Question

我正在开发一个JavaScript游戏，我的一个对象的两个属性出了问题。
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！

对不起，但我写了一篇新帖子，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

在加载纹理后将分配

obj.w和obj.h，但是，您可以在上面摘录的最后两行中立即使用它们。

您必须将这些行移动到回调函数.onload中，这样才能确保变量width和height的存在。

此外，关于this - 上下文存在一个陷阱。 this回调函数中的onload未引用 Spacecraft 的实例。
由于您已经有一个变量obj，它引用了您的 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  
}

另一个选择是bind针对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不再引用 Image 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);

NaN的JavaScript问题

1 个答案: