上下文不是上下文？ （不启动对话框）

Question

- 这可能不是问题 - 我现在无法在修复设备并尝试恢复之后重现此设备。

所以我有点困惑，但这里有一些代码。我更改了代码以简化它，所以我可能有一个错字，但这是我所看到的“要点”。但基本上在某些设备上，示例1不会启动对话框，而选项2会在所有设备上启动它：

1. 为什么设备/ android版本的区别？
2. 上下文应该不一样吗？我知道Application Context＆amp;查看上下文不同。但不确定为什么这应该有效。我知道我不应该使用getApplicationContext()作为视图..但仍然有点困惑。

（请注意MyTestActivity是MyActivity的子类）

编辑：在选项＃1中，MyActivity从类变量中获取Context并使用它来启动对话框。 在选项＃2中，MyTestActivity获取上下文（应该与MyActivity相同的上下文？）并将其传递给showMessage。 （showMessage在示例＃2中采用了一个新参数）

示例＃1

///////////////////////////////////////////
// THIS Example works on Galaxy Nexus, but not on some other devices, such as : 
// SAMSUNG-SGH-I997. (infuse) - this just simply does not display anything.

public class MyTestActivity extends MyActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.my_layout);
        Intent i = new Intent(MyTestActivity.this, ActivityTwo.class);
        startActivityForResult(i, Constants.FROM_ACTIVITY);
    }

    @Override 
    public void onActivityResult(int requestCode, int resultCode, Intent data) {     
        super.onActivityResult(requestCode, resultCode, data); 
        IntentResult intentResult = IntentIntegrator.parseActivityResult(requestCode, 
                   resultCode, data);
        if ((intentResult != null) && (resultCode == RESULT_OK)) {
             playsound(SOUND_DING); // User hears this - SoundPool
             showMessage("Test Title","Test Message"); // User does not see message
        }
    }
}

public class MyActivity extends Activity {
    private Context mContext;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mContext=this;
    }

    public void showMessage(String title, String message) {
        AlertDialog.Builder builder = new AlertDialog.Builder(mContext);
    builder.setTitle(title);
    builder.setMessage(message)
               .setPositiveButton(mContext.getString(R.string.ok), 
                new DialogInterface.OnClickListener() {
                   public void onClick(DialogInterface dialog, int id) {

                            finish();

                   }
               });
    AlertDialog alert = builder.create();
    alert.show();
    }
}

示例＃2

/////////////////////////////////////////////////////////////
// This works on all devices
    public class MyTestActivity extends MyActivity {
        private Context context;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.my_layout);
            context = this;
        Intent i = new Intent(MyTestActivity.this, ActivityTwo.class);
        startActivityForResult(i, Constants.FROM_ACTIVITY);
        }

        @Override 
        public void onActivityResult(int requestCode, int resultCode, Intent data) {     
            super.onActivityResult(requestCode, resultCode, data); 
            IntentResult intentResult = IntentIntegrator.parseActivityResult(requestCode, 
                       resultCode, data);
            if ((intentResult != null) && (resultCode == RESULT_OK)) {
                 playsound(SOUND_DING); // user hears this
                 showMessage("Test Title","Test Message", context); // user sees this
            }
        }
    }

    public class MyActivity extends Activity {
        private Context mContext;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            mContext=this;
        }

        public void showMessage(String title, String message, Context context) {
            AlertDialog.Builder builder = new AlertDialog.Builder(context);
        builder.setTitle(title);
        builder.setMessage(message)
                   .setPositiveButton(context.getString(R.string.ok), 
                    new DialogInterface.OnClickListener() {
                       public void onClick(DialogInterface dialog, int id) {

                                finish();

                       }
                   });
        AlertDialog alert = builder.create();
        alert.show();
        }
    }

Answer 1

答案很简单。如果您未在覆盖super.onCreate(savedInstanceState);中拨打onCreate()，则会收到垃圾而不是context。如果您致电super.onCreate(savedInstanceState);，则可以使用context。