It seems like for WatchOS extensions, it is more often the case where code needs to be explicitly placed on the main thread, as opposed to explicitly put in the background or another queue as in IOS. What activities need to be explicitly put on the main thread in WatchOS extensions? What I have seen is that this is being done when either updating the user interface or when changing the state of a workout with HealthKit.
For example, before I update the label values on my watchOS interface view controller I am calling the main queue:
func locationUpdate(locationDict: [String:AnyObject]) {
dispatch_async(dispatch_get_main_queue()) {
if let first = locationDict["firstValue"] as? String {
self.firstValue.setText(first)
}
if let second = locationDict["second"] as? String {
self.secondValue.setText(second)
}
}
Is this required? I would not do this in iOS. Are there other common cases? Is there a good reference regarding special main queue considerations for WatchOS?
Related
I am trying to design an observable task-like entity which would have the following properties:
Reports its current state changes reactively
Shares state and result events: new subscribers will also be notified if the change happens after they've subscribed
Has a lifecycle (backed by CoroutineScope)
Doesn't have suspend functions in the interface (because it has a lifecycle)
The very basic code is something like this:
class Worker {
enum class State { Running, Idle }
private val state = MutableStateFlow(State.Idle)
private val results = MutableSharedFlow<String>()
private val scope = CoroutineScope(Dispatchers.Default)
private suspend fun doWork(): String {
println("doing work")
return "Result of the work"
}
fun start() {
scope.launch {
state.value = State.Running
results.emit(doWork())
state.value = State.Idle
}
}
fun state(): Flow<State> = state
fun results(): Flow<String> = results
}
The problems with this arise when I want to "start the work after I'm subscribed". There's no clear way to do that. The simplest thing doesn't work (understandably):
fun main() {
runBlocking {
val worker = Worker()
// subscriber 1
launch {
worker.results().collect { println("received result $it") }
}
worker.start()
// subscriber 2 can also be created "later" and watch
// for state()/result() changes
}
}
This prints only "doing work" and never prints a result. I understand why this happens (because collect and start are in separate coroutines, not synchronized in any way).
Adding a delay(300) to coroutine inside doWork "fixes" things, results are printed, but I'd like this to work without artificial delays.
Another "solution" is to create a SharedFlow from results() and use its onSubscription to call start(), but that didn't work either last time I've tried.
My questions are:
Can this be turned into something that works or is this design initially flawed?
If it is flawed, can I take some other approach which would still hit all the goals I have specified in the beginning of the post?
Your problem is that your SharedFlow has no buffer set up, so it is emitting results to its (initially zero) current collectors and immediately forgetting them. The MutableSharedFlow() function has a replay parameter you can use to determine how many previous results it should store and replay to new collectors. You will need to decide what replay amount to use based on your use case for this class. For simply displaying latest results in a UI, a common choice is a replay of 1.
Depending on your use case, you may want to give your CoroutineScope a SupervisorJob() in its context so it isn't destroyed by any child job failing.
Side note, your state() and results() functions should be properties by Kotlin convention, since they do nothing but return references. Personally, I would also have them return read-only StateFlow/SharedFlow instead of just Flow to clarify that they are not cold.
I'm exploring Kotlin Native and have a program with a bunch of Workers doing concurrent stuff
(running on Windows, but this is a general question).
Now, I wanted to add simple logging. A component that simply logs strings by appending them as new lines to a file that is kept open in 'append' mode.
(Ideally, I'd just have a "global" function...
fun log(text:String) {...} ]
...that I would be able to call from anywhere, including from "inside" other workers and that would just work. The implication here is that it's not trivial to do this because of Kotlin Native's rules regarding passing objects between threads (TLDR: you shouldn't pass mutable objects around. See: https://github.com/JetBrains/kotlin-native/blob/master/CONCURRENCY.md#object-transfer-and-freezing ).
Also, my log function would ideally accept any frozen object. )
What I've come up with are solutions using DetachedObjectGraph:
First, I create a detached logger object
val loggerGraph = DetachedObjectGraph { FileLogger("/foo/mylogfile.txt")}
and then use loggerGraph.asCPointer() ( asCPointer() ) to get a COpaquePointer to the detached graph:
val myPointer = loggerGraph.asCPointer()
Now I can pass this pointer into the workers ( via the producer lambda of the Worker's execute function ), and use it there. Or I can store the pointer in a #ThreadLocal global var.
For the code that writes to the file, whenever I want to log a line, I have to create a DetachedObjectGraph object from the pointer again,
and attach() it in order to get a reference to my fileLogger object:
val fileLogger = DetachedObjectGraph(myPointer).attach()
Now I can call a log function on the logger:
fileLogger.log("My log message")
This is what I've come up with looking at the APIs that are available (as of Kotlin 1.3.61) for concurrency in Kotlin Native,
but I'm left wondering what a better approach would be ( using Kotlin, not resorting to C ). Clearly it's bad to create a DetachedObjectGraph object for every line written.
One could pose this question in a more general way: How to keep a mutable resource open in a separate thread ( or worker ), and send messages to it.
Side comment: Having Coroutines that truly use threads would solve this problem, but the question is about how to solve this task with the APIs currently ( Kotlin 1.3.61 ) available.
You definitely shouldn't use DetachedObjectGraph in the way presented in the question. There's nothing to prevent you from trying to attach on multiple threads, or if you pass the same pointer, trying to attach to an invalid one after another thread as attached to it.
As Dominic mentioned, you can keep the DetachedObjectGraph in an AtomicReference. However, if you're going to keep DetachedObjectGraph in an AtomicReference, make sure the type is AtomicRef<DetachedObjectGraph?> and busy-loop while the DetachedObjectGraph is null. That will prevent the same DetachedObjectGraph from being used by multiple threads. Make sure to set it to null, and repopulate it, in an atomic way.
However, does FileLogger need to be mutable at all? If you're writing to a file, it doesn't seem so. Even if so, I'd isolate the mutable object to a separate worker and send log messages to it rather than doing a DetachedObjectGraph inside an AtomicRef.
In my experience, DetachedObjectGraph is super uncommon in production code. We don't use it anywhere at the moment.
To isolate mutable state to a Worker, something like this:
class MutableThing<T:Any>(private val worker:Worker = Worker.start(), producer:()->T){
private val arStable = AtomicReference<StableRef<T>?>(null)
init {
worker.execute(TransferMode.SAFE, {Pair(arStable, producer).freeze()}){
it.first.value = StableRef.create(it.second()).freeze()
}
}
fun <R> access(block:(T)->R):R{
return worker.execute(TransferMode.SAFE, {Pair(arStable, block).freeze()}){
it.second(it.first.value!!.get())
}.result
}
}
object Log{
private val fileLogger = MutableThing { FileLogger() }
fun log(s:String){
fileLogger.access { fl -> fl.log(s) }
}
}
class FileLogger{
fun log(s:String){}
}
The MutableThing uses StableRef internally. producer makes the mutable state you want to isolate. To log something, call Log.log, which will wind up calling the mutable FileLogger.
To see a basic example of MutableThing, run the following test:
#Test
fun goIso(){
val mt = MutableThing { mutableListOf("a", "b")}
val workers = Array(4){Worker.start()}
val futures = mutableListOf<Future<*>>()
repeat(1000) { rcount ->
val future = workers[rcount % workers.size].execute(
TransferMode.SAFE,
{ Pair(mt, rcount).freeze() }
) { pair ->
pair.first.access {
val element = "ttt ${pair.second}"
println(element)
it.add(element)
}
}
futures.add(future)
}
futures.forEach { it.result }
workers.forEach { it.requestTermination() }
mt.access {
println("size: ${it.size}")
}
}
The approach you've taken is pretty much correct and the way it's supposed to be done.
The thing I would add is, instead of passing around a pointer around. You should pass around a frozen FileLogger, which will internally hold a reference to a AtomicRef<DetachedObjectGraph>, the the attaching and detaching should be done internally. Especially since DetachedObjectGraphs are invalid once attached.
Running ARKit 2.0 with an ARSCNView. iOS12
The application uses multithreading, that's why these functions are being performed on the main thread (just to be sure). I also tried without explicitly performing the functions on the main thread too, with no avail.
I'm using an .aiff sound file but have also tried a .wav. No joy.
I even tried removing audioNode_alarm from the node hierarchy & the sound still plays. I even remove the ARSCNView from the view hierarchy and the sound STILL plays. FFS
From what I can see, I'm doing things EXACTLY as I'm supposed to, to stop the audio from playing. The audio simply will not stop no matter what I try. Can anyone think why?!
weak var audioNode_alarm: SCNNode!
weak var audioPlayer_alarm: SCNAudioPlayer?
func setupAudioNode() {
let audioNode_alarm = SCNNode()
addChildNode(audioNode_alarm)
self.audioNode_alarm = audioNode_alarm
}
func playAlarm() {
DispatchQueue.main.async { [unowned self] in
self.audioNode_alarm.removeAllAudioPlayers()
if let audioSource_alarm = SCNAudioSource(fileNamed: "PATH_TO_MY_ALARM_SOUND.aiff") {
audioSource_alarm.loops = true
audioSource_alarm.load()
audioSource_alarm.isPositional = true
let audioPlayer_alarm = SCNAudioPlayer(source: audioSource_alarm)
self.audioNode_alarm.addAudioPlayer(audioPlayer_alarm)
self.audioPlayer_alarm = audioPlayer_alarm
}
}
}
func stopAlarm() {
DispatchQueue.main.async { [unowned self] in
self.audioNode_alarm?.removeAudioPlayer(self.audioPlayer_alarm!)
self.audioNode_alarm?.removeAllAudioPlayers()
}
}
What I ended up doing is stopping the sound and removing the player by
yourNode.audioPlayers.forEach { audioLocalPlayer in
audioLocalPlayer.audioNode?.engine?.stop()
yourNode.removeAudioPlayer(audioLocalPlayer)
}
According to the documentation SCNAudioPlayer has audioNode, which is supposed to be used "to vary parameters such as volume and reverb in real time during playback".
audioNode is of AVAudioNode type, so if we jump to engine prop and its type definition, we'll find all the controls we need.
For multithread concurency editing objects in Swift I use:
import Foundation
func lockForEdit(object: NSObject, closure: () -> Void) {
objc_sync_enter(object)
closure()
objc_sync_exit(object)
}
// In each thread
lockForEdit (object: threadsDictionarie as NSObject) {
threadsDictionarie.append(dict)
}
But in Linux Ubuntu 14.04 with Swift 3.0.1 I get:
use of unresolver identifier 'objc_sync_enter',
use of unresolver identifier 'objc_sync_exit'
What to use for Swift in Linux for concurency editing objects?
There is no per-object locking in Swift. You could use NSLock or pthread locks as a replacement, but you need to maintain the lock/object mapping on your own.
Also, you may want to use a serial DispatchQueue instead of a lock in the first place (checkout: About Dispatch Queues). But this obviously depends on what you are doing.
A way to do this is to add something like a ThreadsafeDictionary and wrap the real dictionary inside it. Like so:
class ThreadsafeDictionary<T> : ... all the protocols... {
let lock = NSLock()
var values = Dictionary<T>()
... all the methods you need ...
}
Presumably you can find an implementation of this on GitHub.
I get strange errors when I am trying to pass around NSManagedObject through several functions. (all are in the same VC).
Here are the two functions in question:
func syncLocal(item:NSManagedObject,completionHandler:(NSManagedObject!,SyncResponse)->Void) {
let savedValues = item.dictionaryWithValuesForKeys([
"score",
"progress",
"player"])
doUpload(savedParams) { //do a POST request using params with Alamofire
(success) in
if success {
completionHandler(item,.Success)
} else {
completionHandler(item,.Failure)
}
}
}
func getSavedScores() {
do {
debugPrint("TRYING TO FETCH LOCAL SCORES")
try frc.performFetch()
if let results = frc.sections?[0].objects as? [NSManagedObject] {
if results.count > 0 {
print("TOTAL SCORE COUNT: \(results.count)")
let incomplete = results.filter({$0.valueForKey("success") as! Bool == false })
print("INCOMPLETE COUNT: \(incomplete.count)")
let complete = results.filter({$0.valueForKey("success") as! Bool == true })
print("COMPLETE COUNT: \(complete.count)")
if incomplete.count > 0 {
for pendingItem in incomplete {
self.syncScoring(pendingItem) {
(returnItem,response) in
let footest = returnItem.valueForKey("player") //only works if stripping syncScoring blank
switch response { //response is an enum
case .Success:
print("SUCCESS")
case .Duplicate:
print("DUPLICATE")
case .Failure:
print("FAIL")
}
}
} //sorry for this pyramid of doom
}
}
}
} catch {
print("ERROR FETCHING RESULTS")
}
}
What I am trying to achieve:
1. Look for locally saved scores that could not submitted to the server.
2. If there are unsubmitted scores, start the POST call to the server.
3. If POST gets 200:ok mark item.key "success" with value "true"
For some odd reason I can not access returnItem at all in the code editor - only if I completely delete any code in syncLocal so it looks like
func syncLocal(item:NSManagedObject,completionHandler:(NSManagedObject!,SyncResponse)->Void) {
completionHandler(item,.Success)
}
If I do that I can access .syntax properties in the returning block down in the for loop.
Weirdly if I paste the stuff back in, in syncLocal the completion block keeps being functional, the app compiles and it will be executed properly.
Is this some kind of strange XCode7 Bug? Intended NSManagedObject behaviour?
line 1 was written with stripped, line 2 pasted rest call back in
There is thread confinement in Core Data managed object contexts. That means that you can use a particular managed object and its context only in one and the same thread.
In your code, you seem to be using controller-wide variables, such as item. I am assuming the item is a NSManagedObject or subclass thereof, and that its context is just one single context you are using in your app. The FRC context must be the main thread context (a NSManagedObjectContext with concurrency type NSMainThreadConcurrencyType).
Obviously, the callback from the server request will be on a background thread. So you cannot use your managed objects.
You have two solutions. Either you create a child context, do the updates you need to do, save, and then save the main context. This is a bit more involved and you can look for numerous examples and tutorials out there to get started. This is the standard and most robust solution.
Alternatively, inside your background callback, you simply make sure the context updates occur on the main thread.
dispatch_async(dispatch_get_main_queue()) {
// update your managed objects & save
}