Flutter 80: 初识 Flutter Stream (一)

      小菜在之前尝试 EventChannel 时曾经用到过 Stream 流数据，现在准备学习一下 flutter_bloc 时也主要用到 Stream 来做异步处理，于是简单学习一下何为 Stream？

A source of asynchronous data events.

      Stream 主要应用于 Flutter 的异步操作，在其他编程语言中也存在；Stream 提供了一种接受事件队列的方法，可通过 listen 进行数据监听，通过 error 接收失败状态，通过 done 来接收结束状态；

1. Stream 创建

      Flutter 提供了多种创建 Stream 的方式；

1.1 Stream.fromFuture(Future future)

      Stream 通过 Future 创建新的单订阅流，当 Future 完成时会触发 data / error，然后以 done 事件结束；

Future getData() async {
  await Future.delayed(Duration(seconds: 3));
  return '当前时间为：${DateTime.now()}';
}

_streamFromFuture() {
  Stream.fromFuture(getData())
      .listen((event) => print('Stream.fromFuture -> $event'))
      .onDone(() => print('Stream.fromFuture -> done 结束'));
}

1.2 Stream.fromFutures(Iterable> futures)

      Stream 通过一系列的 Future 创建新的单订阅流，每个 Future 都会有自身的 data / error 事件，当这一系列的 Future 均完成时，Stream 以 done 事件结束；若 Futures 为空，则 Stream 会立刻关闭；其分析源码，很直接的看到是将每一个 Future 事件监听完之后才会执行的微事件结束；

====================================== 源码 ======================================
factory Stream.fromFutures(Iterable> futures) {
    _StreamController controller =
        new _SyncStreamController(null, null, null, null);
    int count = 0;
    var onValue = (T value) {
      if (!controller.isClosed) {
        controller._add(value);
        if (--count == 0) controller._closeUnchecked();
      }
    };
    var onError = (error, StackTrace stack) {
      if (!controller.isClosed) {
        controller._addError(error, stack);
        if (--count == 0) controller._closeUnchecked();
      }
    };
    for (var future in futures) {
      count++;
      future.then(onValue, onError: onError);
    }
    // Use schedule microtask since controller is sync.
    if (count == 0) scheduleMicrotask(controller.close);
    return controller.stream;
}
====================================== 测试 ======================================
_streamFromFutures() {
  var data = [getData(), getData(), getData()];
  Stream.fromFutures(data)
      .listen((event) => print('Stream.fromFutures -> $event'))
      .onDone(() => print('Stream.fromFutures -> done 结束'));
}

1.3 Stream.fromIterable(Iterable elements)

      Stream 通过数据集合中获取并创建单订阅流，通过 listen 监听迭代器中每一个子 element，当 Stream 监听到取消订阅或 Iterator.moveNext 返回 false / throw 异常 时停止迭代；

_streamFromIterable() {
  var data = [1, 2, '3.toString()', true, false, 6];
  Stream.fromIterable(data)
      .listen((event) => print('Stream.fromIterable -> $event'))
      .onDone(() => print('Stream.fromIterable -> done 结束'));
}

1.4 Stream.periodic(Duration period, [T computation(int computationCount)])

      Stream 通过 Duration 对象作为参数创建一个周期性事件流，其中若不设置 computation 时 onData 获取数据为 null；若没有事件结束则会一直周期性执行；

_streamFromPeriodic() {
  Duration interval = Duration(seconds: 1);
  // onData 获取数据为 null
  Stream stream = Stream.periodic(interval);
  stream.listen((event) {
  print('Stream.periodic -> $event');
  }).onDone(() {
  print('Stream.periodic -> done 结束');
  });

  // onData 获取数据为 int 类型 data
  Stream streamData = Stream.periodic(interval, (data) => data);
  streamData.listen((event) {
    print('Stream.periodic -> $event');
    if (event >= 10) {}
  }).onDone(() {
    print('Stream.periodic -> done 结束');
  });
}

2. Stream 基本操作

2.1 Stream take(int count)

      take() 对于单订阅方式，可以提供 take 设置之前的 Stream 订阅数据，例如设置中断 Stream.periodic 周期展示次数；小菜粗略理解为 take 可以作为中断订阅，如果 take 设置次数大于 onDone 之前的订阅数据次数，Stream 依旧获取所有 onDone 之前的订阅数据；

_streamFromPeriodic() {
  Duration interval = Duration(seconds: 1);
  Stream streamData = Stream.periodic(interval, (data) => data + 1);
  streamData.take(5).listen((event) {
    print('Stream.periodic -> $event');
  }).onDone(() {
    print('Stream.periodic -> done 结束');
  });
}

_streamFromIterable() {
  var data = [1, 2, '3.toString()', true, false, 6];
  Stream.fromIterable(data)
      .take(8)
      .listen((event) => print('Stream.fromIterable -> $event'))
      .onDone(() => print('Stream.fromIterable -> done 结束'));
}

2.2 Stream takeWhile(bool test(T element))

      takeWhile 也可以实现上述相同效果，通过 test 返回一个 boolean 类型，如果为 false 则中断订阅；

_streamFromPeriodic() {
  Duration interval = Duration(seconds: 1);
  Stream streamData = Stream.periodic(interval, (data) => data + 1);
  streamData.takeWhile((element) {
    print('Stream.periodic.takeWhile -> $element');
    return element <= 5;
  }).listen((event) {
    print('Stream.periodic -> $event');
  }).onDone(() {
    print('Stream.periodic -> done 结束');
  });
}

2.3 Stream where(bool test(T event))

      where 用于在当前 Stream 中创建一个新的 Stream 用来丢弃不符合 test 的数据；小菜简单理解为类似数据库查询一样，仅过滤符合需求的数据流；且 where 可以设置多次；

Stream streamData = Stream.periodic(interval, (data) => data + 1);
streamData.takeWhile((element) {
  print('Stream.periodic.takeWhile -> $element');
  return element <= 5;
}).where((event) {
  print('Stream.periodic.where -> $event');
  return event > 3;
}).listen((event) {
  print('Stream.periodic -> $event');
}).onDone(() {
  print('Stream.periodic -> done 结束');
});

2.4 Stream distinct([bool equals(T previous, T next)])

      distinct 小菜理解为相邻两个数据滤重；

var data = [1, 2, '3.toString()', true, true, false, true, 6];
Stream.fromIterable(data)
    .distinct()
    .listen((event) => print('Stream.fromIterable -> $event'))
    .onDone(() => print('Stream.fromIterable -> done 结束'));

2.5 Stream skip(int count)

      skip 用于跳过符合条件的订阅数据次数；

Stream streamData = Stream.periodic(interval, (data) => data + 1);
streamData.takeWhile((element) {
      print('Stream.periodic.takeWhile -> $element');
      return element <= 6;
    }).where((event) {
      print('Stream.periodic.where -> $event');
      return event > 2;
    })
    .skip(2).listen((event) {
      print('Stream.periodic -> $event');
    }).onDone(() {
      print('Stream.periodic -> done 结束');
    });

2.6 Stream skipWhile(bool test(T element))

      skipWhile 用于跳过在 where 符合条件下满足设置 test 条件的订阅数据；即当 test 为 true 时跳过当前订阅数据监听；

Stream streamData = Stream.periodic(interval, (data) => data + 1);
streamData.takeWhile((element) {
  print('Stream.periodic.takeWhile -> $element');
  return element <= 6;
}).where((event) {
  print('Stream.periodic.where -> $event');
  return event > 2;
}).skipWhile((element) {
  print('Stream.periodic.skipWhile -> $element');
  return element <= 4;
}).listen((event) {
  print('Stream.periodic -> $event');
}).onDone(() {
  print('Stream.periodic -> done 结束');
});

2.7 Stream map(S convert(T event))

      在当前 Stream 基础上创建一个新的 Stream 并对当前 Stream 进行数据操作，onData 监听到的是 map 变更后的新的数据流；

Stream streamData = Stream.periodic(interval, (data) => data + 1);
streamData.takeWhile((element) {
  print('Stream.periodic.takeWhile -> $element');
  return element <= 6;
}).where((event) {
  print('Stream.periodic.where -> $event');
  return event > 2;
}).skipWhile((element) {
  print('Stream.periodic.skipWhile -> $element');
  return element <= 4;
}).map((event) {
  print('Stream.periodic.map -> $event -> ${event * 100}');
  return event * 100;
}).listen((event) {
  print('Stream.periodic -> $event');
}).onDone(() {
  print('Stream.periodic -> done 结束');
});

2.8 Stream expand(Iterable convert(T element))

      在当前 Stream 基础上创建新的 Stream 并将当前订阅数据转为新的订阅数据组，onData 监听 数据组 中每个新的订阅数据元素；

Stream streamData = Stream.periodic(interval, (data) => data + 1);
streamData.takeWhile((element) {
  print('Stream.periodic.takeWhile -> $element');
  return element <= 6;
}).where((event) {
  print('Stream.periodic.where -> $event');
  return event > 2;
}).skipWhile((element) {
  print('Stream.periodic.skipWhile -> $element');
  return element <= 4;
}).expand((element) {
  print('Stream.periodic.expand -> $element -> ${element * 10} -> ${element * 100}');
  return [element, element * 10, element * 100];
}).listen((event) {
  print('Stream.periodic -> $event');
}).onDone(() {
  print('Stream.periodic -> done 结束');
});

2.9 Future get length

      Stream 监听订阅事件结束后，符合 where 条件的数量；

_streamLength(index) async {
  Duration interval = Duration(seconds: 1);
  Stream streamData = Stream.periodic(interval, (data) => data + 1);
  streamData = streamData.takeWhile((element) {
    print('Stream.periodic.takeWhile -> $element');
    return element <= 6;
  }).where((event) {
    print('Stream.periodic.where -> $event');
    return event > 2;
  }).skipWhile((element) {
    print('Stream.periodic.skipWhile -> $element');
    return element <= 4;
  });
  switch (index) {
    case 1:
      var length = await streamData.length;
      print('Stream.length -> $length');
      break;
    case 2:
      var isEmpty = await streamData.isEmpty;
      print('Stream.isEmpty -> $isEmpty');
      break;
    case 3:
      var isBroadcast = await streamData.isBroadcast;
      print('Stream.isBroadcast -> $isBroadcast');
      break;
    case 4:
      var first = await streamData.first;
      print('Stream.first -> $first');
      break;
    case 5:
      var last = await streamData.last;
      print('Stream.last -> $last');
      break;
  }
}

2.10 Future get isEmpty

      Stream 监听订阅事件结束后，统计是否符合 where 条件的订阅数据是否为空；

_streamLength(2);

2.11 Future get first

      获取 Stream 符合条件的第一个订阅数据；

_streamLength(4);

2.12 Future get last

      获取 Stream 符合条件的最后一个订阅数据；

_streamLength(5);

2.13 Future> toList()

      在 Stream 监听结束之后，将订阅数据存储在 List 中，该操作为异步操作；

_streamToList() async {
  var data = [1, 2, '3.toString()', true, true, false, true, 6];
  Stream stream = Stream.fromIterable(data).distinct();
  List list = await stream.toList();
  if (list != null) {
    print('Stream.toList -> ${list}');
    for (int i = 0; i < list.length; i++) {
      print('Stream.toList -> ${i + 1} -> ${list[i]}');
    }
  }
}

2.14 Future> toSet()

      在 Stream 监听结束之后，将订阅数据存储在 Set 中，Set 可以过滤重复数据；

_streamToSet() async {
  var data = [1, 2, '3.toString()', true, true, false, true, 6];
  Stream stream = Stream.fromIterable(data);
  Set set = await stream.toSet();
  if (set != null) {
    print('Stream.toSet -> ${set}');
  }
}

2.15 Future forEach(void action(T element))

      监听 Stream 中订阅数据，是对 listen 方式的一种监听；

_streamForEach() {
  var data = [1, 2, '3.toString()', true, true, false, true, 6];
  Stream stream = Stream.fromIterable(data).distinct();
  stream.forEach((element) => print('Stream.forEach -> $element'));
}

---

      小菜对 Stream 的尝试才刚刚开始，还有众多方法未曾尝试，对 Stream 的理解还很浅显，如有错误请多多指导！

来源： 阿策小和尚