OnActivityResult()与OnNewIntent()的区别及自定义调用场景咨询

Hey there! Let's tackle your questions one by one, nice and clear.

These two methods serve completely different purposes in Android Activity lifecycle:

• onActivityResult(): This is the go-to method for handling results from a child Activity that you started with startActivityForResult() (or the newer AndroidX ActivityResultContracts). It’s built for a request-response pattern—you explicitly launch a child Activity expecting data back, and when the child calls setResult() and finishes, this method fires in the parent to process the returned data.
• onNewIntent(): This method kicks in when you’re reusing an existing Activity instance (thanks to launch modes like singleTop, singleTask, or singleInstance). Instead of creating a new Activity instance when it’s requested again, the system passes the new Intent to the existing instance via this method. It’s all about reusing Activity instances, not handling results from a child that’s finishing.

You want onActivityResult() to trigger when returning from MenuActivity, and onNewIntent() for other child pages. Here’s how to make that happen:

For MenuActivity (trigger onActivityResult()) #

We’ll use the standard result-based launch flow:

1. Launch MenuActivity with a result request:
   If you’re using the older API:

   // In MainActivity
   private static final int REQUEST_CODE_MENU = 1001;
   
   // To launch MenuActivity
   Intent menuIntent = new Intent(this, MenuActivity.class);
   startActivityForResult(menuIntent, REQUEST_CODE_MENU);
   

   For AndroidX (recommended, as startActivityForResult() is deprecated):

   // In MainActivity
   private ActivityResultLauncher<Intent> menuLauncher = registerForActivityResult(
       new ActivityResultContracts.StartActivityForResult(),
       result -> {
           if (result.getResultCode() == RESULT_OK) {
               // Handle the result from MenuActivity here (equivalent to onActivityResult logic)
               Intent data = result.getData();
               if (data != null) {
                   // Extract any passed data
               }
           }
       }
   );
   
   // To launch MenuActivity
   menuLauncher.launch(new Intent(this, MenuActivity.class));
   

2. Return a result from MenuActivity:
   When the user clicks the back button in MenuActivity, send a result before finishing:

   // In MenuActivity's back button click listener
   backButton.setOnClickListener(v -> {
       // Optional: Pass data back to MainActivity
       Intent resultIntent = new Intent();
       resultIntent.putExtra("menu_selected", "some_value");
       setResult(RESULT_OK, resultIntent);
       finish();
   });
   

   This will trigger onActivityResult() (or the AndroidX callback) in MainActivity.

For other child pages (trigger onNewIntent()) #

We’ll use Activity launch mode to reuse the existing MainActivity instance:

1. Set MainActivity’s launch mode to singleTop:
   Update your AndroidManifest.xml:

   <activity
       android:name=".MainActivity"
       android:launchMode="singleTop">
       <intent-filter>
           <action android:name="android.intent.action.MAIN" />
           <category android:name="android.intent.category.LAUNCHER" />
       </intent-filter>
   </activity>
   

   singleTop means if MainActivity is already at the top of the task stack, a new launch request will reuse the existing instance instead of creating a new one.
2. Launch MainActivity from other child pages:
   Instead of just finishing the child Activity, explicitly launch MainActivity with a new Intent:

   // In OtherActivity's back button click listener
   backButton.setOnClickListener(v -> {
       Intent mainIntent = new Intent(this, MainActivity.class);
       // Pass data to MainActivity if needed
       mainIntent.putExtra("from_child", "OtherActivity");
       startActivity(mainIntent);
       finish(); // Close the child Activity
   });
   

3. Handle the new Intent in MainActivity:
   Override onNewIntent() to process the incoming Intent:

   // In MainActivity
   @Override
   protected void onNewIntent(Intent intent) {
       super.onNewIntent(intent);
       // Update the Activity's current Intent (important for future references)
       setIntent(intent);
       // Handle the data from the child page
       String fromChild = intent.getStringExtra("from_child");
       if ("OtherActivity".equals(fromChild)) {
           // Execute your desired logic here
       }
   }
   

This setup ensures that returning from MenuActivity triggers the result flow, while returning from other child pages reuses the MainActivity instance and fires onNewIntent().

内容的提问来源于stack exchange，提问作者Jung Jaehoon