I'm just learning Ios programming for the first time, with Swift and Xcode 6 beta.
I am making a simple test app that should call an API, and then segue programmatically to a different view to present the information that was retrieved.
The problem is the segue. In my delegate method didReceiveAPIResults, after everything has been successfully retrieved, I have:
println("--> Perform segue")
performSegueWithIdentifier("segueWhenApiDidFinish", sender: nil)
When the app runs, the console outputs --> Perform segue, but then there is about a 5-10 second delay before the app actually segues to the next view. During this time all the UI components are frozen.
I'm a little stuck trying to figure out why the segue doesn't happen immediately, or how to debug this!
Heres The Full View controller:
import UIKit
class ViewController: UIViewController, APIControllerProtocol {
#lazy var api: APIController = APIController(delegate: self)
override func viewDidLoad() {
super.viewDidLoad()
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}
func didReceiveAPIResults(results: NSDictionary) {
println(results)
println("--> Perform segue")
performSegueWithIdentifier("segueWhenApiDidFinish", sender: nil)
}
#IBAction func getData(sender : AnyObject){
println("--> Get Data from API")
api.getInfoFromAPI()
}
}
And my API controller:
import UIKit
import Foundation
protocol APIControllerProtocol {
func didReceiveAPIResults(results: NSDictionary)
}
class APIController: NSObject {
var delegate: APIControllerProtocol?
init(delegate: APIControllerProtocol?) {
self.delegate = delegate
}
func getInfoFromAPI(){
let session = NSURLSession.sharedSession()
let url = NSURL(string: "https://itunes.apple.com/search?term=Bob+Dylan&media=music&entity=album")
let task = session.dataTaskWithURL(url, completionHandler: {data, response, error -> Void in
if(error) {
println("There was a web request error.")
return
}
var err: NSError?
var jsonResult = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions. MutableContainers, error: &err) as NSDictionary
if(err?) {
println("There was a JSON error.")
return
}
self.delegate?.didReceiveAPIResults(jsonResult)
})
task.resume()
}
}
UPDATE: Got this working based on Ethan's answer. Below is the exact code that ended up getting the desired behavior. I needed assign that to self to have access to self inside the dispatch_async block.
let that = self
if(NSThread.isMainThread()){
self.delegate?.didReceiveAPIResults(jsonResult)
}else
{
dispatch_async(dispatch_get_main_queue()) {
println(that)
that.delegate?.didReceiveAPIResults(jsonResult)
}
}
Interestingly, this code does not work if I remove the println(that) line! (The build fails with could not find member 'didReceiveAPIResults'). This is very curious, if anyone could comment on this...
I believe you are not on the main thread when calling
self.delegate?.didReceiveAPIResults(jsonResult)
If you ever are curious whether you are on the main thread or not, as an exercise, you can do NSThread.isMainThread() returns a bool.
Anyway, if it turns out that you are not on the main thread, you must be! Why? Because background threads are not prioritized and will wait a very long time before you see results, unlike the mainthread, which is high priority for the system. Here is what to do... in getInfoFromAPI replace
self.delegate?.didReceiveAPIResults(jsonResult)
with
dispatch_sync(dispatch_get_main_queue())
{
self.delegate?.didReceiveAPIResults(jsonResult)
}
Here you are using GCD to get the main queue and perform the UI update within the block on the main thread.
But be wear, for if you are already on the main thread, calling dispatch_sync(dispatch_get_main_queue()) will wait FOREVER (aka, freezing your app)... so be aware of that.
I have a delay problem with segue from a UITableView. I have checked and I appear to be on the main thread. I checked "NSThread.isMainThread()" during prepareForSegue. It always returns true.
I found a solution on Apple Developer forums! https://forums.developer.apple.com/thread/5861
This person says it is a bug in iOS 8.
I followed their suggestion to add a line of code to didSelectRowAtIndexPath...... Despatch_async.....
It worked for me, hopefully you too.
Related
I have a Button on First VC which is directed to two active states.
1) SecondVC
override func viewDidLoad() {
super.viewDidLoad()
subjectPickerView.dataSource = self
subjectPickerView.delegate = self
SwiftyRequest()
// Used the text from the First View Controller to set the label
}
func SwiftyRequest(){
print("SecondViewController METHOD BEGINS")
let jsonobj = UserDefaults.standard.object(forKey: "PostData")
let json = JSON(jsonobj as Any)
for i in 0 ..< json.count{
let arrayValue = json[i]["name"].stringValue
print(arrayValue)
self.subjects.append(arrayValue)
self.subjectPickerView.reloadAllComponents()
}
print(self.subjects)
}
2) IBAction of FirstVC
#IBAction func buttonPressed(_ sender: Any) {
Alamofire.request("http://localhost/AIT/attempt3.php",method: .post, parameters: ["something": semValue, "branch" : streamValue])
.responseJSON { response in
print(response.result)
if let JSON1 = response.result.value {
print("Did receive JSON data: \(JSON1)")
// JSONData.someData = JSON1 as AnyObject?
UserDefaults.standard.set(JSON1, forKey: "PostData")
UserDefaults.standard.synchronize()
}
else {
print("JSON data is nil.")
}
}
}
NOW, Whenever i pressed the button it calls the viewDidLoad of SecondVC before IBAction of FirstVC which is a bit problematic for my app! How can i decide the priority between these two function.
You have to think about what you want to happen. Clearly the Alamofire call is going to take some time. What do you want to do with the 2nd VC while that time elapses? What do you want to do if the call does not return at all?
This is a common problem when dependent on external resources. How do you manage the UI? Do you present the UI in a partial state? Do you put a popover saying something like "loading". Or do you wait for the resource to complete before presenting the 2nd VC at all?
We cannot make that decision for you, since it depends on your requirement. There are ways to implement each one, though. If the resource usually responds quickly you could show the VC in a partial state and then populate it on some kind of callback. Typically call backs are either (1) blocks (2) delegate methods or (3) notifications. There is also (less commonly) (4) KVO. You should probably research the pros and cons of each.
I am trying to make an API call in my Swift project. I just started implementing it and i am trying to return a Swift Dictionary from the call.
But I think i am doing something wrong with the completion handler!
I am not able to get the returning values out of my API call.
import UIKit
import WebKit
import SafariServices
import Foundation
var backendURLs = [String : String]()
class ViewController: UIViewController, WKNavigationDelegate, WKUIDelegate {
#IBOutlet var containerView : UIView! = nil
var webView: WKWebView!
override func viewDidLoad() {
super.viewDidLoad()
self.getBackendURLs { json in
backendURLs = self.extractJSON(JSON: json)
print(backendURLs)
}
print(backendURLs)
}
func getBackendURLs(completion: #escaping (NSArray) -> ()) {
let backend = URL(string: "http://example.com")
var json: NSArray!
let task = URLSession.shared.dataTask(with: backend! as URL) { data, response, error in
guard let data = data, error == nil else { return }
do {
json = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as? NSArray
completion(json)
} catch {
#if DEBUG
print("Backend API call failed")
#endif
}
}
task.resume()
}
func extractJSON(JSON : NSArray) -> [String : String] {
var URLs = [String : String]()
for i in (0...JSON.count-1) {
if let item = JSON[i] as? [String: String] {
URLs[item["Name"]! ] = item["URL"]!
}
}
return URLs
}
}
The first print() statements gives me the correct value, but the second is "nil".
Does anyone have a suggestion on what i am doing wrong?
Technically #lubilis has answered but I couldn't fit this inside a comment so please bear with me.
Here's your viewDidLoad
override func viewDidLoad() {
super.viewDidLoad()
self.getBackendURLs { json in
backendURLs = self.extractJSON(JSON: json)
print(backendURLs)
}
print(backendURLs)
}
What will happen is the following:
viewDidLoad is called, backendURLs is nil
you call getBackendURLs, which starts on another thread in the background somewhere.
immediately after that your code continues to the outer print(backendURLs), which prints nil as backendURLs is still nil because your callback has not been called yet as getBackendURLs is still working on another thread.
At some later point your getBackendURLs finishes retrieving data and parsing and executes this line completion(json)
now your callback is executed with the array and your inner print(backendURLs) is called...and backendURLs now has a value.
To solve your problem you need to refresh your data inside your callback method.
If it is a UITableView you could do a reloadData() call, or maybe write a method that handles updating the UI for you. The important part is that you update the UI inside your callback, because you don't have valid values until then.
Update
In your comments to this answer you say:
i need to access the variable backendURLs right after the completionHandler
To do that you could make a new method:
func performWhateverYouNeedToDoAfterCallbackHasCompleted() {
//Now you know that backendURLs has been updated and can work with them
print(backendURLs)
//do what you must
}
In the callback you then send to your self.getBackendURLs, you invoke that method, and if you want to be sure that it happens on the main thread you do as you have figured out already:
self.getBackendURLs { json in
backendURLs = self.extractJSON(JSON: json)
print(backendURLs)
DispatchQueue.main.async {
self.performWhateverYouNeedToDoAfterCallbackHasCompleted()
}
}
Now your method is called after the callback has completed.
As your getBackendURLs is an asynchronous method you can not know when it has completed and therefore you cannot expect values you get from getBackedURLs to be ready straight after calling getBackendURLs, they are not ready until getBackendURLs has actually finished and is ready to call its callback method.
Hope that makes sense.
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 know that there are multiple approaches to pass data back from one controller to another like Delegates, NSNotifications. I am using another way using Closures to pass data data back. I just want to know is it safe way how I pass any data using blocks like below or should I avoid this approach.
First ViewController (where I make object of Second ViewController)
#IBAction func push(sender: UIButton) {
let v2Obj = storyboard?.instantiateViewControllerWithIdentifier("v2ViewController") as! v2ViewController
v2Obj.completionBlock = {(dataReturned) -> ()in
//Data is returned **Do anything with it **
print(dataReturned)
}
navigationController?.pushViewController(v2Obj, animated: true)
}
Second ViewController (where data is passed back to First VC)
import UIKit
typealias v2CB = (infoToReturn :NSString) ->()
class v2ViewController: UIViewController {
var completionBlock:v2CB?
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
}
#IBAction func returnFirstValue(sender: UIButton) {
guard let cb = completionBlock else {return}
cb(infoToReturn: returnFirstValue)
}
#IBAction func returnSecondValue(sender: UIButton) {
guard let cb = completionBlock else {return}
cb(infoToReturn: returnSecondValue)
}
}
That's a very good and reasonable approach and much better than notifications.
Looking at the evolution of Cocoa API you will notice that Apple has replaced more and more delegate API with blocks / closures over the years.
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.