将其作为顶层小部件，可能放在build函数中：

bool canNavigateBack = true;//This should come from a Stream/bloc/etc.

return WillPopScope(
        onWillPop: () {
          if (Platform.isAndroid) {
            if (canNavigateBack) {
              //Navigate back actions
            } else {
              //Show exit dialog, etc.
            }
          }
          return Future.value(false);
        },
        child: Platform.isIOS
            ? GestureDetector(
                onHorizontalDragEnd: (details) {
                  if (details.velocity.pixelsPerSecond.dx > 50) {
                    if (canNavigateBack) {
                      //Navigate back actions
                    } else {
                      //Show exit dialog, etc.
                    }
                  }
                },
                child: _anotherWidget())
            : _anotherWidget(),
      );