_raiseTiledImageDrawnEvent(tiledImage, tiles){
        if(!this.viewer) {
            return;
        }

        /**
        *  Raised when a tiled image is drawn to the canvas. Used internally for testing.
        *  The update-viewport event is preferred if you want to know when a frame has been drawn.
        *
        * @event tiled-image-drawn
        * @memberof OpenSeadragon.Viewer
        * @type {object}
        * @property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised the event.
        * @property {OpenSeadragon.TiledImage} tiledImage - Which TiledImage is being drawn.
        * @property {Array} tiles - An array of Tile objects that were drawn.
        * @property {?Object} userData - Arbitrary subscriber-defined object.
        * @private
        */
        this.viewer.raiseEvent( 'tiled-image-drawn', {
            tiledImage: tiledImage,
            tiles: tiles,
        });
    }
Called by implementations to fire the tiled-image-drawn event (used by tests)
@private