Firebase(角度)承诺在循环中循环 [英] Firebase (Angular) Promises with Loops in Loops

问题描述

只有在所有promise都解决了之后，才能在循环内等待异步调用的情况下，如何执行功能?

How do you execute a function only after all the promises are resolved where you have to wait for async calls in loops within loops?

代码已简化到最小

$scope.book = function(input) {

  //Get the cart items from DB
  ref.child('/cart/' + userId).once('value').then(function(cartSnap) {

    //Loop through the cart items
    cartSnap.forEach(function(cartItemSnap) {

      var itemId = cartItemSnap.key();

      //Get the inventory items from DB that are in the cart
      ref.child('/inventory/' + itemId).once('value').then(function(inventoryItem) {

        //Loop through booking IDs of inventoryItem
        inventoryItem.child('rentals').forEach(function(rentalSnap) {

          var rentalId = rentalSnap.key();

          //Get bookings from rental/active
          ref.child('/rental/'+ rentalId).once('value').then(function(activeRentals) {

            checkIfBookingIsAllowed();

          });
        });
      });
    });

    //Once everything was checked
    bookRental();
  });
};

为了提高速度，所有请求都可以并行进行，但最终功能bookRental()仅在所有问题解决后才能调用.

To improve speed all the requests can be made in parallel but the final function bookRental() can only be called when everything is resolved.

感谢您的帮助.

另一个尝试失败了.Promise.all('collector')在所有诺言被兑现之前就被解雇了.因此，在控制台中的所有检查"之前都会显示完成".

EDITED: Another try that failed. The Promise.all('collector') gets fired before all the promises are resolved. So 'done' shows up before all the 'checks' in the console.

$scope.book = function(input) {

  //Get the cart items from DB
  ref.child('/cart/' + userId).once('value').then(function(cartSnap) {

    //Promise collector
    var collector = [];

    //Loop through the cart items
    cartSnap.forEach(function(cartItemSnap) {

      var itemId = cartItemSnap.key();

      //Get the inventory items from DB that are in the cart
      var promise1 = ref.child('/inventory/' + itemId).once('value').then(function(inventoryItem) {

        //Loop through booking IDs of inventoryItem
        inventoryItem.child('rentals').forEach(function(rentalSnap) {

          var rentalId = rentalSnap.key();

          //Get bookings from rental/active
          var promise2 = ref.child('/rental/'+ rentalId).once('value').then(function(activeRentals) {

            console.log('check');
            collector.push(promise2);

          });
        });

        collector.push(promise1);
      });
    });

    //Once everything was checked
    Promise.all(collector).then(function() {
      console.log('Done');
    });
  });
};

推荐答案

您收拾承诺的时间太晚了.立即收集它们，现在.中的代码.然后 s稍后运行.

You're collecting the promises too late. Collect them now, not later. Code inside .thens runs later.

这是立即运行的内容:

1. 两个嵌套的for循环都运行完毕，减去 .then s中的所有代码.
2. Promise.all(collector).

1. Both nested for-loops run to completion, minus all code inside .thens.
2. Promise.all(collector).

此时， collector 仍然为空，因此 Promise.all()可以快速完成.

At this point collector is still empty, so Promise.all() completes real fast.

将 collector.push(promise)调用移到之外.然后 s.

使用上述修复程序，您的代码应该可以工作，但是更干净的方法是退还所有诺言.尽管 forEach 不允许返回值，但 map 允许返回值，所以可以将其重写为(简化):

With the above fix, your code should work, but a cleaner approach is to return all promises. While forEach doesn't allow return values, map does, so this can be re-written as (simplifying):

Promise.all(cartSnap.map(snap => ref.child(itemUrl).once('value').then(item =>
  Promise.all(item.child('rentals').map(snap => ref.child(url).once('value'))))))
.then(allActiveRentals => console.log('Done'));

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