//list为原图片数组列表，index表示当前图片下标，
//imgList表示已经通过canvas转化的图片列表
  trasformImgType(list,index,imgList){
    this.setData({
      isSignCanvasShow:true
    });
    index=index?index:0;
    const that=this;
    let img=list[index].fileUrl;
    img=img.replace(/http/,'https');
    tip.loading('正在打开图片');
    //获取图片信息，
    wx.getImageInfo({
      src: img,
      success (res) {
        //画入canvas
        const context = wx.createCanvasContext('picCanvas');
        that.resetCanvas(context);
        context.drawImage(res.path,0, 0);
        context.draw(false,function(drawed){
          // console.log(drawed);
          wx.canvasToTempFilePath({
            x: 0,
            y: 0,
            width: 414,
            height: 300,
            destWidth: 414,
            destHeight:300,
            fileType: 'jpg',
            canvasId: 'picCanvas',
            success(imgRes) {
              tip.loaded();
              imgList.push(imgRes.tempFilePath);
              if(index<list.length-1){
                that.trasformImgType(list,index+1,imgList)
                return;
              }
              that.setData({
                isSignCanvasShow:false
              })
              wx.previewImage({
                current: '', //图标当前下标
                urls: imgList, // 需要预览的图片http链接列表
                fail:function(res){
                  tip.alert('图片过期需刷新');
                },
              })
            },
            fail() {
              that.setData({
                isSignCanvasShow:false
              })
              tip.loaded();
              tip.alert('图片过期需刷新');
            }
          })
        } )
      }
    })
  },  
  //重绘画板
  resetCanvas(context){
    context.rect(0, 0, this.data.screenWidth, this.data.screenHeight - 4); //画板大小
    context.setFillStyle('#fff');//背景填充
    context.fill() //设置填充
    context.draw()  //开画
 
  },

<view hidden="{{!isSignCanvasShow}}">
　　<canvas canvas-id="picCanvas" id='picCanvas' class="pic-canvas" ></canvas>
</view>