Android Extending View and creating from XML Null pointer exception

Question

Hi all,

I've created a custom view by extending View, it draws fine but When i try and refer to the object in the xml so I can say add a touch listener if reports a null pointer exception. My xml file is extremely simple

<com.projector.interaction.layout.MapMouseView2
    xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/mouse"
    android:layout_width="fill_parent" android:layout_height="fill_parent">
</com.projector.interaction.layout.MapMouseView2>

I then try and referene the View by

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    try {
        setContentView(R.layout.testlayout);
        MapMouseView2 view = (MapMouseView2)findViewById(R.id.mouse);
        view.setOnTouchListener(new OnTouchListener() {

            @Override
            public boolean onTouch(View v, MotionEvent event) {
                Log.d("Debug","Touch Not Working");
                return true;
            }
        }); 

    } catch (Exception e) {
        Log.e("Error","Error in TestActivity",e);
    }

}

But it gives me a null pointer exception. Then only fix I can find is to wrap my view in the xml file inside a LinearLayout and reference the view by

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    try {
        setContentView(R.layout.testlayout);

        LinearLayout lay = (LinearLayout)findViewById(R.id.lin);
        MapMouseView2 mouseView = (MapMouseView2)lay.getChildAt(0);
        mouseView.setOnTouchListener(new OnTouchListener() {

            @Override
            public boolean onTouch(View v, MotionEvent event) {
                Log.d("Debug","Touch Not Working");
                return true;
            }
        }); 

    } catch (Exception e) {
        Log.e("Error","Error in TestActivity",e);
    }

}

Can anybody please tell me what I am doing wrong???

The View class is here

public class MapMouseView2 extends View{


WindowManager mWinMgr;
Rect zoom;
Rect panLeftRight;
Rect panUpDown;



public MapMouseView2(Context context,AttributeSet attrs){
    super(context);
    Log.d("Debug","Context,AttribyteSet");
    try {
        mWinMgr = (WindowManager) context
        .getSystemService(Context.WINDOW_SERVICE);
        zoom = new Rect(0, 0, 45, mWinMgr.getDefaultDisplay().getHeight());
        panLeftRight = new Rect(0, 458, mWinMgr.getDefaultDisplay().getWidth(),
                mWinMgr.getDefaultDisplay().getHeight());
        panUpDown = new Rect(mWinMgr.getDefaultDisplay().getWidth() - 45, 0,
                mWinMgr.getDefaultDisplay().getWidth(), mWinMgr
                .getDefaultDisplay().getHeight());
        Log.d("Debug","Leaving Constructor");
    } catch (Exception e) {
        Log.e("Error","Error in MapMouseView",e);
    }
}




protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    try {
        Paint paint = new Paint();
        paint.setStyle(Paint.Style.FILL);
        // make the entire canvas white
        paint.setColor(Color.BLACK);
        canvas.drawPaint(paint);
        paint.setColor(Color.BLUE);
        canvas.drawRect(zoom, paint);
        paint.setColor(Color.RED);
        canvas.drawRect(panLeftRight, paint);
        paint.setColor(Color.RED);
        canvas.drawRect(panUpDown, paint);
        //canvas.drawCircle(x,y,30,threadPaint);

    } catch (Exception e) {
        Log.e("Error","Exception in OnDraw",e);
    }


}

}

Answer 1

I think you should call super(context, attrs); in constructor of MapMouseView2.

edit:

If you don't do that your view doesn't get id.

In the second approach you refer to the view by position not name, so it works.