對話框投擲「無法添加窗口 - 令牌null不是一個應用程序」

Question

我的代碼：

    public class MainActivity extends Activity { 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     NoticeBoard.init(MainActivity.this); 
     NoticeBoard.getInstance().setButton(); 
     setContentView(NoticeBoard.getInstance()); 
    } 

} 

    public class NoticeBoard extends ImageView { 
    private static NoticeBoard instance = null; 
    private static Paint paint; 
    protected Context mContext; 
    private static WindowManager mWindowManager; 

    public NoticeBoard(Context context, AttributeSet attrs) { 
     super(context, attrs); 
    } 

    public static NoticeBoard getInstance() { 
     return instance; 
    } 

    public static void init(Context context) { 
     instance = new NoticeBoard(context); 
     instance.setImageResource(R.drawable.ic_launcher); 
     instance.invalidate(); 
    } 

    public NoticeBoard(Context context) { 
     super(context); 
     mWindowManager = (WindowManager) getContext().getApplicationContext().getSystemService(Context.WINDOW_SERVICE); 
     mContext = context; 
     setBackgroundColor(Color.BLACK); 
    } 

    @Override 
    protected void onDraw(Canvas canvas) { 
     super.onDraw(canvas); 
     paint = new Paint(Paint.LINEAR_TEXT_FLAG); 
     paint.setColor(Color.GRAY); 
     paint.setTextSize(12.0F); 
     canvas.drawText("Hello World", 100, 200, paint); 
    } 

    @Override 
    public boolean onTouchEvent(MotionEvent event) { 
     return super.onTouchEvent(event); 
    } 

    public void setButton() { 
     Activity a =(Activity) mContext; 
     ImageButton btn = new ImageButton(a); 
     btn.setBackgroundResource(R.drawable.ic_launcher); 
     android.view.WindowManager.LayoutParams paramsSplit = new android.view.WindowManager.LayoutParams(); 
     paramsSplit.width=2; 
     paramsSplit.height=2; 
     instance.mWindowManager.addView(btn, paramsSplit); 
    } 
} 

我試圖找到答案，我改變getApplicationContext（）來MainActivity.this，並改變上下文？（活動）上下文，但它仍然不能正常工作有什麼不妥的地方 Ps.I'm中國人，我的英語不太好感謝您的幫助

Answer 1

在下面一行：。

mWindowManager = (WindowManager) getContext().getApplicationContext().getSystemService(Context.WINDOW_SERVICE); 

刪除對應用上下文的引用

mWindowManager = (WindowManager) getContext().getSystemService(Context.WINDOW_SERVICE); 

活動的上下文就足夠了。