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这是一个非常简单的幻灯片的代码，它应该在 4 秒内显示 4 张图像，每秒一张图像。相反，我得到了 4 秒的延迟，然后所有图像都被绘制在彼此之上。我究竟做错了什么？

<html>
<head>
<script langugage="javascript">
// 4 images
var image0 = new Image();
image0.src = "img/image0.png";
var image1 = new Image();
image1.src = "img/image1.png";
var image0 = new Image();
image2.src = "img/image2.png";
var image3 = new Image();
image3.src = "img/image3.png";
// array of 4 images
images = new Array(image0, image1, image2, image3);

// this is the main function
function draw(){
    myCanvas = document.getElementById('myCanvas');
    ctx = myCanvas.getContext('2d');
    counter=0; // this is the index of the next image to be shown
    for (var i=0;i<images.length;i++){
        setTimeout(draw_next_image, 1000);
        ctx.clearRect(0, 0, myCanvas.width, myCanvas.height)
    }
}
// this is the function called after each timeout to draw next image
function draw_next_image(){
    ctx.drawImage(images[counter], 0, 0);
    counter++;
    if (counter>images.length) {counter=0;}
}

window.onload = draw;
</script>
</head>
<body>
    <canvas id="myCanvas" width="800" height="600"></canvas>
</body>
</html>

更新：答案是：

在上面的代码中，我错误地假设getTimeout函数是同步的，即我预计，在调用它时程序执行将停止，等待 1000 毫秒，然后调用 ，然后draw_next_image才执行ctx.clearRect。

实际上，Javascript 不是那样工作的。实际上getTimeout是异步的，即getTimeout设置一个 Timeout 并几乎立即返回并且代码继续执行，因此它ctx.clearRect被立即调用并且实际上在draw_next_image. 因此，到 Timeout 到期并调用draw_next_image时，代码的执行可能会到达任意行代码。在我的情况下，所有 4clearRect将几乎同时被调用，早在超时到期之前。然后 1000 毫秒后，所有 4 个超时将几乎一个接一个地立即到期，并且所有 4 个图像也将几乎同时绘制，没有clearRects，这在很久以前就已经执行了。

4

3 回答 3

3

问题是在您的代码中，您将异步函数视为同步函数。

要点在这里：

image0.src = "img/image0.png";
image1.src = "img/image1.png";
image2.src = "img/image2.png";
image3.src = "img/image3.png";
...
setTimeout(draw_next, delayInMilliseconds);

由于这些调用一旦被调用就会失败，并且您的代码在这些结果（可能）准备好之前开始执行下一步。

因此，您需要根据事件链接您的呼叫，例如：

//image counter as there is no guarantee that the last images loaded
//is the last one to finish
var loaded = 0, numOfImages = 4;

//first part of chain, invoke async load
var image0 = document.createElement('img'); //this will work in new Chrome
var image1 = document.createElement('img'); //instead of new Image
var image2 = document.createElement('img');
var image3 = document.createElement('img');

//common event handler when images has loaded with counter
//to know that all images has loaded
image0.onload = image1.onload = 
image2.onload = image3.onload = function(e) {
    loaded++;
    if (loaded === numOfImages)
        draw();   // <-- second part of chain, invoke loop
}

//show if any error occurs
image0.onerror = image1.onerror = 
image2.onerror = image3.onerror = function(e) {
    console.log(e);
}

//invoke async loading... you can put these four into your
//window.onload if you want to
image0.src = "img/image0.png";
image1.src = "img/image1.png";
image2.src = "img/image2.png";
image3.src = "img/image3.png";

// this is the main function
function draw() {

    var images = new Array(image0, image1, image2, image3),
        counter = 0,
        delayInMilliseconds = 4000,
        maxNum = images.length - 1,

        myCanvas = document.getElementById('myCanvas'),
        ctx = myCanvas.getContext('2d'),

        me = this; //this we need for setTimeout()

    //third part of chain, have a function to invoke by setTimeout
    this._draw = function() {

        //if the next image will cover the canvas
        //there is no real need to clear the canvas first.
        //I'll leave it here as you ask for this specifically
        ctx.clearRect(0, 0, myCanvas.width, myCanvas.height)
        ctx.drawImage(images[counter++], 0, 0);
        if (counter > maxNum) counter = 0;

        setTimeout(me._draw, delayInMilliseconds); //use me instead of this
    }
    this._draw(); //START the loop
}

在这里工作演示：http:
//jsfiddle.net/AbdiasSoftware/dhxNz/

被_draw()包裹起来draw()以本地化变量并确保它_draw()不会最终出现在window对象上。出于同样的原因，当调用它的代码时，我们存储对thisas的引用将其this更改为object。（或者你想怎么称呼它）确保我们调用的是正确的对象，以便我们可以访问局部变量（canvas、ctx、counter 等）。windowme

于 2013-06-05T04:14:29.477 回答
1

你有一些问题，包括错误地命名你的图像变量。

尝试这个：

// 4 images
var image0 = new Image();
image0.src = "http://placekitten.com/200/300";
var image1 = new Image();
image1.src = "http://placekitten.com/205/305";
var image2 = new Image();
image2.src = "http://placekitten.com/300/400";
var image3 = new Image();
image3.src = "http://placekitten.com/800/600";
// array of 4 images
images = new Array(image0, image1, image2, image3);

// global counter and canvas
var counter = 0, ctx, interval;

// this is the main function
function draw(){
    myCanvas = document.getElementById('myCanvas');
    ctx = myCanvas.getContext('2d');
    interval = setInterval(draw_next_image, 1000);
}

// this is the function called after each timeout to draw next image
function draw_next_image(){
    ctx.clearRect(0, 0, myCanvas.width, myCanvas.height);
    ctx.drawImage(images[counter], 0, 0);
    counter++;
    if (counter==images.length) {counter=0;}
}

window.onload = draw;

参见： http: //jsfiddle.net/8c9MM/1/例如。您也可以暂停幻灯片，因为我们正在为setInterval()函数分配间隔

于 2013-06-05T04:10:40.533 回答
1

这个怎么样。

HTML

<!DOCTYPE HTML>
<html>
    <head>
        <title>Slider</title>
        <meta charset="utf-8">
        <link type="text/css" rel="stylesheet" href="css/main.css">
    </head>
    <body>
        <canvas id="canvas" width="600" height="400"></canvas>
        <script type="text/javascript" src="js/slider.js"></script>
    </body>
</html>

Javascript

// The 3 images
var im1 = new Image();
im1.src = "img/kitten1.jpg";
var im2 = new Image();
im2.src = "img/kitten2.jpg";
var im3 = new Image();
im3.src = "img/kitten3.jpg";

// Starting position of the 3 images
var x1 = 0;
var x2 = -600;
var x3 = -1200;

var counter = 0;
var img_count = 0;

// Canvas
canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
//This draws the first image when the page is loaded
ctx.drawImage(im1, x1, 0);

function sliderMove() {
    if(counter <= 590) {
        x1+=10;
        ctx.drawImage(im1,x1,0);
        x2+=10;
        ctx.drawImage(im2,x2,0);
        x3+=10;         
        ctx.drawImage(im3,x3,0);


        counter+=10;
    }
    else {
        counter = 0;
        img_count++

        if(img_count == 1) {
            x1 = -1200;
        }else if(img_count == 2) {
            x2 = -1200;
        }else if(img_count == 3) {
            x3 = -1200;
        }
        // This stops move_loop once counter gets to 600
        clearInterval(move_loop);
    }
}
function moveImg() {
//This part moves all of the images 20px to the right every 10ms
    move_loop = setInterval(sliderMove, 10);
    if(img_count > 2) {
        img_count = 0;
    }
}
// This executes the moveImg function every 5 seconds
image_loop = setInterval(timer, 5000);

抱歉，如果代码不是很有条理。这是我第一次使用 javascript 创建任何东西。

于 2015-03-03T21:19:59.077 回答