# 自定义事件与事件模拟
# 一、自定义事件
# 1、js自定义事件有三种方式
- new CustomEvent('wheelup', {});
- new Event('wheelup', {})
- document.createEvent('HTMLEvents'); initEvent('wheelup', false, false) 其中第三中方式必须进行初始化
// 以实现监听鼠标滚轮上滚或者下滚动事件为例子
// 1、自定义事件实现滚轮上下滚动事件
// 第一种
// let wheelupEvent = new CustomEvent('wheelup', {});
// let wheeldownEvent = new CustomEvent('wheeldown', {});
// 第二种
// let wheelupEvent = new Event('wheelup', {});
// let wheeldownEvent = new Event('wheeldown', {});
// 第三种 这种必须初始化 一般用于编程模拟鼠标等已有事件
// createEvent参数 HTMLEvents MouseEvents UIEvents
let wheelupEvent = document.createEvent('HTMLEvents');
let wheeldownEvent = document.createEvent('HTMLEvents');
wheelupEvent.initEvent('wheelup', false, false);
wheeldownEvent.initEvent('wheeldown', false, false);
let preY = null;
window.onwheel = function(e) {
// console.log('onwheel11:', e);
if (e.wheelDelta > 0) {
console.log('onwheelup:')
window.dispatchEvent(wheelupEvent, {data: e.wheelDelta});
} else {
console.log('onwheeldown:')
window.dispatchEvent(wheeldownEvent, {data: e.wheelDelta});
};
}
window.addEventListener('wheelup', function(e) {
alert('wheelupEvent 触发', e);
})
window.addEventListener('wheeldown', function(e) {
alert('wheeldownEvent 触发', e);
})
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# 2、createEvent()
1、createEvent()方法返回新创建的Event对象，支持一个参数，表示事件类型，具体见下表：
2、initEvent()方法用于初始化通过DocumentEvent接口创建的Event的值。
支持三个参数：initEvent(eventName, canBubble, preventDefault)
分别表示：
- 事件名称
- 是否可以冒泡
- 是否阻止事件的默认操作
# 二、事件模拟
1、事件模拟通常是用来兼容wap端和pc端的效果
以wap端兼容pc端的canvas画板效果为例子:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>事件模拟</title>
<style>
* {
margin: 0;
padding: 0;
}
#canvas_box {
position: relative;
width: 100vw;
height: 100vh;
}
</style>
</head>
<body>
<!-- <div style="height: 2000px;width: 100%;background-color: gray;"></div> -->
<div id="canvas_box">
<canvas id="canvas"></canvas>
</div>
<button id="btn">下载</button>
</body>
</html>
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// 2、通过事件模拟处理移动端的canvas场景
const canvasEle = document.querySelector('#canvas');
if (canvasEle.getContext) {
let ctx = canvasEle.getContext('2d');
let container = document.querySelector('#canvas_box');
canvasEle.width = container.clientWidth;
canvasEle.height = container.clientHeight;
let startPoint = null;
let movePoint = null;
let drawFlag = false;
function draw() {
ctx.lineWidth = 1;
ctx.lineCap = 'round';
ctx.lineJoin = 'round';
ctx.moveTo(startPoint.x, startPoint.y);
ctx.lineTo(movePoint.x, movePoint.y);
ctx.stroke();
}
canvasEle.addEventListener('mousedown', function(e) {
drawFlag = true;
startPoint = {
x: e.clientX,
y: e.clientY
};
});
canvasEle.addEventListener('mousemove', function(e) {
if (drawFlag) {
if (!drawFlag) return;
movePoint = {
x: e.clientX,
y: e.clientY
}
draw();
startPoint= movePoint;
}
});
canvasEle.addEventListener('mouseup', function(e) {
if (!drawFlag) return;
drawFlag = false;
});
// 通过事件模拟来兼容wrap端
canvasEle.addEventListener('touchstart', onTouch);
canvasEle.addEventListener('touchmove', onTouch);
canvasEle.addEventListener('touchend', onTouch);
function onTouch(e) {
let type = null;
let touch = e.touches[0] || e.changedTouches[0];
// console.log(e);
switch(e.type) {
case 'touchstart':
type = 'mousedown';
break;
case 'touchmove':
type = 'mousemove';
break;
case 'touchend':
type = 'mouseup';
break;
default:
break;
};
const newEvent = new MouseEvent(type, touch);
e.target.dispatchEvent(newEvent);
}
};
document.querySelector('#btn').onclick = function() {
let a = document.createElement('a');
// image/png、image/jpeg、image/webp等等，默认为image/png格式；
a.href = canvasEle.toDataURL();
a.download = 'canvas' + new Date().getTime();
a.target = '_blank';
a.click();
}
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