：在Google调用之外的值为null

Question

我正在为一个特定事件调用Google，并且能够获得递归值。

  function getRecurrence(payload) {
    console.log('payload = ' + payload);
    var recurrence = '';
    if (payload != undefined) {
      var requestRecurringEvent = window.gapi.client.calendar.events.get({
        'calendarId': 'primary',
        'eventId': payload
      });
      requestRecurringEvent.execute(function(resp) {
        console.log('requestRecurringEvent = ' + resp);
        console.log('requestRecurringEvent.recurrence = ' + resp.recurrence);
        recurrence = resp.recurrence;

        console.log('recurrence (inside execute)= ' + recurrence); //NO ISSUE (YET): recurrence (inside execute) = RRULE:FREQ=WEEKLY;COUNT=10

        return recurrence;
      });
    } else {
      return 'no recurrence value';
    }
  }

但是，当我将递归值返回给变量时

var recurrence = getRecurrence(action.payload.calendarEventRecurringEventId);
console.log('recurrence (outside execute) = ' + recurrence);  //ISSUE: recurrence (outside execute) = undefined

注意：action.payload.calendarEventRecurringEventId是因为我在redux中传递来自有效负载的值。

为什么在api调用中定义了值，但在api调用之外没有定义？？。

谢谢!

更新

由于console.log('recurrence (outside execute) = ' + recurrence)打印了一个值，所以承诺似乎把我带到了正确的方向，但是它似乎还没有完全解决这个问题，因为仍在为redux中的值设置一个空字符串。

var recurrencePromise = new Promise(function(resolve, reject) {
      var payload = action.payload.calendarEventRecurringEventId;
      console.log('action.payload.calendarEventRecurringEventId = ' + payload);
      var recurrence = '';
      if (payload != undefined) {
        console.log('getRecurrence payload != undefined');
        var requestRecurringEvent = window.gapi.client.calendar.events.get({
          'calendarId': 'primary',
          'eventId': payload
        });
        requestRecurringEvent.execute(function(resp) {
          console.log('requestRecurringEvent = ' + resp);
          console.log('requestRecurringEvent.recurrence = ' + resp.recurrence);
          recurrence = resp.recurrence;
          console.log('recurrence (inside execute)= ' + recurrence);
          resolve(recurrence);
        });
      } else {
        reject(Error("It broke"));
      }
    });

    recurrencePromise.then(function(recurrence) {
      console.log('recurrence (outside execute) = ' + recurrence);
      var recurrenceTemp = 'RRULE:FREQ=WEEKLY;COUNT=10';
      return {
        ...state,
        calendarEventEndDate: action.payload.calendarEventEndDate,
        calendarEventEndDateTime: action.payload.calendarEventEndDateTime,
        calendarEventEndTime: action.payload.calendarEventEndTime,
        calendarEventID: action.payload.calendarEventID,
        calendarEventTitle: action.payload.calendarEventTitle,
        calendarEventRecurringEventId: defaultIfUndefined(action.payload.calendarEventRecurringEventId, ''),
        calendarEventRecurrence: recurrence, //ISSUE: this is where the value should be something like `RRULE:FREQ=WEEKLY;COUNT=10` instead of `''`
      }
    }, function(err) {
      console.log(err); // Error: "It broke"
    });

为什么仍然没有在api调用之外定义正确的值？

Answer 1

由于requestRecurringEvent.execute函数是异步的，这意味着您永远无法判断异步函数何时完成其执行(返回)，而不像保证返回值的同步函数那样，异步不知道。如果您想使用外部的递归值，您需要做的是，A)使用回调函数B)使用承诺(我相信默认情况下google调用提供了这些承诺)

我建议您在javascript中阅读更多有关异步性的内容，以了解它的工作原理。