flutter webview 允许下拉刷新的方法

WebView(
              key: UniqueKey(),
              initialUrl: 'https://...',
              javascriptMode: JavascriptMode.unrestricted,
              onWebViewCreated: (WebViewController webViewController) {
                _controller = webViewController;
              },
              onPageFinished: (String str) {
                myModel.stopLoading();
              },
              onPageStarted: (String str) {
                myModel.startLoading();
              },
              gestureRecognizers: Set()
                ..add(Factory<VerticalDragGestureRecognizer>(
                    () => VerticalDragGestureRecognizer()
                      ..onDown = (DragDownDetails dragDownDetails) {
                        _controller.getScrollY().then((value) {
                          if (value == 0 &&
                              dragDownDetails.globalPosition.direction < 1) {
                            _controller.reload();
                          }
                        });
                      })),
            ),