JavaScript 与 NaN 的问题

Question

我正在开发一款 JavaScript 游戏，但我的一个对象的两个属性出现了问题。
属性 x 和 y 是 NaN，但我不明白为什么。现在我发布代码:

var canvas;
var ctx;
var canvasH = 480;
var canvasW = 960;

window.onload = function () {
canvas  = document.getElementById("canvas");
ctx     = canvas.getContext('2d');

canvas.width  = canvasW;
canvas.height = canvasH;

drawCanvas();
}

函数drawCanvas():

function drawCanvas () {

ctx.fillStyle = "#000000"
ctx.fillRect(0, 0, canvasW, canvasH);
}

这里是我的对象的构造函数:

function SpaceCraft () {

var obj = this;

this.texture = new Image();
this.texture.src = "img/spacecraft.png";
this.texture.onload = function () {
    obj.w = this.width;
    obj.h = this.height;
}

this.x = canvasW / 2 - obj.w / 2; //PROBLEM IS NaN
this.y = canvasH - obj.h;         //PROBLEM IS NaN

//Methods

//Draw

this.draw = function () {
    ctx.drawImage(this.texture, this.x, this.y);
}
}

感谢您的帮助!

抱歉，我写了一篇新帖子，因为没有人回答旧帖子。

Answer 1

obj.w 和 obj.h 在您使用它们时未设置。

您的代码:

this.texture = new Image();
this.texture.src = "img/spacecraft.png";
this.texture.onload = function () {
    obj.w = this.width;
    obj.h = this.height;
}

// the following lines won't get executed after the onload event
// they will be executed immediately after the assignment of the onload event 
this.x = canvasW / 2 - obj.w / 2; //PROBLEM IS NaN
this.y = canvasH - obj.h;         //PROBLEM IS NaN

obj.w 和 obj.h 将在纹理加载后分配，但是，您可以立即在上面摘录的最后两行中使用它们。

您必须将这些行移至回调函数.onload中，以便确保变量 width 和 height 的存在。

此外，还有一个关于 this 的陷阱——上下文。 onload 回调函数中的 this 并不引用 Spacecraft 的实例。
由于您已经有一个变量 obj 引用您的 Spacecraft 实例，因此只需使用它:

this.texture = new Image();
this.texture.src = "img/spacecraft.png";

this.texture.onload = function () {
    obj.w = this.width;
    obj.h = this.height;
    obj.x = canvasW / 2 - obj.w / 2; //PROBLEM IS NaN
    obj.y = canvasH - obj.h;         //PROBLEM IS NaN  
}

<小时/>

另一个选择是 bind特定上下文的回调函数。
请注意，您必须将 this.width/height 替换为 this.texture.width/height，因为 this 并不引用 图像不再是对象了!

this.texture = new Image();
this.texture.src = "img/spacecraft.png";

this.texture.onload = (function () {
    // obj = this due to the use of bind()

    this.w = this.texture.width;
    this.h = this.texture.height;
    this.x = canvasW / 2 - this.w / 2; //PROBLEM IS NaN
    this.y = canvasH - this.h;         //PROBLEM IS NaN  
}).bind(this);