Question

I’m having issues using my class to create a mouse listener. First I create the class (new mouseEventManager()), then I call the init() function, which I want to set up my zoomHandler function. It must use the class’ variable scale_level in order to appropriately scale the screen. However I’m told that scale_level is NaN (not a number).

Here is the code:

function mouseEventManager() {

this.designManager;
this.canvasManager;
this.scale_level; 

this.init = function(designManager, canvasManager) {

this.designManager = designManager;
this.canvasManager = canvasManager; 
this.scale_level = 1; alert(this.scale_level);

var designManager = this.designManager;

//general mousemove listener
$(document).mousemove(function(e){
    designManager.HAND_X = e.pageX;
    designManager.HAND_Y = e.pageY;
}); 


//create scroll wheel listener
if(window.addEventListener) { document.addEventListener('DOMMouseScroll', this.zoomHandler, false); } 
document.onmousewheel = this.zoomHandler;

}


this.zoomHandler = function() {

var delta = 0;

if (!event) event = window.event;
// normalize the delta
if (event.wheelDelta) {
        // IE and Opera
        delta = event.wheelDelta / 60;
} else if (event.detail) {
        // W3C
        delta = -event.detail / 2;
}

if (delta > 0) {
    //scroll wheel up
    this.scale_level = this.scale_level * 1.25; alert(this.scale_level);
    this.canvasManager.mainLayer.setScale(this.scale_level);
    this.canvasManager.mainLayer.draw();
} else if (delta < 0) {
    //scroll wheel down
    this.scale_level = this.scale_level * 0.8;
    this.canvasManager.mainLayer.setScale(this.scale_level);
    this.canvasManager.mainLayer.draw();
}

alert(delta);

}


}

It looks like within zoomHandler, none of the mouseEventManager’s internal members are defined. So I think I’m doing something wrong with variable scoping or something related.

Thank you for any help.

Answer 1

The problem is that this in your handler won’t be what you need it to be when the handler is actually invoked.

Try changing things as follows:

var manager = this;
this.zoomHandler = function() {
   // ...

then where you use this in the function, instead use manager, the “saved” reference to the object.

if (delta > 0) {
    //scroll wheel up
    manager.scale_level = this.scale_level * 1.25; alert(manager.scale_level);
    manager.canvasManager.mainLayer.setScale(manager.scale_level);
    manager.canvasManager.mainLayer.draw();
} else if (delta < 0) {
    //scroll wheel down
    manager.scale_level = manager.scale_level * 0.8;
    manager.canvasManager.mainLayer.setScale(manager.scale_level);
    manager.canvasManager.mainLayer.draw();
}