var canvas,ctx;

wx.createSelectorQuery().select('#myCanvas').node(function(res){

    canvas=res.node;

    ctx=canvas.getContext("2d");

    const CameraContext = wx.createCameraContext()

    const listener = CameraContext.onCameraFrame((frame) => {

        ctx.putImageData(frame, 0, 0);

    })

    listener.start();

}).exec()