IOS中html5上传图片方向问题解决方法

用html5编写图片裁切上传，在iphone手机上可能会遇到图片方向错误问题，在此把解决方法和大家分享一下，
用到了html5的 filereader和canvas，如果还没有接触的同学，先了解一下其方法。

 //此方法为file input元素的change事件
 function change(){
  var file = this.files[0];
  var orientation;
  //exif js 可以读取图片的元信息 https://github.com/exif-js/exif-js
  exif.getdata(file,function(){
    orientation=exif.gettag(this,'orientation');
  });
  var reader = new filereader();
  reader.onload = function(e) {  
    getimgdata(this.result,orientation,function(data){
      //这里可以使用校正后的图片data了 
    }); 
  }
  reader.readasdataurl(file);
}

// @param {string} img 图片的base64
// @param {int} dir exif获取的方向信息
// @param {function} next 回调方法，返回校正方向后的base64
function getimgdata(img,dir,next){
 var image=new image();
 image.onload=function(){
  var degree=0,drawwidth,drawheight,width,height;
  drawwidth=this.naturalwidth;
  drawheight=this.naturalheight;
  //以下改变一下图片大小
  var maxside = math.max(drawwidth, drawheight);
  if (maxside > 1024) {
    var minside = math.min(drawwidth, drawheight);
    minside = minside / maxside * 1024;
    maxside = 1024;
    if (drawwidth > drawheight) {
      drawwidth = maxside;
      drawheight = minside;
    } else {
      drawwidth = minside;
      drawheight = maxside;
    }
  }
  var canvas=document.createelement('canvas');
  canvas.width=width=drawwidth;
  canvas.height=height=drawheight; 
  var context=canvas.getcontext('2d');
  //判断图片方向，重置canvas大小，确定旋转角度，iphone默认的是home键在右方的横屏拍摄方式
  switch(dir){
    //iphone横屏拍摄，此时home键在左侧
    case 3:
      degree=180;
      drawwidth=-width;
      drawheight=-height;
      break;
    //iphone竖屏拍摄，此时home键在下方(正常拿手机的方向)
    case 6:
      canvas.width=height;
      canvas.height=width; 
      degree=90;
      drawwidth=width;
      drawheight=-height;
      break;
    //iphone竖屏拍摄，此时home键在上方
    case 8:
      canvas.width=height;
      canvas.height=width; 
      degree=270;
      drawwidth=-width;
      drawheight=height;
      break;
  }
  //使用canvas旋转校正
  context.rotate(degree*math.pi/180);
  context.drawimage(this,0,0,drawwidth,drawheight);
  //返回校正图片
  next(canvas.todataurl("image/jpeg",.8));
 }
 image.src=img;
}