Android自用-----WindowManager$BadTokenException: Unable to add window -- token null is not for an application

private Context mcontext;

    @Override
    protected void onCreate(Bundle savedInstanceState) {mcontext = getApplicationContext();
        System.out.println("mcontext=" + mcontext);

    }

new AlertDialog.Builder(mcontext)
            .setIcon(android.R.drawable.ic_dialog_alert)
            .setTitle("Warnning")
            .setMessage(
                    "You forget to write the message. Do you want to fill out it ??")
            .setPositiveButton("Yes", positiveListener).setNegativeButton(
                    "No", negativeListener).create().show();

導致報這個錯是在於new AlertDialog.Builder(mcontext)，雖然這里的參數是AlertDialog.Builder(Context context)但我們不能使用getApplicationContext()獲得的Context,而必須使用Activity,因為只有一個Activity才能添加一個窗體。

 

 解決方法：

將new AlertDialog.Builder(Context context)中的參數用Activity.this（Activity是你的Activity的名稱）來填充就可以正確的創建一個Dialog了。

new AlertDialog.Builder(MyActivity.this)
                .setIcon(android.R.drawable.ic_dialog_alert)
                .setTitle("Warnning")
                .setMessage(
                        "You forget to write the message. Do you want to fill out it ??")
                .setPositiveButton("Yes", positiveListener).setNegativeButton(
                        "No", negativeListener).create().show();