下面代码，无论that放哪里，怎么整都提示：fcanvasToTempFilePath: fail canvas is empty，真是无语，麻烦求解，谢谢。

onReady: function ()

  {

    var that = this;

    var context = wx.createCanvasContext('mycanvas',that);

    wx.chooseImage

    ({

      success: function (res)

      {

        context.drawImage(res.tempFilePaths[0], 0, 0, 150, 100);

        context.fillStyle = "#FF0000";

        context.fillRect(0, 0, 50, 50);

        context.draw(true, setTimeout(function()

        {

          wx.canvasToTempFilePath

          ({

            destWidth: 150,

            destHeight: 100,

            canvasId: 'mycanvas',

            success: function (res)

            {

              console.log('1212' + res.tempFilePath);

            },

            fail: function (res) {console.log(res);}

          },that);

        },300));

      }

    });

  }