCurrentActivity != null) {
+      Assertions.assertCondition(
+          activity == mCurrentActivity,
+          "Pausing an activity that is not the current activity, this is incorrect! "
+              + "Current activity: "
+              + mCurrentActivity.getClass().getSimpleName()
+              + " "
+              + "Paused activity: "
+              + activity.getClass().getSimpleName());
+    }
     onHostPause();
   }
 
@@ -583,7 +592,8 @@ public void onHostPause(Activity activity) {
    *     this instance of {@link ReactInstanceManager}.
    */
   @ThreadConfined(UI)
-  public void onHostResume(Activity activity, DefaultHardwareBackBtnHandler defaultBackButtonImpl) {
+  public void onHostResume(
+      @Nullable Activity activity, DefaultHardwareBackBtnHandler defaultBackButtonImpl) {
     UiThreadUtil.assertOnUiThread();
 
     mDefaultBackButtonImpl = defaultBackButtonImpl;
@@ -592,7 +602,7 @@ public void onHostResume(Activity activity, DefaultHardwareBackBtnHandler defaul
 
   /** Use this method when the activity resumes. */
   @ThreadConfined(UI)
-  public void onHostResume(Activity activity) {
+  public void onHostResume(@Nullable Activity activity) {
     UiThreadUtil.assertOnUiThread();
 
     mCurrentActivity = activity;
@@ -631,8 +641,8 @@ public void onViewDetachedFromWindow(View v) {
           // activity is attached to window, we can enable dev support immediately
           mDevSupportManager.setDevSupportEnabled(true);
         }
-      } else {
-        // there is no activity, we can enable dev support
+      } else if (!mRequireActivity) {
+        // there is no activity, but we can enable dev support
         mDevSupportManager.setDevSupportEnabled(true);
       }
     }
@@ -666,7 +676,9 @@ public void onHostDestroy() {
    * @param activity the activity being destroyed
    */
   @ThreadConfined(UI)
-  public void onHostDestroy(Activity activity) {
+  public void onHostDestroy(@Nullable Activity activity) {
+    // In some cases, Activity may (correctly) be null.
+    // See mRequireActivity flag.
     if (activity == mCurrentActivity) {
       onHostDestroy();
     }

ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManagerBuilder.java

@@ -45,6 +45,7 @@ public class ReactInstanceManagerBuilder {
   private @Nullable NotThreadSafeBridgeIdleDebugListener mBridgeIdleDebugListener;
   private @Nullable Application mApplication;
   private boolean mUseDeveloperSupport;
+  private boolean mRequireActivity;
   private @Nullable LifecycleState mInitialLifecycleState;
   private @Nullable UIImplementationProvider mUIImplementationProvider;
   private @Nullable NativeModuleCallExceptionHandler mNativeModuleCallExceptionHandler;
@@ -171,6 +172,16 @@ public ReactInstanceManagerBuilder setUseDeveloperSupport(boolean useDeveloperSu
     return this;
   }
 
+  /**
+   * When {@code false}, indicates that correct usage of React Native will NOT involve an Activity.
+   * For the vast majority of Android apps in the ecosystem, this will not need to change. Unless
+   * you really know what you're doing, you should probably not change this!
+   */
+  public ReactInstanceManagerBuilder setRequireActivity(boolean requireActivity) {
+    mRequireActivity = requireActivity;
+    return this;
+  }
+
   /**
    * Sets the initial lifecycle state of the host. For example, if the host is already resumed at
    * creation time, we wouldn't expect an onResume call until we get an onPause call.
@@ -283,6 +294,7 @@ public ReactInstanceManager build() {
         mJSMainModulePath,
         mPackages,
         mUseDeveloperSupport,
+        mRequireActivity,
         mBridgeIdleDebugListener,
         Assertions.assertNotNull(mInitialLifecycleState, "Initial lifecycle state was not set"),
         mUIImplementationProvider,

ReactAndroid/src/main/java/com/facebook/react/ReactNativeHost.java

@@ -68,6 +68,7 @@ protected ReactInstanceManager createReactInstanceManager() {
             .setApplication(mApplication)
             .setJSMainModulePath(getJSMainModuleName())
             .setUseDeveloperSupport(getUseDeveloperSupport())
+            .setRequireActivity(getShouldRequireActivity())
             .setRedBoxHandler(getRedBoxHandler())
             .setJavaScriptExecutorFactory(getJavaScriptExecutorFactory())
             .setUIImplementationProvider(getUIImplementationProvider())
@@ -124,6 +125,11 @@ protected UIImplementationProvider getUIImplementationProvider() {
     return null;
   }
 
+  /** Returns whether or not to treat it as normal if Activity is null. */
+  public boolean getShouldRequireActivity() {
+    return true;
+  }
+
   /**
    * Returns the name of the main module. Determines the URL used to fetch the JS bundle from Metro.
    * It is only used when dev support is enabled. This is the first file to be executed once the