Android(安卓)2.2 API Demos -- 通过调用子Activity返回值

我们使用Intent可以将数据从一个Activity传递到下一个Activity，同样，在Android中我们可以将数据从一个Activity返回给前一个Activity。

参考API Demo示例：

1. 定义父Activity，ReceiveResult.java。在这个Activity中我们通过startActivityForResult(intent, GET_CODE)启动子Activity。
Java代码

package com.example.android.apis.app;import java.util.Map;import com.example.android.apis.R;import android.app.Activity;import android.content.Intent;import android.text.Editable;import android.os.Bundle;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.TextView;import java.util.Map;public class ReceiveResult extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {    // Be sure to call the super class.    super.onCreate(savedInstanceState);    // See assets/res/any/layout/hello_world.xml for this    // view layout definition, which is being set here as    // the content of our screen.    setContentView(R.layout.receive_result);    // Retrieve the TextView widget that will display results.    mResults = (TextView)findViewById(R.id.results);    // This allows us to later extend the text buffer.    mResults.setText(mResults.getText(), TextView.BufferType.EDITABLE);    // Watch for button clicks.    Button getButton = (Button)findViewById(R.id.get);    getButton.setOnClickListener(mGetListener);  } @Override    protected void onActivityResult(int requestCode, int resultCode,        Intent data) {    // You can use the requestCode to select between multiple child    // activities you may have started. Here there is only one thing    // we launch.    if (requestCode == GET_CODE) {      // We will be adding to our text.      Editable text = (Editable)mResults.getText();      // This is a standard resultCode that is sent back if the      // activity doesn't supply an explicit result. It will also      // be returned if the activity failed to launch.      if (resultCode == RESULT_CANCELED) {        text.append("(cancelled)");      // Our protocol with the sending activity is that it will send      // text in 'data' as its result.      } else {        text.append("(okay ");        text.append(Integer.toString(resultCode));        text.append(") ");        if (data != null) {          text.append(data.getAction());        }      }      text.append("\n");    }  }  // Definition of the one requestCode we use for receiving resuls.  static final private int GET_CODE = 0;  private OnClickListener mGetListener = new OnClickListener() {    public void onClick(View v) {      // Start the activity whose result we want to retrieve. The      // result will come back with request code GET_CODE.      Intent intent = new Intent(ReceiveResult.this, SendResult.class);      startActivityForResult(intent, GET_CODE);    }  };  private TextView mResults;}

我们需要为startActivityForResult传递一个Intent和一个请求码。

Intent决定启动哪个Activity。请求码是对子Activity标记的唯一ID。假如从一个父Activity中可能启动的子Activity有多个，那么通过请求码我们就可以知道是从哪个子Activity返回的。

2. 定义子Activity，SendResult.java。在这个Activity中我们通过setResult(RESULT_OK, (new Intent()).setAction("Corky!"))将结果返回给父Activity。
Java代码

package com.example.android.apis.app;import com.example.android.apis.R;import android.app.Activity;import android.content.Intent;import android.os.Bundle;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;public class SendResult extends Activity{  @Override    protected void onCreate(Bundle savedInstanceState)  {    // Be sure to call the super class.    super.onCreate(savedInstanceState);    // See assets/res/any/layout/hello_world.xml for this    // view layout definition, which is being set here as    // the content of our screen.    setContentView(R.layout.send_result);    // Watch for button clicks.    Button button = (Button)findViewById(R.id.corky);    button.setOnClickListener(mCorkyListener);    button = (Button)findViewById(R.id.violet);    button.setOnClickListener(mVioletListener);  }  private OnClickListener mCorkyListener = new OnClickListener()  {    public void onClick(View v)    {      // To send a result, simply call setResult() before your      // activity is finished.      setResult(RESULT_OK, (new Intent()).setAction("Corky!"));      finish();    }  };  private OnClickListener mVioletListener = new OnClickListener()  {    public void onClick(View v)    {      // To send a result, simply call setResult() before your      // activity is finished.      setResult(RESULT_OK, (new Intent()).setAction("Violet!"));      finish();    }  };}

在调用finish()之前，要先调用setResult方法，将结果码和数据返回给父Activity。在android.app.Activity中定义两个标准结果码，RESULT_OK和RESULT_CANCELED。

如果子Activity启动失败或者没有显式的在setResult()中设置结果码，子Activity会默认返回RESULT_CANCELED。

3. 在父Activity中，我们重写onActivityResult(int requestCode, int resultCode, Intent data)方法，接收子Activity返回的数据。参考1中的代码。

请求码（requestCode）：通过它我们可以对多个子Activity进行处理。

结果码（resultCode）：通过它我们可以判断子Activity的处理结果，对不同的结果码进行相应的操作。

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