listeners don't work correctly. What is the problem? Please help me.

The examples don't work as below.

------------

....

xtype: 'xxx',

...

listeners: {

tap: this.onTapA,

scope: this

}

....

-------------

listeners: {

tap: 'onTapA',

scope: this

}

-----------

listeners: {

tap: 'onTapA',

scope: 'this'

}

listeners don't work correctly. What is the problem? Please help me.