Swift – Wait until swift for loop with asynchronous network requests finishes executing

asynchronousgrand-central-dispatchnsoperationswift

I would like a for in loop to send off a bunch of network requests to firebase, then pass the data to a new view controller once the the method finishes executing. Here is my code:

var datesArray = [String: AnyObject]()

for key in locationsArray {       
    let ref = Firebase(url: "http://myfirebase.com/" + "\(key.0)")
    ref.observeSingleEventOfType(.Value, withBlock: { snapshot in

        datesArray["\(key.0)"] = snapshot.value
    })
}
// Segue to new view controller here and pass datesArray once it is complete

I have a couple concerns. First, how do I wait until the for loop is finished and all the network requests are complete? I can't modify the observeSingleEventOfType function, it is part of the firebase SDK. Also, will I create some sort of race condition by trying to access the datesArray from different iterations of the for loop (hope that makes sense)? I've been reading about GCD and NSOperation but I'm a bit lost as this is the first app I've built.

Note: Locations array is an array containing the keys I need to access in firebase. Also, it's important that the network requests are fired off asynchronously. I just want to wait until ALL the asynchronous requests complete before I pass the datesArray to the next view controller.

Best Answer

You can use dispatch groups to fire an asynchronous callback when all your requests finish.

Here's an example using dispatch groups to execute a callback asynchronously when multiple networking requests have all finished.

override func viewDidLoad() {
    super.viewDidLoad()

    let myGroup = DispatchGroup()

    for i in 0 ..< 5 {
        myGroup.enter()

        Alamofire.request("https://httpbin.org/get", parameters: ["foo": "bar"]).responseJSON { response in
            print("Finished request \(i)")
            myGroup.leave()
        }
    }

    myGroup.notify(queue: .main) {
        print("Finished all requests.")
    }
}

Output

Finished request 1
Finished request 0
Finished request 2
Finished request 3
Finished request 4
Finished all requests.

Related Solutions

Python – Asynchronous Requests with Python requests

Note

The below answer is not applicable to requests v0.13.0+. The asynchronous functionality was moved to grequests after this question was written. However, you could just replace requests with grequests below and it should work.

I've left this answer as is to reflect the original question which was about using requests < v0.13.0.

To do multiple tasks with async.map asynchronously you have to:

Define a function for what you want to do with each object (your task)
Add that function as an event hook in your request
Call async.map on a list of all the requests / actions

Example:

from requests import async
# If using requests > v0.13.0, use
# from grequests import async

urls = [
    'http://python-requests.org',
    'http://httpbin.org',
    'http://python-guide.org',
    'http://kennethreitz.com'
]

# A simple task to do to each response object
def do_something(response):
    print response.url

# A list to hold our things to do via async
async_list = []

for u in urls:
    # The "hooks = {..." part is where you define what you want to do
    # 
    # Note the lack of parentheses following do_something, this is
    # because the response will be used as the first argument automatically
    action_item = async.get(u, hooks = {'response' : do_something})

    # Add the task to our list of things to do via async
    async_list.append(action_item)

# Do our list of things to do via async
async.map(async_list)

Entity-framework – Creating object with nested object list using LINQ to Entities

Here is a working example of something similar. In this case it is the nested Taxes collection that is being populated.

public async Task<IEnumerable<ServiceLayer.DTO.Vendor>> GetAllVendors()
{
    return await (

        from vendor in _db.Vendors

        select new ServiceLayer.DTO.Vendor()
        {
            Id = vendor.Id,
            Name = vendor.Name,
            Taxes = (from tax in _db.VendorTaxes
                     where tax.VendorId.Equals(vendor.Id)
                     select new DTO.VendorTax() { Id = tax.Id, Rate = tax.Rate })
        })

    .ToListAsync();
}

Related Topic

Swift – How to iterate a loop with index and element in Swift