在Android中控制后退“按钮”

Question

我希望能够从子活动中获取文本数据值。当我尝试时，一切都很顺利。但是，当子活动被打开后，我只需在手机上点击“返回”按钮，就会抛出异常错误。我在Android NotePad生命周期控制教程中找到了一些信息，但很难理解。有人可以帮助我吗？这是我的代码：

public class SBooks extends ListActivity {
private String title_raw;
private SBooksDbAdapter mDbHelper;  
private static final int ACTIVITY_SEARCH = 0;   

private static final int SEARCH_ID = Menu.FIRST;     

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.sbooks_list);       
    mDbHelper = new SBooksDbAdapter(this);
    mDbHelper.open();                    

}

// Create Menu Option
@Override
public boolean onCreateOptionsMenu(Menu menu) {        
    boolean result = super.onCreateOptionsMenu(menu);
    menu.add(0, SEARCH_ID, 0, R.string.menu_search);            
    return result;      
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch(item.getItemId()){
    case SEARCH_ID:
        Intent intent = new Intent(this, SBooksSearch.class);
        startActivityForResult(intent, ACTIVITY_SEARCH);            

    return super.onOptionsItemSelected(item);
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent intent){
    super.onActivityResult(requestCode, resultCode, intent);
        switch(requestCode){
        case ACTIVITY_SEARCH:
            Bundle bundle = intent.getExtras();
            title_raw = bundle.getString(SBooksDbAdapter.KEY_TITLE_RAW);
            if(title_raw!=null){
                Cursor cursor = mDbHelper.searchData(title_raw);
                String[] from = new String[]{ SBooksDbAdapter.KEY_ROWID,
                        SBooksDbAdapter.KEY_TITLE, SBooksDbAdapter.KEY_LYRICS };
                int[] to = new int[]{ R.id.id, R.id.title, R.id.lyrics };
                SimpleCursorAdapter adapter = 
                    new SimpleCursorAdapter(this, R.layout.sbooks_row, cursor, from, to );
                setListAdapter(adapter);

        }

    }   
}

这是我的子活动:

}

这是我的子活动:

public class SBooksSearch extends Activity {
private EditText mTextSearch;
private Button searchButton;
private SBooksDbAdapter mDbHelper;

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);     
    setContentView(R.layout.sbooks_search); 

    mTextSearch = (EditText)findViewById(R.id.text_search);     
    searchButton = (Button)findViewById(R.id.btn_search);       

    searchButton.setOnClickListener(new View.OnClickListener(){
        public void onClick(View v){                
            Intent intent = new Intent();
            intent.putExtra(SBooksDbAdapter.KEY_TITLE_RAW, mTextSearch.getText().toString());               
            setResult(RESULT_OK, intent);
            finish();

        }
    });
}   

@Override
protected void onSaveInstanceState(Bundle outState){
    super.onSaveInstanceState(outState);

}
@Override
protected void onPause(){
    super.onPause();
    checkState();

}
@Override
protected void onResume(){
    super.onResume();       
}


private void checkState() {     

} }

这是Logcat：

08-12 18:44:39.211: WARN/InputManagerService(581): Starting input on non-focused client com.android.internal.view.IInputMethodClient$Stub$Proxy@43719140 (uid=10004 pid=623)
08-12 18:44:39.510: INFO/ActivityManager(581): Displayed activity com.original.sbooks/.SBooks: 4934 ms
08-12 18:44:40.821: WARN/KeyCharacterMap(730): No keyboard for id 0
08-12 18:44:40.821: WARN/KeyCharacterMap(730): Using default keymap: /system/usr/keychars/qwerty.kcm.bin
08-12 18:44:41.412: INFO/ARMAssembler(581): generated scanline__00000177:03515104_00001A01_00000000 [ 64 ipp] (89 ins) at [0x297608:0x29776c] in 952924 ns
08-12 18:44:41.520: INFO/ARMAssembler(581): generated scanline__00000177:03515104_00000A01_00000000 [ 46 ipp] (70 ins) at [0x297798:0x2978b0] in 517061 ns
08-12 18:44:42.171: INFO/ActivityManager(581): Starting activity: Intent { comp={com.original.sbooks/com.original.sbooks.SBooksSearch} }
08-12 18:44:52.196: WARN/ActivityManager(581): Launch timeout has expired, giving up wake lock!
08-12 18:44:52.279: WARN/ActivityManager(581): Activity idle timeout for HistoryRecord{436bfc80 {com.original.sbooks/com.original.sbooks.SBooksSearch}}
08-12 18:44:57.350: DEBUG/dalvikvm(620): GC freed 4103 objects / 231808 bytes in 75ms
08-12 18:45:05.130: DEBUG/dalvikvm(649): GC freed 2604 objects / 150112 bytes in 88ms
08-12 18:45:10.120: DEBUG/dalvikvm(623): GC freed 2750 objects / 149592 bytes in 71ms
08-12 18:45:11.920: INFO/ActivityManager(581): Displayed activity com.original.sbooks/.SBooksSearch: 29748 ms
08-12 18:45:17.263: WARN/ActivityManager(581): Activity pause timeout for HistoryRecord{436bfc80 {com.original.sbooks/com.original.sbooks.SBooksSearch}}
08-12 18:45:26.762: WARN/ActivityManager(581): Launch timeout has expired, giving up wake lock!
08-12 18:45:27.272: WARN/ActivityManager(581): Activity idle timeout for HistoryRecord{435a7760 {com.original.sbooks/com.original.sbooks.SBooks}}
08-12 18:46:04.905: DEBUG/AndroidRuntime(730): Shutting down VM
08-12 18:46:04.905: WARN/dalvikvm(730): threadid=3: thread exiting with uncaught exception (group=0x4000fe70)
08-12 18:46:04.905: ERROR/AndroidRuntime(730): Uncaught handler: thread main exiting due to uncaught exception
08-12 18:46:04.991: ERROR/AndroidRuntime(730): java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=0, result=0, data=null} to activity {com.original.sbooks/com.original.sbooks.SBooks}: java.lang.NullPointerException
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.deliverResults(ActivityThread.java:3005)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.handleSendResult(ActivityThread.java:3047)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.access$2300(ActivityThread.java:112)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1721)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.os.Looper.loop(Looper.java:123)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.main(ActivityThread.java:3948)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at java.lang.reflect.Method.invokeNative(Native Method)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at java.lang.reflect.Method.invoke(Method.java:521)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:782)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:540)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at dalvik.system.NativeStart.main(Native Method)
08-12 18:46:04.991: ERROR/AndroidRuntime(730): Caused by: java.lang.NullPointerException
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at com.original.sbooks.SBooks.onActivityResult(SBooks.java:111)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.Activity.dispatchActivityResult(Activity.java:3595)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.deliverResults(ActivityThread.java:3001)

- Dennie

---

如果你想在出现“异常错误”时得到别人的帮助，那么最好附上与该异常相关的Java堆栈跟踪信息。这些信息可以通过adb logcat、DDMS或Eclipse中的DDMS透视图找到。 - CommonsWare

我发布了logcat，这是你想要的吗？抱歉，我没有关于在Android上进行调试的经验。感谢您的帮助！ - Dennie

Answer 1

我认为Nate关于intent.getExtras()的NPE是正确的。

在执行任何操作之前，您应该检查结果代码的值。您也可能想要检查一下intent.hasExtra(SBooksDbAdapter.KEY_TITLE_RAW)是否存在，然后再获取该值。因此，代码应该像这样：

switch(requestCode){
case ACTIVITY_SEARCH:
    if (resultCode == RESULT_OK && intent.hasExtra(SBooksDbAdapter.KEY_TITLE_RAW)) {
        title_raw = intent.getStringExtra(SBooksDbAdapter.KEY_TITLE_RAW);
        ... do all that other stuff ...
    }
}

- Shaun K.

Answer 2

尝试向主活动的onActivityResult(...)方法中添加一些日志记录。子活动中的“返回”按钮可能会给您一个RESULT_CANCELED的结果代码，在这种情况下，意图可以为null。因此，你将在

Bundle bundle = intent.getExtras();

处得到NullPointerException。

- Nate

---

你说的“logging”是什么意思？我不知道怎么做。 - Dennie

简单的方法是使用 System.out.println("++++++++++++ " + resultCode); 然后在控制台中观察 "adb -e logcat" 并查找加号。 - Nate

推荐（同样简单）的方法是使用Log类：Log.i("your-log-tag", "your message");。你可以在LogCat中看到你的日志，并且还可以设置一个筛选器来只看到自己的消息。 - Ridcully