Flutter Builder & FutureBuilder组件

Builder组件

Builder组件起闭包作用，可以限制期使用范围。

问题

class BuilderPage extends StatelessWidget {
    const BuilderPage({Key? key}) : super(key: key);

    @override
    Widget build(BuildContext context) {
        return Scaffold(
            appBar: AppBar(
                title: const Text("Builder组件"),
            ),
            body: Center(
                child: RaisedButton(
                    color: Colors.red,
                    textColor: Colors.white,
                    onPressed: () {
                        showSnackBar(context);
                    },
                    child: const Text('show SnackBar'),
                ),
            ),
        );
    }

    showSnackBar(BuildContext context) {
        const snackBar = SnackBar(content: Text('老孟'));
        Scaffold.of(context).showSnackBar(snackBar);
    }
}

点击按钮后，会提示异常信息：Scaffold.of() called with a context that does not contain a Scaffold.。

这是因为你传入的BuildContext是当前parent Widget的，不是Scaffold的，所以会报错。

解决

可以借助Builder组件解决。

@override
Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
            title: const Text("Builder组件"),
        ),
        body: Builder(
            builder: (context) {
                return Center(
                    child: RaisedButton(
                        color: Colors.red,
                        textColor: Colors.white,
                        onPressed: () {
                            showSnackBar(context);
                        },
                        child: const Text('show SnackBar'),
                    ),
                );
            }
        ),
    );
}

FutureBuilder组件

展示异步任务状态，当有一个Future异步任务需要展示给用户时，可以使用FutureBuilder组件来实现。

final Future<String> _futureSuccess = Future.delayed(const Duration(seconds: 3), () {
    return "成功了";
});
final Future<dynamic> _futureError = Future.delayed(const Duration(seconds: 3), () {
    return Future.error("失败了");
});
String _msg = "加载中";
String _msg2 = "加载中";

FutureBuilder(
    future: _futureSuccess,
    builder: (context, snapshot) {
        late Widget widget;
        if (snapshot.connectionState == ConnectionState.done) {
            if (snapshot.hasError) {
                widget = const Icon(Icons.error, color: Colors.red, size: 50);
                _msg = snapshot.error.toString();
            } else {
                widget = const Icon(Icons.check_circle, color: Colors.green, size: 50);
                _msg = snapshot.data.toString();
            }
        } else {
            widget = const CircularProgressIndicator();
            _msg = "加载中";
        }
        return Container(
            height: 100,
            width: 100,
            decoration: BoxDecoration(
                border: Border.all(color: Colors.grey),
                borderRadius: const BorderRadius.all(Radius.circular(10)),
            ),
            child: Padding(
                padding: const EdgeInsets.all(8.0),
                child: Column(
                    children: [
                        widget,
                        Text(_msg),
                    ],
                ),
            ),
        );
    },
),
FutureBuilder(
    future: _futureError,
    builder: (context, snapshot) {
        late Widget widget;
        if (snapshot.connectionState == ConnectionState.done) {
            if (snapshot.hasError) {
                widget = const Icon(Icons.error, color: Colors.red, size: 50);
                _msg2 = snapshot.error.toString();
            } else {
                widget = const Icon(Icons.check_circle, color: Colors.green, size: 50);
                _msg2 = snapshot.data.toString();
            }
        } else {
            widget = const CircularProgressIndicator();
            _msg2 = "加载中";
        }
        return Container(
            height: 100,
            width: 100,
            decoration: BoxDecoration(
                border: Border.all(color: Colors.grey),
                borderRadius: const BorderRadius.all(Radius.circular(10)),
            ),
            child: Padding(
                padding: const EdgeInsets.all(8.0),
                child: Column(
                    children: [
                        widget,
                        Text(_msg2),
                    ],
                ),
            ),
        );
    },
)