HTML5 canvas中最简单的幻灯片显示canvas.context.clearRect无法与setTimeout一起使用

Question

这是一个非常简单的幻灯片演示的代码，该演示应在4秒钟内显示4张图像，每秒显示一张图像。 取而代之的是，我得到了4秒的延迟，然后所有图像相互叠加绘制。 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，也就是说，我期望在调用该函数时，程序执行将停止，等待1000毫秒，然后调用draw_next_image ，然后才执行ctx.clearRect 。

实际上，JavaScript不能那样工作。 实际上， getTimeout是异步的，即getTimeout设置Timeout并几乎立即返回，并且代码执行继续，因此ctx.clearRect被调用，并且实际上在 draw_next_image 之前被draw_next_image 。 因此，在Timeout到期并调用draw_next_image ，代码的执行可能会到达任意代码行。 在我的情况下，所有4个clearRect几乎都将在超时到期之前几乎同时被调用。 然后1000毫秒后，所有4个超时将几乎立即一个接一个地到期，并且所有4个图像也几乎都将在同一时间绘制，而没有clearRects ，该对象早已执行。

Answer 1

问题是在代码中，您将异步函数视为同步函数。

要点如下：

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掉线，并且您的代码将开始执行下一步，然后（可能）准备好结果。

因此，您需要根据事件链接呼叫，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
}

这里的工作演示：
http://jsfiddle.net/AbdiasSoftware/dhxNz/

_draw()包装在draw()以对变量进行本地化，并确保_draw()不会最终出现在window对象上。 出于同样的原因，我们存储的参考this因为this改变为window调用它的代码时对象。 me （或您要调用的名称）确保我们正在调用正确的对象，以便可以访问局部变量（画布，ctx，计数器等）。

Answer 2

您有一些问题，包括错误命名图像变量。

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;

例如，请参阅： http ： //jsfiddle.net/8c9MM/1/ 。 您还可以暂停幻灯片放映，因为我们正在为setInterval()函数分配间隔

Answer 3

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>

使用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);

很抱歉，如果代码不是很整齐。 这是我第一次用javascript创建任何东西。

HTML5 canvas中最简单的幻灯片显示canvas.context.clearRect无法与setTimeout一起使用

问题描述

3 个解决方案

解决方案1
3 已采纳

解决方案2
1 2013-06-05 04:10:40

解决方案3
1 2015-03-03 21:19:59

HTML5 canvas中最简单的幻灯片显示canvas.context.clearRect无法与setTimeout一起使用

问题描述

3 个解决方案

解决方案1 3 已采纳

解决方案2 1 2013-06-05 04:10:40

解决方案3 1 2015-03-03 21:19:59

解决方案1
3 已采纳

解决方案2
1 2013-06-05 04:10:40

解决方案3
1 2015-03-03 21:19:59