AS3 Hold mouseDown to increase speed/power - performance

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;