override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
submitTapped()
if let scheduleController = segue.destination as? ScheduleController {
scheduleController.jsonObject = self.info
}
}
In submitTapped(), self.info is assigned a value. But when I run my app, self.info is reported as "nil". I tried setting breakpoints at each of the three lines, and it seems that submitTapped() doesn't execute until after this function is finished.
Why is this? Does it have to deal with threads? How can I get submitTapped() to execute before the rest? I'm just trying to move from one view controller to another while also sending self.info to the next view controller.
UPDATE:
I ended up figuring it out (for the most part) thanks to the answer below + my own testing.
#IBAction func submitTapped() {
update() { success in
if success {
DispatchQueue.main.async {
self.performSegue(withIdentifier: "showScheduler", sender: nil)
}
}
}
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
// I'll probably check the segue identifier here once I have more "actions" implemented
let destinationVC = segue.destination as! ScheduleController
destinationVC.jsonObject = self.info
}
public func update(finished: #escaping (Bool) -> Void) {
...
self.info = jsonObject //get the data I need
finished(true)
...
}
The network request is an asynchronous task that occurs in the background and takes some time to complete. Your prepareForSegue method call will finish before the data comes back from the network.
You should look at using a completionHandler and also only triggering the segue once you have the data.
so your submitTapped function (probably best to rename this to update or something) will make the network request and then when it gets the data back will set the self.info property and then call performSegueWithIdentifier.
func update(completion: (Bool) -> Void) {
// setup your network request.
// perform network request, then you'll likely parse some JSON
// once you get the response and parsed the data call completion
completion(true)
}
update() { success in
// this will run when the network response is received and parsed.
if success {
self.performSegueWithIdentifier("showSchedular")
}
}
UPDATE:
Closures, Completion handlers an asynchronous tasks can be very difficult to understand at first. I would highly recommend looking at this free course which is where I learnt how to do it in Swift but it takes some time.
This video tutorial may teach you basics quicker.
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I want to pass simple String from oneVC to secondVC. I'm new to iOS.
I declared String nameOfFilm.
When I'm clicking on CollectionViewItem I'm downloading JSON, taking one value of it(String) and assign it to nameOfFilm. Then I'm starting segue to the secondVC.
in the PrepareForSegue method with proper identifier I compare the secondVC value to the nameOfFlim.(nextScene.name = nameOfFilm)
In the result, when the secondVC comes in, nextScene.name is empty.enter image description here
JSON i parsed properly. I think it may be mismatch in time and some functions are too fast. What is causing the problem?
var nameOfFilm = String()
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
INDEX_NUMBER_BEFORE = indexPath.row
let filmID = arrayWithID[INDEX_NUMBER_BEFORE]
downloadFilm(url: "https://api.themoviedb.org/3/movie/\(filmID)\(key)&append_to_response=videos,images")
performSegue(withIdentifier: "toTheDetail", sender: self)
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "toTheDetail" {
let nextScene = segue.destination as! FilmDetailViewController
nextScene.name = nameOfFilm
}
}
The issue here will be that downloadFilm will be an asynchronous task that occurs in the background and takes some time to make the request and respond with the value that you need.
Your downloadFilm function should accept a callback so that you can wait for the response value and then perform an action based on that response, a typical way of using this would be something like:
downloadFilm(url: "https://api.themoviedb.org/3/movie/\(filmID)\(key)&append_to_response=videos,images") { filmName in
self.nameOfFilm = filmName
self.performSegue(withIdentifier: "toTheDetail", sender: self)
}
To do this you would need to update your downloadFilm function, so that its similar to this...
func downloadFilm(url: String, completion: #escaping (_ filmName: String) -> Void) {
// do work here to setup network request
let task = URLSession.shared.dataTask(with: request) { data, response, error in
// parse response and get name
completion(name)
}
}
Here is a guide on Swift completion handlers that will help understand the concept.
I have two viewControllers that are giving me trouble. One is called notificationAccessViewController.swift and the other is called classChooseViewController.swift. I have a button in the notificationAccessViewController that triggers a segue to the classChooseViewController. What I need to do is within the prepareForSegue function, perform an Alamofire request and then pass the response to the classChooseViewController. That works! BUT, not fast enough.
Below is my prepareForSegue function within the notificationAccessViewController
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
let DestViewController = segue.destination as! classChooseViewController
Alamofire.request("MYURL.com").responseJSON { (response) in
if let JSON : NSDictionary = response.result.value as! NSDictionary?{
let classNameArray = JSON["className"] as! NSArray
print("---")
print(classNameArray)
DestViewController.classNameArray = classNameArray
}
}
}
I have it printing out classNameArray to the console, which it does SUCCESSFULLY. Although, in my classChooseViewController.swift, I also print out the classNameArray to the console and it prints with nothing inside of it. This is because the viewDidLoad() function of the classChooseViewController.swift is running before the prepareForSegue function has finished. I want to know what I can do to ensure that the classChooseViewController does not load until the prepareForSegue function has FINISHED running.
Below is my code for classChooseViewController
class classChooseViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {
var classNameArray: NSArray = []
override func viewDidLoad() {
super.viewDidLoad()
print(classNameArray)
}
}
I have attached a picture of what the console reads. You can see that the classNameArray prints from the classChooseViewControllers viewDidLoad() function BEFORE the prepareForSegue() function because the "---" prints out after.
Your design is wrong.
The Alamofire request function is asynchronous. It returns immediately, before the results are ready. Then it invokes the completion handler that you pass in.
I tend to agree with Paul that you should make the request in the destination view controller, not in the current view controller.
If you DO want to fetch the data in Alamofire before you segue to the other view controller then you'll need to write a method that starts the request, then invokes the segue to the other view controller from the Alamofire.request() call's completion handler.
If you're invoking the new view controller from a button press, you need to not link the button directly to a segue, but instead write an IBAction method that triggers the AlamoFire request call, then invokes the segue (or instantiates the view controller directly and pushes/presents it manually) from the completion handler.
Something like this:
#IBAction buttonAction(sender: UIButton) {
Alamofire.request("MYURL.com").responseJSON {
(response) in
if let JSON : NSDictionary = response.result.value as! NSDictionary? {
let classNameArray = JSON["className"] as! NSArray
print("---")
print(classNameArray)
let theClassChooseViewController = storyboard.instantiateViewController(withIdentifier:"ClassChooseViewController" as ClassChooseViewController
theClassChooseViewController.classNameArray = classNameArray
presentViewController(theClassChooseViewController,
animated: true,
completion: nil)
}
}
}
By the way, class names and types should start with an upper case letter, and variable names should start with a lower-case letter. This is a very strong convention in both Swift and Objective-C, and you'll confuse the hell out of other iOS/Mac developers unless you follow the convention.
Please check the below code:
#IBAction func sendActivationCode(_ sender: UIButton) {
service.Register(phoneNumber: self.mobileNumberTxt.text!, callback: { (response) in
self.setCustomerValues(response: response)
})
}
func setCustomerValues(response: [String:Any]) {
registrationToken = (response["token"]! as! String)
code = response["code"] as! Int
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "toStep2" {
let vc = segue.destination as! Step2ViewController
vc.registrationToken = registrationToken
}
}
The problem is: prepare function is executed before setCustomerValues and I cannot use registrationToken variable in Step2ViewController.swift because it's nil.
Instead of connecting your segue from the button to Step2ViewController, connect it from the view controller. This way the segue will not automatically be performed when the button is touched.
Then call performSegue from within your setCustomerValues callback to perform the segue explicitly after getting the registration token. Note that if the callback is not on the main thread, you will need to dispatch_async to the main thread before calling performSegue.
You should push viewcontroller after self.setCustomerValues(response: response). Don't push viewcontroller when sendActivationCode
The best way to come out of this problem is to create an IBAction method from your button on a Touch Up Inside Event and not create any Segues on 'action' of your button.
Use the following code:
#IBAction func sendActivationCode(_ sender: UIButton) {
service.Register(phoneNumber: self.mobileNumberTxt.text!, callback: {
(response) in
let vc = self.storyboard?.instantiateViewControllerWithIdentifier("Step2ViewController") as! Step2ViewController
vc.registrationToken = (response["token"]! as! String)
vc.code = response["code"] as! Int
self.navigationController?.pushViewController(vc!, animated: true)
})
}
My Issue:
I am trying to load data from Server, through Alamofire, before SubViewController Load its view. After writing the code, I failed to solve the problem of Async Feature of Alamofire. The view is always be loaded in the SubViewController before Alamofire finished its job.
Part Of My Code:
ParentViewController:
Leading the way to SubViewController through PrepareForSegue().
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if segue.identifier == "CellDetailSegue" {
if let indexPaths = self.dateCollectionView.indexPathsForSelectedItems() {
let subViewController = segue.destinationViewController as! SubViewConroller
}
}
SubViewController:
Test whether the data has been loaded by print() in the its viewDidLoad() and load the data by dataRequest() in viewWillAppear()
class SubViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {
var availablePeriods401 = [String]()
var availablePeriods403 = [String]()
var availablePeriods405 = [String]()
override func viewDidLoad() {
super.viewDidLoad()
self.dataRequest(self.availablePeriods401)
self.dataRequest(self.availablePeriods403)
self.dataRequest(self.availablePeriods405)
print(self.availablePeriods401.count)
print(self.availablePeriods403.count)
print(self.availablePeriods405.count)
}
func dataRequest(_ target: [String]) {
Alamofire.request(.POST, "http://httpbin.org/get", parameters: ["foo": "bar"]).responseJSON {
.
.
.
target = Result
}
}
}
Problem Description:
Three variables in the SubViewController can not be assigned the valid values after view was loaded.
Three Outputs' results are all 0.
But I can get valid count if I set print() in the dataRequest().
My Question:
How to make sure that Alamofire finishes its job?
Where Shall I put the Alamofire Request Function? viewWillApper()? viewDidApper()?
Should I even finished requesting job in ParentViewController's PrepareForSegue() ?
Please teach me how to solve this problem.
A big appreciation for your guide and time.
Ethan Joe
You should call Alamofire Request Function in viewDidLoad function. and you should reload table data when you got response from completion block(from where you print the data).
You can reload tableview like,
self.tableView.reloadData()
hope this will help :)
The first thing I noticed is that you are doing 3 asynchronous requests, not one. You could use a completion handler but which one? I think you have 2 options.
Nest the network calls so that the completion of one starts the next one. The downside to this approach is that they will run sequentially and if you add more, you have to continue nesting. An approach like this might be OK if you are only doing 2 calls but beyond that it will get more and more difficult.
Use a semaphore to wait until all the data is loaded from all the remote calls. Use the completion handler to signal the semaphore. If you are going to use this approach, then it must be done on a background thread because use of a semaphore will block the thread and you don't want that happening on the main thread.
These three calls will all happen simultaneously. And the functions will return even though AlamoFire has not completed.
self.dataRequest(self.availablePeriods401)
self.dataRequest(self.availablePeriods403)
self.dataRequest(self.availablePeriods405)
These will execute, whether AlamoFire has completed or not.
print(self.availablePeriods401.count)
print(self.availablePeriods403.count)
print(self.availablePeriods405.count)
Using semaphores would look something like this:
override func viewWillAppear(animated: Bool) {
// maybe show a "Please Wait" dialog?
loadMyData() {
(success) in
// hide the "Please Wait" dialog.
// populate data on screen
}
}
func loadMyData(completion: MyCompletionHandler) {
// Do this in an operation queue so that we are not
// blocking the main thread.
let queue = NSOperationQueue()
queue.addOperationWithBlock {
let semaphore = dispatch_semaphore_create(0)
Alamofire.request(.POST, "http://httpbin.org/get", parameters: ["foo": "bar1"]).responseJSON {
// This block fires after the results come back
// do something
dispatch_semaphore_signal(semaphore);
}
Alamofire.request(.POST, "http://httpbin.org/get", parameters: ["foo": "bar2"]).responseJSON {
// This block fires after the results come back
// do something
dispatch_semaphore_signal(semaphore);
}
Alamofire.request(.POST, "http://httpbin.org/get", parameters: ["foo": "bar3"]).responseJSON {
// This block fires after the results come back
// do something
dispatch_semaphore_signal(semaphore);
}
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
completion(true)
}
}
Apple Docs - Grand Central Dispatch
How to use semaphores
The question I have for you is what are you going to do if some, bit not all of the web calls fail?
First create a global bool variable with false
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if segue.identifier == "CellDetailSegue" && boolVar {
if let indexPaths = self.dateCollectionView.indexPathsForSelectedItems() {
let subViewController = segue.destinationViewController as! SubViewConroller
}
}
call prepare segue with segue name and boolVar true from almofire block.
I'm loading some XML from a webservice (car data), create some car objects and would like to display them in a TableViewController.
When the user has selected start and destination location, I'm making an async call to the webservice, show an activity indicator and as soon as the data is loaded, I go to a new view. So I have something like this:
class NewReservationViewController : UIViewController {
#IBAction func searchCarsClicked(sender: UIBarButtonItem) {
//show load cars activity indicator
loadingCarsActivityIndicator.startAnimating()
//load available cars from webservice asyncronously
DataManager.getAvailableCars([...parameter list...], carsLoadedCallback: carsLoaded)
}
func carsLoaded(loadedCars: [Car]) {
//dismiss the waiting widget
//trigger the segue and advance to the next screen
loadingCarsActivityIndicator.stopAnimating()
print("stopped cars loading activity indicator")
print("cars loaded callback called")
print("loaded \(loadedCars.count) distinct cars")
self.cars = loadedCars
performSegueWithIdentifier("showAvailableCars", sender: self)
}
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if segue.identifier == "showAvailableCars" {
let carTableViewController = segue.destinationViewController as! CarTableViewController
carTableViewController.cars = self.cars
presentViewController(carTableViewController, animated: false, completion: nil)
}
}
}
class DataManager {
class func getAvailableCars([...parameter list...], carsLoadedCallback: ([Car]) -> Void){
Webservice.getAvailability([...parameter list...], completionHandler: { (data, response, error) in
//parse xml
//in the end I get an Array of Car objects
var cars: [Car] = ...
carsLoadedCallback(cars)
})}
}
}
When I populate the TableView with some DummyData I create within the CarTableViewController class, it works fine. However when I try to pass the car arrays to my TableViewController I get a EXC_BAD_ACCESS code = 2 exception. As far as I know this is some kind of Memory exception that is usually caused by a corrupt pointer. So I guess that the car array I created in my static DataManager class within the static method I called gets destroyed. However I'm not sure about that because automatic reference counting should avoid that.
The table view even displays the data but then immediately crashes with the EXC_BAD_ACCESS exception. I tried to set a general breakpoint in the XCode's breakpoints tab but however I don't get a reasonable error message on why the app crashes.
Do you have any ideas on why this happens. How can I get a better error message?
Thanks for your help in advance.
First of all check this function:
func carsLoaded(loadedCars: [Car])
It is returned as callback - what thread is it running? main or backround?
You should call on main thread
dispatch_async(dispatch_get_main_queue(), {
performSegueWithIdentifier("showAvailableCars", sender: self)
})
If doesn't help - provide the line, where it breaks in debugger, so I can help and see more.
UPD: Didn't notice, why do you do manual present?
presentViewController(carTableViewController, animated: false, completion: nil)
Your segue automatically shows view controller, you can change it's style (modal, push) in storyboard.
There are two potential issues that I can see. The first is that you are triggering your segue from a method that might not be on the main thread, you need to ensure that this is done on the main thread. The other issue is that in your prepare for segue you are unwrapping the new ViewController without checking it, so try this instead:
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if segue.identifier == "showAvailableCars" {
if let carTableViewController = segue.destinationViewController as? CarTableViewController {
carTableViewController.cars = self.cars
presentViewController(carTableViewController, animated: false, completion: nil)
}
}
}
As you said the table view displays the data and then immediately crashes I would assume that the problem is in your CarTableViewController. One thing you could check is when your cells are rendered, if you are trying to access some information that is null from the Car objects