Enemy Patrolling system

Question

cc.Class({
    extends: cc.Component,

    properties: {

        flags: {
          default: [],
          type: [cc.Node],
        },
        speed: 2,
        _currentMove: 0,
        _forward: true,
    },

    // use this for initialization
    onLoad: function () {

    },

    start: function () {
        this.node.position = this.flags[0].position;

    },
    // called every frame, uncomment this function to activate update callback
    update: function (dt) {
         this.movement();
    },

        movement: function () {
// comparePos is a custome helper method, check bottom of script
            if (this.comparePos(this.node.position,  this.flags[this._currentMove].position)
            && this._forward) {
                this._currentMove++;
                this.moveActions();

        }
        else if (this.comparePos(this.node.position, this.flags[this._currentMove].position)
        && !this._forward) {
            this._currentMove--;
            this.moveActions();
        }  

        if (this._currentMove >= this.flags.length - 1) {
            this._currentMove = this.flags.length - 1;
            this._forward = !this._forward;
        }
        else if (this._currentMove <= 0) {
            this._currentMove = 0;
            this._forward = !this._forward;
        }
    },

    moveActions: function () {
        var move = cc.moveTo(this.speed, this.flags[this._currentMove].position);
        this.node.runAction(move);
    },

    comparePos: function (a, b) {
        return Math.round(a.x) == Math.round(b.x) && 
        Math.round(a.y) == Math.round(b.y)
    },

});

I'm using Cocos Creator, basically I have an array of empty objects, and I want my enemy to patrol back and forth towards those objects. The problem is the enemy would complete a full round ( move towards all objects and back) and then gives me error when it goes back to the first object, strangely enough, sometimes it will complete more than 1 round before giving the error:

Uncaught TypeError: Cannot read property 'position' of undefined

It might be because the position comparison is not accurate enough, but I don't know how else to do it.

Update: I finally fixed it, the problem was here:

this._forward = !this._forward;

I changed it to this:

this._forward = true; // and false down below

That's very very strange, but it's now working fine :/

Laurel · Answer 1 · 2023-07-25T11:19:31.260

This seems to be a JavaScript problem. See JavaScript "cannot read property "bar" of undefined or Detecting an undefined object property.

According to the one answer this happens when:

The object has the property and its value is not undefined.
The object has the property and its value is undefined.
The object does not have the property.

(I don't know JS so I can't tell you what exactly is causing this in your code).

score 0 · Answer 2 · answered Mar 23 '16 at 08:21

0

It's either this line:

          if (this.comparePos(this.node.position,  this.flags[this._currentMove].position)

or this line:

  else if (this.comparePos(this.node.position, this.flags[this._currentMove].position)

Your this._currentMove is out of array size bounds, or this.flags[] array has undefined on that position. Add additional logging to get the exact scenario

answered Mar 23 '16 at 08:21

Роман Гуйван

1,118
6
14

I don't know what else should I log out, the weird thing is that it sometimes work and sometimes doesn't, which made me question the position comparison. Have you tried the code yourself? or do you have a different approach to achieving this ? – Abdou023 Mar 24 '16 at 20:23
If you're stuck that badly perhaps you can upload your game somewhere and I'll debug it for you :) Just throw the link in, man – Роман Гуйван Mar 25 '16 at 10:39

Enemy Patrolling system

2 Answers2