Question

我正在尝试使DialogFragment包含一个警告对话框。它包含一个接口，该接口持有一个方法，该方法应该在按下对话框的取消/确定按钮时调用。

现在的问题是我想在用户点击OK / Cancel时显示Toast.makeText，但我不知道我应该向Toast传递什么Context，以免崩溃。当用户单击对话框按钮时我的应用程序崩溃，我猜上下文导致异常：

片段类：

public class AlertDFragment extends DialogFragment {

dialogListener ds;
Context con;

public interface dialogListener {
    public void onOK(Context c);
    public void onCancel(Context c);
}

@Override
public Dialog onCreateDialog(Bundle savedInstances) {
    Builder b = new AlertDialog.Builder(getActivity());

    b.setIcon(R.drawable.abc_ic_search);
    b.setTitle("DialogFragment instance");
    b.setMessage("Choose an option");
    b.setPositiveButton("OK", new DialogInterface.OnClickListener() {

        public void onClick(DialogInterface dialog, int which) {
            ds.onOK(con);
        }
    });

    b.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {

        public void onClick(DialogInterface arg0, int arg1) {
            ds.onCancel(con);
        }
    });

    return b.create();
}

public AlertDFragment() {

}

protected void OnAttach(Activity activity) {
    super.onAttach(activity);
    con = activity;
    try {
        ds = (dialogListener) activity;
    } catch(ClassCastException e) {
        throw new ClassCastException(activity.toString()+" must implement the dialogListener interface");
    }
}

}

主要活动（片段的父级）：

public class MainActivity extends FragmentActivity implements AlertDFragment.dialogListener {

Button dialog2;
FragmentManager fm = getSupportFragmentManager();


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

    dialog2 = (Button) findViewById(R.id.btn2);
    dialog2.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View arg0) {
            AlertDFragment adf = new AlertDFragment();
            adf.show(fm, "MY TAG");
        }
    });

}

//interface methods
public void onOK(Context c) {
    Toast.makeText(MainActivity.this, "OK pressed", Toast.LENGTH_SHORT).show();     
}

public void onCancel(Context c) {
    Toast.makeText(MainActivity.this, "Cancel pressed", Toast.LENGTH_SHORT).show();
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

}

LogCat：

11-04 10:23:30.959: E/AndroidRuntime(2014): FATAL EXCEPTION: main
11-04 10:23:30.959: E/AndroidRuntime(2014): Process: com.apex.dialogtest, PID: 2014
11-04 10:23:30.959: E/AndroidRuntime(2014): java.lang.NullPointerException
11-04 10:23:30.959: E/AndroidRuntime(2014):     at com.apex.dialogtest.AlertDFragment$1.onClick(AlertDFragment.java:33)
11-04 10:23:30.959: E/AndroidRuntime(2014):     at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166)
11-04 10:23:30.959: E/AndroidRuntime(2014):     at android.os.Handler.dispatchMessage(Handler.java:102)
11-04 10:23:30.959: E/AndroidRuntime(2014):     at android.os.Looper.loop(Looper.java:136)
11-04 10:23:30.959: E/AndroidRuntime(2014):     at android.app.ActivityThread.main(ActivityThread.java:5017)
11-04 10:23:30.959: E/AndroidRuntime(2014):     at java.lang.reflect.Method.invokeNative(Native Method)
11-04 10:23:30.959: E/AndroidRuntime(2014):     at java.lang.reflect.Method.invoke(Method.java:515)
11-04 10:23:30.959: E/AndroidRuntime(2014):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
11-04 10:23:30.959: E/AndroidRuntime(2014):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
11-04 10:23:30.959: E/AndroidRuntime(2014):     at dalvik.system.NativeStart.main(Native Method)

提前致谢！

Answer 1

ds未初始化，因为protected void OnAttach(Activity activity)需要小写字母o。请务必始终使用@Override注释，否则会出错。

Answer 2

您在代码中有以下内容

dialogListener ds;

但你永远不会实例化它。然后，您可以在引用ds.onOK(con);上调用ds方法。这会导致空指针异常。

尝试在AlertDialog按钮上显示Toast时应用程序崩溃单击（在片段中）

2 个答案: