小程序canvas的使用

现在开发小程序都会有保存图片到本地，这个保存的图片我们可以用canvas画布来画出这么图片

我们首先在wxml中触发点击事件

          <view bindtap="canvas">
            <text>保存到本地</text>
          </view>
    
<view class="model" hidden="{{canvasHide}}" bindtap="clickhid">
      <canvas canvas-id="Canvas" class="canvas"></canvas>
      <view class="btnsave" catchtap="save">保存到相册</view>
    </view>

　　

　　wxss中注入样式

.model {
   100%;
  height: 100%;
  position: fixed;
  z-index: 99;
  background: rgba(0, 0, 0, 0.7);
  left: 0;
  top: 0;
}

.canvas{
   240px;
  height: 255px;
  position: fixed;
  left: 0;
  right: 0;
  top: 30%;
  z-index: 999;
  margin: 0 auto;
  background: #fff;
}
.btnsave{
   100%;
  height: 60px;
  line-height: 60px;
  background: #FE2B54;
  color: #fff;
  text-align: center;
  position: fixed;
  z-index: 99;
  left: 0;
  bottom: 0;
}

　　js中写入逻辑代码，如下：

 data:{ 
 canvasHide:true, //控制画布是否显示
 canvasImg:"",
 canvasWidth: 140
})
//画布生成图片
  canvas() {
   this.setData({
        canvasHide:false,  
  })
    wx.showToast({
      title: '分享图片生成中...',
      icon: 'loading',
      duration: 1000
    });
    var item = {
      headImg:"https://t8.baidu.com/it/u=3571592872,3353494284&fm=79&app=86&size=h300&n=0&g=4n&f=jpeg?sec=1586336426&t=06ceca9a464a8b57ba350c632f4a9dfd",
      cover:"http://img5.imgtn.bdimg.com/it/u=3720661285,1522818110&fm=26&gp=0.jpg",
      nickName:"微笑时很美n",
      title:"111111111"
    };
    let promise1 = new Promise(function(resolve, reject) {
      wx.getImageInfo({
        src: item.cover,
        success: function(res1) {
          resolve(res1);
        }
      })
    });
    let promise2 = new Promise(function(resolve, reject) {
      wx.getImageInfo({
        src: item.headImg,
        success: function(res1) {
          resolve(res1);

        }
      })
    });
    let promise3 = new Promise(function(resolve, reject) {
      wx.getImageInfo({
        src: 'http://img4.imgtn.bdimg.com/it/u=1707295955,795233957&fm=26&gp=0.jpg',
        success: function(res1) {
          resolve(res1);
        }
      })
    });
    Promise.all([
      promise1, promise2, promise3
    ]).then(res1 => {
      const ctx = wx.createCanvasContext('Canvas', this)
      //主要就是计算好各个图文的位置
      ctx.setFillStyle('white');
      ctx.fillRect(0, 0, 600, 880);
      // ctx.drawImage(res1[0].path, 0, 0, 240, 135)
      that.drawImage(ctx, res1[0].path, res1[0].width, res1[0].height);
      var avatarurl_width = 23.5; //绘制的头像宽度
      var avatarurl_heigth = 23.5; //绘制的头像高度
      var avatarurl_x = 17; //绘制的头像在画布上的位置
      var avatarurl_y = 145; //绘制的头像在画布上的位置
      ctx.save();
      ctx.beginPath(); //开始绘制
      ctx.arc(avatarurl_width / 2 + avatarurl_x, avatarurl_heigth / 2 + avatarurl_y, avatarurl_width / 2, 0, Math.PI * 2, false);
      ctx.clip();
      ctx.drawImage(res1[1].path, avatarurl_x, avatarurl_y, avatarurl_width, avatarurl_heigth); // 推进去图片，必须是https图片
      ctx.restore();

      ctx.drawImage(res1[2].path, 166, 180, 68, 68)

      ctx.setFontSize(15)
      ctx.setFillStyle('#000000')
      ctx.fillText(item.nickName, 50, 163)

      ctx.setFontSize(12)
      ctx.setFillStyle('#888888')
      that.drawText(ctx, (item.title || ''), 193, 10, 110);

      ctx.setFontSize(9)
      ctx.setFillStyle('#888888')
      ctx.fillText('长按识别二维码，查看新鲜事儿~', 10, 245)

      ctx.stroke()

      //绘制图片
      ctx.draw(false, setTimeout(() => {
        wx.canvasToTempFilePath({
          x: 0,
          y: 0,
           310,
          height: 355,
          destWidth: 310 * 2,
          destHeight: 355 * 2,
          canvasId: 'Canvas',
          fileType: 'png',
          quality: 1,
          success: function(res) {
            that.setData({
              canvasImg: res.tempFilePath,
            })
            wx.hideToast()
          },
          fail: function(res) {

          }
        }, this)
      }, 200));
    })
  },
  // 描绘图片
  drawImage(ctx, imgPath, imgWidth, imgHeight) {
    let cWidth = this.data.canvasWidth;
    let cHeight = this.data.canvasWidth;
    let dWidth = cWidth / imgWidth;
    let dHeight = cHeight / imgHeight;
    if (imgWidth > cWidth && imgHeight > cHeight || imgWidth < cWidth && imgHeight < cHeight) {
      if (dWidth > dHeight) {
        ctx.drawImage(imgPath, 0, (imgHeight - cHeight / dWidth) / 2, imgWidth, cHeight / dWidth, cWidth / 3 + 5, 0, cWidth, cHeight)
      } else {
        ctx.drawImage(imgPath, (imgWidth - cWidth / dHeight) / 2, 0, cWidth / dHeight, imgHeight, cWidth / 3 + 5, 0, cWidth, cHeight)
      }
    } else {
      if (imgWidth < cWidth) {
        ctx.drawImage(imgPath, 0, (imgHeight - cHeight / dWidth) / 2, imgWidth, cHeight / dWidth, cWidth / 3 + 5, 0, cWidth, cHeight)
      } else {
        ctx.drawImage(imgPath, (imgWidth - cWidth / dHeight) / 2, 0, cWidth / dHeight, imgHeight, cWidth / 3 + 5, 0, cWidth, cHeight)
      }
    }
  },
  //绘制多行文字
  drawText: function(ctx, str, initHeight, titleHeight, canvasWidth) {
    var lineWidth = 0;
    var lastSubStrIndex = 0; //每次开始截取的字符串的索引
    for (let i = 0; i < str.length; i++) {
      lineWidth += ctx.measureText(str[i]).width;
      if (lineWidth > canvasWidth) {
        ctx.setFontSize(14)
        ctx.setFillStyle('#888888')
        ctx.fillText(str.substring(lastSubStrIndex, i), 20, initHeight); //绘制截取部分
        initHeight += 20; //20为字体的高度
        lineWidth = 0;
        lastSubStrIndex = i;
        titleHeight += 30;
      }
      if (i == str.length - 1) { //绘制剩余部分
        ctx.fillText(str.substring(lastSubStrIndex, i + 1), 20, initHeight);
      }
    }
    // 标题border-bottom 线距顶部距离
    titleHeight = titleHeight + 10;
    return titleHeight
  },

　　保存到相册

  save: function() {
    var that = this
    //生产环境时 记得这里要加入获取相册授权的代码
    wx.saveImageToPhotosAlbum({

      filePath: that.data.canvasImg,
      success(res) {
        wx.showModal({
          content: '图片已保存到相册，赶紧晒一下吧~',
          showCancel: false,
          confirmText: '好',
          confirmColor: '#000000',
          success: function(res) {
            if (res.confirm) {
              that.setData({
                canvasHide: false
              })
            }
          }
        })
      }
    })
  },