在触摸设备上平移视口

我想在触摸设备上平移画布的视口,我编写了以下代码:

canvas.on('touch:drag', function (opt) {
        var e = opt.e;
        if (this.isDragging) {
            if (e.clientX === undefined ) {
                this.isDragging = false;
            } else {
                this.viewportTransform[4] += e.clientX - this.lastPosX;
                this.viewportTransform[5] += e.clientY - this.lastPosY;
                this.requestRenderAll();
                this.lastPosX = e.clientX;
                this.lastPosY = e.clientY;
            }
        } else {
            this.isDragging = true;
            this.selection = false;
            this.lastPosX = e.clientX;
            this.lastPosY = e.clientY;
        }
    });