下面是使用JS和canvas实现gif动图的停止和播放的完整攻略：
1. 了解Canvas画布和Image对象
Canvas是HTML5新增的标签，允许通过JS脚本来实现动态绘制图像。而Image对象则是JS中常见的图像对象，可以用于显示一张图片。
2. 载入GIF动图并绘制到Canvas画布上
使用Image对象来载入本地存储路径下的GIF动图文件，并将其绘制到Canvas画布上，Canvas的context对象提供了一个用于绘制图片的方法 drawImage()。代码类似于以下这段：
let canvas = document.getElementById("mycanvas");
let ctx = canvas.getContext("2d");
let img = new Image();
img.onload = function() {
ctx.drawImage(img, 0, 0);
}
img.src = "mygif.gif";
3. GIF动图的暂停
要实现GIF动图的暂停，我们需要重新绘制一张空白的画布来覆盖原有的动图。对于这种情况，我们可以使用绘制一个与Canvas画布大小相等的矩形来覆盖之前的动图。代码类似于以下这段：
function pauseGif() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.fillRect(0, 0, canvas.width, canvas.height);
}
4. GIF动图的播放
要实现GIF动图的播放，我们可以利用Image对象的自定义属性和requestAnimationFrame()方法来让动图进行连续播放。我们需要记录GIF动图的每一帧，然后将其逐一绘制到Canvas画布上。代码类似于以下这段：
let canvas = document.getElementById("mycanvas");
let ctx = canvas.getContext("2d");
let img = new Image();
img.onload = function() {
let gif_frames = [];
let frame = 0;
let is_paused = false;
// 获得GIF动图的每一帧
let duration_sum = 0;
let canvas_temp = document.createElement("canvas");
canvas_temp.width = canvas.width;
canvas_temp.height = canvas.height;
let ctx_temp = canvas_temp.getContext("2d");
let draw_frame = function(i) {
duration_sum += img.frames[i].duration;
img.seek(i);
ctx_temp.clearRect(0, 0, canvas.width, canvas.height);
ctx_temp.drawImage(img, 0, 0);
gif_frames.push({
duration: img.frames[i].duration,
image: canvas_temp.toDataURL("image/png")
});
if(i < img.frames.length - 1) {
draw_frame(i+1);
}
}
draw_frame(0);
function playGif() {
if(is_paused) return;
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.drawImage(
new Image(),
0, 0, canvas.width, canvas.height,
0, 0, canvas.width, canvas.height
);
ctx.drawImage(new Image(gif_frames[frame].image), 0, 0);
frame = (frame + 1) % gif_frames.length;
setTimeout(playGif, gif_frames[frame].duration);
}
playGif();
}
img.src = "mygif.gif";
5. 示例说明
以下是两个示例：
示例一： 使用按钮实现GIF动图播放和暂停
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>GIF动图播放和暂停</title>
</head>
<body>
<canvas id="mycanvas" width="500" height="500"></canvas>
<button id="play-button">播放</button>
<button id="pause-button">暂停</button>
<script>
let canvas = document.getElementById("mycanvas");
let ctx = canvas.getContext("2d");
let img = new Image();
img.onload = function() {
let gif_frames = [];
let frame = 0;
let is_paused = false;
let play_button = document.getElementById("play-button");
let pause_button = document.getElementById("pause-button");
// 获得GIF动图的每一帧
let duration_sum = 0;
let canvas_temp = document.createElement("canvas");
canvas_temp.width = canvas.width;
canvas_temp.height = canvas.height;
let ctx_temp = canvas_temp.getContext("2d");
let draw_frame = function(i) {
duration_sum += img.frames[i].duration;
img.seek(i);
ctx_temp.clearRect(0, 0, canvas.width, canvas.height);
ctx_temp.drawImage(img, 0, 0);
gif_frames.push({
duration: img.frames[i].duration,
image: canvas_temp.toDataURL("image/png")
});
if(i < img.frames.length - 1) {
draw_frame(i+1);
}
}
draw_frame(0);
// 播放GIF动图
function playGif() {
if(is_paused) return;
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.drawImage(
new Image(),
0, 0, canvas.width, canvas.height,
0, 0, canvas.width, canvas.height
);
ctx.drawImage(new Image(gif_frames[frame].image), 0, 0);
frame = (frame + 1) % gif_frames.length;
setTimeout(playGif, gif_frames[frame].duration);
}
play_button.addEventListener("click", function() {
is_paused = false;
playGif();
});
pause_button.addEventListener("click", function() {
is_paused = true;
});
}
img.src = "mygif.gif";
</script>
</body>
</html>
示例二： 使用鼠标事件实现GIF动图暂停
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>GIF动图暂停</title>
</head>
<body>
<canvas id="mycanvas" width="500" height="500"></canvas>
<script>
let canvas = document.getElementById("mycanvas");
let ctx = canvas.getContext("2d");
let img = new Image();
img.onload = function() {
let gif_frames = [];
let frame = 0;
// 获得GIF动图的每一帧
let duration_sum = 0;
let canvas_temp = document.createElement("canvas");
canvas_temp.width = canvas.width;
canvas_temp.height = canvas.height;
let ctx_temp = canvas_temp.getContext("2d");
let draw_frame = function(i) {
duration_sum += img.frames[i].duration;
img.seek(i);
ctx_temp.clearRect(0, 0, canvas.width, canvas.height);
ctx_temp.drawImage(img, 0, 0);
gif_frames.push({
duration: img.frames[i].duration,
image: canvas_temp.toDataURL("image/png")
});
if(i < img.frames.length - 1) {
draw_frame(i+1);
}
}
draw_frame(0);
// 暂停GIF动图
canvas.addEventListener("mousedown", function() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.fillRect(0, 0, canvas.width, canvas.height);
});
// 播放GIF动图
function playGif() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
let img_temp = new Image();
img_temp.src = gif_frames[frame].image;
img_temp.onload = function() {
ctx.drawImage(img_temp, 0, 0);
frame = (frame + 1) % gif_frames.length;
setTimeout(playGif, gif_frames[frame].duration);
};
}
playGif();
}
img.src = "mygif.gif";
</script>
</body>
</html>
以上就是使用JS和canvas实现GIF动图的停止和播放的完整攻略了。希望对你有所帮助。
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