小程序canvas生成分享图

1.图片绘制到canvas

<canvas type="2d" id="canvas"></canvas>
<view catchtap="canvasSave">保存到相册</view>

onLoad: function (options) {
  wx.createSelectorQuery()
    .select('#canvas')
    .fields({
      node: true,
      size: true,
    })
    .exec(this.drawInit.bind(this))
},
drawInit: function (res) {
  const width = res[0].width
  const height = res[0].height

  const canvas = res[0].node
  const ctx = canvas.getContext('2d')

  const dpr = wx.getSystemInfoSync().pixelRatio
  const canvasWidth = width * dpr
  const canvasHeight = height * dpr
  canvas.width = canvasWidth
  canvas.height = canvasHeight
  ctx.scale(dpr, dpr)

  this.data.canvas = canvas
  this.data.ctx = ctx
  this.data.canvasWidth = canvasWidth
  this.data.canvasHeight = canvasHeight
  this.data.dpr = dpr
},
getData:function () {
  //异步接口获取数据，回掉函数中调用draw,这里的setTimeout很关键,开发工具中不加也正常，一旦真机执行就会报错
  setTimeout(()=>{
    this.draw(res.data[0])
  },1000)
}
draw: function (product) {
  var canvas = this.data.canvas
  var ctx = this.data.ctx
  var canvasWidth = this.data.canvasWidth
  var canvasHeight = this.data.canvasHeight
  var dpr = this.data.dpr
  var url = product.url[0]
  var title = product.title

  //白色背景
  ctx.fillStyle="#FFFFFF";
  ctx.fillRect(0,0,canvasWidth,canvasHeight)

  ctx.fillStyle="#000000"
  ctx.fillText(title, 20, canvasHeight/dpr/2+20)

  //因为使用云开发，用云文件 ID 换取真实链接
  wx.cloud.getTempFileURL({
    fileList: [url],
    success: res => {
      var imgUrl = res.fileList[0].tempFileURL
      const img = canvas.createImage()
      img.onload = (res) => {
        ctx.drawImage(img, 0, 0, canvasWidth/dpr, canvasHeight/dpr/2)
      }
      img.src = imgUrl
    }
  })
}

流程:
onload阶段进行canvas的初始化
远程接口获取数据，获取到图片后进行渲染
注意setTimeout如果不加手机上测试会报错

2.将canvas内容生成图片并保存

canvasSave: function () {
  var _this = this
  var canvasWidth = this.data.canvasWidth
  var canvasHeight = this.data.canvasHeight
  var dpr = this.data.dpr
  wx.canvasToTempFilePath({
    x: 0,
    y: 0,
    width: canvasWidth,
    height: canvasHeight,
    destWidth: canvasWidth*dpr,
    destHeight: canvasHeight*dpr,
    canvas: this.data.canvas,                    //注意此参数
    success: res => {
      wx.saveImageToPhotosAlbum({
        filePath:res.tempFilePath,
        success(res) { 
          wx.showToast({
            icon:'success',
            title:'保存成功!'
          })
        }
      })
    }
  })
}

注意文档中canvasId和canvas两个参数的不同传值
保存图片前，要进行权限检查

3.ActionSheet中如何添加分享button

<action-sheet hidden="{{actionSheetHidden}}" bindchange="listenerActionSheet">
  <action-sheet-item>分享</action-sheet-item>
  <action-sheet-cancel>取消</action-sheet-cancel>
</action-sheet>

原生ActionSheet中结构固定，不能添加别的元素进去，其实基于功能而言，完全可以自己重写一个，但是它的一些元素是可以复用的，基于ui交互体验的一致性，代码可维护性，我们在原生组件上进行局部修改
保留action-sheet组件可以获得原生蒙版效果，内部元素都可删除

4.弹出层蒙版，在点击透明区域时候隐藏

<view class="mask" bindtap="hideMask">
  <view catchtap="stopBubbling"></view>
</view>

stopBubbling: function () {
  //内容为空，确保函数存在,目的阻止蒙版内部事件传播
}

内部元素用了catchtap

铁血汉子 2020年3月30日

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