I am trying to make async. request using Promisekit's promises. I have the following code in a subclass of UITableViewController, to reload tableview with data that is fetched from async. request.
my_promise.then { asynly_fetched_data in
self.data = asyncly_fetched_data
self.tableView.reloadData()
}
However, the following statement (self.tableView.reloadData()) is causing the following build error.
Missing return in a closure expected to return 'AnyPromise'
Is it because we cannot call reloadData() inside a closure. IF that is the case, what is the best practice to reload tableview after asynchronous request is complete.
It's a swift bug. But you can fix it by adding -> Void in your closure:
my_promise.then { asynly_fetched_data -> Void in
self.data = asyncly_fetched_data
self.tableView.reloadData()
}
That way, Swift knows that the return is Void.
Related
I am new to iOS objective C and currently am enhancing some features to an existing project. I would like to implement a callback method to return the results from an API call in the viewDidLoad method as I need the results to determine the number of icons to display on the screen.
I have looked around for various answers, however, I am not sure what has went wrong and I am unable to return the responses from the APIs to the NSMutableArray in the ViewDidLoad method. I would greatly appreciate any help on how to go about get the response into the customerArray in the viewDidLoad method
This is the Service Delegate header file that defines the method:
I made use of the first method requestCompleted in my ViewController file to retrieve the response returned from the API. Within this method, customerArray which is a globally declared array contains the items that was returned from the API, however when this array was called in the viewDidLoad method, it appears to be null.
I tried the following way to retrieve using the dispatch async method, but the customer Array is still empty
You are printing customerArray, before dispatch_async executed. Try putting breakpoint in requestCompleted method or put print statement in requestCompleted method. and check the value.
You can't execute the callback method in viewDidLoad function. And then this attempt is not good pattern in iOS.
I understand what you want. You want to display various icons when view controller is appeared.
For this, you may need to use UITableView or CollectionView.
In this case, i have been doing following.
First, Run API call by using Alamofire
Second. Show loading icon (by using HUD)
Third, In API callback, fetch the list information and reload tableview.
*Following is swift code but it is very similar with Objective C.
override func viewDidLoad() {
super.viewDidLoad()
HUD.show(.rotatingImage(UIImage(named: "rotateLogo")), onView: nil)
APIManager.request(Constant.sdkCredential,
parameters: [:],
fullResponse:true)
{ (response, success, error) in
if success == true {
guard (response as? Dictionary<String,Any>) != nil else {
self.showAlert(withTitle: "Error", message: "Invalid Credential")
return
}
if let dict = response as? Dictionary<String,Any> {
print(dict)
if let dictData = dict["data"] as? Dictionary<String,Any>{
self.tableView.reloadData();
}
}
} else {
HUD.hide()
self.showAlertWithError(error)
}
}
}
}
Hey guys, I have a problem setting a value for the label. The label should display the number of elements in the array inside my JSON (link - followers_url variable). I call alamo and make a request with that url. When I print the value inside parseData() method I get the right result. When I print it inside configureView() and viewDidLoad() I always get 0.
Setting the label text also works only inside parseData() method. Any ideas how I can get it to work?
Alamofire.request(url).validate().responseJSON { response in
self.parseData(data: response.data!)
}
This above request runs on another background thread.
So when you call the function callAlamo the response is received in the completion block ( { response in ). So when you call print() after callAlamo. the response has not yet been received and print is called so value is not updated. So please perform the operation on the response only through completion block.
If you want to set a label write you set label code after self.parseData in completion block ({response in). Make sure you set it in main queue as the UI operation needs to be performed on main queue only
Following question will help to set label on main thread.
In Swift how to call method with parameters on GCD main thread?
You need to understand multithreading concept to get a better understanding of this. Follow this https://medium.com/#gabriel_lewis/threading-in-swift-simply-explained-5c8dd680b9b2
You should learn something about iOS Parsing techniques. Then learn how to create Model using class or struct. Then you will get Idea.
You should look into Object Mapper as well.
You're dealing with an asynchronous operation. Asynchronous operations are "actions" that are dispatched and require you to wait before they complete. Think about loading a website in Safari. Once you type, let's say, stackoverflow.com in your browser, a loading spinner will notify that something is loading. While the page is loading, you obviously cannot see what's on the webpage. There's only an empty, white page.
The same is happening with your request. When you call the callAlamo function you're telling the app to start loading something. This is requiring you to wait until the task is done. If you count the elements in the followersAndFollowingArray right after the server call, then you'll get it empty, because the request is still waiting to be completed. It's like pretending to view the stackoverflow.com website immediately after having typed the URL. You can't.
That's where closures come in handy. You can use closures to execute something when another action has been completed. In this case, I would fire the web request, display a loading spinner to notify the user that something is loading, and finally populate the followersLabel along with stopping the animation. You can do something like that
func callAlamo(url: String, completion: #escaping ([User]) -> Void) {
if Connectivity.isConnectedToInternet {
Alamofire.request(url).validate().responseJSON { response in
let userData = self.parseData(data: response.data!)
completion(userData)
}
}
}
Additionally you need to let the parseData method to return the parsed array of Users, so the callAlamo function could use it.
func parseData(data : Data) -> [User] {
do {
return try JSONDecoder().decode([User].self, from: data)
} catch let jsonErr {
print("Error serializing", jsonErr)
return [User]()
}
}
Finally, you can execute the callAlamo function on inside the configureView method, performing an action when the server request has been completed. In our case, we want to populate the label.
private func configureView(){
followersLabel.text = String(followers)
// Starting the loading animation
startAnimation()
callAlamo(url: "Hello") { userData in
// Assigning the callAlamo result to your followers array
// once the server request has been completed
self.followersAndFollowingArray = userData
// This will return the number you'd expect
print(self.followersAndFollowingArray.count)
// Stopping the loading animation
stopAnimation()
}
}
Right now you probably won't have the startAnimation and stopAnimation methods, but you can feel free to implement them, I just wanted to give you an idea of a classic implementation.
I need to load data from rss and display it in tableview. I decided to do it in some other class rather than in ViewController. So I created DataFetch class. The problem is that I use third party library to work with rss, and it works like this:
func fetchPodcastFeed() -> [RSSFeedItem]{
let feedURL = NSURL(string: feedURLString)!
var feedItems = [RSSFeedItem]()
dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), {
// Run parsing in a background thread
FeedParser(URL: feedURL)?.parse({ (result) in
feedItems = (result.rssFeed?.items)!
dispatch_async(dispatch_get_main_queue(), { () -> Void in
//reload table
})
})
})
return feedItems
}
Of course when I call fetchItems = dataFetcher.fetchPodcastFeed() from ViewController I don't get any data. Should I use notifications and if so, how to pass fetched data to ViewController through them? Or is there better approach?
So I assume the fetchPodcastFeed() function is your code, right?
Then I'd suggest defining it with a callback like so:
func fetchPodcastFeed(onCompletion:(result:[RSSFeedItem]) -> Void) {
// setup as you have it ...
dispatch_async(dispatch_get_main_queue(), { () -> Void in
// call the completion closure
onCompletion(feedItems)
})
})
})
}
Then, wherever you're calling the method, do it like this:
fetchPodcastFeed({(result) in
// reload your table with result, which is of type [RSSFeedItem]
})
Theoretically you could also simply pass the completion handler directly into the parse method you're calling on the FeedParser object, but since you are on a background thread it's probably wiser to first send it to the main thread again. Otherwise you'd have to put the reload table stuff in a dispatch_asynccall, it's nicer the way you started.
My question is very similar to several others here but I just can't get it to work. I'm making an API call via a helper class that I wrote.
First I tried a standard function with a return value and the result was as expected. The background task completed after I tired to assign the result.
Now I'm using a closure and I can get the value back into my view controller but its still stuck in the closure, I have the same problem. I know I need to use GCD to get the assignment to happen in the main queue.
this is what I have in my view controller
var artists = [String]()
let api = APIController()
api.getArtistList("foo fighters") { (thelist) -> Void in
if let names = thelist {
dispatch_async(dispatch_get_main_queue()) {
artists = names
print("in the closure: \(artists)")
}
}
}
print ("method 1 results: \(artists)")
as the results are:
method 1 results: []
in the closure: [Foo Fighters & Brian May, UK Foo Fighters, John Fogerty with Foo Fighters, Foo Fighters, Foo Fighters feat. Norah Jones, Foo Fighters feat. Brian May, Foo Fighters vs. Beastie Boys]
I know why this is happening, I just don't know how to fix it :( The API calls need to be async, so what is the best practice for capturing these results? Based on what the user selects in the table view I'll be making subsequent api calls so its not like I can handle everything inside the closure
I completely agree with the #Craig proposal of the use of the GCD, but as your question involves the request of the API call every time you select a row, you can do the following:
Let's suppose you use the tableView:didSelectRowAtIndexPath: method to handle the selection, then you can do the following inside it:
func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
// it is just a form to get the item
let selectedItem = items.objectAtIndex(indexPath.row) as String
api.getArtistList(selectedItem) { (thelist) -> Void in
if let names = thelist {
dispatch_async(dispatch_get_main_queue()) {
artists = names
}
}
}
}
And then you can observe the property and handle do you want inside it :
var artists: [String] = [] {
didSet {
self.tableView.reloadData() // or anything you need to handle.
}
}
It just another way to see it. I hope this help you.
The easy solution is to do whatever you're doing at your print(), inside the closure.
Since you're already dispatch_asyncing to the main queue (the main/GUI thread), you can complete any processing there. Push a new view controller, present some modal data, update your current view controller, etc.
Just make sure that you don't have multiple threads modifying/accessing your local/cached data that is being displayed. Especially if it's being used by UITableViewDelegate / UITableViewDataSource implementations, which will throw fits if you start getting wishy-washy or inconsistent with your return values.
As long as you can retrieve the data in the background, and the only processing that needs to occur on the main thread is an instance variable reassignment, or some kind of array appending, just do that on the main thread, using the data you retrieved on the back end. It's not heavy. If it is heavy, then you're going to need more sophisticated synchronization methods to protect your data.
Normally the pattern looks like:
dispatch_async(getBackgroundQueue(), {
var theData = getTheDataFromNetwork();
dispatch_async(dispatch_get_main_queue() {
self.data = theData // Update the instance variable of your ViewController
self.tableView.reloadData() // Or some other 'reload' method
});
})
So where you'd normally refresh a table view or notify your ViewController that the operation has completed (or that local data has been updated), you should continue your main-thread processing.
When the post button is pressed, the function below executes. In the function, all the objects that are retrieved using the Parse backend are appended to the groupConversation array, which is a global array. However, when I reference the array in the UITableViewController that is popped to towards the end of the function and use println() to print the content of the array, the array is empty. However, when I use println() in the UIViewController that contains this function the array is shown to contain one object. In the console, the println() of the UITableViewController that is popped to once the button is pressed, is executed before the println() of the UIViewController that contains the function below. How can I make the functon below execute completely before popping to the UITableViewController.
#IBAction func postButtonPressed(sender: AnyObject) {
//Adds Object To Key
var name=PFObject(className:currentScreen)
name["userPost"] = textView.text
name.saveInBackgroundWithBlock {
(success: Bool!, error: NSError!) -> Void in
if success == true {
self.textView.text=""
} else {
println("TypeMessageViewController Error")
}
}
//Gets all objects of the key
var messageDisplay = PFQuery(className:currentScreen)
messageDisplay.selectKeys(["userPost"])
messageDisplay.findObjectsInBackgroundWithBlock {
(objects: [AnyObject]!, error: NSError!) -> Void in
if error == nil{
for object in objects {
var textObject = object["userPost"] as String
groupConversation.append(textObject)
}
} else {
// Log details of the failure
}
println("Type message \(groupConversation)")
}
navigationController!.popToViewController(navigationController!.viewControllers[1] as UIViewController, animated: true)
}
The problem is here messageDisplay.findObjectsInBackgroundWithBlock. As you are doing this in background thread, it will be separated from main thread. And your main thread will execute as it should be.
So it before finishing the task you main thread popping the view.
messageDisplay.findObjectsInBackgroundWithBlock {
(objects: [AnyObject]!, error: NSError!) -> Void in
if error == nil{
for object in objects {
var textObject = object["userPost"] as String
groupConversation.append(textObject)
}
} else {
// Log details of the failure
}
println("Type message \(groupConversation)")
dispatch_async(dispatch_get_main_queue()) {
self.navigationController!.popToViewController(navigationController!.viewControllers[1] as UIViewController, animated: true)
return
}
}
Pushing and popping in background thread may cause problem. So get the main thread after executing the task in background and then pop in main thread.
In swift single statement closures automatically return the statement return value. In your specific case, it's attempting to return an instance of [AnyObject]?, which is the return value of popToViewControllerAnimated. The closure expected by dispatch_afteris Void -> Void instead. Since the closure return type doesn't match, the compiler complains about that.
Hope this helps.. ;)
You are running into a very common issue with asynchronous code. Both your ...InBackgroundWithBlock {} methods run something in the background (async).
The best example I have found to explain it is this:
When you start an async code block, it is like putting eggs on to boil. You also get to include something that should be done when they finish boiling (the block). This might be something like remove the shell and slice the eggs.
If your next bit of code is "butter bread, put eggs on bread" you might get unexpected results. You don't know if the eggs have finished boiling yet, or if the extra tasks (removing shell, slicing) has finished yet.
You have to think in an async way: do this, then when it is finished do this, etc.
In terms of your code, the call to popToViewController() should probably go inside the async block.