I've found so many solutions for progress bar update within the same thread and view controller, however they seemed to be not similar cases as mine.
In my application, the main view controller calls loadIntoCoreData()(implemented in class MyLoadingService) which asynchronously loads data into core data by another thread. This function has to continuously update the loading percentage (which is written in NSUserDefaults.standardUserDefaults()) to the main thread so that it could be shown on the progress bar in main view controller. I had ever used a while loop in MainViewController to continuously fetch the current percentage value, like below:
class MainViewController {
override func viewDidLoad() {
MyLoadingService.loadIntoCoreData() { result in
NSUserDefaults.standardUserDefaults().setBool(false, forKey: "isLoading")
// do something to update the view
}
self.performSelectorInBackground("updateLoadingProgress", withObject: nil)
}
func updatingLoadingProgress() {
let prefs = NSUserDefaults.standardUserDefaults()
prefs.setBool(true, forKey: "isLoading")
// here I use a while loop to listen to the progress value
while(prefs.boolForKey("isLoading")) {
// update progress bar on main thread
self.performSelectorOnMainThread("showLoadingProcess", withObject: nil, waitUntilDone: true)
}
prefs.setValue(Float(0), forKey: "loadingProcess")
}
func showLoadingProcess() {
let prefs = NSUserDefaults.standardUserDefaults()
if let percentage = prefs.valueForKey("loadingProcess") {
self.progressView.setProgress(percentage.floatValue, animated: true)
}
}
}
And in the class of function loadIntoCoreData:
class MyLoadingService {
let context = (UIApplication.sharedApplication()delegate as! AppDelegate).managedObjectContext!
func loadIntoCoreData(source: [MyModel]) {
var counter = 0
for s in source {
//load into core data using the class context
NSOperationQueue.mainQueue.addOperationWithBlock({
// updating the value of "loadingProcess" in NSUserDefaults.standardUserDefaults()
// and synchronize it on main queue
})
counter++
}
}
}
The above code can successfully run the progress bar, however it often encounter BAD_ACCESS or some other exceptions(like "Cannot update object that was never inserted") due to the conflicts on core data context (thought it seems that managedObjectContext isn't touched by the main thread). Therefore, instead of using a while loop listening on the main thread, I consider using NSOperationQueue.performSelectorOnMainThread to acknowledge the main thread after each entry. Therefore I put my view controller as an argument sender into loadCoreData and call performSelectorOnMainThread("updateProgressBar", withObject: sender, waitUntilDone: true) but failed with error "unrecognized selector sent to class 'XXXXXXXX'". So I would like to ask if is it possible to update an UI object between threads? Or, how to modify my previous solution so that the core data context conflicts could be solved? Any solutions are appreciated.
class MyLoadingService {
func loadIntoCoreData(sender: MainViewController, source: [MyModel]) {
var counter = 0
for s in source {
//load into core data using the class context
NSOperationQueue.mainQueue.addOperationWithBlock({
// updating the value of "loadingProcess" in NSUserDefaults.standardUserDefaults()
// and synchronize it on main queue
})
NSOperationQueue.performSelectorOnMainThread("updateProgressBar", withObject: sender, waitUntilDone: true)
counter++
}
}
func updateProgressBar(sender: MainViewController) {
sender.progressView.setProgress(percentage, animated: true)
}
}
class MainViewController {
override func viewDidLoad() {
MyLoadingService.loadIntoCoreData(self) { result in
// do something to update the view
}
}
}
