make main component name nullable

Summary: Sometimes the main component name is not known at activity construction time and depends on e.g. reading shared preferences. To support this use case, make `(Fragment)ReactActivity#getMainComponentName()` nullable and return `null` by default. In this case, the app will not be loaded in `onCreate` by default and the user has to call `loadApp` manually once the component name is known.

Reviewed By: andreicoman11

Differential Revision: D3722517

fbshipit-source-id: 062eed158798606e4160f1c142b23fd98ca618c8

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

@@ -9,6 +9,8 @@
 
 package com.facebook.react;
 
+import javax.annotation.Nullable;
+
 import android.app.Activity;
 import android.content.Intent;
 import android.os.Bundle;
@@ -35,7 +37,9 @@ public abstract class ReactActivity extends Activity
    * This is used to schedule rendering of the component.
    * e.g. "MoviesApp"
    */
-  protected abstract String getMainComponentName();
+  protected @Nullable String getMainComponentName() {
+    return null;
+  }
 
   /**
    * Called at construction time, override if you have a custom delegate implementation.
@@ -120,4 +124,8 @@ public abstract class ReactActivity extends Activity
   protected final ReactInstanceManager getReactInstanceManager() {
     return mDelegate.getReactInstanceManager();
   }
+
+  protected final void loadApp(String appKey) {
+    mDelegate.loadApp(appKey);
+  }
 }

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

@@ -33,19 +33,21 @@ public class ReactActivityDelegate {
 
   private final @Nullable Activity mActivity;
   private final @Nullable FragmentActivity mFragmentActivity;
-  private final String mMainComponentName;
+  private final @Nullable String mMainComponentName;
 
   private @Nullable ReactRootView mReactRootView;
   private @Nullable DoubleTapReloadRecognizer mDoubleTapReloadRecognizer;
   private @Nullable PermissionListener mPermissionListener;
 
-  public ReactActivityDelegate(Activity activity, String mainComponentName) {
+  public ReactActivityDelegate(Activity activity, @Nullable String mainComponentName) {
     mActivity = activity;
     mMainComponentName = mainComponentName;
     mFragmentActivity = null;
   }
 
-  public ReactActivityDelegate(FragmentActivity fragmentActivity, String mainComponentName) {
+  public ReactActivityDelegate(
+    FragmentActivity fragmentActivity,
+    @Nullable String mainComponentName) {
     mFragmentActivity = fragmentActivity;
     mMainComponentName = mainComponentName;
     mActivity = null;
@@ -85,13 +87,22 @@ public class ReactActivityDelegate {
       }
     }
 
+    if (mMainComponentName != null) {
+      loadApp(mMainComponentName);
+    }
+    mDoubleTapReloadRecognizer = new DoubleTapReloadRecognizer();
+  }
+
+  protected void loadApp(String appKey) {
+    if (mReactRootView != null) {
+      throw new IllegalStateException("Cannot loadApp while app is already running.");
+    }
     mReactRootView = createRootView();
     mReactRootView.startReactApplication(
       getReactNativeHost().getReactInstanceManager(),
-      mMainComponentName,
+      appKey,
       getLaunchOptions());
     getPlainActivity().setContentView(mReactRootView);
-    mDoubleTapReloadRecognizer = new DoubleTapReloadRecognizer();
   }
 
   protected void onPause() {

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

@@ -9,6 +9,8 @@
 
 package com.facebook.react;
 
+import javax.annotation.Nullable;
+
 import android.content.Intent;
 import android.os.Bundle;
 import android.support.v4.app.FragmentActivity;
@@ -36,7 +38,9 @@ public abstract class ReactFragmentActivity extends FragmentActivity implements
    * This is used to schedule rendering of the component.
    * e.g. "MoviesApp"
    */
-  protected abstract String getMainComponentName();
+  protected @Nullable String getMainComponentName() {
+    return null;
+  }
 
   /**
    * Called at construction time, override if you have a custom delegate implementation.
@@ -121,4 +125,8 @@ public abstract class ReactFragmentActivity extends FragmentActivity implements
   protected final ReactInstanceManager getReactInstanceManager() {
     return mDelegate.getReactInstanceManager();
   }
+
+  protected final void loadApp(String appKey) {
+    mDelegate.loadApp(appKey);
+  }
 }