I am using AWS to host images for my iOS app. Right now I am trying to list all the objects in an S3 bucket.
Here is my code:
var description = ""
AWSS3.registerS3WithConfiguration(serviceConfig2, forKey: "s3")
AWSS3.S3ForKey("s3").listObjects(objectList).continueWithBlock { (task: AWSTask!) -> AnyObject! in
if task.error != nil {
println(task.error)
}
if task.result != nil {
description = task.result!.description
println(description)
}
return nil
}
println(description == "")
The output is true followed by the correct contents of task.result!.description. In other words, the println outside of the continueWithBlock is printing first and description has not been updated at that time.
How am I supposed to do things with description outside of the continueWithBlock?
You can assign a value that you need to another variable inside the scope of your class or function, then you can call didSet on the variable and carry out another function if you need to, like this:
var someVariableInScopeOfWhereItsNeeded = "abc" {
didSet {
self.maybeSomeOtherFunctionNow
}
}
You asked:
How am I supposed to do things with description outside of the
continueWithBlock
Short answer: You're not.
The whole point of an async method is that it continues immediately, before the time-consuming task has even begun processing. You put the code that depends on the results inside your block. See my answer on this thread for a detailed explanation, including an example project:
Why does Microsoft Azure (or Swift in general) fail to update a variable to return after a table query?
(Don't be fooled by the fact that it mentions MS Azure. It actually has nothing to do with Azure.)
#thefredelement 's solution of using a didSet method on the variable that gets set would work too.
Related
I'm building a Kotlin library to use in my iOS app using Kotlin/Native. After I call some methods in the library from Swift, which works, I also want to call methods in Swift from the library. To accomplish this I implemented an interface in the library:
class Outbound {
interface HostInterfaceForTracking {
fun calcFeatureVector(bitmap: Any?): Array<Array<FloatArray>>?
}
var hostInterface: HostInterfaceForTracking? = null
fun registerInterface(hostInterface: HostInterfaceForTracking) {
this.hostInterface = hostInterface
instance.hostInterface = hostInterface
}
}
This is implemented on the Swift side like this:
class HostInterfaceForTracking : OutboundHostInterfaceForTracking {
var t : Outbound? = nil
init() {
TrackingWrapper.instance?.runOnMatchingLibraryThread {
self.t = Outbound()
self.t!.registerInterface(hostInterface: self)
}
}
func calcFeatureVector(bitmap: Any?) -> KotlinArray<KotlinArray<KotlinFloatArray>>? {
do {
var test : Any? = (bitmap as! Bitmap).bitmap
return nil
} catch {
return nil
}
}
}
The TrackingWrapper looks like this:
class TrackingWrapper : NSObject {
static var instance: TrackingWrapper? = nil
var inbound: Inbound? = nil
var worker: Worker
override init() {
self.worker = Worker()
super.init()
initInboundInterface()
}
func initInboundInterface() {
runOnMatchingLibraryThread {
TrackingWrapper.instance = self
self.inbound = Inbound()
HostInterfaceForTracking()
}
}
func runOnMatchingLibraryThread(block: #escaping() -> Void) {
worker.enqueue {
block()
}
}
}
The function runOnMatchingLibraryThread is needed because every call to the TrackingLibrary needs to be called from the exact same thread, so the Worker class initializes a thread and enqueues every method to that thread.
The Bitmap in this case is simply a wrapper for an UIImage, which I already accessed with the .bitmap call, so I've tried to access the wrapped UIImage and save it in the test variable. The library gets the current camera frame from the Swift side every few frames and sends the current image wrapped as a Bitmap to the method calcFeatureVector depicted here.
Problem: My memory load starts increasing as soon as the app starts until the point it crashes. This is not the case if I don't access the wrapped UIImage (var test : Any? = (bitmap as! Bitmap)). So there is a huge memory leak, just by accessing the wrapped variable on the Swift side. Is there anything I've missed or is there any way to release the memory?
Looks like you have a circular dependency here:
TrackingWrapper.instance?.runOnMatchingLibraryThread {
self.t = Outbound()
self.t!.registerInterface(hostInterface: self)
}
You are asking a property inside HostInterfaceForTracking to maintain a strong reference to the same instance of HostInterfaceForTracking. You should be using [weak self] to avoid the circular reference.
EDIT:
Ok after seeing the rest of you code theres a lot to unpack. There is a lot of unnecessary bouncing back and forth between classes, functions and threads.
There is no need to use runOnMatchingLibraryThread to just create an instance of something. You only need to use that for the code processing the image itself (I would assume, I haven't seen anything so far that requires being split off into another thread). Inside TrackingWrapper, you can create a singleton more easily, and matching the swift pattern by simply doing this as the first line:
static let shared = TrackingWrapper()
And everywhere you want to use it, you can just call TrackingWrapper.shared. This is more common and will avoid one of the levels of indirection in the code.
I'm not sure what Worker or Inbound are, but again these can and should be created inside the TrackingWrapper init, rather than branching Inbound's init, to use another thread.
Inside initInboundInterface you are creating an instance of HostInterfaceForTracking() which doesn't get stored anywhere. The only reason HostInterfaceForTracking is continuing to stay in memory after its creation, is because of the internal circular dependency inside it. This is 100% causing some form of a memory issue for you. This should probably also be a property on TrackingWrapper, and again, its Init should not be called inside runOnMatchingLibraryThread.
Having HostInterfaceForTracking's init, also using runOnMatchingLibraryThread is problematic. If we inline all the code whats happening is this:
TrackingWrapper
init() {
self.runOnMatchingLibraryThread {
TrackingWrapper.instance = self
self.inbound = Inbound()
TrackingWrapper.instance?.runOnMatchingLibraryThread {
self.t = Outbound()
self.t!.registerInterface(hostInterface: self)
}
}
}
Having all these classes unnecessarily keep coming back to TrackingWrapper is going to cause issues.
Inside HostInterfaceForTracking 's init, no need to be creating Outbound on a separate thread. First line in this class can simply be:
var t : Outbound = OutBound()
Or do it in the init if you prefer. Either way will also remove the issue of needing to unwrap Outbound before using it.
Inside Outbound you are storing 2 references to the hostInterface instance:
this.hostInterface = hostInterface
instance.hostInterface = hostInterface
I would have imagined there should only be 1. If there are now multiple copies of a class that has a circular dependency, which has multiple calls to separate threads. This again will cause issues.
I'm still not sure on the differences between Swift and Kotlin. In Swift when passing self into a function to be stored, the class storing it would mark the property as weak, like so:
weak var hostInterface: ......
Which will avoid any circular dependency from forming. A quick google says this isn't how things work in Kotlin. It might be better to look into the swift side passing in a closure (lambda on kotlin) and the kotlin side executing that. This might avoid the need to store a strong reference. Otherwise you need to be looking into some part of your code setting hostInterface back to null. Again its a bit hard to say only seeing some of the code and not knowing how its working.
In short, it looks like the code is very over complicated, and needs to be simplified, so that all these moving pieces can be tracked easier.
So, I am new to cloudKit and to working with multiple threads in general, which I think is the source of the problem here, so if I simply need to research more, please just comment so and I will take that to heart.
Here is my question:
I am working in Swift 3 Xcode 8.1
I have in my view controller this variable:
var contactsNearby: [String:CLLocation]?
Then at the end of ViewDidLoad I call one of my view controllers methods let's call it:
populateContactsNearby()
inside that method I call:
container.discoverAllIdentities(completionHandler: { (identities, error) in
for userIdentity in identities! {
self.container.publicCloudDatabase.fetch(withRecordID: userIdentity.userRecordID!, completionHandler: { (userRecord, error) in
let contactsLocation = userRecord?.object(forKey: "currentLocation")
if self.closeEnough(self.myLocation!, contactLocation: contactsLocation as! CLLocation) {
var contactsName = ""
contactsFirstName = userIdentity.nameComponents?.givenName
if contactsName != "" && contactsLocation != nil {
self.contactsNearby?["\(contactsName)"] = contactsLocation as? CLLocation
}
}
})
}
})
}
I apologize if I am missing or have an extra bracket somewhere. I have omitted some error checking code and so forth in order to get this down to bare-bones. So the goal of all that is to populate my contactsNearby Dictionary with data from CloudKit. A name as the key a location as the value. I want to use that data to populate a tableview. In the above code, the call to closeEnough is a call to another one of my view controllers methods to check if the contact from CloudKit has a location close enough to my user to be relevant to the apps purposes. Also myLocation is a variable that is populated before the segue. It holds the CLLocation of the app users current location.
The Problem:
The if statement:
if contactsName != "" && contactsLocation != nil { }
Appears to succeed. But my view controllers variable:
var contactsNearby: [String:CLLocation]?
Is never populated and I know there is data available in cloudKit.
If it's relevant here is some test code that I have in cellForRowAtIndexPath right now:
let contact = self.contactsNearby?.popFirst()
let name = contact?.key
if name != nil {
cell.textLabel?.text = name
}else {
cell.textLabel?.text = "nothing was there"
}
My rows alway populate with "nothing was there". I have seen answers where people have done CKQueries to update the UI, but in those answers, the user built the query themselves. That seems different from using a CloudKit function like discoverAllIdentities.
I have tried to be as specific as possible in asking this question. If this question could be improved please let me know. I think it's a question that could benefit the community.
Okay, I need to do some more testing, but I think I got it working. Thank you Paulw11 for your comment. It got me on the right track.
As it turns out there were 2 problems.
First, as pointed out I have an asynchronous call inside a for loop. As recommended, I used a dispatchGroup.
Inside the cloudKit call to discoverAllIdentities I declared a dispatchGroup, kind of like so:
var icloudDispatchGroup = DispatchGroup()
Then just inside the for loop that is going to make an async call, I enter the dispatchGroup:
icloudDispatchGroup.enter()
Then just before the end of the publicCloudDatabase.fetch completion handler I call:
icloudDispatchGroup.leave()
and
icloudDispatchGroup.wait()
Which, I believe, I'm still new to this remember, ends the dispatchGroup and causes the current thread to wait until that dispatchGroup finishes before allowing the current thread to continue.
The Above took care of the multithreading issue, but my contactsNearby[String:CLLocation]? Dictionary was still not being populated.
Which leads me to the 2nd problem
At the top of my view controller I declared my Dictionary:
var contactsNearby: [String: CLLocation]?
This declared a dictionary, but does not initialize it, which I don't think I fully realized, so when I attempted to populate it:
self.contactsNearby?["\(contactsName)"] = contactsLocation as? CLLocation
It quietly failed because it is optional and returned nil
So, in viewDidLoad before I even call populateContactsNearby I initialize the dictionary:
contactsNearby = [String:CLLocation]()
This does not make it cease to be an optional, which Swift being strongly typed would not allow, but merely initializes contactsNearby as an optional empty Dictionary.
At least, that is my understanding of what is going on. If anyone has a more elegant solution, I am always trying to improve.
In case you are wondering how I then update the UI, I do so with a property observer on the contactsNearby Dictionary. So the declaration of the dictionary at the top of the view controller looks like this:
var contactsNearby: [String: CLLocation]? {
didSet {
if (contactsNearby?.isEmpty)! || contactsNearby == nil {
return
}else{
DispatchQueue.main.sync {
self.nearbyTableView.reloadData()
}
}
}
}
I suppose I didn't really need to check for empty and nil. So then in cellForRowAtIndexPath I have something kind of like so:
let cell = tableview.dequeueReusableCell(withIdentifier: "nearbyCell", for: indexPath)
if contactsNearby?.isEmpty == false {
let contact = contactsNearby?.popFirst()
cell.textLabel?.text = contact?.key
}else {
cell.textLabel?.text = "Some Placeholder Text Here"
}
return cell
If anyone sees an error in my thinking or sees any of this heading for disaster, feel free to let me know. I still have a lot of testing to do, but I wanted to get back here and let you know what I have found.
I want to write a function to reverse geocode a location and assign the resulting string into a variable. Following this post i've got something like this:
extension CLLocation {
func reverseGeocodeLocation(completion: (answer: String?) -> Void) {
CLGeocoder().reverseGeocodeLocation(self) {
if let error = $1 {
print("[ERROR] \(error.localizedDescription)")
return
}
if let a = $0?.last {
guard let streetName = a.thoroughfare,
let postal = a.postalCode,
let city = a.locality else { return }
completion(answer: "[\(streetName), \(postal) \(city)]")
}
}
}
}
For calling this function i've just got something like this:
location.reverseGeocodeLocation { answer in
print(answer)
}
But instead i want to assign the string value of answer to a variable and i don't know how to pass that data out of the closure. What is the best way to do something like this?
The problem is that it runs asynchronously, so you can't return the value. If you want to update some property or variable, the right place to do that is in the closure you provide to the method, for example:
var geocodeString: String?
location.reverseGeocodeLocation { answer in
geocodeString = answer
// and trigger whatever UI or model update you want here
}
// but not here
The entire purpose of the closure completion handler pattern is that is the preferred way to provide the data that was retrieved asynchronously.
Short answer: You can't. That's not how async programming works. The function reverseGeocodeLocation returns immediately, before the answer is available. At some point in the future the geocode result becomes available, and when that happens the code in your closure gets called. THAT is when you do something with your answer. You could write the closure to install the answer in a label, update a table view, or some other behavior. (I don't remember if the geocoding methods' closures get called on the main thread or a background thread. If they get called on a background thread then you would need to wrap your UI calls in dispatch_async(dispatch_get_main_queue()).)
I'm working with Parse and I'm trying to store an objects unique identifier that Parse gives you. Here's the code to help make sense of this.
var objectID = ""
var object = PFObject(className: "class")
object["columnTitle"] = "String Data"
object.saveInBackgroundWithBlock { (success, error) -> Void in
guard error == nil else {
print("Error")
return
}
objectID = object.objectId!
}
print(objectID)
So I thought when I ran the app the print statement would print out the objectId that Parse gives you but it doesn't. If I add the print statement inside the closure it is definitely getting the value I'm expecting. It seems like the print statement is running before the value can be changed. Any idea how to fix this? I'm assuming using GCD?
All of this is inside viewDidLoad()
saveInBackgroundWithBlock is an asynchronous call. If you print the object ID with in the block, it gets printed while the block is executed in a different thread.
If you place the print outside the block, it will be the immediate next statement executed after the call to saveInBackgroundWithBlock. You should place break points in these two statements and understand the behavior.
You could define a method which contains the object ID related processing code and call that method at the end of your saveInBackgroundWithBlock
Here's my code:
var goBack: String!
if (goBack == "yes")
{
firstName.text = passFirstName1
lastName.text = passLastName1
}
All I want to do is execute the if-statement if 'goBack' is undefined. How can I do that? (I don't know what to put in the blank)
The overall program is more complicated which is why I need the variable to be undefined at first. In short, I'm declaring 'goBack', asking the user to type in their first and last name, then continuing to the next view controller. That view controller has a back button that brings us back to the first view controller (where I declared 'goBack'). When the back button is pressed, a 'goBack' string is also passed of "yes". I also passed the first and last name to the next view controller but now I want to pass it back. I'm able to pass it back, its just a matter of making the text appear.
EDIT: firstName and lastName are labels while passFirstName1 and passLastName1 are variables from the second view controller.
"All I want to do is execute the if-statement if 'goBack' is undefined. How can I do that?"
To check whether a variable equals nil you can use a pretty cool feature of Swift called an if-let statement:
if let goBackConst = goBack {
firstName.text = passFirstName1
lastName.text = passLastName1
}
It's essentially the logical equivalent of "Can we store goBack as a non-optional constant, i.e. can we "let" a constant = goBack? If so, perform the following action."
It's really interesting, you can define a variable as optional, which means it may or may not be defined, consider the following scenerio:
you want to find out if the app has been installed before...
let defaults = NSUserDefaults()
let testInstalled : String? = defaults.stringForKey("hasApplicationLaunchedBefore")
if defined(testInstalled) {
NSLog("app installed already")
NSLog("testAlreadyInstalled: \(testInstalled)")
defaults.removeObjectForKey("hasApplicationLaunchedBefore")
} else {
NSLog("no app")
defaults.setValue("true", forKey: "hasApplicationLaunchedBefore")
}
Then all you need to do is write a function to test for nil...
func defined(str : String?) -> Bool {
return str != nil
}
And you've got it. A simpler example might be the following:
if let test : String? = defaults.stringForKey("key") != nil {
// test is defined
} else {
// test is undefined
}
The exclamation mark at the end is to for unwrapping the optional, not to define the variable as optional or not
"All I want to do is execute the if-statement if 'goBack' is undefined"
The guard statement (new in Swift 2) allows exactly this. If goBack is nil then the else block runs and exits the method. If goBack is not nil then localGoBack is available to use following the guard statement.
var goBack:String?
func methodUsingGuard() {
guard let localGoBack = goBack else {
print("goBack is nil")
return
}
print("goBack has a value of \(localGoBack)")
}
methodUsingGuard()
From The Swift Programming Language (Swift 3.1):
Constants and variables created with optional binding in an if
statement are available only within the body of the if statement. In
contrast, the constants and variables created with a guard statement
are available in the lines of code that follow the guard statement, as
described in Early Exit.