We are loading data via REST call:
let task = NSURLSession.sharedSession().dataTaskWithRequest(request, completionHandler: {data, response, error -> Void in
//Update data store….
self.tableView.reloadData()
})
task.resume()
The data in the grid is not shown however. You have to click on the grid in the simulator - when you do this the data is shown. What are we missing here to get the data to automatically load without any UI interaction?
The reloadData() method must run in the main thread. dataTaskWithRequest(_ request: NSURLRequest) runs in a background thread:
let task = NSURLSession.sharedSession().dataTaskWithRequest(request, completionHandler: {data, response, error -> Void in
dispatch_async(dispatch_get_main_queue(),{
//Update data store….
self.tableView.reloadData()
})
})
task.resume()
The completion handler gets called on a background thread. You must call reloadData on the main thread/queue.
Related
I'm building a macOS framework and at some point, I need to make a request to some API
When I got the response I want to update the UI. I'm using URLSession.shared.dataTask to make the call and as I know the call is made in the background thread
For some reason when I try to go back to the main thread nothing happens
I'm using a virtual machine to run my framework
Any help?
Thanks
Here how I doing the request:
URLSession.shared.dataTask(with: request) { data, response, error in
if error != nil {
DispatchQueue.main.async {
//Display error message on the UI
//This never happens
//Never go back to the main thread
//Framework stop working
}
}
}.resume()
Are you sure that your task is called?
let dataTask = URLSession.shared.dataTask(with: request, completionHandler: { (data, response, error) -> Void in
....
DispatchQueue.main.async {
completion(nil, nil)
}
}
dataTask.resume() // You should add this.
I created two views and then connect one to the other by ctrl-dragging and creating a segue. Gave this segue the identifier "functionOutput1"
I then created a function and programmatically triggered the segue created above based on certain output in a function.
performSegue(withIdentifier: "functionOuput1", sender: self)
The view segues okay but the contents of the view don't appear until a minute later.
Is there something else I need to be doing here?
EDIT:
I'm performing this segue in a callback function for a URLRequest
let task = session.dataTask(with: request as URLRequest, completionHandler: {
data, response, error -> Void in
if (error != nil) {
// there is an error with the request
print("error: ", error)
} else {
// Request was successful. Check the contents of the response.
print("response: ", response)
let httpResponse = response as! HTTPURLResponse
if httpResponse.statusCode != 200 {
print("problems with server request")
} else {
print("request successful")
//go to next page
performSegue(withIdentifier: "functionOuput1", sender: self)
}
}
})
This could be a threading issue. Because the data is returned asynchronously. You need to run the view updates on the main thread. You can do this with Swift 3.0 with the following code:
DispatchQueue.main.async {
// Your view updates here
}
Using Facebook Graph API, I retrieved a string URL to a 200x200 profile picture that I want to display in a UIImageView. I'm successfully able to do this, but I notice that it can take as long as 10 seconds for the image to display on the screen. Can anyone give me some pointers (no pun intended) on how to optimize it?
override func viewDidAppear(animated: Bool) {
super.viewDidAppear(animated)
NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: self.profilePictureUrl)!, completionHandler: { (data, response, error) ->
Void in
self.profilePictureImageView.image = UIImage(data: data!)
self.profilePictureImageView.layer.cornerRadius = self.profilePictureImageView.frame.size.width / 2;
self.profilePictureImageView.clipsToBounds = true
dispatch_async(dispatch_get_main_queue(), { () -> Void in
self.view.addSubview(self.profilePictureImageView)
})
}).resume()
}
You should move all UIView calls (so anything you set on the UIImageView) onto the main thread as UIKit for the most part isn't thread-safe. You can instantiate the UIImage on the background thread though for performance optimization, so try this:
override func viewDidAppear(animated: Bool) {
super.viewDidAppear(animated)
let url = NSURL(string: self.profilePictureUrl)!
NSURLSession.sharedSession().dataTaskWithURL(
url,
completionHandler: { [weak self] (data, response, error) -> Void in
guard let strongSelf = self else { return }
// create the UIImage on the background thread
let image = UIImage(data: data!)
// then jump to the main thread to modify your UIImageView
dispatch_async(dispatch_get_main_queue(), { [weak self] () -> Void in
guard let strongSelf = self else { return }
let profilePictureImageView = strongSelf.profilePictureImageView
profilePictureImageView.image = image
profilePictureImageView.layer.cornerRadius = profilePictureImageView.frame.size.width / 2;
profilePictureImageView.clipsToBounds = true
strongSelf.view.addSubview(profilePictureImageView)
})
}
).resume()
}
Note also that I've weak-ified your references to self. There is no guarantee the user hasn't dismissed the view controller that is initiating this code by the time the completion routines get called so you want to make sure you're not keeping a strong reference to self. This allows the view controller to deallocate if the user dismisses it and the completion routines then return early without doing any unnecessary work.
This code is illegal:
NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: self.profilePictureUrl)!, completionHandler: { (data, response, error) ->
Void in
self.profilePictureImageView.image = UIImage(data: data!)
Stop! you are setting a UIImageView's image on a background thread. No no no. UIKit is not thread-safe. You must get onto the main thread to do this. (You do eventually get onto the main thread in your code, but you are doing it too late.)
What it's supposed to be
I have a username field. When the username is entered and the sendButton is clicked, the userdata is fetched with a asynchronousRequest as a JSON file.
After the sendButton is clicked, I want to display an ActivityIndicator.
The UI shall still be responsive, while the request is made.
How it is now
I click the sendButton and the UI freezes. Even the ActivityIndicator does NOT get displayed.
The code
LoginVC:
func buttonTouchedUpInside(sender: UIButton) {
toggleActivityIndicatorVisibilityOn(true)
LoginManager.sharedInstance.checkUserForCredentials(username: textFieldLogin.text, password: "")
toggleActivityIndicatorVisibilityOn(false)
}
func loginManagerDidFinishAuthenticationForUser(userData: [String:String]?){
// Delegate Method, which works with the fetched userData.
}
LoginManager
func checkUserForCredentials(#username: String ,password: String) -> Void {
let url = NSURL(string: "\(Config.checkCredentialsUrl)username=\(username)")
let request = NSURLRequest(URL: url!)
NSURLConnection.sendAsynchronousRequest(request, queue: .mainQueue()) { (response, data, error) -> Void in
if error != nil {
//Display error-message
}
var error : NSError?
let json = NSJSONSerialization.JSONObjectWithData(data, options: .MutableContainers, error: &error) as? [String:String]
self.delegate?.loginManagerDidFinishAuthenticationForUser(json)
}
}
In short: I want the request to be made in background, that the Activityindicator is shown and the UI stays responsive. After the asynchronous request successfully fetched the json, the delegate method shall be called
The second line of code in the buttonTouchedUpInside method, which reads LoginManager.sharedInstance.checkUserForCredentials(username: textFieldLogin.text, password: "") is calling an asynchronous function within it, which means that it is not blocking the next line of code... which is the one that (I am guessing) triggers your loading screen to become invisible again.
Basically, your loading screen is showing up, but it is immediately being hidden again. To fix at least the part with your loading screen, put the third line of code in the buttonTouchedUpInside function in the callback method loginManagerDidFinishAuthenticationForUser instead.
My problem is when I try to go another view after some api call finishes, it wont go to the next view. Here is the code below. Thanks in advance.
var task = NSURLSession.sharedSession().dataTaskWithURL(url!, completionHandler: {
(data, response, error) in //println(response)
//println(error)
self.myWeather.apiCallData = JSON(data: data)
self.myWeather.apiCallError = error
println("api call finished")
if error == nil {
println("no error")
let weatherView = self.storyboard?.instantiateViewControllerWithIdentifier("WeatherView") as WeatherViewController
weatherView.myWeather = self.myWeather
self.navigationController?.pushViewController(weatherView, animated: false)
}
}).resume()
It does print api call finished and no error on console. But it doesn't go to the other scene.
The problem is that the completion handler code of the dataTaskWithURL method runs in a background secondary thread, not in the main thread (where view controller transition can happen).
Wrap the call to pushViewController in a main thread queue closure:
...
dispatch_async(dispatch_get_main_queue(), {
let navigationVC = self.navigationController
navigationVC?.pushViewController(weatherView, animated: false)
})
...
I have written the code in two lines to avoid a swift compiler bug (single statement and closure return value: see this). You can also write it as:
...
dispatch_async(dispatch_get_main_queue(), {
(self.navigationController)?.pushViewController(weatherView, animated: false)
return
})
...