关于 FutureBuilder 的使用，我在之前的公众号文章中有写过，

如果没看过的可以跳转：Flutter FutureBuilder 异步UI神器.

FutureBuilder

首先看 FutureBuilder<T> 类。

构造函数

const FutureBuilder({
  Key key,
  this.future,
  this.initialData,
  @required this.builder,
}) : assert(builder != null),
super(key: key);

构造函数很简单，上一篇文章也说过，主要就是三个参数：

•future：是我们的异步请求，该异步请求必须不能在 build 方法中初始化！•initialData：如果Future 没有完成的情况下展示该数据•builder：构建我们的UI

AsyncWidgetBuilder

其中 builder 的类型为 AsyncWidgetBuilder，我们来看一下：

typedef AsyncWidgetBuilder<T> = Widget Function(BuildContext context, AsyncSnapshot<T> snapshot);

其中 typedef 是为函数起别名用的，

也就是说 builder 是一个方法，从而在定义builder的时候就要实现这个方法。

AsyncSnapshot

接着看一下 snapshot：

@immutable
class AsyncSnapshot<T> {
  /// Creates an [AsyncSnapshot] with the specified [connectionState],
  /// and optionally either [data] or [error] (but not both).
  const AsyncSnapshot._(this.connectionState, this.data, this.error)
    : assert(connectionState != null),
      assert(!(data != null && error != null));

  /// Creates an [AsyncSnapshot] in [ConnectionState.none] with null data and error.
  const AsyncSnapshot.nothing() : this._(ConnectionState.none, null, null);

  /// Creates an [AsyncSnapshot] in the specified [state] and with the specified [data].
  const AsyncSnapshot.withData(ConnectionState state, T data) : this._(state, data, null);

  /// Creates an [AsyncSnapshot] in the specified [state] and with the specified [error].
  const AsyncSnapshot.withError(ConnectionState state, Object error) : this._(state, null, error);

  /// Current state of connection to the asynchronous computation.
  final ConnectionState connectionState;

  /// The latest data received by the asynchronous computation.
  ///
  /// If this is non-null, [hasData] will be true.
  ///
  /// If [error] is not null, this will be null. See [hasError].
  ///
  /// If the asynchronous computation has never returned a value, this may be
  /// set to an initial data value specified by the relevant widget. See
  /// [FutureBuilder.initialData] and [StreamBuilder.initialData].
  final T data;

  /// Returns latest data received, failing if there is no data.
  ///
  /// Throws [error], if [hasError]. Throws [StateError], if neither [hasData]
  /// nor [hasError].
  T get requireData {
    if (hasData)
      return data;
    if (hasError)
      throw error;
    throw StateError('Snapshot has neither data nor error');
  }

  /// The latest error object received by the asynchronous computation.
  ///
  /// If this is non-null, [hasError] will be true.
  ///
  /// If [data] is not null, this will be null.
  final Object error;

  /// Returns a snapshot like this one, but in the specified [state].
  ///
  /// The [data] and [error] fields persist unmodified, even if the new state is
  /// [ConnectionState.none].
  AsyncSnapshot<T> inState(ConnectionState state) => AsyncSnapshot<T>._(state, data, error);

  /// Returns whether this snapshot contains a non-null [data] value.
  ///
  /// This can be false even when the asynchronous computation has completed
  /// successfully, if the computation did not return a non-null value. For
  /// example, a [Future<void>] will complete with the null value even if it
  /// completes successfully.
  bool get hasData => data != null;

  /// Returns whether this snapshot contains a non-null [error] value.
  ///
  /// This is always true if the asynchronous computation's last result was
  /// failure.
  bool get hasError => error != null;

}

前面定义了一个私有的构造函数 const AsyncSnapshot._(this.connectionState, this.data, this.error)，

后面用命名构造函数来调用私有构造函数返回一个 snapshot。

也可以看到 hasData hasError 其实就是判断 data/error 是否等于 null。

_FutureBuilderState

重点是 _FutureBuilderState<T>，还是从上往下看，

首先定义了两个私有变量：

/// An object that identifies the currently active callbacks. Used to avoid
/// calling setState from stale callbacks, e.g. after disposal of this state,
/// or after widget reconfiguration to a new Future.
Object _activeCallbackIdentity;
AsyncSnapshot<T> _snapshot;

_activeCallbackIdentity 根据注释来解释大概就是：标记当前还存活的对象，用于避免已经dispose了还调用setState。

_snapshot 就是我们刚才说用来返回数据的。

initState()

接着是初始化方法：

@override
void initState() {
  super.initState();
  _snapshot = AsyncSnapshot<T>.withData(ConnectionState.none, widget.initialData);
  _subscribe();
}

首先根据传入的 initialData初始化_snapshot，

然后调用_subscribe()

_subscribe()

看一下 _subscribe() 方法 ：

void _subscribe() {
  if (widget.future != null) {
    final Object callbackIdentity = Object();
    _activeCallbackIdentity = callbackIdentity;
    widget.future.then<void>((T data) {
      if (_activeCallbackIdentity == callbackIdentity) {
        setState(() {
          _snapshot = AsyncSnapshot<T>.withData(ConnectionState.done, data);
        });
      }
    }, onError: (Object error) {
      if (_activeCallbackIdentity == callbackIdentity) {
        setState(() {
          _snapshot = AsyncSnapshot<T>.withError(ConnectionState.done, error);
        });
      }
    });
    _snapshot = _snapshot.inState(ConnectionState.waiting);
  }
}

这里做了如下几件事：

1. 判断 future 是否为null;

2.如果不为null，则初始化 _activeCallbackIdentity 为 Object(); 3.变更 _snapshot 的状态为 ConnectionState.waiting; 4.接着对 Future 调用 then 方法，这里主要就是先判断了 callbackIdentity是否相等，如果不相等，那么这个 Future肯定是更改了，或者已经 dispose 了。如果 callbackIdentity 相等，则继续判断是有错误还是有数据，有数据就调用 AsyncSnapshot<T>.withData，有错误就调用 AsyncSnapshot<T>.withError，并传入状态。

didUpdateWidget

接着下面是 didUpdateWidget 方法，该方法主要是用来判断是否需要更新 widget：

@override
void didUpdateWidget(FutureBuilder<T> oldWidget) {
  super.didUpdateWidget(oldWidget);
  if (oldWidget.future != widget.future) {
    if (_activeCallbackIdentity != null) {
      _unsubscribe();
      _snapshot = _snapshot.inState(ConnectionState.none);
    }
    _subscribe();
  }
}

这里更新的逻辑是判断 future 是否一样，如果不一样则：

1.判断 _activeCallbackIdentity 是否为 null2.unsubscribe()，取消订阅。这里就一行代码：_activeCallbackIdentity = null;，3.把 _snapshot 的状态置为 ConnectionState.none4.subscribe()，重新订阅。

dispose()

最后就是 dispose()方法：

@override
void dispose() {
  _unsubscribe();
  super.dispose();
}

FutureBuilder 重写该方法来达到 dispose 时自动取消订阅。

总结

Future 的状态无非三种：

1.未开始2.进行中3.已完成

其中 已完成 又分为两种：

1.有数据2.有异常

其实可以看到，FutureBuilder 大体上的思路就是对 Future 状态的封装，从而达到我们想要的效果。

在 Flutter 中，我们可以通过查看源码来获取很多的灵感，因为 Flutter 的 注释写的简直不要太到位！