Android：跨类启动监听器

Question

Hey I am currently working myself my way into custom views on Android but I now run into a problem setting a listener. If I call the listener from within the custom view class, everything works, but I want to set the listener from my MainActivity which is where I am running into problems right now. Every time, I call the Listener from my MainActivity I just get a nullpointer, no matter what I pass to the method. Here the code of the custom class

public class RoundKnobButton extends RelativeLayout implements OnGestureListener {

public interface RoundKnobButtonListener {
    public void onStateChange(boolean newstate) ;
    public void onRotate(int percentage);
}


public RoundKnobButtonListener m_listener;

public void SetListener(RoundKnobButtonListener l) {
    m_listener = l;
} 

and if I call the Listener from within this class like

     SetListener(new RoundKnobButtonListener() {
            public void onStateChange(boolean newstate) {

            }

            public void onRotate(final int percentage) {
            System.out.println(percentage); 
     }
                }); 

It returns the value perfectly. Now, when I want to do something like

rv=(RoundKnobButton) findViewById(R.id.jogView);
rv.SetListener(new RoundKnobButton.RoundKnobButtonListener() {
public void onStateChange(boolean newstate) {
}

public void onRotate(final int percentage) {
    System.out.println(percentage); 
        }
    }); 

in my MainActivity, I get

02-10 14:31:45.354: W/dalvikvm(19006): threadid=1: thread exiting with uncaught exception (group=0x418259a8)
02-10 14:31:45.360: E/AndroidRuntime(19006): FATAL EXCEPTION: main
02-10 14:31:45.360: E/AndroidRuntime(19006): java.lang.NullPointerException
02-10 14:31:45.360: E/AndroidRuntime(19006):    at  .buildUI( .java:191)
02-10 14:31:45.360: E/AndroidRuntime(19006):    at  .access$6( .java:125)
02-10 14:31:45.360: E/AndroidRuntime(19006):    at  .onPostExecute( .java:334)

Where line 191 refers to rv.SetListener(new RoundKnobButton.RoundKnobButtonListener()

I already tried passing all kinds of parameters with all kinds of casts, but nothing seems to help here.

Thanks in advance, Alex

Answer 1

Thanks to KevDev, I found the problem. I called

super(context);

but I shold've called super(context,attrs); in my constructer as it is public RoundKnobButton(Context context, AttributeSet attrs) otherwise it of course can't return the ID

Thanks