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;
}