In Scala and other programming languages one can use Futures and Await.
(In real code one would use e.g. zip+map instead of Await)
def b1() = Future { 1 }
def b2() = Future { 2 }
def a() = Future {
Await.result(b1(),Duration.inf) + Await.result(b2(),Duration.inf)
}
What is the difference to Async/Await in Javascript/Scala?
async function b1() { return 1 }
async function b2() { return 3 }
async function a() {
return await b1() + await b2()
}
The "Await.result" function in Scala is "blocking", which means that the calling thread will be paused until the awaited Future is completed, at which point it will resume with the returned value.
Pausing a thread can be expensive in a system under high load, as the thread context has to be saved in memory, and it can cause cache misses etc.. Blocking threads is considered poor practice in concurrent programming for that reason.
The async / await syntax in Javascript is non-blocking. When an async function invokes "await", it is converted into a Future, and placed into the execution queue. When the awaited future is complete, the calling function is marked as ready for execution and it will be resumed at some later point. The important difference is that no Threads need to be paused in this model.
There are a number of libraries which implement the async / await syntax in Scala, including https://github.com/scala/scala-async
Further reading
The Futures and Promises book by PRASAD, PATIL, AND MILLER has a good introduction to blocking and non-blocking ops.
Await is blocking while async is non blocking. Await waits in the current thread to finish the task while async won't block the current thread and runs in background.
I don't have idea about JavaScript. In Scala Await from scala.concurrent package is used for blocking main thread. There's another library called scala-async what is used for using await inside async block.
If you are using scala-async, then you need to call await inside async
Related
I have a function that calls a Future in it. Now I am not sure if the first function needs to be a future too to await for the data. Here is my code:
FireBaseHandler handler = FireBaseHandler();
saveRows() {
handler.saveRows(plan.planId, plan.rows); ///this is a future
}
In my FireBaseHandler class I have this Future:
final CollectionReference usersCol =
FirebaseFirestore.instance.collection('users');
Future saveRows(String id, data) async {
return await usersCol.doc(myUser.uid).collection('plans').doc(id)
.update({'rows': data});
}
So does the first function needs to be a Future too?
You can have async function inside a sync function. But this way you lose ability to await for the result. And await is allowed only in functions marked as async, which leads us to Future as a the result of that function. So, yes, if you need to wait for the result, both functions have to be Future functions.
Edit:
If you need your wrapper function to be sync, but still be able to retrieve the result from inner async function, you can use a callback:
saveRows(Function(dynamic) callback) {
handler.saveRows(plan.planId, plan.rows).then((result){
callback(result);
});
}
This way the result will be retrieved at any point of time after calling the function (the code is not awaited)
I'm doing the Advanced coroutines with Kotlin flow and LiveData code lab and encountered this function in CacheOnSuccess.kt.
There is a comment that says "// Note: mutex is not held in this async block". What does this mean exactly? Why wouldn't the mutex be held in the async block? And what is the significance of that?
suspend fun getOrAwait(): T {
return supervisorScope {
// This function is thread-safe _iff_ deferred is #Volatile and all reads and writes
// hold the mutex.
// only allow one coroutine to try running block at a time by using a coroutine-base
// Mutex
val currentDeferred = mutex.withLock {
deferred?.let { return#withLock it }
async {
// Note: mutex is not held in this async block
block()
}.also {
// Note: mutex is held here
deferred = it
}
}
// await the result, with our custom error handling
currentDeferred.safeAwait()
}
}
according to withLock implementation, mutex is held on the just stack-frame, which means, after withLock execution the mutex is released, but code inside async may not execute right in that frame (maybe in another thread according to current Dispatchers), so probably when the block of async get executed, withLock call could have already returned, as for the also call, it is marked as inline, so it is executed in the current frame, right before withLock returned
The mutex is held by at most one coroutine at any time. async launches a coroutine which doesn't attempt to acquire the mutex. The significance of that is the same as for any other mutex -- the code inside the async block isn't guarded by the mutex, so it must not touch the state that is required to be guarded by it.
I have an async method. Can I call it from an non-async method? Like the following
My method
void method() async{
await 'something'
}
Case 1
onPressed:() {
method();
}
Case 2
onPressed:() async{
await method();
}
Which of the above is correct? It seems to me two of them is OK. However, the second one I think works much more slower, am I wrong?
In general, the caller of an async function must also be asynchronous if it wants to wait for the call to complete. This makes asynchronous-ness contagious.
In your case, your async function is a "fire-and-forget" function; callers cannot wait for it to complete, so it doesn't matter. Your second case (with await method()) is wrong because you should use await only on Future/FutureOr, but method returns void, so there nothing to wait for. (The Dart analyzer would warn you about this if you have the await_only_futures lint enabled.)
You also could simplify your code further by using a tear-off instead of creating an unnecessary closure:
onPressed: method
From the documentation for the Future.microtask constructor, it says:
* Creates a future containing the result of calling [computation]
* asynchronously with [scheduleMicrotask].
and the documentation for the regular Future constructor states:
* Creates a future containing the result of calling [computation]
* asynchronously with [Timer.run].
I am wondering, what kind of implications do they have on coding, and when should we use one or another?
All microtasks are executed before any other Futures/Timers.
This means that you will want to schedule a microtask when you want to complete a small computation asynchronously as soon as possible.
void main() {
Future(() => print('future 1'));
Future(() => print('future 2'));
// Microtasks will be executed before futures.
Future.microtask(() => print('microtask 1'));
Future.microtask(() => print('microtask 2'));
}
You can run this example on DartPad.
The event loop will simply pick up all microtasks in a FIFO fashion before other futures. A microtask queue is created when you schedule microtasks and that queue is executed before other futures (event queue).
There is an outdated archived article for The Event Loop and Dart, which covers the event queue and microtask queue here.
You can also learn more about microtasks with this helpful resource.
Here is a simple example of how code would run in sequence in terms of how Futures are executed. In the example below, the resulting print statements wouldn't be in alphabetical order.
void main() async{
print("A");
await Future((){
print("B");
Future(()=>print("C"));
Future.microtask(()=>print("D"));
Future(()=>print("E"));
print("F");
});
print("G");
}
The resulting print statements would end up in the order shown below. Notice how B, F, and G gets printed first, then C, then E. This is because B,F, and G are synchronous. D then gets called before C and E because of it being a microtask.
I saw many people's code use async keyword in a function without an await keyword in the function body. Even some official flutter example code do this. I have no idea why. What is the point? Is this a mistake or having a purpose?
Normally, I just remove the async keyword from those code and everything will run without any problems. Can some dart expert clarify that if there is a purpose for a function which has the async keyword but NO await keyword? Or is this just their mistake?
async is sometimes used to simplify code.
Here are some examples:
Future<int> f1() async => 1;
Future<int> f1() => Future.value(1);
Future<void> f2() async {
throw Error();
}
Future<void> f2() {
return Future.error(Error());
}
According to official Dart Language tour, async executes synchronously until it finds await keyword :
Note: Although an async function might perform time-consuming operations, it doesn’t wait for those operations. Instead, the async function executes only until it encounters its first await expression (details). Then it returns a Future object, resuming execution only after the await expression completes.