Home > Flash Games, Game Update, Programming > Programming A Click and Drag Function

Programming A Click and Drag Function

Last night I decided to program in a function to allow easier map scrolling when using mouse controls. Previously I had made it similar to the Wii’s turning functionality in first-person shooters. When you got near the edge of the screen, the game would start to scroll in that direction.

The problems arose when you were just trying to access buttons that were off screen because the game would try to scroll in the direction of the buttons. It was just too easy to accidentally scroll. And if you wanted to scroll to the other side of the screen from where you were, you needed to move your mouse all the way to the other side.

Enter Click and Drag
So last night, I decided I was going to make my game scroll when I clicked and dragged. This was a more intuitive interface, similar to the iPhone’s scrolling functionality.

My first attempt at setting this up had mixed results. The scrolling was jittery and it wasn’t 1:1 for some reason. Check out my code below to see if you can spot the error. For reference, the mouseDown function is called when the user presses the left mouse button while on the map, the mouseUp function is called when the user releases the left mouse button while on the map and the mouseMove function is called when the user moves the mouse around the playField, but only after the mouse button is pressed.

private function mouseDown(e:MouseEvent):void {
	this.startMouseX = this.mouseX;
	this.startMouseY = this.mouseY;
	this.startX = this.x;
	this.startY = this.y;
		
	this.addEventListener(MouseEvent.MOUSE_UP, mouseUp);
	this.addEventListener(MouseEvent.MOUSE_MOVE, mouseMove);
	Main.stageSprite.addEventListener(Event.DEACTIVATE, mouseUp);		
}

private function mouseUp(e:Event):void {
	this.removeEventListener(MouseEvent.MOUSE_UP, mouseUp);
	this.removeEventListener(MouseEvent.MOUSE_MOVE, mouseMove);
	Main.stageSprite.removeEventListener(Event.DEACTIVATE, mouseUp);	
}

private function mouseMove(e:Event):void {
	this.x = (this.startX - this.startMouseX + this.mouseX);
	this.y = (this.startY - this.startMouseY + this.mouseY);
}

Ignore the fact that the game will keep scrolling if you happen to release the mouse button while your mouse is outside the flash screen. I haven’t yet figured out how to fix that, so anyone that has a solution should let me know. If you can spot the other error in my code, then you’re a better programmer than I was last night.

The Solution
The problem, it turned out, was caused by my use of this.mouseX and this.mouseY. You see, as you move a sprite around, it’s mouseX and mouseY coordinates change to reflect the relative position of the mouse from the sprite’s (0,0) point, usually the upper-left hand corner.

What was happening was the game would update the mouse coordinates at the same time it tried to update the map’s coordinates. Resulting in what some would call hilarity, but what I called annoyance. To fix the problem, all I had to do was update the references to this.mouseX and this.mouseY to Main.stageSprite.mouseX and Main.stageSprite.mouseY.

Main is my initializing class and stageSprite is a static reference to a sprite that never moves. So as the map scrolled around, the stageSprite stays where it is and provides an accurate reference point to calculate how far the mouse has moved. Here are the results:

Starting Position

The ending point after one click and drag

So overall, I’m pretty happy with the results. I’m trying to reintegrate my old classes for moving units around and attacking other units now. The only problem is that I changed many of the variable names for my units and actually implemented private variables instead of public variables, so I have a lot of updates left to make before I can even run the game without compiler errors. Maybe I just need to program those classes from scratch too. As I write this I’m buzzing from my 2nd coffee this week (I only drink coffee 2-3 times per week to avoid developing a tolerance) and I’m probably feeling way too ambitious. Maybe that’s for the best.

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