h5上传多张 照片太大会出现耗时久，或是卡死问题

//html

//js

var formdata = getFormData();

var filesData = [],
sendData = [];

//formdata 上传图片
function uploadFile() {
$.ajax({
url: '上传路径',
type: 'POST',
data: formdata,
async: false,
cache: false,
contentType: false,
processData: false,
success: function(returndata) {

},
error: function(returndata) {

}
});
} 
}
$(function() {
var filechooser = document.getElementById("choose");
//    用于压缩图片的canvas
var canvas = document.createElement("canvas");
var ctx = canvas.getContext('2d');
//    瓦片canvas
var tCanvas = document.createElement("canvas");
var tctx = tCanvas.getContext("2d");
var maxsize = 100 * 1024;
$("#upload").on("click", function() {
filechooser.click();
})
.on("touchstart", function() {
$(this).addClass("touch")
})
.on("touchend", function() {
$(this).removeClass("touch")
});
filechooser.onchange = function() {
if (!this.files.length) return;
var files = Array.prototype.slice.call(this.files);
if (files.length > 4 || sendData.length > 4) {
alert("最多同时只可上传4张图片");
return;
}
$(".img-list li").eq(0).siblings().remove()
for (var i = 0; i < files.length; i++) {
sendData.push(files[i])
}
sendData.forEach(function(file, i) {
if (!/\/(?:jpeg|png|gif)/i.test(file.type)) return;
var reader = new FileReader();
var li = document.createElement("li");
//获取图片大小
var size = file.size / 1024 > 1024 ? (~~(10 * file.size / 1024 / 1024)) / 10 + "MB" : ~~(file.size / 1024) + "KB";
li.innerHTML = '

' + size + '

';
$(".img-list").append($(li));
reader.onload = function() {
var result = this.result;
var img = new Image();
img.src = result;
$(li).css("background-image", "url(" + result + ")");
//如果图片大小小于100kb，则直接上传
if (result.length <= maxsize) {
img = null;
upload(result, file.type, $(li), i);
return;
}
//      图片加载完毕之后进行压缩，然后上传
if (img.complete) {
callback();
} else {
img.onload = callback;
}


function callback() {
var data = compress(img);
upload(data, file.type, $(li), i);
img = null;
}
};
reader.readAsDataURL(file);
})
};
//    使用canvas对大图片进行压缩
function compress(img) {
var initSize = img.src.length;
var width = img.width;
var height = img.height;
//如果图片大于四百万像素，计算压缩比并将大小压至400万以下
var ratio;
if ((ratio = width * height / 4000000) > 1) {
ratio = Math.sqrt(ratio);
width /= ratio;
height /= ratio;
} else {
ratio = 1;
}
canvas.width = width;
canvas.height = height;
//        铺底色
ctx.fillStyle = "#fff";
ctx.fillRect(0, 0, canvas.width, canvas.height);
//如果图片像素大于100万则使用瓦片绘制
var count;
if ((count = width * height / 1000000) > 1) {
count = ~~(Math.sqrt(count) + 1); //计算要分成多少块瓦片
//            计算每块瓦片的宽和高
var nw = ~~(width / count);
var nh = ~~(height / count);
tCanvas.width = nw;
tCanvas.height = nh;
for (var i = 0; i < count; i++) {
for (var j = 0; j < count; j++) {
tctx.drawImage(img, i * nw * ratio, j * nh * ratio, nw * ratio, nh * ratio, 0, 0, nw, nh);
ctx.drawImage(tCanvas, i * nw, j * nh, nw, nh);
}
}
} else {
ctx.drawImage(img, 0, 0, width, height);
}
//进行最小压缩
var ndata = canvas.toDataURL('image/jpeg', 0.1);
// console.log('压缩前：' + initSize);
// console.log('压缩后：' + ndata.length);
// console.log('压缩率：' + ~~(100 * (initSize - ndata.length) / initSize) + "%");
tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;
return ndata;
}
//    图片上传，将base64的图片转成二进制对象，塞进formdata上传
function upload(basestr, type, $li, num) {
$li = $li
var text = window.atob(basestr.split(",")[1]);
var buffer = new Uint8Array(text.length);


for (var i = 0; i < text.length; i++) {
buffer[i] = text.charCodeAt(i);
}
var blob = getBlob([buffer], type);
if (num == 0) {
formdata.append('FirstPic', blob)
} else if (num == 1) {
formdata.append('SecondPic', blob)
} else if (num == 2) {
formdata.append('ThirdPic', blob)
} else if (num == 3) {
formdata.append('FourthPic', blob)
}
}
/**
* 获取blob对象的兼容性写法
* @param buffer
* @param format
* @returns {*}
*/
function getBlob(buffer, format) {
try {
return new Blob(buffer, {
type: format
});
} catch (e) {
var bb = new(window.BlobBuilder || window.WebKitBlobBuilder || window.MSBlobBuilder);
buffer.forEach(function(buf) {
bb.append(buf);
});
return bb.getBlob(format);
}
}
/**
* 获取formdata


/**
* formdata 补丁, 给不支持formdata上传blob的android机打补丁
* @constructor
*/
function FormDataShim() {
console.warn('using formdata shim');
var o = this,
parts = [],
boundary = Array(21).join('-') + (+new Date() * (1e16 * Math.random())).toString(36),
oldSend = XMLHttpRequest.prototype.send;
this.append = function(name, value, filename) {
parts.push('--' + boundary + '\r\nContent-Disposition: form-data; name="' + name + '"');
if (value instanceof Blob) {
parts.push('; filename="' + (filename || 'blob') + '"\r\nContent-Type: ' + value.type + '\r\n\r\n');
parts.push(value);
} else {
parts.push('\r\n\r\n' + value);
}
parts.push('\r\n');
};
// Override XHR send()
XMLHttpRequest.prototype.send = function(val) {
var fr,
data,
oXHR = this;
if (val === o) {
// Append the final boundary string
parts.push('--' + boundary + '--\r\n');
// Create the blob
data = getBlob(parts);
// Set up and read the blob into an array to be sent
fr = new FileReader();
fr.onload = function() {
oldSend.call(oXHR, fr.result);
};
fr.onerror = function(err) {
throw err;
};
fr.readAsArrayBuffer(data);
// Set the multipart content type and boudary
this.setRequestHeader('Content-Type', 'multipart/form-data; boundary=' + boundary);
XMLHttpRequest.prototype.send = oldSend;
} else {
oldSend.call(this, val);
}
};
}
})




function getFormData() {
var isNeedShim = ~navigator.userAgent.indexOf('Android') &&
~navigator.vendor.indexOf('Google') &&
!~navigator.userAgent.indexOf('Chrome') &&
navigator.userAgent.match(/AppleWebKit\/(\d+)/).pop() <= 534;
return isNeedShim ? new FormDataShim() : new FormData()
}


function stopPropagation(e) {
if (e.stopPropagation)
e.stopPropagation();
else
e.cancelBubble = true;
}