微信小程序canvas拖拽、截图组件功能

先看下微信小程序canvas拖拽功能

组件地址

github.com/jasondu/wx-… readme近期补上

实现效果

如何实现

1. 使用canvas
2. 使用movable-view标签

由于movable-view无法实现旋转，所以选择使用canvas

需要解决的问题

• 如何将多个元素渲染到canvas上
• 如何知道手指在元素上、如果多个元素重叠如何知道哪个元素在最上层
• 如何实现拖拽元素
• 如何缩放、旋转、删除元素

看起来挺简单的嘛，就把上面这几个问题解决了，就可以实现功能了；接下来我们一一解决。

如何将多个元素渲染到canvas上

定义一个DragGraph类，传入元素的各种属性（坐标、尺寸…）实例化后推入一个 渲染数组 里，然后再循环这个数组调用实例中的渲染方法，这样就可以把多个元素渲染到canvas上了。

如何知道手指在元素上、如果多个元素重叠如何知道哪个元素在最上层

在DragGraph类中定义了判断点击位置的方法，我们在canvas上绑定touchstart事件，将手指的坐标传入上面的方法，我们就可以知道手指是点击到元素本身，还是删除图标或者变换大小的图标上了，这个方法具体怎么判断后面会讲解。

通过循环 渲染数组 判断是非点击到哪个元素到，如果点击中了多个元素，也就是多个元素重叠，那第一个元素就是最上层的元素啦。

###如何实现拖拽元素

通过上面我们可以判断手指是否在元素上，当touchstart事件触发时我们记录当前的手指坐标，当touchmove事件触发时，我们也知道这时的坐标，两个坐标取差值，就可以得出元素位移的距离啦，修改这个元素实例的x和y，再重新循环渲染 渲染数组 就可以实现拖拽的功能。

如何缩放、旋转、删除元素

这一步相对比较难一点，我会通过示意图跟大家讲解。

我们先讲缩放和旋转

通过touchstart和touchmove我们可以获得旋转前的旋转后的坐标，图中的线A为元素的中点和旋转前点的连线；线B为元素中点和旋转后点的连线；我们只需要求A和B两条线的夹角就可以知道元素旋转的角度。缩放尺寸为A和B两条线长度之差。

计算旋转角度的代码如下：

const centerX = (this.x + this.w) / 2; // 中点坐标
const centerY = (this.y + this.h) / 2; // 中点坐标
const diffXBefore = px - centerX;  // 旋转前坐标
const diffYBefore = py - centerY;  // 旋转前坐标
const diffXAfter = x - centerX;   // 旋转后坐标
const diffYAfter = y - centerY;   // 旋转后坐标
const angleBefore = Math.atan2(diffYBefore, diffXBefore) / Math.PI * 180;
const angleAfter = Math.atan2(diffYAfter, diffXAfter) / Math.PI * 180;
// 旋转的角度
this.rotate = currentGraph.rotate + angleAfter - angleBefore;

计算缩放尺寸的代码如下：

// 放大 或 缩小
this.x = currentGraph.x - (x - px);
this.y = currentGraph.y - (x - px);

下面介绍下小程序canvas截图组件

最近做一个小程序的过程中，需要用到截图功能，网上搜了一下，发现没有符合要求的，就自己搞了个组件，方便复用。

目前功能很简单，传入宽高和图片路径即可，宽高是为了计算截图的比例，只支持缩放和移动。

实现思路是：

1.模拟一个截取框；

2.移动图片位置，缩放图片；

3.获取图片在其中的位置(left，top，width，height)；

4.使用canvas绘制图片，然后截取就ok了。

其中第二步的缩放图片比较麻烦，缩放中心点以及平滑缩放

以下是我的实现方式

wxml：

<!--component/picPro/picPro.wxml-->
<scroll-view class='body' hidden="{{hidden}}">
<view class='flex-column flex-between full-height full-width' bindtouchstart="touchstart" bindtouchmove="touchmove" bindtouchend="touchend">
<view class='bg_dark out_item'></view>
<view class='flex-row main flex-between' style='height:{{(windowWidth - margin.left - margin.right)/ratio + "px"}}'>
<view class='bg_dark main_item full-height' style='width:{{margin.left + "px"}}'></view>
<view class='inner relative full-width' id='showArea'>
<image class='absolute img' src='{{src}}' style="width:{{img.width}}px;height:{{img.height}}px;left:{{img.left}}px;top:{{img.top}}px;"></image>
<canvas canvas-id='imgCanvas' class='absolute img_canvas full-height full-width' />
<view class='absolute inner_item left_top'></view>
<view class='absolute inner_item right_top'></view>
<view class='absolute inner_item right_bottom'></view>
<view class='absolute inner_item left_bottom'></view>
</view>
<view class='bg_dark main_item full-height' style='width:{{margin.right + "px"}}'></view>
</view>
<view class='bg_dark out_item flex-column flex-end'>
 <view class='flex-around text_white text_bg'>
<view catchtap='outputImg' data-type='1'><text>重新上传</text></view>
<view catchtap='getImg'><text>选择图片</text></view>
</view>
</view>
<!-- -->
<view class='absolute full-width full-height bg_black'></view>
</view>
</scroll-view>

wxss:(其中引入了一个公共样式，关于flex布局的，看样式名也能猜到)

/* component/picPro/picPro.wxss */
@import '../../resource/style/flex.wxss';
.body{
 position: fixed;
 top: 0;
 right: 0;
 bottom: 0;
 left: 0;
}
.text_white{
 color: white;
}
.main{
}
.out_item{
 width: 100%;
 height: 100%;
 flex: 1;
}
.bg_dark{
 background-color: rgba(0, 0, 0, 0.85)
}
.main_item{
 width: 15px;
}
.inner{
 outline: 3rpx solid white;
 background-color: rgba(0, 0, 0, 0.12);
 box-shadow: 0 0 4px rgba(0, 0, 0, 0.5) inset;
}
.inner_item{
 width: 8px;
 height: 8px;
}
.inner_item.left_top{
 border-left: 3px solid white;
 border-top: 3px solid white;
 left: -3px;
 top: -3px;
}
.inner_item.right_top{
 border-right: 3px solid white;
 border-top: 3px solid white;
 right: -3px;
 top: -3px;
}
.inner_item.right_bottom{
 border-right: 3px solid white;
 border-bottom: 3px solid white;
 right: -3px;
 bottom: -3px;
}
.inner_item.left_bottom{
 border-left: 3px solid white;
 border-bottom: 3px solid white;
 left: -3px;
 bottom: -3px;
}
.img{
 z-index: -1;
}
.bg_black{
 background-color:black;
 z-index: -2;
}
.text_bg{
 padding-bottom: 2em;
 font-size: 0.9em;
}
.img_canvas{
 opacity: 0.5;
}
.newImg{
 z-index: 2
}

js：

// component/picPro/picPro.js
const state = {
 // 可用区域body
 window: { width: 0, height: 0 },
 // 原始图片信息
 originImg: { width: 0, height: 0 },
 // 第一次图片缩放信息
 firstScaleImg: { width: 0, height: 0 },
 // 截取区域信息
 interArea: { width: 0, height: 0 },
 // 单手触摸位置
 touchLast: { x: 0, y: 0 },
 // 滑动距离
 touchMove: { x: 0, y: 0 },
 // 滑动离开时图片状态
 moveImgState: {
  width: 0,
  height: 0,
  top: 0,
  left: 0,
 },
 // 双手触摸位置
 touchList: [{ x: 0, y: 0 }, { x: 0, y: 0 }],
 // 图片缩放比例
 scale: 1,
}
Component({
 /**
  * 组件的属性列表
  */
 properties: {
  //宽（非实际值）
  width: {
   type: Number,
   value: 600
  },
  //高
  height: {
   type: Number,
   value: 300
  },
  //图片路径
  src: {
   type: String,
   value: ""
  },
  //显示隐藏
  hidden: {
   type: Boolean,
   value: false
  },
  //截取框的信息
  margin: {
   type: Object,
   value: {
    left: 15,
    right: 15,
    top: 200,
    bottom: 200,
   }
  }
 },

 ready() {
  this.initialize();
  // const canvas = wx.createCanvasContext('imgCanvas', this);
  // canvas.draw(false, () => { console.log('ccc') }, this);
 },

 /**
  * 组件的初始数据
  */
 data: {
  touchRange: 8,
  img: {
   width: 0,
   height: 0,
   top: 0,
   left: 0,
  },
  canvas: {},
  ratio: 0,
  originImg: {
   width: 0,
   height: 0
  }
 },

 /**
  * 组件的方法列表
  */
 methods: {
  touchstart(e) {
   // console.log("touchstart", e);

  },
  touchmove(e) {
   if (e.touches.length === 1) { this.singleSlip(e.touches[0]) } else {
    this.doubleSlip(e.touches)
   }
  },
  touchend(e) {
   // console.log("touchend", e);
   const x = 0, y = 0;
   state.touchLast = { x, y };
   state.touchMove = { x, y };
   state.touchList = [{ x, y }, { x, y }];
   state.moveImgState = this.data.img;
   // console.log(this.data.img);
  },
  // 单手滑动操作
  singleSlip(e) {
   const { clientX: x, clientY: y } = e;
   const that = this;
   if (state.touchLast.x && state.touchLast.y) {
    state.touchMove = { x: x - state.touchLast.x, y: y - state.touchLast.y };
    state.touchLast = { x, y };
    const move = (_x = false, _y = false) => {
     const bottom = that.data.img.height + that.data.img.top;
     const right = that.data.img.width + that.data.img.left;
     const h = state.interArea.height;
     const w = state.interArea.width;
     const param = {};
     if (_x) {
      if (right > w && that.data.img.left < 0) {
       param.left = that.data.img.left + state.touchMove.x * 0.1
      } else if (right <= w && state.touchMove.x > 0) {
       param.left = that.data.img.left + state.touchMove.x * 0.1
      } else if (that.data.img.left >= 0 && state.touchMove.x < 0) {
       param.left = that.data.img.left + state.touchMove.x * 0.1
      }
     };
     if (_y) {
      if (bottom > h && that.data.img.top < 0) {
       param.top = that.data.img.top + state.touchMove.y * 0.1
      } else if (bottom <= h && state.touchMove.y > 0) {
       param.top = that.data.img.top + state.touchMove.y * 0.1
      } else if (that.data.img.top >= 0 && state.touchMove.y < 0) {
       param.top = that.data.img.top + state.touchMove.y * 0.1
      }
     };
     // console.log(param);
     that.setImgPos(param)
    };
    if (state.scale == 1) {
     if (that.data.img.width == state.interArea.width) {
      move(false, true)
     } else {
      move(true, false)
     }
    } else {
     move(true, true)
    }
   } else {
    state.touchLast = { x, y }
   }
  },
  // 双手缩放操作
  doubleSlip(e) {
   const that = this;
   const { clientX: x0, clientY: y0 } = e[0];
   const { clientX: x1, clientY: y1 } = e[1];
   if (state.touchList[0].x && state.touchList[0].y) {
    let changeScale = (Math.sqrt((x1 - x0) * (x1 - x0) + (y1 - y0) * (y1 - y0)) - Math.sqrt((state.touchList[1].x - state.touchList[0].x) * (state.touchList[1].x - state.touchList[0].x) + (state.touchList[1].y - state.touchList[0].y) * (state.touchList[1].y - state.touchList[0].y))) * 0.0005;
    changeScale = changeScale >= 1.5 ? 1.5 : (changeScale <= -1 ? -1 : changeScale);
    state.scale = that.data.img.width / state.firstScaleImg.width < 1 ? 1 : (state.scale > 2.5 ? 2.5 : 1 + changeScale);
    let width = state.firstScaleImg.width * (state.scale - 1) + state.moveImgState.width;
    width = width < state.firstScaleImg.width ? state.firstScaleImg.width : width;
    let height = state.firstScaleImg.height * (state.scale - 1) + state.moveImgState.height;
    height = height < state.firstScaleImg.height ? state.firstScaleImg.height : height;
    let left = width * (1 - state.scale) / 4 + state.moveImgState.left;
    left = left * (-1) > width - state.interArea.width ? state.interArea.width - width: left > 0 ? 0 : left;
    let top = height * (1 - state.scale) / 4 + state.moveImgState.top;
    top = top * (-1) > height - state.interArea.height ?state.interArea.height - height : top > 0 ? 0 : top;
    const setImgObj = { width, height, left, top };
    that.setImgPos(setImgObj)
   } else {
    state.touchList = [{ x: x0, y: y0 }, { x: x1, y: y1 }]
   }
  },
  // 获取可用区域宽高
  getScreenInfo() {
   const that = this;
   return new Promise((resolve, reject) => {
    wx.getSystemInfo({
     success: function (res) {
      const { windowHeight, windowWidth } = res;
      state.window = { windowHeight, windowWidth };
      that.setData({ windowHeight, windowWidth })
      // console.log(state.window);
      resolve(res);
     },
    })
   })
  },
  setShowArea() {
   const that = this;
   const w = state.window.windowWidth - that.data.margin.left - that.data.margin.right;
   const h = (that.data.height / that.data.width) * w;
  },
  outputImg() {
   this.setData({
    hidden: true,
   })
  },
  getImgInfo(path) {
   return new Promise((resolve, reject) => {
    wx.getImageInfo({
     src: path,
     success(res) {
      console.log(res);
      resolve(res);
     },
     fail(err) {
      reject(err)
     }
    })
   })
  },
  // 设置图片
  setImgPos({ width, height, top, left }) {
   width = width || this.data.img.width;
   height = height || this.data.img.height;
   top = top || this.data.img.top;
   left = left || this.data.img.left
   this.setData({
    img: { width, height, top, left }
   })
  },
  // 初始化图片位置大小
  initialize() {
   const that = this;
   const ratio = that.data.width / that.data.height;
   this.getScreenInfo().then(res => {
    console.log(res);
    state.interArea = { width: res.windowWidth - that.data.margin.left - that.data.margin.right + 2, height: (res.windowWidth - that.data.margin.left - that.data.margin.right) / ratio };
    console.log("interArea", state.interArea)
    that.getImgInfo(that.data.src).then(imgInfo => {
     const { width, height } = imgInfo;
     const imgRatio = width / height;
     state.originImg = { width, height };
     that.setData({
      ratio: ratio
     });
     if (imgRatio > ratio) {
      that.setImgPos({
       height: state.interArea.height,
       width: state.interArea.height * imgRatio
      })
     } else {
      that.setImgPos({
       height: state.interArea.width / imgRatio,
       width: state.interArea.width,
      })
     };
     state.firstScaleImg = { width: that.data.img.width, height: that.data.img.height }
    });
   });
  },
  // 截图
  getImg(){
   const that = this;
   // console.log('dudu', that.data.img);
   const canvas = wx.createCanvasContext('imgCanvas', this);
   const {width,height,left,top} = that.data.img;
   const saveImg = ()=>{
    console.log('开始截取图片');
    wx.canvasToTempFilePath({
     canvasId:"imgCanvas",
     success(res){
      // console.log(res);
      that.setData({
       hidden:true,
       // src:""
      });
      that.triggerEvent("putimg", { imgUrl: res.tempFilePath},{});
     },
     fail(err){
      console.log(err)
     }
    },that)
   };
   canvas.drawImage(that.data.src, left, top, width, height);
   canvas.draw(false, () => { saveImg() }, that)
  }
 }
})

引用的时候除了宽高路径以外，需要wx:if；如果不卸载组件，会出现只能截一次的bug

因为小程序里面没有类似vue中catch的观测数据变化的东西，也不想为了个组件专门去搞一个，就用这种方式代替了，嘻嘻，好敷衍。。

总结

以上所述是小编给大家介绍的微信小程序canvas拖拽、截图组件功能，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对我们网站的支持！