问题
I need to monitor the direction a user is indicating using the four directional arrow keys on a keyboard in ActionScript 3.0 and I want to know the most efficient and effective way to do this.
I've got several ideas of how to do it, and I'm not sure which would be best. I've found that when tracking Keyboard.KEY_DOWN events, the event repeats as long as the key is down, so the event function is repeated as well. This broke the method I had originally chosen to use, and the methods I've been able to think of require a lot of comparison operators.
The best way I've been able to think of would be to use bitwise operators on a uint variable. Here's what I'm thinking
var _direction:uint = 0x0; // The Current Direction
That's the current direction variable. In the Keyboard.KEY_DOWN event handler I'll have it check what key is down, and use a bitwise AND operation to see if it's already toggled on, and if it's not, I'll add it in using basic addition. So, up would be 0x1 and down would be 0x2 and both up and down would be 0x3, for example. It would look something like this:
private function keyDownHandler(e:KeyboardEvent):void
{
switch(e.keyCode)
{
case Keyboard.UP:
if(!(_direction & 0x1)) _direction += 0x1;
break;
case Keyboard.DOWN:
if(!(_direction & 0x2)) _direction += 0x2;
break;
// And So On...
}
}
The keyUpHandler wouldn't need the if operation since it only triggers once when the key goes up, instead of repeating. I'll be able to test the current direction by using a switch statement labeled with numbers from 0 to 15 for the sixteen possible combinations. That should work, but it doesn't seem terribly elegant to me, given all of the if statements in the repeating keyDown event handler, and the huge switch.
private function checkDirection():void
{
switch(_direction)
{
case 0:
// Center
break;
case 1:
// Up
break;
case 2:
// Down
break;
case 3:
// Up and Down
break;
case 4:
// Left
break;
// And So On...
}
}
Is there a better way to do this?
回答1:
You can keep track of whether each key is down or not by listening for all KEY_DOWN and KEY_UP events, and storing each key state in an array. I wrote a class a while ago to do just that (included at the end of my answer).
Then you are no longer tied to the event model to know if a certain key is down or not; you can periodically check every frame (or every timer interval). So you could have a function like:
function enterFrameCallback(e:Event):void
{
var speed:Number = 1.0; // net pixels per frame movement
thing.x += (
-(int)Input.isKeyDown(Keyboard.LEFT)
+(int)Input.isKeyDown(Keyboard.RIGHT)
) * speed;
thing.y += (
-(int)Input.isKeyDown(Keyboard.UP)
+(int)Input.isKeyDown(Keyboard.DOWN)
) * speed;
}
which would take into account all possible combinations of arrow key presses. If you want the net displacement to be constant (e.g. when going right and down at same time, the object moves X pixels diagonally, as opposed to X pixels in both horizontal and vertical directions), the code becomes:
function enterFrameCallback(e:Event):void
{
var speed:Number = 1.0; // net pixels per frame movement
var displacement:Point = new Point();
displacement.x = (
-(int)Input.isKeyDown(Keyboard.LEFT)
+(int)Input.isKeyDown(Keyboard.RIGHT)
);
displacement.y = (
-(int)Input.isKeyDown(Keyboard.UP)
+(int)Input.isKeyDown(Keyboard.DOWN)
);
displacement.normalize(speed);
thing.x += displacement.x;
thing.y += displacement.y;
}
Here is the Input class I wrote (don't forget to call init from the document class). Note that it also keeps track of mouse stuff; you can delete that if you don't need it:
/*******************************************************************************
* DESCRIPTION: Defines a simple input class that allows the programmer to
* determine at any instant whether a specific key is down or not,
* or if the mouse button is down or not (and where the cursor
* is respective to a certain DisplayObject).
* USAGE: Call init once before using any other methods, and pass a reference to
* the stage. Use the public methods commented below to query input states.
*******************************************************************************/
package
{
import flash.events.KeyboardEvent;
import flash.events.MouseEvent;
import flash.display.Stage;
import flash.geom.Point;
import flash.display.DisplayObject;
public class Input
{
private static var keyState:Array = new Array();
private static var _mouseDown:Boolean = false;
private static var mouseLoc:Point = new Point();
private static var mouseDownLoc:Point = new Point();
// Call before any other functions in this class:
public static function init(stage:Stage):void
{
stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDown, false, 10);
stage.addEventListener(KeyboardEvent.KEY_UP, onKeyUp, false, 10);
stage.addEventListener(MouseEvent.MOUSE_DOWN, onMouseDown, false, 10);
stage.addEventListener(MouseEvent.MOUSE_UP, onMouseUp, false, 10);
stage.addEventListener(MouseEvent.MOUSE_MOVE, onMouseMove, false, 10);
}
// Call to query whether a certain keyboard key is down.
// For a non-printable key: Input.isKeyDown(Keyboard.KEY)
// For a letter (case insensitive): Input.isKeyDown('A')
public static function isKeyDown(key:*):Boolean
{
if (typeof key == "string") {
key = key.toUpperCase().charCodeAt(0);
}
return keyState[key];
}
// Property that is true if the mouse is down, false otherwise:
public static function get mouseDown():Boolean
{
return _mouseDown;
}
// Gets the current coordinates of the mouse with respect to a certain DisplayObject.
// Leaving out the DisplayObject paramter will return the mouse location with respect
// to the stage (global coordinates):
public static function getMouseLoc(respectiveTo:DisplayObject = null):Point
{
if (respectiveTo == null) {
return mouseLoc.clone();
}
return respectiveTo.globalToLocal(mouseLoc);
}
// Gets the coordinates where the mouse was when it was last down or up, with respect
// to a certain DisplayObject. Leaving out the DisplayObject paramter will return the
// location with respect to the stage (global coordinates):
public static function getMouseDownLoc(respectiveTo:DisplayObject = null):Point
{
if (respectiveTo == null) {
return mouseDownLoc.clone();
}
return respectiveTo.globalToLocal(mouseDownLoc);
}
// Resets the state of the keyboard and mouse:
public static function reset():void
{
for (var i:String in keyState) {
keyState[i] = false;
}
_mouseDown = false;
mouseLoc = new Point();
mouseDownLoc = new Point();
}
///// PRIVATE METHODS BEWLOW /////
private static function onMouseDown(e:MouseEvent):void
{
_mouseDown = true;
mouseDownLoc = new Point(e.stageX, e.stageY);
}
private static function onMouseUp(e:MouseEvent):void
{
_mouseDown = false;
mouseDownLoc = new Point(e.stageX, e.stageY);
}
private static function onMouseMove(e:MouseEvent):void
{
mouseLoc = new Point(e.stageX, e.stageY);
}
private static function onKeyDown(e:KeyboardEvent):void
{
keyState[e.keyCode] = true;
}
private static function onKeyUp(e:KeyboardEvent):void
{
keyState[e.keyCode] = false;
}
}
}
来源：https://stackoverflow.com/questions/2501424/what-is-the-most-effective-way-to-test-for-combined-keyboard-arrow-direction-in