Detecting Direction of Mouse Movement
       code by Macaroni Ted, write-up by kirupa   |  30 July 2005

In Flash, you have the onMouseMove event that executes any code when your mouse is moving. In some cases, executing code simply based on mouse movement may not be enough. How would you like to learn how to determine the direction of mouse movement so that you can execute different pieces of code depending on which direction the mouse is moving?

Note

For the ActionScript 3 version of this code, please see the following blog post.

In this short tutorial, I will explain just that. The code is based on Macaroni Ted's (see thread) implementation of mouse movement direction. Here is the code:

checkX = function (dx, oldVal, newVal) {
if (oldVal<newVal) {
trace("moving right");
} else if (oldVal>newVal) {
trace("moving left");
}
return newVal;
};
checkY = function (dy, oldVal, newVal) {
if (oldVal<newVal) {
trace("down");
} else if (oldVal>newVal) {
trace("up");
}
return newVal;
};
this.watch("xdir", checkX);
this.watch("ydir", checkY);
this.onMouseMove = function() {
xdir = _xmouse;
ydir = _ymouse;
};

Simply paste the above code onto a frame in Flash, and preview the animation in Flash. Notice that the code traces the direction your mouse is moving!

Code Explanation
As always, here is an explanation of why the code does what it does.

this.onMouseMove = function() {

Any code contained in the function referred by the onMouseMove event handler will execute when the mouse cursor moves.


xdir = _xmouse;
ydir = _ymouse;

I create two new variables, xdir and ydir to store the x and y position of our mouse. Since they are both contained within the onMouseMove event handler, the values of xdir and ydir only change when the mouse cursor is moved.


this.watch("xdir", checkX);
this.watch("ydir", checkY);

This brings us to the most interesting part of the tutorial, and to a topic that has not been covered on this site at all: the watch function.

The watch function is pretty neat. What it does, is if a property or variable it is interested in is changed, it calls on a callback function to do some "stuff". I'm being intentionally vague, but you will be overwhelmed with details shortly.

In our code, the variables our watch function is watching is xdir and ydir variables. If you remember, they both store the x and y position of your mouse cursor. When either of those variables change due to you moving the mouse, the watch function is calls either the checkX or checkY callback function depending on whether xdir or ydir changed.

The callback functions that the watch function calls on have to fit the following format for most cases:

callback(prop, oldVal, newVal)

To recap, our watch function checks to see whether the xdir or ydir variables have been changed. If any of these variables did change, then it calls on either the checkX or checkY functions.


checkX = function (dx, oldVal, newVal) {
if (oldVal<newVal) {
trace("moving right");
} else if (oldVal>newVal) {
trace("moving left");
}
return newVal;
};

The above is our callback function for checking movement in the x direction, and notice that it takes in the three arguments I mentioned earlier.

When the checkX function is called initially, oldVal is undefined  and newVal contains the x position of your mouse cursor. As you keep moving the mouse, you will notice that your new mouse position replaces the old newVal, and that the old newVal becomes your new oldVal.

To reduce the confusion of my above paragraph, here is a series of trace actions that highlight what happens as the mouse moves:

oldVal is: undefined and newVal is: 102
oldVal is: 102 and newVal is: 135
oldVal is: 135 and newVal is: 137
oldVal is: 137 and newVal is: 141
oldVal is: 141 and newVal is: 143
oldVal is: 143 and newVal is: 145
oldVal is: 145 and newVal is: 147

Notice how the initial value for oldVal is undefined, and the values for newVal and oldVal alternated as I kept moving the mouse. The reason oldVal and newVal alternate, is because of your return statement:

return newVal;

Because you return newVal, the watch function's oldVal variable is replaced with the value from newVal.


if (oldVal<newVal) {
trace("down");
} else if (oldVal>newVal) {
trace("up");
}

This code is fairly straightforward. You check to see if oldVal is less than newVal or if oldVal is greater than newVal. oldVal is analogous to your old mouse position and newVal is analogous to your new mouse position. I simply have a trace function that traces which direction the mouse is moving, but you would probably replace it with code that does something...useful!

For more information and gritty details of the watch function, take a look at Macromedia's LiveDoc article on it.

Other Versions

There are several versions of the mouse direction detection script that I and others created before settling on Marconi Ted's version, and you can find them all here: //www.kirupa.com/forum/showthread.php?t=186469 

If you have a question about this or any other topic, the easiest thing is to drop by our forums where a bunch of the friendliest people you'll ever run into will be happy to help you out!

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