例子

为了在上下文中了解整个过程，这里有一个补充的答案。更多的解释请看我的完整回答。

MainActivity.java

public class MainActivity extends AppCompatActivity {

    // Add a different request code for every activity you are starting from here
    private static final int SECOND_ACTIVITY_REQUEST_CODE = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    // "Go to Second Activity" button click
    public void onButtonClick(View view) {

        // Start the SecondActivity
        Intent intent = new Intent(this, SecondActivity.class);
        startActivityForResult(intent, SECOND_ACTIVITY_REQUEST_CODE);
    }

    // This method is called when the second activity finishes
    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        // check that it is the SecondActivity with an OK result
        if (requestCode == SECOND_ACTIVITY_REQUEST_CODE) {
            if (resultCode == RESULT_OK) { // Activity.RESULT_OK

                // get String data from Intent
                String returnString = data.getStringExtra("keyName");

                // set text view with string
                TextView textView = (TextView) findViewById(R.id.textView);
                textView.setText(returnString);
            }
        }
    }
}

SecondActivity.java

public class SecondActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);
    }

    // "Send text back" button click
    public void onButtonClick(View view) {

        // get the text from the EditText
        EditText editText = (EditText) findViewById(R.id.editText);
        String stringToPassBack = editText.getText().toString();

        // put the String to pass back into an Intent and close this activity
        Intent intent = new Intent();
        intent.putExtra("keyName", stringToPassBack);
        setResult(RESULT_OK, intent);
        finish();
    }
}

对于那些在onActivityResult中有错误requestCode问题的人

如果你在Fragment中调用startActivityForResult()， requestCode会被拥有Fragment的Activity改变。

如果你想在你的活动中得到正确的resultCode，试试这个:

变化:

startActivityForResult(意图,1);:

getActivity()。startActivityForResult(意图,1);

ActivityResultRegistry是推荐的方法

ComponentActivity现在提供了一个ActivityResultRegistry，它可以让你处理startActivityForResult()+onActivityResult()以及requestPermissions()+onRequestPermissionsResult()流，而无需覆盖你的Activity或Fragment中的方法，通过ActivityResultContract带来了更高的类型安全性，并提供了用于测试这些流的钩子。

强烈建议使用Android 10 Activity 1.2.0-alpha02和Fragment 1.3.0-alpha02中引入的Activity Result api。

将此添加到build.gradle中

def activity_version = "1.2.0-beta01"

// Java language implementation
implementation "androidx.activity:activity:$activity_version"
// Kotlin
implementation "androidx.activity:activity-ktx:$activity_version"

如何使用预建合同

这个新的API具有以下预构建的功能

TakeVideo PickContact GetContent GetContents OpenDocument opendocument OpenDocumentTree CreateDocument 刻度盘 TakePicture RequestPermission RequestPermissions

下面的例子使用了takePicture合约:

private val takePicture = prepareCall(ActivityResultContracts.TakePicture()) { bitmap: Bitmap? ->
    // Do something with the Bitmap, if present
}

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    button.setOnClickListener { takePicture() }
}

So what’s going on here? Let’s break it down slightly. takePicture is just a callback which returns a nullable Bitmap - whether or not it’s null depends on whether or not the onActivityResult process was successful. prepareCall then registers this call into a new feature on ComponentActivity called the ActivityResultRegistry - we’ll come back to this later. ActivityResultContracts.TakePicture() is one of the built-in helpers which Google have created for us, and finally invoking takePicture actually triggers the Intent in the same way that you would previously with Activity.startActivityForResult(intent, REQUEST_CODE).

如何编写自定义合同

一个简单的契约，它接受一个Int作为输入，并返回一个被请求的Activity在结果Intent中返回的字符串。

class MyContract : ActivityResultContract<Int, String>() {

    companion object {
        const val ACTION = "com.myapp.action.MY_ACTION"
        const val INPUT_INT = "input_int"
        const val OUTPUT_STRING = "output_string"
    }

    override fun createIntent(input: Int): Intent {
        return Intent(ACTION)
            .apply { putExtra(INPUT_INT, input) }
    }

    override fun parseResult(resultCode: Int, intent: Intent?): String? {
        return when (resultCode) {
            Activity.RESULT_OK -> intent?.getStringExtra(OUTPUT_STRING)
            else -> null
        }
    }
}

class MyActivity : AppCompatActivity() {

    private val myActionCall = prepareCall(MyContract()) { result ->
        Log.i("MyActivity", "Obtained result: $result")
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        ...
        button.setOnClickListener {
            myActionCall(500)
        }
    }
}

查看官方文档了解更多信息。

从你的FirstActivity，使用startActivityForResult()方法调用SecondActivity。

例如:

int LAUNCH_SECOND_ACTIVITY = 1
Intent i = new Intent(this, SecondActivity.class);
startActivityForResult(i, LAUNCH_SECOND_ACTIVITY);

在你的SecondActivity中，设置你想返回给FirstActivity的数据。如果你不想返回，就不要设置任何。

例如:在SecondActivity中，如果你想发回数据:

Intent returnIntent = new Intent();
returnIntent.putExtra("result",result);
setResult(Activity.RESULT_OK,returnIntent);
finish();

如果你不想返回数据:

Intent returnIntent = new Intent();
setResult(Activity.RESULT_CANCELED, returnIntent);
finish();

现在在FirstActivity类中，为onActivityResult()方法编写以下代码。

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == LAUNCH_SECOND_ACTIVITY) {
        if(resultCode == Activity.RESULT_OK){
            String result=data.getStringExtra("result");
        }
        if (resultCode == Activity.RESULT_CANCELED) {
            // Write your code if there's no result
        }
    }
} //onActivityResult

要在Kotlin中以更好的方式实现两个活动之间的数据传递，请参见“在活动之间传递数据的更好方式”。

首先，你在第一个活动中使用startActivityForResult()和参数，如果你想从第二个活动发送数据到第一个活动，然后使用意图通过setResult()方法传递值，并在第一个活动中的onActivityResult()方法中获取数据。

如何检查主要活动的结果?

你需要重写Activity.onActivityResult()，然后检查它的参数:

requestCode标识哪个应用程序返回了这些结果。这是在调用startActivityForResult()时由您定义的。 resultCode告诉你这个应用程序是成功了、失败了还是其他什么 data保存此应用程序返回的任何信息。这可能是空的。