链式期货不按顺序执行

Posted 2023-03-07

【中文标题】链式期货不按顺序执行

【英文标题】：Chaining Futures Do Not Execute In Order

【发布时间】：2018-07-17 21:11:19

【问题描述】：

我目前正在从蓝牙设备读取变量。这显然需要不确定的时间，所以我使用期货（这个方法在我下面的代码中是 readCharacteristic）。

一次不能进行多个读取操作 - 如果在第一个操作仍在进行时启动了第二个读取操作，Flutter 将抛出错误。

我的理解是，使用 .then() 将 future 链接在一起只会允许下一个语句在上一个调用完成时执行。这个想法似乎是正确的，直到我尝试读取第三个值 - 即由于重叠的读取事件而引发错误时。

这是我的代码：

readCharacteristic(scanDurationCharacteristic)
    .then((list) => sensorScanDuration = list[0].toDouble())
    .then((_) 
  readCharacteristic(scanPeriodCharacteristic)
      .then((list) => sensorScanPeriod = list[0].toDouble());
).then((_) 
  readCharacteristic(aggregateCharacteristic)
      .then((list) => sensorAggregateCount = list[0].toDouble());
).then((_) 
  readCharacteristic(appEUICharacteristic)
      .then((list) => appEUI = decimalToHexString(list));
).then((_) 
  readCharacteristic(devEUICharacteristic)
      .then((list) => devEUI = decimalToHexString(list));
).then((_) 
  readCharacteristic(appKeyCharacteristic)
      .then((list) => appKey = decimalToHexString(list));
);

有什么更好的方法来确保这些读取事件不会重叠？

【问题讨论】：

为什么要连锁？为什么不把它们全部并行呢？

并行读取会引发异常，所以我不能并行执行。

【参考方案1】：

虽然 R.C Howell 的答案是正确的，但还是更喜欢使用 async/await 关键字。这更具可读性，并且您出错的可能性更小

Future<void> scanBluetooth() async 
  sensorScanDuration = (await readCharacteristic(scanDurationCharacteristic))[0].toDouble();
  sensorScanPeriod = (await readCharacteristic(scanPeriodCharacteristic))[0].toDouble();
  sensorAggregateCount = (await readCharacteristic(aggregateCharacteristic))[0].toDouble();
  appEUI = await readCharacteristic(appEUICharacteristic).then(decimalToHexString);
  devEUI = await readCharacteristic(devEUICharacteristic).then(decimalToHexString);
  appKey = await readCharacteristic(appKeyCharacteristic).then(decimalToHexString);

【参考方案2】：

如果你想链接 Futures，你必须在前一个 Future 的 then 方法中 return 前一个 Future。

文档说要这样链，

expensiveA()
    .then((aValue) => expensiveB())
    .then((bValue) => expensiveC())
    .then((cValue) => doSomethingWith(cValue));

与，

expensiveA()
    .then((aValue) 
        return expensiveB();
    ).then((bValue) 
        return expensiveC();
    ).then((cValue) => doSomethingWith(cValue));

由于这适用于您的情况，

readCharacteristic(scanDurationCharacteristic)
    .then((list) 
        sensorScanDuration = list[0].toDouble();
        return readCharacteristic(scanPeriodCharacteristic);
    ).then((list) 
        sensorScanPeriod = list[0].toDouble());
        return readCharacteristic(aggregateCharacteristic);
    ).then((list) 
        sensorAggregateCount = list[0].toDouble());
        return readCharacteristic(appEUICharacteristic);
    ).then((list) 
        appEUI = decimalToHexString(list));
        return readCharacteristic(devEUICharacteristic);
    ).then((list) 
        devEUI = decimalToHexString(list));
        return readCharacteristic(appKeyCharacteristic);
    ).then((list) => appKey = decimalToHexString(list));