Question

我很困惑如何定义和调用JavaScript对象方法。我试图用JavaScript创建游戏，并希望创建具有控制每个元素的函数的对象。目前，game.update函数在尝试调用player.update时调用错误。它找不到方法。我该如何定义和调用方法。我来自使用java。

//INIT
var game;
var paper;
var key = [0, 0, 0, 0, 0]; // left, right, up, down

window.onload = function () {
    paper = Raphael("canvas", 640, 480);
    game = new Game();
    game.init();
};

//GAME
function Game() {
    this.player = new Player();
    this.score = 0;

    this.drawCanvas = function () {
        paper.clear();
        paper.rect(0, 0, 640, 480, 10).attr({
            fill: "#fff",
            stroke: "none"
        });
        paper.text(40, 10, "Score " + this.score);
        paper.ellipse(this.player.positionX, this.player.positionY, 10, 10);
    }

    this.update = function () {
        this.player.update.call();
        this.drawCanvas.call()();
    }

    this.init = function () {
        this.drawCanvas();
        setInterval(this.update, 35);
    }
}

//UNITS

//PLAYER
function Player() {
    this.positionX = 100;
    this.positionY = 100;

    this.update = function () {
        if (key[0]) { //left
            this.positionX -= 3;
            game.score += 3;
        }
        if (key[1]) { //right
            this.positionX += 3;
            game.score += 3;
        }
        if (key[2]) { //up
            this.positionY -= 3;
            game.score += 3;
        }
        if (key[3]) { //down
            this.positionY += 3;
            game.score += 3;
        }
        if (key[4]) { //space

        }

        if (this.positionX > 640) {
            this.positionX = 640;
        } else if (this.positionX < 0) {
            this.positionX = 0;
        } else if (this.positionY > 480) {
            this.positionY = 480;
        } else if (this.positionY < 0) {
            this.positionY = 0;
        }
    }
}

function changeKey(which, to) {
    switch (which) {
        case 65:
        case 37:
            key[0] = to;
            break; // left
        case 87:
        case 38:
            key[2] = to;
            break; // up
        case 68:
        case 39:
            key[1] = to;
            break; // right
        case 83:
        case 40:
            key[3] = to;
            break; // down
        case 32:
            key[4] = to;
            break; // space bar;
        case 17:
            key[5] = to;
            break; // ctrl
    }
}
document.onkeydown = function (e) {
    changeKey((e || window.event).keyCode, 1);
};
document.onkeyup = function (e) {
    changeKey((e || window.event).keyCode, 0);
};

我收到以下错误：

Uncaught TypeError: Cannot read property 'update' of undefined      main.js:25

Answer 1

这是无效的语法：

this.update : function(){

你想要这个

this.update = function(){

Answer 2

问题在于您从update分离了this方法。

this.init = function () {
    this.drawCanvas();
    setInterval(this.update, 35); // <-------- passing `update` without `this`
}

所以当你到update()

的这一行时

this.player.update.call();

this的值是全局对象，没有player属性，因此您尝试访问.update.call()上的undefined。

你需要保持它们的约束力：

this.init = function () {
    this.drawCanvas();
    var self = this;
    setInterval(function() {
       self.update();
    }, 35);
}

或者使用Function.prototype.bind创建一个新功能，this永久绑定到该功能。

this.init = function () {
    this.drawCanvas();
    setInterval(this.update.bind(this), 35);
}

调用对象函数

2 个答案: