最近在帮工作室做一个小程序，在换背景图的时候需要预览图片，并且需要裁剪成固定的尺寸。因为小程序并不支持原生的dom操作，导致很多现有的插件都无法使用，所以花了半天专门做了一个小程序的预览裁剪插件。下面贴上代码和效果图。

wxml：

<canvas hidden='{{hide_canvas}}' id='cover-preview' bindtouchstart='canvas_start' bindtouchmove='canvas_move' bindtouchend='canvas_end' disable-scroll='true' canvas-id='cover-preview'>
  <cover-view catchtap='upload_bg' id='croper-sure'>确定</cover-view>
  <cover-view catchtap='cancel_croper' id='croper-cancel'>取消</cover-view>
  <cover-view id='croper'></cover-view>
 </canvas>

注意：canvas里面一定要用cover-view，否则无法覆盖canvas

js：

const ctx = wx.createCanvasContext('cover-preview');
var start_position = {};//移动图片时手指起始坐标
var tempFilePath;//图片路径
var tempWidth;//图片初始宽度
var tempHeight;//图片初始高度
var old_x = 0;//图片初始x坐标
var old_y = 0;//图片初始y坐标
var _touches = 1;//触屏的手指数
var old_scale = 1;//原始放大倍数
var new_scale = 1;//新的放大倍数
var is_move = false;//是否移动
var bg_url;
Page({
data: {
 hide_canvas:true,//绘图层显示控制变量
}，
//选择并将图片输出到canvas
change_cover:function(){
  var that = this;
  wx.showModal({
   title: '提示',content: '更改我的封面',confirmColor: '#39bae8',success: function (res) {
    if (res.confirm) {
 
     wx.chooseImage({
      count: 1,// 默认9
      sizeType: ['original','compressed'],// 可以指定是原图还是压缩图，默认二者都有
      sourceType: ['album','camera'],// 可以指定来源是相册还是相机，默认二者都有
      success: function (res0) {
 
       tempFilePath = res0.tempFilePaths[0];
       that.setData({
        hide_canvas: false,// edit_url: tempFilePath
       })
       wx.getImageInfo({
        src: tempFilePath,success: function (res) {
         // console.log(res.width)
         // console.log(res.height)
         tempWidth = res.width;
         tempHeight = res.height;
         ctx.drawImage(tempFilePath,375,res.height/res.width*375);
         ctx.draw();
        }
       })
 
      }
     })
    } else if (res.cancel) {
     console.log('用户点击取消')
    }
   }
  })
 },//监听手指触摸事件，并判断是移动还是缩放，并记录初始状态
 canvas_start:function(e){
  // console.log(e);
  var touches = e.touches.length;
  if(touches == 1){
   _touches = 1;
   start_position = { x: e.touches[0].x,y: e.touches[0].y,timeStamp:e.timeStamp}
  }else if(touches == 2){
   _touches = 2;
   start_position = { x: e.touches[0].x,x1: e.touches[1].x,y1: e.touches[1].y,timeStamp: e.timeStamp }
  }else{
   _touches = 1;
  }
 },//监听手指移动事件，并做出相应调整
 canvas_move: function (e) {
  // console.log(e);
  var touches = e.touches.length;
  if (_touches == 1 && e.timeStamp - start_position.timeStamp > 150) {
   ctx.drawImage(tempFilePath,old_x + e.touches[0].x - start_position.x,old_y + e.touches[0].y - start_position.y,375 * new_scale,tempHeight / tempWidth * 375 * new_scale);
   ctx.draw();
   is_move = true;
  } else if (_touches == 2 && e.timeStamp - start_position.timeStamp > 150) {
   var change_x = Math.abs(Math.abs(e.touches[0].x - e.touches[1].x) - Math.abs(start_position.x - start_position.x1));
   var change_y = Math.abs(Math.abs(e.touches[0].y - e.touches[1].y) - Math.abs(start_position.y - start_position.y1));
   if(change_x - change_y > 10){
    old_scale = Math.abs(e.touches[0].x - e.touches[1].x) / Math.abs(start_position.x - start_position.x1);
   }else{
    old_scale = Math.abs(e.touches[0].y - e.touches[1].y) / Math.abs(start_position.y - start_position.y1);
   }
   ctx.drawImage(tempFilePath,old_x,old_y,375 * old_scale * new_scale,tempHeight / tempWidth * 375 * old_scale * new_scale);
   ctx.draw();
   is_move = true;
  }else{
   is_move = false;
  }
 },//监听手指离开动作，并保存当前状态数据
 canvas_end: function (e) {
  // console.log(e);
  if (_touches == 1 && is_move) {
   old_x = old_x + e.changedTouches[0].x - start_position.x;
   old_y = old_y + e.changedTouches[0].y - start_position.y;
  } else if (_touches == 2 && is_move) {
   new_scale = old_scale * new_scale;
  }
 
 },//确定并上传背景图
 upload_bg:function(){
  var that = this;
  var screenWidth = wx.getSystemInfoSync().screenWidth;
  // console.log(screenWidth);
  wx.canvasToTempFilePath({
   x: 0,y: screenWidth / 750 * 400,width: screenWidth,height: screenWidth / 750 * 526,destWidth: screenWidth,screenHeight: screenWidth / 750 * 526,quality:1,canvasId: 'cover-preview',success: function (res) {
    that.setData({
     hide_canvas: true,})
    //res.tempFilePath即为生成的图片路径
    console.log(res.tempFilePath)
 
   }
  })
 },//取消图片预览编辑
 cancel_croper:function(){
  ctx.clearActions();
  this.setData({
   hide_canvas: true,// edit_url: tempFilePath
  })
 },})

wxss:

#cover-preview{
 width: 100%;
 height: 100%;
 background-color: black;
}
#croper{
 width: 750rpx;
 height: 526rpx;
 position: absolute;
 top: 400rpx;
 left: 0;
 background-color: rgba(135,206,250,0.5);
}
 
#croper-sure{
 width: 120rpx;
 height: 50rpx;
 border-radius: 10rpx;
 color: black;
 background-color: rgba(135,0.8);
 font-size: 40rpx;
 position: absolute;
 top: 946rpx;
 right: 10rpx;
 text-align: center
}
#croper-cancel{
 width: 120rpx;
 height: 50rpx;
 border-radius: 10rpx;
 color: black;
 background-color: rgba(135,0.8);
 font-size: 40rpx;
 position: absolute;
 top: 946rpx;
 right: 150rpx;
 text-align: center
}

效果图

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