onclick(){draw()}

  async draw(){

    let screenshot = wx.createImage();

    let promiseObj1 = this.creatPromise();

    screenshot.src = "....";

    screenshot.onload = (img)=>{

      canvas.getContext('2d').darw(img)

      ...

      //保存图片

      promiseObj1.resolve()

    }

    await promiseObj1.promise;//使用await时 会在resolve调用后再执行后面的语句

    //第二张图片

  }

  creatPromise(){

    let resolve ,reject;

    let executor= (_resolve, _reject)=>{

      resolve = _resolve;

      reject = _reject;

    }

    let promise =  new Promise<any>(executor);

    return {promise, resolve, reject};

  }