flutter appbar bottom 底部view

appBar: AppBar(
          title: const Text('AppBar Bottom Widget'),
          leading: IconButton(
            tooltip: 'Previous choice',
            icon: const Icon(Icons.arrow_back),
            onPressed: () {
              _nextPage(-1);
            },
          ),
          actions: <Widget>[
            IconButton(
              icon: const Icon(Icons.arrow_forward),
              tooltip: 'Next choice',
              onPressed: () {
                _nextPage(1);
              },
            ),
          ],
          bottom: PreferredSize(
            preferredSize: const Size.fromHeight(48.0),
            child: Theme(
              data: Theme.of(context).copyWith(accentColor: Colors.white),
              child: Container(
                height: 48.0,
                alignment: Alignment.center,
                child: TabPageSelector(controller: _tabController),
              ),
            ),
          ),
        ),
        body: TabBarView(
          controller: _tabController,
          children: choices.map((Choice choice) {
            return Padding(
              padding: const EdgeInsets.all(16.0),
              child: ChoiceCard(choice: choice),
            );
          }).toList(),
        ),
      ),
    );
  }