add(item, callback) {
		if (this._stopped) return callback(new WebpackError("Queue was stopped"));
		this.hooks.beforeAdd.callAsync(item, err => {
			if (err) {
				callback(
					makeWebpackError(err, `AsyncQueue(${this._name}).hooks.beforeAdd`)
				);
				return;
			}
			const key = this._getKey(item);
			const entry = this._entries.get(key);
			if (entry !== undefined) {
				if (entry.state === DONE_STATE) {
					if (inHandleResult++ > 3) {
						process.nextTick(() => callback(entry.error, entry.result));
					} else {
						callback(entry.error, entry.result);
					}
					inHandleResult--;
				} else if (entry.callbacks === undefined) {
					entry.callbacks = [callback];
				} else {
					entry.callbacks.push(callback);
				}
				return;
			}
			const newEntry = new AsyncQueueEntry(item, callback);
			if (this._stopped) {
				this.hooks.added.call(item);
				this._root._activeTasks++;
				process.nextTick(() =>
					this._handleResult(newEntry, new WebpackError("Queue was stopped"))
				);
			} else {
				this._entries.set(key, newEntry);
				this._queued.enqueue(newEntry);
				const root = this._root;
				root._needProcessing = true;
				if (root._willEnsureProcessing === false) {
					root._willEnsureProcessing = true;
					setImmediate(root._ensureProcessing);
				}
				this.hooks.added.call(item);
			}
		});
	}
@param {T} item an item
@param {Callback<R>} callback callback function
@returns {void}