API Demos 2.2 研读笔记（9）——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。

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参考API Demo示例：

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

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 {       @Overrideprotected 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);    }  @Overrideprotected 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;}

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

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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。

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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。

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{    @Overrideprotected 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();        }    };}

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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。

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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。

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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。

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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。