如何通过扩展Button的类调用MainActivity方法

Question

I want to call a method showMessage() which is defined in my main activity from a TestButton class that extend Button. The code I use below throw a null pointer exception. I am not sure how to solve this. Can you please help me?

MainActivity

public class MainActivity extends MultiTouchActivity {

TestButton btn;
@Override
public void onCreate(final Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    btn = (TestButton)findViewById(R.id.button4);
    btn.setOnTouchListener(this);
}
public void showMessage()
{
    Toast.makeText(this, "hello", Toast.LENGTH_SHORT).show();
}

TestButton class

public class TestButton extends Button {

MainActivity m;
public TestButton(final Context context, final AttributeSet attrs)
{
    super(context, attrs);

    // TODO Auto-generated constructor stub
}

@Override
public boolean onTouchEvent(final MotionEvent event)
{
    if (event.getAction() == MotionEvent.ACTION_UP)
    {
        m = new MultitouchtestActivity();
        m.showMessage();
    }
    return super.onTouchEvent(event);
}

}

EDIT Thank you @ΦXoce 웃 Пepeúpa this solve my issue but I made a little bit changes.

btn.setCallback(this); from btn.setCallback();

and

public void setCallback(final ICallback iCallback)

Answer 1

2 things:

1. m = new MultitouchtestActivity(); is not the way ANDROID wants you to create / instantiate Activities.. so you have more chances to break your code than doing something right by calling the constructor directly..
2. On the other hand trying to call a method from the Activity inside of a button class is forcing you to make circle-dependencies... now your Activity needs a button Object and your button needs an Activity Object..(not a good approach)

How to solve it

Implement a callback like doing:

interface ICallback{
    void showMessage();
}

public class MainActivity extends MultiTouchActivity implements ICallback{

TestButton btn;

@Override
public void onCreate(final Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    btn = (TestButton)findViewById(R.id.button4);
    btn.setOnTouchListener(this);
    //register here the callback
    btn.setCallback(this);
}

@Override
public void showMessage()
{
    Toast.makeText(this, "hello", Toast.LENGTH_SHORT).show();
}

public class TestButton extends Button {

MainActivity m;
ICallback iCallback;
public setCallback(final ICallback iCallback)
{
    this.iCallback = iCallback; 
}
public TestButton(final Context context, final AttributeSet attrs)
{
    super(context, attrs);

    // TODO Auto-generated constructor stub
}

@Override
public boolean onTouchEvent(final MotionEvent event)
{
    if (event.getAction() == MotionEvent.ACTION_UP)
    {
        iCallback.showMessage();
    }
    return super.onTouchEvent(event);
}
}