Why we should use suspend?
Are they used only to restrict a function not to be used outside coroutine scope or other suspend functions?
Are they used only to restrict a function not to be used outside co routine scope or other suspend functions?
No. The main purpose of a suspend function is to suspend a coroutine it is called in to eliminate callback hell. Consider the following code with a callback:
fun interface Callback {
fun call()
}
fun executeRequest(c: Callback) {
// ... create new Thread to execute some request and call callback after the request is executed
c.call()
}
On the caller side there will be:
executeRequest {
// ... do sth after request is competed.
}
Imagine you need to make another request after that one:
executeRequest {
// make another request
executeRequest {
// and another one
executeRequest {
// and another one ...
executeRequest {
}
}
}
}
That is a Callback Hell. To avoid it we can get rid of a Callback code and use only suspend functions:
// withContext(Dispatchers.IO) switches a coroutine's context to background thread
suspend fun executeRequest() = withContext(Dispatchers.IO) {
// just execute request, don't create another Thread, it is already in the background Thread
// if there is some result, return it
}
And on the caller side if there are a couple of requests, that should be executed one by one, we can call them without the Callback Hell by launching a coroutine and calling those request in the coroutine:
viewModelScope.launch {
executeRequest()
executeRequest()
executeRequest()
}
If you wonder what it does, I guess you will find the answer in this near-duplicate question's answer.
It basically allows to use the syntax of synchronous calls to call an asynchronous function.
If the question is "Why use async programming?", then there are probably plenty of resources explaining this on the internet. It usually allows to use threads more effectively and avoids using too many resources.
Now as to why you would want to use suspend to do that instead of callbacks, there is probably several reasons. Probably the biggest of them is to avoid the infamous "callback hell" (well known in JS): using actual callbacks creates nesting in the code. It makes the language harder to manipulate because you can't use local variables or loops as easily as with regular sequential code. With suspend functions, the code reads sequentially even though some asynchronous mechanisms are used behind the scenes to resume executing a piece of code at a later point.
Another big reason to use suspend (and more precisely the coroutines library in general) is structured concurrency. It allows to organize your asynchronous work so you don't leak anything.
Under the hood, suspend functions provide the extra functionality (involving a hidden Continuation object) that allow their code to be stopped and resumed later from where it left off, instead of the typical limitation that all function calls in a function are called in sequence and block the thread the entire time. The keyword is there to make the syntax very simple for doing a long-running task in the background.
Three different benefits of this:
No need for callbacks for long-running tasks ("callback hell")
Without suspend functions, you would have to use callbacks to prevent a long-running task from blocking the thread, and this leads to complicated-looking code that is not written in sequential order, especially when you need to do a number of long-running tasks in sequence and even more-so if you need to do them in parallel.
Structured concurrency
Built-in features for automatically cancelling long-running tasks. Guarantees about shared state being correct whenever accessed despite thread switching.
No need for state machines for lazy iterators
A limited set of suspend functions can be used in lazy iterator functions like the iterator { } and sequence { } builders. These are a very different kind of coroutine than the ones launched with CoroutineScopes, because they don't do anything with thread switching. The yield() suspend function in these builders' lambdas allows the code to be paused until the next item is requested in iteration. No thread is being blocked, but not because it's doing some task in the background. It instead enables code to be written sequentially and concisely without a complicated state machine.
Related
I'm a little confused.
I know that if a function wants to work with coroutines, it should be declared as suspend
For example:
private suspend fun doSomething() {
withContext(Dispatchers.IO) {
//do something
} }
And I also know that there is such a way to use coroutines without the function being suspend.
like:
private fun doSomething1() {
CoroutineScope(Dispatchers.IO).launch {
//do something
} }
What is the difference between the two functions?
When to use the first example and when to use the second example?
What is the difference between the two functions?
There are 2 major differences between the 2:
the usage is different: the suspend one "feels" synchronous, while the launch is explicitly asynchronous
the second function breaks structured concurrency, and shouldn't be written this way
Let me elaborate.
The suspend function appears synchronous from the usage perspective: when you call it, the next line of code is only executed when the function is done (like with any other regular function). This makes it easy to reason about. You can even assign the return value of a suspend function to a variable, and go on with your life as if the function wasn't suspend. That is, when you're in a suspend context already of course. When you're not, you have to start the "root" coroutine with an explicit coroutine builder (like launch, async or runBlocking).
When using launch, you're explicitly starting an asynchronous task, and thus the code after launch runs concurrently with what's inside the launch. So in turn, when calling doSomething1(), the code after it will run concurrently with whatever is in the launch inside. However, it is really not clear from the API's perspective that this function will launch a task that outlives it. This also goes with the fact that you shouldn't create "free" coroutine scopes like this. I'll elaborate below.
When to use the first example and when to use the second example?
Use suspend functions as much as possible to keep things simple. Most of the time, you don't need to start tasks that outlive the function call, so this is perfectly fine. You can still do some work concurrently inside your suspend function by using coroutineScope { ... } to launch some coroutines. This doesn't require an externally-provided scope, and all the computation will happen within the suspend function call from the caller's perspective, because coroutineScope {} will wait for the child coroutines to complete before it returns.
The function using launch as written here is very poorly behaved, you should never write things like this:
CoroutineScopes should not be created on the spot and left for dead. You should keep a handle on it and cancel it when appropriate
if you're already in the suspending world when calling this function, the existing coroutine context and jobs will be ignored
To avoid these problems, you can make the API explicit by making the CoroutineScope a receiver instead of creating one on the spot:
private fun CoroutineScope.doSomething1() {
launch(Dispatchers.IO) {
//do something
}
}
But only use this approach if the essence of the function is to start something that will keep going after the function returns.
The shortest answer is that suspend function is a block that can be executed in CoroutineScope. So it's not the first example vs the second example.
By combining those blocks you can start your own scope, and execute suspend functions using different contexts.
private suspend fun doSomething() {
withContext(Dispatchers.IO){
// task executed in io thread
}
}
private suspend fun doSomethingUI() {
withContext(Dispatchers.Main) {
// task executed in ui thread
}
}
private fun ioOperation() {
CoroutineScope(Dispatchers.IO).launch {
doSomething()
doSomethingUI()
}
}
Edit: This is just a basic sample made with simplicity in mind. It doesn't handle the proper lifecycle of the Coroutine Scope, and should not be directly used.
I would like to ask you why does it work?
Normally when I used collectLatest with flow my data wasn't collected on time and the return value was empty. I have to use async-await coroutines, but I have read it blocks main thread, so it is not efficient. I've made my research and find the solution using sharedflow.
Previously:
suspend fun getList: List<Items> {
CoroutineScope(Dispatchers.Main).launch {
async {
flow.collectLatest {
myItems = it
}
}.await()
}
return myItems
}
or without await-async and it returns emptyList
now:
suspend fun getList: List<Items> {
val sharedFlow = flow.conflate().shareIn(
coroutineScopeIO,
replay = 1,
started = SharingStarted.WhileSubscribed()
)
return sharedFlow.first()
}
conflate means:
Conflates flow emissions via conflated channel and runs collector in a separate coroutine. The effect of this is that emitter is never suspended due to a slow collector, but collector always gets the most recent value emitted.
I'm not sure I understand it clearly. When I conflate flow, I just create seperate coroutine to emit what will be inside my another function as in my example shareIn().first() and using this variablesharedFlow which is suspended so will give the same effect I made asnyc-await, but in that case I do not block main thread, but only my exact *parentCoroutine-or-suspendFunction?
SharingStarted.WhileSubscribed()
It just means to start emit when subcribed.
conflate() has nothing to do with why this is working. The separate coroutine it talks about is run under the hood and you don't need to think about it. It's just to make sure your flow never causes the upstream emitter to have to wait for a slow collector, and your collector skips values if they are coming faster than it can handle them. conflate() makes it safe to have a slow collector without a buffer.
In your first code block, you are launching a new coroutine in a new CoroutineScope, so it is not a child coroutine and will not be waited for before the function returns. (Incidentally, this new coroutine will only finish when the Flow completes, and most types of Flows never complete.)
In the second code block, you are calling first() on the Flow, which suspends and gets the next value emitted by the flow and then returns that value without waiting for the Flow to complete.
Some other notes:
You should never use async { /*...*/ }.await() where await() is called immediately on the Deferred, because it is just a more convoluted version of withContext(/*...*/) { /*...*/ }.
It's a code smell to create a CoroutineScope that you never assign to a property, because the point of creating a scope is so you can manage the scope, and you obviously aren't managing it if you have no reference to it to work with.
You said you are worried about blocking the main thread, but nothing in the code you showed looks suspicious of blocking the main thread. But it's possible your flow that you are basing this on has blocking code in it. By convention it shouldn't. If that flow blocks, you should use the flowOn(Dispatchers.IO) operator on it at the source so downstream users don't have to worry about it.
Although your code worked, it doesn't make sense to create a SharedFlow in a function and immediately collect from it. It's not being shared with anything! Your code could be simplified to this equivalent code:
suspend fun getList: List<Items> {
return flow.first()
}
I've looked at plenty of articles online but I'm still a bit confused as to what happens specifically, step by step, when suspend functions are suspended in coroutines.
I know that a suspend function, under the hood, is just a regular function with a Continuation parameter that allows it to resume, but my confusion is regarding where that suspend function or coroutine goes and where it comes back once resumed.
I've heard a few people saying "they don't necessarily come back to the same thread" and I don't get it, can someone please explain this to me step by step?
TLDR;
There is no guarantee, it may or may not,
it really depends that on the following points:
Is the dispatcher is multi-threaded?
Is there any dispatcher override in between?
LONG Answer
A coroutine has a CoroutineContext that specify how it behaves, where it run.
A CoroutineContext is mainly build up with four elements: Job, CoroutineName, CoroutineExceptionHandler and Dispatcher.
Its responsibility of the dispatcher to dispatch the coroutine. A dispatcher can be paused to stop coroutines to even run (this is useful in unit testing) mentioned here in the android conference talk, it may be a single-threaded dispatcher just like Dispatchers.Main, it has an event-loop like javascript has.
So, it really depends that on the following points:
Is the dispatcher is multi-threaded?
For example: This will run on single thread.
suspend fun main() {
val dispatcherScope = CoroutineScope(Executors.newSingleThreadExecutor().asCoroutineDispatcher())
val job = dispatcherScope.launch {
repeat(10) {
launch {
println("I'm working in thread ${Thread.currentThread().name}")
// every coroutine on same thread
}
}
}
job.join()
}
Run it here, other single threaded dispatchers: Dispatchers.Main
Is there any dispatcher override in between?
With the same context, if we override the dispatcher before launch, it will change the thread even if original context is based on single-threaded event-loop, each coroutine will run on different thread creating 10 different thread:
dispatcherScope.launch {
repeat(10) {
launch(Dispatchers.IO) {
println("I'm working in thread ${Thread.currentThread().name}")
// every coroutine on same thread
}
}
}
Run it here, other multi-threaded Dispatchers: Dispatchers.Default, Executor based dispatcher, Dispatchers.Unconfined (this launch coroutine in any free thread).
The short answer is: they execute somewhere and come back with the result to somewhere.
The long(er) explanation for the short answer:
"Somewhere" might be the same thread, it might be a different thread - it depends on the dispatcher and in many cases, the current state of the dispatcher. For instance, the contents of a SequenceScope will (by default) run on the same thread.
Another case where a suspend function might run on the same thread is if it's using the same dispatcher as the calling function.
Dispatchers can also share threads between them to save on thread creation (and just keep a count of the maximum number of parallel operations for their own tasks), so even switching between different dispatchers that each use thread pools may not result in using a different thread.
As far as people saying "they don't necessarily come back to the same thread," this is correct, for similar reasons. Keep in mind that your dispatcher may have many threads, and the one that you were using before you got suspended might be occupied with a different function right now, so the dispatcher will simply pick a different thread to run your code on.
When the coroutine suspends, the underlying Java method returns a special COROUTINE_SUSPENDED value. If the calling function is also suspendable, it also returns the object, and so the execution returns to the innermost plain, non-suspendable function. This function typically runs an event loop, where event handlers are calls to continuation.resume(). So now it is ready to take the next handler from the queue and resume another coroutine.
When you call continuation.resume(), the continuation itself knows about the dispatcher in charge of the coroutine and delegates to it. If the current thread isn't owned by that dispatcher, it dispatches an event to another event loop, the one served by the dispatcher's thread pool. This way the dispatcher controls the thread where the corutine resumes.
If the operations inside CoroutineScope(job+Dispatchers.Main){...} run on the main thread then how come it does not violate Android's requirement that slow (blocking) operations (Networking etc.) are not allowed to run on the main/UI thread? I can run blocking operations with this scope and the UI does not freeze at all.
I would be grateful if someone could explain what is happening under the hood. My guess is that it is similar to how JavaScript manages blocking operations with the event loop, but I struggle to find any relevant materials.
My guess is that it is similar to how JavaScript manages blocking operations with the event loop
Yes, this is correct, the event loop is essential to making coroutines work. Basically, when you write this:
uiScope.launch {
delay(1000)
println("A second has passed")
}
it compiles into code that has the same effect as this:
Handler(Looper.mainLooper()).postDelayed(1000) { println("A second has passed") }
The main concept is the continuation, an object that implements a state machine that corresponds to the sequential code you wrote in a suspendable function. When you call delay or any other suspendable function, the continuation's entry-point method returns a special COROUTINE_SUSPENDED value. Later on, when some outside code comes up with the return value of the suspendable function, it must call continuation.resume(result). This call will be intercepted by the dispatcher in charge, which will post this call as an event on the GUI event loop. When the event handler is dequeued and executed, you are back inside the state machine which figures out where to resume the execution.
You can review this answer for a more fleshed-out example of using the Continuation API.
Running blocking operations and running suspending operations on CoroutineScope(Dispatchers.Main) are two different things.
delay() is a suspending function and it is non blocking
CoroutineScope(Dispatchers.Main){
delay(6000)
}
While Thread.sleep() is blocking and calling code below will cause ANR
CoroutineScope(Dispatchers.Main){
Thread.sleep(6000)
}
I suggest you checking Kotlin coroutines talk by Roman Elizarov on Kotlinconf 2017, especially the part where he runs 100,000 delay()
I am trying to make an application that constantly hits the google API to fetch the distance between two points and then process the distance and add it in db inside a loop.
I was previously using the volley but it does not waits and my function proceeds with the rest of lines of code. I need help so that the line of code waits for that method to return only if some result is received once I hit google API, then use that result in next line of code.
I'd recommend looking into coroutines. More specifically take a look at the async launcher which should return the value to an awaiting variable. Your code runs asynchronously (starting a new thread so your main UI thread doesn't block and the user can freely do as they wish) and when the call returns your code is called once again at the point where the variable will be awaiting. You could set a timer right before starting the async call and stop it after await.
You could also use withContext{} to await for a result. The code will be continued in the calling function when the coroutine returns a result.
startTimer()
val job = coroutineScope{
async{
networkCall()
}
}
job.await()
stopTimer()
or
startTimer()
coroutineScope{
launch{
withContext{
networkCall()
}
//code will continue when networkCall() has returned a result
stopTimer() //be careful, this may be called through a background thread.
}
}
Coroutines are thread-safe and lightweight so you can start as many as you want without any problems. Also, by using coroutineScope you will avoid lifecycle problems like your ui being rotated which would normally cause a network call to be resent. With coroutineScope your call will have the life span of the calling activity/fragment thus it will be cancelled or killed if your app unexpectedly decides to stop and not care for the result any longer.
Hope this helped, Panos.
If you are beginner use retrofit library to make API calls in there is option enque will call API background for you.
https://www.journaldev.com/13639/retrofit-android-example-tutorial
If you are pro go for Rxjava with Retrofit.
https://medium.com/3xplore/handling-api-calls-using-retrofit-2-and-rxjava-2-1871c891b6ae
You cannot do synchronous calls on the main thread. Blocking the UI thread on a network call for more than a specified period of time would trigger an ANR.
A couple of options would be to use an AysncTask or AsyncTaskLoader. Blocking the main thread is definitely not recommended.
AsyncTasks create a separate thread of execution while the UI thread can continue with its work.
Android applications usually start with a single thread of execution so if you block this thread then an ANR would result.
Take a look here
If you don't mind the ANRs which will not be acceptable to a user then go with using a simple HttpURLConnection but this is not recommended.
If you do not prefer the AsyncTask approach you can create a Runnable to do the background processing and then update the UI thread.
More here
You can implement a jobservice to get distance in an interval.You can view the implementation .here