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
Related
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.
Context: iOS App written in Swift 3 powered by Firebase 3.0
Challenge: On my app, the user's currentScore is stored on Firebase. This user can complete/un-complete tasks (that will increase/decrease its currentScore) from several ViewControllers.
Overview of the App's architecture:
ProfileVC - where I fetch the currentUser's data from Firebase & display the currentScore.
⎿ ContainerView
⎿ CollectionViewController - users can update their score from here
⎿ DetailsVC - (when users tap on a collectionView cell) - again users can update their score from here.
Question: I need to pass the currentScore to the VCs where the score can be updated. I thought about using prepare(forSegue) in cascade but this doesn't work since it passes "nil" before the query on ProfileVC is finished.
I want to avoid having a global variable as I've been told it's bad practice.
Any thoughts would be much appreciated.
Why don't you create a function that will pull all data before you do anything else.
So in ViewDidLoad call...
pullFirebaseDataASYNC()
Function will look like below...
typealias CompletionHandler = (_ success: Bool) -> Void
func pullFirebaseDataASYNC() {
self.pullFirebaseDataFunction() { (success) -> Void in
if success {
// Perform all other functions and carry on as normal...
Firebase function may look like...
func pullFirebaseDataFunction(completionHandler: #escaping CompletionHandler) {
let refUserData = DBProvider.instance.userDataRef
refUserData.observeSingleEvent(of: .value, with: { snapshot in
if let dictionary = snapshot.value as? [String: AnyObject] {
self.userCurrentScore = dictionary["UserScore"] as! Int
completionHandler(true)
}
})
}
Then when you segue the information across...
In ProfileVC
Create 2 properties
var containerVC: ContainerVC!
var userCurrentScore = Int()
Include the below function in ProfileVC...
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "ProfileToContainerSegue" {
let destination = segue.destination as! ContainerVC
containerVC = destination
containerVC.userCurrentScore = userCurrentScore
}
}
In ContainerVC create a property
var userCurrentScore = Int()
Ways to improve could be an error message to make sure all the information is pulled from Firebase before the user can continue...
Then the information can be segued across the same way as above.
Try instantiation, first embed a navigation controller to your first storyboard, and then give a storyboardID to the VC you are going to show.
let feedVCScene = self.navigationController?.storyboard?.instantiateViewController(withIdentifier: "ViewControllerVC_ID") as! ViewController
feedVCScene.scoreToChange = current_Score // scoreToChange is your local variable in the class
// current_Score is the current score of the user.
self.navigationController?.pushViewController(feedVCScene, animated: true)
PS:- The reason why instantiation is much healthier than a modal segue storyboard transfer , it nearly removes the memory leaks that you have while navigating to and fro, also avoid's stacking of the VC's.
I'm trying to perform a segue to a new view controller, but the segue is being called twice and the new view controller appears twice.I'm using a method that performs a GET request to an API to retrieve data.That method uses a completion handler.
func getSearchResultsForQuery(_ query: String, completionHandlerForSearchResultsForQuery: #escaping (_ success: Bool, _ error: NSError?) -> Void)
When the method completes successfully my segue is called, from within the main queue as is required.
I've set breakpoints so I could see what was going on and the execution jumps from the performSegue back up to the conditional that checks if the method was successful and then continues until the segue is called a second time. I've tried a purely programatic segue, but the result was the same.I also added a print statement, and if I comment out the segue the print statement is only called once.
I've used this same pattern a number of times before and never had a problem with it and I just can't figure out why this is happening.The only thing I'm doing different this time is using Swift 3 and using DispatchQueue.main.async instead of dispatch_async(dispatch_get_main_queue(). Here is the function which is giving me this problem:
#IBAction func search(_ sender: UIButton) {
let searchQuery = searchField.text
TIClient.sharedInstance().getSearchResultsForQuery(searchQuery!) { (success, error) in
if success {
print("Food items fetch successful")
DispatchQueue.main.async {
print("Perorming segue for food item: \(searchQuery)")
self.performSegue(withIdentifier: "showFoodItems", sender: self)
}
} else {
print("error: \(error)")
}
}
}
Edit: I never found out what the problem was, but completely deleting the story board and recreating it solved it.
I know this isn't a great way to fix this issue, Also I can't leave a comment due to low reputation but what happens if you wrap the whole if statement in DispatchQueue.main?
#IBAction func search(_ sender: UIButton) {
let searchQuery = searchField.text
TIClient.sharedInstance().getSearchResultsForQuery(searchQuery!) { (success, error) in
DispatchQueue.main.async {
if success {
print("Food items fetch successful")
self.performSegue(withIdentifier: "showFoodItems", sender: self)
} else {
print("error")
}
}
}
Would that yield a different result or still the same result? checking for Bool doesn't require too much processing power so I don't think putting it in a main queue is a bad thing but I'd do this to trouble shoot. Sorry I can't just comment on this.
Check in storyboard, maybe you set segue from your button action instead of controller.
I've created a user profile screen, and when loaded, the data takes a few seconds to appear. I'm looking for a more elegant solution.
Here's the view controller in the storyboard:
As you can see, there is placeholder text. The issue is that this text appears very briefly when this screen is loaded (before the data is retrieved from the database).
Here is what the screen looks like when the data is fully loaded:
I've seen that an Activity Indicator on the previous screen may be a good solution. Can anyone verify this and point me to a good SWIFT tutorial or Stack Overflow solution?
UPDATE:
I'm loading the data on the previous View Controller as suggested. The issue that I'm running into is that the constants which I store my data are not accessible in prepareForSegue -
override func performSegueWithIdentifier(identifier: String, sender: AnyObject?) {
let query = PFQuery(className:"UserProfileData")
query.whereKey("username", equalTo: (PFUser.currentUser()?.username)!)
query.findObjectsInBackgroundWithBlock { (objects: [PFObject]?, error: NSError?) -> Void in
if error == nil {
if let objects = objects! as? [PFObject] {
for object in objects {
let yourselfObject = object["yourself"] as! String?
let brideGroomObject = object["brideGroom"] as! String?
}
}
} else {
print(error)
}
}
}
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if segue.identifier == "RSVPToUserProfile" {
if let destination = segue.destinationViewController as? UserProfileViewController {
destination.yourselfPassed = yourselfObject
destination.brideGroomPassed = brideGroomObject
}
}
}
This one is actually very simple... Just remove the placeholder text from the storyboard. That way it won't "appear very briefly."
If you don't like the idea of a blank screen, then move the loading code to the place where the view is being presented. Don't present the view until after the loading code is complete. You will have to pass the data to the view controller at presentation time if you do this.
-- EDIT --
Do not override performSegueWithIdentifier:sender:! Doing so will not accomplish your goal.
I say again. Move the loading code to the place where you are calling perform segue, and delay the call until after your data is loaded. (I.E. move the perform segue call into the findObjectsInBackgroundWithBlock block after you get the items.
I am trying to switch view controllers after a user successfully logs in to their account, but it is not working correctly. I cant use a segue directly because if the login button is clicked it will go to that view controller regardless if the information is correct or not. I have tried everything that I know of with no success. This is the code I am trying.
#IBAction func loginTapped(sender: AnyObject) {
let username = usernameField.text
let password = passwordField.text
if username.isEmpty || password.isEmpty {
var emptyFieldsError:UIAlertView = UIAlertView(title: "Please try again", message: "Please fill in all the fields we can get you logged in to your account.", delegate: self, cancelButtonTitle: "Try again")
emptyFieldsError.show()
}
PFUser.logInWithUsernameInBackground(username, password:password) {
(user: PFUser?, error: NSError?) -> Void in
if user != nil {
self.performSegueWithIdentifier("Klikur", sender: self)
} else {
if let errorString = error!.userInfo?["error"] as? String {
self.errorMessage = errorString
}
self.alertView("Please try again", message: "The username password combiation you have given us does not match our records, please try again.", buttonName: "Try again")
}
}
}
I have the storyboard ID set to "Test" and it is not switching view controller when the correct information is entered. Can somebody help me resolve my problem?
[Assuming that your code is not crashing, but rather just failing to segue]
At least one problem is:
self.performSegueWithIdentifier("Test", sender: self)
should be:
dispatch_async(dispatch_get_main_queue()) {
[unowned self] in
self.performSegueWithIdentifier("Test", sender: self)
}
Remember that all UI operations must be performed on the main thread's queue. You can prove to yourself you're on the wrong thread by checking:
NSThread.isMainThread() // is going to be false in the PF completion handler
ADDENDUM
If there's any chance self might become nil, such as getting dismissed or otherwise deallocated because it's not needed, you should capture self weakly as [weak self] not unowned, and use safe unwrapping: if let s = self { s.doStuff() } or optional chaining: self?.doStuff(...)
ADDENDUM 2
This seems to be a popular answer so it's important to mention this newer alternative here:
NSOperationQueue.mainQueue().addOperationWithBlock {
[weak self] in
self?.performSegueWithIdentifier("Test", sender: self)
}
Note, from https://www.raywenderlich.com/76341/use-nsoperation-nsoperationqueue-swift:
NSOperation vs. Grand Central Dispatch (GCD)
GCD [dispatch_* calls] is a lightweight way to represent units of work that are going to be executed concurrently.
NSOperation adds a little extra overhead compared to GCD, but you can add dependency among various operations and re-use, cancel or suspend them.
ADDENDUM 3
Apple hides the single-threaded rule here:
NOTE
For the most part, use UIKit classes only from your app’s main thread.
This is particularly true for classes derived from UIResponder or that
involve manipulating your app’s user interface in any way.
SWIFT 4
DispatchQueue.main.async(){
self.performSegue(withIdentifier: "Test", sender: self)
}
Reference:
https://developer.apple.com/documentation/uikit
Make sure you're putting your:
self.performSegue(withIdentifier: ..., ...)
in viewDidAppear or later. It won't work in viewWillAppear or viewDidLoad.
I've got the same problem with login issue. probably we do the same tutorial. After naming your segue identifier you need to replace:
performSegueWithIdentifier("Klikur", sender: self)
with:
dispatch_async(dispatch_get_main_queue()){
self.performSegueWithIdentifier("Klikur", sender: self)
}
type of seque needs to be set as "show (e.g. Push)" in the storyboard segue.
Hope it will work.
The segue identifier that you pass to performSegueWithIdentifier(_:sender:) must exactly match the ID you've given the segue in the storyboard. I assume that you have a segue between the login view controller and the success view controller, which is as it should be; if not, ctrl+drag from the first to the second view controller, then select the segue's icon in the storyboard and set its ID to Klikur. Don't perform the navigation on the button click, as one commenter said, because that defeats the main purpose of having segues, which is to give a visual indication of the application flow in the storyboard.
EDIT: Here's the code for a login view controller:
import UIKit
class ViewController: UIViewController {
#IBOutlet weak var usernameField: UITextField!
#IBOutlet weak var passwordField: UITextField!
#IBAction func attemptLogin(sender: AnyObject) {
if !usernameField!.text!.isEmpty && !passwordField!.text!.isEmpty {
performSegueWithIdentifier("Klikur", sender: self)
}
}
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if "Klikur" == segue.identifier {
// Nothing really to do here, since it won't be fired unless
// shouldPerformSegueWithIdentifier() says it's ok. In a real app,
// this is where you'd pass data to the success view controller.
}
}
}
And a screenshot of the segue properties that I'm talking about:
swift 3.x
DispatchQueue.main.async(){
self.performSegue(withIdentifier: "Klikur", sender: self)
}
DispatchQueue.main.async() {
self.performSegue(withIdentifier: "GoToHomeFromSplash", sender: self)`
}
Check to make sure you are running the perform segue on a visible view controller.
This is an edge case, but my perform segue failed when I attempted to run it on the view controller belonging to my UIPageViewController that was not currently visible. It also failed if I attempted to do the segue on all view controllers belonging to my UIPageViewController, including the view controller currently visible. The fix was to track which view controller was currently visible in my UIPageViewController, and only perform the segue on that view controller.
An example in a login. When you have success in your login after clicking a button (Action) you can use:
self.performSegue(withIdentifier: "loginSucess", sender: nil)
But if you are launching the app and you got the credentials from your keychain you need to use this as a part of the theard:
DispatchQueue.main.async(){
self.performSegue(withIdentifier: "sessionSuccess", sender: nil)
}