AS3 Hold mouseDown to increase speed/power

Question

this is what i'm trying to accomplish;

With a click on a movieclip (cannon_mc) a shot is being fired (ball_mc)

The longer mouse is down, the speed of wich the ball is fired with should increase. My question to you is;

What is the most efficient way to accomplish this? With a timer or something like this;

    var isMouseDown:Boolean = false;
    var speed= 10;    

    myCannon.addEventListener(MouseEvent.MOUSE_DOWN,buttonPressed);

        function buttonPressed(event:MouseEvent){
            //trace("down");
            isMouseDown == true;

            if(isMouseDown == false)
            {
                speed == +1
            }

        }

Answer 1

The MOUSE_DOWN event is only fired once. To get the effect you want you need the combo of MOUSE_DOWN and MOUSE_UP Event Handlers.

You can set a variable to true in the MOUSE_DOWN event alongwith the current timestamp from flash.utils.getTimer()

Then on MOUSE_UP, if the variable you set in MOUSE_DOWN is true, you compute the time elapsed and set the power accordingly.

Example:

var isMouseDown:Boolean = false;
var mouseDownBegin:int;
var speed = 10; 
var speed_inc = 2; // give it in per second
var speed_max = 100; // max speed possible

// add event handlers
myCannon.addEventListener(MouseEvent.MOUSE_DOWN, buttonPressed);
myCannon.addEventListener(MouseEvent.MOUSE_UP, buttonReleased);

function buttonPressed(event:MouseEvent){
    isMouseDown = true;
    mouseDownBegin = flash.utils.getTimer();
}

function buttonReleased(event:MouseEvent){
    if(isMouseDown == true){
        // get time between press and release
        var timeElapsed = flash.utils.getTimer() - mouseDownBegin;

        // reset isMouseDown
        isMouseDown = false;

        // compute speed
        speed += int(Math.floor(speed_inc * (timeElapsed / 1000.0)));
        speed = Math.min(speed, speed_max);

        // code to fire ball with new speed
        // .......
    }
}

You can also add an ENTER_FRAME event and animate a power gauge or something for visual effect

Update
As pointed to by The_asMan, MOUSE_UP event will not fire if the mouse is dragged and released Outside the stage. To handle this case add and event listener for MOUSE_LEAVE event with the callback as the copy of buttonReleased function but which takes an Event object:

function buttonReleasedOutsideStage(event:Event){
    if(isMouseDown == true){
        // get time between press and release
        var timeElapsed = flash.utils.getTimer() - mouseDownBegin;

        // reset isMouseDown
        isMouseDown = false;

        // compute speed
        speed += int(Math.floor(speed_inc * (timeElapsed / 1000.0)));
        speed = Math.min(speed, speed_max);

        // code to fire ball with new speed
        // .......
    }
}
stage.addEventListener(Event.MOUSE_LEAVE, buttonReleasedOutsideStage);

Answer 2

(in very short pseudocode)
Write some event handlers:

• onMouseDown: sets flag _mouseDown, set power to zero
• onFrame: if (_mouseDown) power++;
• onMouseUp: clears flag _mouseDown and fires ball with accumulated power

Framerate independent version:

• onMouseDown: _loadStart = getTimer(); _mouseDown = true; _power = 0;
• onFrame: if (_mouseDown) delta = getTimer() - _loadStart; _power += delta;
• onMouseUp: shot the ball with _power, _mouseDown = false;