AS3 - Error #1180: Call to a possibly undefined method kill. - actionscript-3

Question

This one is driving me crazy for a couple hours. I try to call a method kill(); (in function takeDamage()) which is in the same class, yet it won't find it.
package classes.ship
{
imports ...
public class Ship extends MovieClip
{
var speed:Number;
var shootLimiter:Number;
public static var health:Number;
public static var maxHealth:Number;
public function initialize()
{
var stageReff:Stage = this.stage as Stage;
stage.addEventListener(KeyboardEvent.KEY_DOWN, reportKeyDown);
stage.addEventListener(KeyboardEvent.KEY_UP, reportKeyUp);
stage.addEventListener("enterFrame", move);
}
//code
public static function takeDamage(d):void
{
health -= d;
if(health <= 0)
{
health = 0;
kill();
}
Main.healthMeter.bar.scaleX = health/maxHealth;
}
public function kill():void
{
var boom = new Explosion();
stage.addChild(boom);
boom.x = this.x;
boom.y = this.y;
this.visible = false;
//Main.gameOver();
}
//code
}
}
Has it to do with var stageReff:Stage = this.stage as Stage; ?
Thanks in advance.

Answer 1

kill() is an instance method, but takeDamage is a static class method. You can't call instance methods from a static class method. You can only call instance methods when you have an instance reference to call it on.

Answer 2

nice simple one for early in the year!
You have declared the function 'takeDamage' as a static method - this means that it does not belong to a particular instance of the class Ship, instead it belongs to the Class itself. Static methods and properties can be a bit confusing if you are new to OOP, but are easily explained through a quick example:
Class Member Property
In this example we declare a new Class definition for a Ship where we can define the speed of the ship instance via setSpeed().
public class Ship {
private var speed : Number;
public function setSpeed(value : Number) : void {
this.speed = value;
}
public function getSpeed() : Number {
return this.speed;
}
}
Now we will create a couple of ships and set their speed:
var scoutShip : Ship = new Ship();
scoutShip.setSpeed(500); // Scout's are fast!
var cargoShip : Ship = new Ship();
cargoShip.setSpeed(10); // Cargo ships are sloooow.
trace("Scout Ship Speed: " + scoutShip.getSpeed()); // 500
trace("Cargo Ship Speed: " + cargoShip.getSpeed()); // 10
As you can see from the above, each new instance of Ship that we create can have its own Speed - this is a fundamental of Object Orientated Programming (where the Ship is the Object and it's speed is the data).
Static Property
Now we will create another class, this time called StaticShip which uses a static property instead, note the use of the static keyword:
public class StaticShip {
private static var speed : Number;
public function setSpeed(value : Number) : void {
this.speed = value;
}
public function getSpeed() : Number {
return this.speed;
}
}
Because the speed property is static it is shared across all instances of StaticShip; for example:
var scoutShip : StaticShip = new StaticShip();
scoutShip.setSpeed(500); // So the scout should move at 500
var cargoShip : StaticShip = new StaticShip();
cargoShip.setSpeed(10); // ... and the Cargo move at 10, as before
trace("Scout Ship Speed: " + scoutShip.getSpeed()); // 10
trace("Cargo Ship Speed: " + cargoShip.getSpeed()); // 10
Notice how both StaticShips move at 10 - this is because we set the Speed of the 'cargoShip' instance last - as the 'speed' property in StaticShip is declared static it is shared across all instances of that Class.
Now, just as you can have static properties in Classes, you can also have static functions. Usually, when you call a Class' method (ie: setSpeed()) you need to invoke that method on an instance (ie: scoutShip.setSpeed(500);), however, Static Methods allow you to interact with other static members of a given class, here's another example:
Static Method Example
public class StaticMethodShip {
private static var speed : Number;
// Note that setSpeed is now declared as static
public static function setSpeed(value : Number) : void {
this.speed = value;
}
public function getSpeed() : Number {
return this.speed;
}
}
Now, we can still create new instances of StaticMethodShip as before, but because we have now declared 'setSpeed' as static, we can't invoke setSpeed on an instance:
var scoutShip : StaticMethodShip = new StaticMethodShip();
// This call will trigger Error #1180 - Call to a possibly undefined Method because
// setSpeed was declared as static.
scoutShip.setSpeed(500);
Instead, we can now only invoke the setSpeed() method on the StaticMethodShip Class, ie:
// Set the speed of all StaticMethodShip instances.
StaticMethodShip.setSpeed(250); // all StaticMethodShips travel at 250.
// Proof!
var shipOne : StaticMethodShip = new StaticMethodShip();
var shipTwo : StaticMethodShip = new StaticMethodShip();
trace("ShipOne Speed: " + shipOne.getSpeed()); // 250
trace("ShipTwo Speed: " + shipTwo.getSpeed()); // 250
Static methods are useful when you want to define behaviour for all instances of a given Class (ie: all StaticMethodShips move at the specified speed, all fade out Tweens last for 0.25 seconds, etc); but they are also used in common design Patterns such as the Static Factory Method
Now, to the reason you are seeing your error - member level methods are able to invoke static methods, ie:
public class StaticExampleOne {
public static function getName() : String {
return "Robbe";
}
public function traceName() : void {
// traces 'Robbe'.
trace(getName());
}
}
In usage (new StaticExampleOne().traceName()) this works just fine - member methods can access static methods without problem, however this doesn't work the other way around:
public class StaticExampleTwo {
private var name : String = "Robbe";
public function getName() : void {
return this.name;
}
public static function traceName() : void {
// Throws Error #1180.
trace(getName());
}
}
This is because static methods have no scope (ie: They do not know which instance of the Class they are referring too because they can only reference other static members) and therefore can not access class level members (methods and properties).
To solve your problem you could introduce a new static property to Ship called 'STAGE' (typically static properties are written in ALL CAPS to differentiate them from member properties) and then make your kill() method static.
Hope this helps and good luck!
Jonny.