close(callback) {
		asyncLib.each(
			this.watchings,
			(watching, finishedCallback) => {
				watching.close(finishedCallback);
			},
			err => {
				this.compiler.hooks.watchClose.call();
				if (typeof callback === "function") {
					this.compiler.running = false;
					callback(err);
				}
			}
		);
	}
@param {Callback<void>} callback signals when the watcher is closed
@returns {void}