I want to retrieve single object from Room database, so i have this method in Dao
// in Dao
#Query("SELECT * FROM table_foo ORDER BY RANDOM()")
fun getSingleFoo(): Flow<FooEntity>
That object then will be mapped into others model, let say PlainFoo.
// in Repository
fun getRandomFoo(): Flow<PlainFoo> = dao.getSingleFoo()
.map(FooEntity::asExternalModel)
But in the first launch of this app, the table is empty. It makes the dao function return null and trigger NPE when being mapped. I try to wrap it inside a sealed interface like this.
// Result.kt as wrapper
sealed interface Result<out T> {
data class Success<T>(val data: T) : Result<T>
data class Error(val exception: Throwable? = null) : Result<Nothing>
}
fun <T> Flow<T>.asResult(): Flow<Result<T>> = this
.map<T, Result<T>> {
Result.Success(it)
}
.catch {
emit(Result.Error(it))
}
And then i call this method in the presentation layer like this.
// in ViewModel
val randomFoo = fooRepository.getRandomFoo().asResult()
// in activity, log only for checking
lifecycleScope.launch {
viewModel.randomFoo.collect {
Timber.tag("RandomFooFlow").d("$it")
}
}
It catches the error, which look like this.
Error(exception=java.lang.NullPointerException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkNotNullParameter, parameter <this>)
But when new data is inserted, it does not get updated unless i reopen the app (which means new Flow is being collected, not the old one). So it seems that the flow is cancelled.
Is there any way to handle this without making my Dao return a
nullable object?
Note: if the data is already populated when opening the app, the flow is able to keep consuming new value).
Instead of dealing with exceptions, I would suggest to return nullable types from your Dao. You can then also update your mapper function to handle the type nullability. You won't need to wrap it into any Result class, just a simple null check on the UI end would suffice.
// Dao
#Query("SELECT * FROM table_foo ORDER BY RANDOM()")
fun getSingleFoo(): Flow<FooEntity?>
// Repo
fun getRandomFoo(): Flow<PlainFoo?> = dao.getSingleFoo().map { it?.asExternalModel() }
Could you please call repository getRandomFoo() method from inside coroutine in view model ? And also you need to call response with data observe like LiveData or StateFlow. By the way, you can wrap your result with wrap inside repository. In code example, I do not care about it because your error is not related with mapping.
View Model
private val _stateFlow = MutableStateFlow()
val stateFlow:StateFlow
fun getRandom(){
fooRepository.getRandomFoo().onEach{
if(it is Result.Success){
stateFlow.value = it
}
}.launchIn(viewModelScope)
}
Fragment or activity
viewLifecycleOwner.lifecycle.repeatOnLifecycle{
stateFlow.collect{
// Listen data for your UI
}
}
Hello dear reactive programmers, I started to learn project reactor but I still struggle to figure out what operator to use when. I figured out, that if I want to have reusable parts to define a reactor flow, I can use the transform operator. What I would like to achieve is to use a certain implementation of such a flow function based on the current observables context. For a Mono flow, I came up with this, but I am very unsure, if it is a good solution:
So here is a part of the flow
class CloudeventOverDelegatorRoute(
val fromHttpToDelegatorRoute: FromHttpToDelegatorRoute,
val delegatorProvider: DelegatorProvider,
val fromDelegatorToHttpRoute: FromDelegatorToHttpRoute
): MessageRoute<HttpBaseMessage, HttpResponseMessage> {
override fun isHandlerFor(context: RouteContext): Boolean {
return fromHttpToDelegatorRoute.isHandlerFor(context)
&& fromDelegatorToHttpRoute.isHandlerFor(context)
}
override fun buildPipeline(input: Mono<RoutableMessage<HttpBaseMessage>>): Mono<RoutableMessage<HttpResponseMessage>> {
var dynamicallyDeterminedDelegator: Delegator? = null
return input.transform {
fromHttpToDelegatorRoute.buildPipeline(input)
}.handle<RoutableMessage<InternalMessage>> { t, u ->
dynamicallyDeterminedDelegator = delegatorProvider.provideDelegatorFor(t.routeContext)
u.next(t)
u.complete()
}.transform {
dynamicallyDeterminedDelegator!!.sendDelegated(it)
}.transform { fromDelegatorToHttpRoute.buildPipeline(it) }
}
}
Here is the dynamic selection logic
interface DelegatorProvider {
fun provideDelegatorFor(context: RouteContext): Delegator
}
class FirstMatchDelegatorProvider(
private val delegators: List<Delegator>
): DelegatorProvider {
override fun provideDelegatorFor(context: RouteContext): Delegator {
return delegators.firstOrNull {
it.isHandlerFor(context)
}?: throw IllegalStateException("No Delegator route available for context: $context")
}
}
And this is the delegator providing an essential sub-part of the whole flow
interface Delegator {
fun isHandlerFor(context: RouteContext): Boolean
fun sendDelegated(input: Mono<RoutableMessage<InternalMessage>>): Mono<RoutableMessage<InternalStatusMessage>>
}
What do you think? How would you solve it?
this approach is problematic because it relies on shared state (the dynamicallyDeterminedDelegator variable). If multiple subscribers subscribe to the returned Mono, they could overwrite each other delegator. Maybe that (multiple subscriptions) can't happen in your application, but this is a very bad habit to get into in any case.
looks like you can derive a delegator out of a RoutableMessage<InternalMessage> , and that you don't really need to retain that delegator.
the easiest way to resolve and apply the delegator to the routableMessage in one go is simply to use flatMap. see the (pseudo) java code below:
.flatMap(routableMessage -> {
val delegator = delegatorProvider.provideDelegatorFor(routableMessage.routeContext);
return delegator.sendDelegated(routableMessage);
})
We often need some request validation before handling it. With arrow v 0.8 a typical message handler looked like:
fun addToShoppingCart(request: AddToShoppingCartRequest): IO<Either<ShoppingCardError, ItemAddedEvent>> = fx {
request
.pipe (::validateShoppingCard)
.flatMap { validatedRequest ->
queryShoppingCart().bind().map { validatedRequest to it } // fun queryShoppingCart(): IO<Either<DatabaseError, ShoppingCart>>
}
.flatMap { (validatedRequest, shoppingCart) ->
maybeAddToShoppingCart(shoppingCart, validatedRequest) // fun maybeAddToShoppingCart(...): Either<DomainError, ShoppingCart>
}
.flatMap { updatedShoppingCart ->
storeShoppingCart(updatedShoppingCart).bind() // fun storeShoppingCart(ShoppingCart): IO<Either<DatabaseError, Unit>>
.map {
computeItemAddedEvent(updatedShoppingCart)
}
}
.mapLeft(::computeShoppingCartError)
}
This seems to be a convenient and expressive definition of a workflow. I tried to define similar function in arrow v 0.10.5:
fun handleDownloadRequest(strUrl: String): IO<Either<BadUrl, MyObject>> = IO.fx {
parseUrl(strUrl) // fun(String): Either<BadUrl,Url>
.map {
!effect{ downloadObject(it) } // suspended fun downloadObject(Url): MyObject
}
}
Which results in a compiler error "Suspension functions can be called only within coroutine body". The reason is both map and flatMap functions of Either and Option are not inline.
Indeed, the blog post about fx says
"Soon you will find that you cannot call suspend functions inside the
functions declared for Either such as the ones mentioned above, and
other fan favorites like map() and handleErrorWith(). For that you
need a concurrency library!"
So the question is why is it so and what is the idiomatic way of such composition?
The idiomatic way is
fun handleDownloadRequest(strUrl: String): IO<Either<BadUrl, MyObject>> =
parseUrl(strUrl)
.fold({
IO.just(it.left()) // forward the error
}, {
IO { downloadObject(it) }
.attempt() // get an Either<Throwable, MyObject>
.map { it.mapLeft { /* Throwable to BadURL */ } } // fix the left side
})
Personally I wouldn't go to the deep end of IO with that one, and rewrite as a suspend function instead
suspend fun handleDownloadRequest(strUrl: String): Either<BadUrl, MyObject> =
parseUrl(strUrl)
.fold(::Left) { // forward the error
Either.catch({ /* Throwable to BadURL */ }) { downloadObject(it) }
}
What happened is, in 0.8.X the functions for Either used to be inlined. An unintended side-effect of this was that you could call suspend functions anywhere. While this is nice, it can lead to exceptions thrown (or jumping threads or deadlocks 🙈) in the middle of a map or a flatMap, which is terrible for correctness. It's a crutch.
In 0.9 (or was it 10?) we removed that crutch and made it into something explicit in the API: Either.catch. We kept fold as inlined because it's the same as when, so there was no real correctness tradeoff there.
So, the recommended thing is to use suspend everywhere and only reach for IO when trying to do threading, parallelism, cancellation, retries and scheduling, or anything really advanced.
For basic use cases suspend and Either.catch is enough. To call into a suspend function at the edge of your program or where you need to bridge with these advanced behaviors then use IO.
If you want to continue using Either you can define suspend/inline versions of regular functions at your own risk; or wait until IO<E, A> in 0.11 where you can use effectEither and effectMapEither.
I'm trying to poll a paginated API and provide new items to the user as they appear.
fun connect(): Sequence<T> = buildSequence {
while (true) {
// result is a List<T>
val result = dataSource.getFirstPage()
yieldAll(/* the new data in `result` */)
// Block the thread for a little bit
}
}
Here's the sample usage:
for (item in connect()) {
// do something as each item is made available
}
My first thought was to use the delay function, but I get this message:
Restricted suspended functions can only invoke member or extension suspending functions on their restricted coroutine scope
This is the signature for buildSequence:
public fun <T> buildSequence(builderAction: suspend SequenceBuilder<T>.() -> Unit): Sequence<T>
I think this message means that I can only use the suspend functions in SequenceBuilder: yield and yieldAll and that using arbitrary suspend function calls aren't allowed.
Right now I'm using this to block the sequence building by one second after every time the API is polled:
val resumeTime = System.nanoTime() + TimeUnit.SECONDS.toNanos(1)
while (resumeTime > System.nanoTime()) {
// do nothing
}
This works, but it really doesn't seem like a good solution. Has anybody encountered this issue before?
Why does it not work? Some research
When we look at buildSequence, we can see that it takes an builderAction: suspend SequenceBuilder<T>.() -> Unit as its argument. As a client of that method, you'll be able to hand on a suspend lambda that has SequenceBuilder as its receiver (read about lambda with receiver here).
The SequenceBuilder itself is annotated with RestrictSuspension:
#RestrictsSuspension
#SinceKotlin("1.1")
public abstract class SequenceBuilder<in T> ...
The annotation is defined and commented like this:
/**
* Classes and interfaces marked with this annotation are restricted
* when used as receivers for extension `suspend` functions.
* These `suspend` extensions can only invoke other member or extension
* `suspend` functions on this particular receiver only
* and are restricted from calling arbitrary suspension functions.
*/
#SinceKotlin("1.1") #Target(AnnotationTarget.CLASS) #Retention(AnnotationRetention.BINARY)
public annotation class RestrictsSuspension
As the RestrictSuspension documentation tells, in the case of buildSequence, you can pass a lambda with SequenceBuilder as its receiver but with restricted possibilities since you'll only be able to call "other member or extension suspend functions on this particular receiver". That means, the block passed to buildSequence may call any method defined on SequenceBuilder (like yield, yieldAll). Since, on the other hand, the block is "restricted from calling arbitrary suspension functions", using delay does not work. The resulting compiler error verifies it:
Restricted suspended functions can only invoke member or extension suspending functions on their restricted coroutine scope.
Ultimately, you need to be aware that the buildSequence creates a coroutine that is an example of a synchronous coroutine. In your example, the sequence code will be executed in the same thread that consumes the sequence by calling connect().
How to delay the sequence?
As we learned, The buildSequence creates a synchronous sequence. It's fine to use regular Thread blocking here:
fun connect(): Sequence<T> = buildSequence {
while (true) {
val result = dataSource.getFirstPage()
yieldAll(result)
Thread.sleep(1000)
}
}
But, do you really want an entire thread to be blocked? Alternatively, you can implement asynchronous sequences as described here. As a result, using delay and other suspending functions will be valid.
Just for an alternate solution...
If what you're really trying to do is asynchronously produce elements, you can use Flows which are basically asynchronous sequences.
Here is a quick table:
Sync
Async
Single
Normal valuefun example(): String
suspendingsuspend fun example(): Stringorfun example(): Deferred<String>
Many
Sequencefun example(): Sequence<String>
Flowfun example(): Flow<String>
You can convert your Sequence<T> to a Flow<T> by replacing the sequence { ... } builder with the flow { ... } builder and then replace yield/yieldAll with emit/emitAll:
fun example(): Flow<String> = flow {
(1..5).forEach { getString().let { emit(it) } }
}
suspend fun getString(): String = { ... }
So, for your example:
fun connect(): Flow<T> = flow {
while (true) {
// Call suspend function to get data from dataSource
val result: List<T> = dataSource.getFirstPage()
emitAll(result)
// _Suspend_ for a little bit
delay(1000)
}
}
I wish to have a good example for each function run, let, apply, also, with
I have read this article but still lack of an example
All these functions are used for switching the scope of the current function / the variable. They are used to keep things that belong together in one place (mostly initializations).
Here are some examples:
run - returns anything you want and re-scopes the variable it's used on to this
val password: Password = PasswordGenerator().run {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
generate()
}
The password generator is now rescoped as this and we can therefore set seed, hash and hashRepetitions without using a variable.
generate() will return an instance of Password.
apply is similar, but it will return this:
val generator = PasswordGenerator().apply {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
}
val pasword = generator.generate()
That's particularly useful as a replacement for the Builder pattern, and if you want to re-use certain configurations.
let - mostly used to avoid null checks, but can also be used as a replacement for run. The difference is, that this will still be the same as before and you access the re-scoped variable using it:
val fruitBasket = ...
apple?.let {
println("adding a ${it.color} apple!")
fruitBasket.add(it)
}
The code above will add the apple to the basket only if it's not null. Also notice that it is now not optional anymore so you won't run into a NullPointerException here (aka. you don't need to use ?. to access its attributes)
also - use it when you want to use apply, but don't want to shadow this
class FruitBasket {
private var weight = 0
fun addFrom(appleTree: AppleTree) {
val apple = appleTree.pick().also { apple ->
this.weight += apple.weight
add(apple)
}
...
}
...
fun add(fruit: Fruit) = ...
}
Using apply here would shadow this, so that this.weight would refer to the apple, and not to the fruit basket.
Note: I shamelessly took the examples from my blog
There are a few more articles like here, and here that are worth to take a look.
I think it is down to when you need a shorter, more concise within a few lines, and to avoid branching or conditional statement checking (such as if not null, then do this).
I love this simple chart, so I linked it here. You can see it from this as written by Sebastiano Gottardo.
Please also look at the chart accompanying my explanation below.
Concept
I think it as a role playing way inside your code block when you call those functions + whether you want yourself back (to chain call functions, or set to result variable, etc).
Above is what I think.
Concept Example
Let's see examples for all of them here
1.) myComputer.apply { } means you want to act as a main actor (you want to think that you're computer), and you want yourself back (computer) so you can do
var crashedComputer = myComputer.apply {
// you're the computer, you yourself install the apps
// note: installFancyApps is one of methods of computer
installFancyApps()
}.crash()
Yup, you yourself just install the apps, crash yourself, and saved yourself as reference to allow others to see and do something with it.
2.) myComputer.also {} means you're completely sure you aren't computer, you're outsider that wants to do something with it, and also wants it computer as a returned result.
var crashedComputer = myComputer.also {
// now your grandpa does something with it
myGrandpa.installVirusOn(it)
}.crash()
3.) with(myComputer) { } means you're main actor (computer), and you don't want yourself as a result back.
with(myComputer) {
// you're the computer, you yourself install the apps
installFancyApps()
}
4.) myComputer.run { } means you're main actor (computer), and you don't want yourself as a result back.
myComputer.run {
// you're the computer, you yourself install the apps
installFancyApps()
}
but it's different from with { } in a very subtle sense that you can chain call run { } like the following
myComputer.run {
installFancyApps()
}.run {
// computer object isn't passed through here. So you cannot call installFancyApps() here again.
println("woop!")
}
This is due to run {} is extension function, but with { } is not. So you call run { } and this inside the code block will be reflected to the caller type of object. You can see this for an excellent explanation for the difference between run {} and with {}.
5.) myComputer.let { } means you're outsider that looks at the computer, and want to do something about it without any care for computer instance to be returned back to you again.
myComputer.let {
myGrandpa.installVirusOn(it)
}
The Way to Look At It
I tend to look at also and let as something which is external, outside. Whenever you say these two words, it's like you try to act up on something. let install virus on this computer, and also crash it. So this nails down the part of whether you're an actor or not.
For the result part, it's clearly there. also expresses that it's also another thing, so you still retain the availability of object itself. Thus it returns it as a result.
Everything else associates with this. Additionally run/with clearly doesn't interest in return object-self back. Now you can differentiate all of them.
I think sometimes when we step away from 100% programming/logic-based of examples, then we are in better position to conceptualize things. But that depends right :)
There are 6 different scoping functions:
T.run
T.let
T.apply
T.also
with
run
I prepared a visual note as the below to show the differences :
data class Citizen(var name: String, var age: Int, var residence: String)
Decision depends on your needs. The use cases of different functions overlap, so that you can choose the functions based on the specific conventions used in your project or team.
Although the scope functions are a way of making the code more concise, avoid overusing them: it can decrease your code readability and lead to errors. Avoid nesting scope functions and be careful when chaining them: it's easy to get confused about the current context object and the value of this or it.
Here is another diagram for deciding which one to use from https://medium.com/#elye.project/mastering-kotlin-standard-functions-run-with-let-also-and-apply-9cd334b0ef84
Some conventions are as the following :
Use also for additional actions that don't alter the object, such as logging or printing debug information.
val numbers = mutableListOf("one", "two", "three")
numbers
.also { println("The list elements before adding new one: $it") }
.add("four")
The common case for apply is the object configuration.
val adam = Person("Adam").apply {
age = 32
city = "London"
}
println(adam)
If you need shadowing, use run
fun test() {
var mood = "I am sad"
run {
val mood = "I am happy"
println(mood) // I am happy
}
println(mood) // I am sad
}
If you need to return receiver object itself, use apply or also
let, also, apply, takeIf, takeUnless are extension functions in Kotlin.
To understand these function you have to understand Extension functions and Lambda functions in Kotlin.
Extension Function:
By the use of extension function, we can create a function for a class without inheriting a class.
Kotlin, similar to C# and Gosu, provides the ability to extend a class
with new functionality without having to inherit from the class or use
any type of design pattern such as Decorator. This is done via special
declarations called extensions. Kotlin supports extension functions
and extension properties.
So, to find if only numbers in the String, you can create a method like below without inheriting String class.
fun String.isNumber(): Boolean = this.matches("[0-9]+".toRegex())
you can use the above extension function like this,
val phoneNumber = "8899665544"
println(phoneNumber.isNumber)
which is prints true.
Lambda Functions:
Lambda functions are just like Interface in Java. But in Kotlin, lambda functions can be passed as a parameter in functions.
Example:
fun String.isNumber(block: () -> Unit): Boolean {
return if (this.matches("[0-9]+".toRegex())) {
block()
true
} else false
}
You can see, the block is a lambda function and it is passed as a parameter. You can use the above function like this,
val phoneNumber = "8899665544"
println(phoneNumber.isNumber {
println("Block executed")
})
The above function will print like this,
Block executed
true
I hope, now you got an idea about Extension functions and Lambda functions. Now we can go to Extension functions one by one.
let
public inline fun <T, R> T.let(block: (T) -> R): R = block(this)
Two Types T and R used in the above function.
T.let
T could be any object like String class. so you can invoke this function with any objects.
block: (T) -> R
In parameter of let, you can see the above lambda function. Also, the invoking object is passed as a parameter of the function. So you can use the invoking class object inside the function. then it returns the R (another object).
Example:
val phoneNumber = "8899665544"
val numberAndCount: Pair<Int, Int> = phoneNumber.let { it.toInt() to it.count() }
In above example let takes String as a parameter of its lambda function and it returns Pair in return.
In the same way, other extension function works.
also
public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }
extension function also takes the invoking class as a lambda function parameter and returns nothing.
Example:
val phoneNumber = "8899665544"
phoneNumber.also { number ->
println(number.contains("8"))
println(number.length)
}
apply
public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }
Same as also but the same invoking object passed as the function so you can use the functions and other properties without calling it or parameter name.
Example:
val phoneNumber = "8899665544"
phoneNumber.apply {
println(contains("8"))
println(length)
}
You can see in the above example the functions of String class directly invoked inside the lambda funtion.
takeIf
public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? = if (predicate(this)) this else null
Example:
val phoneNumber = "8899665544"
val number = phoneNumber.takeIf { it.matches("[0-9]+".toRegex()) }
In above example number will have a string of phoneNumber only it matches the regex. Otherwise, it will be null.
takeUnless
public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predicate(this)) this else null
It is the reverse of takeIf.
Example:
val phoneNumber = "8899665544"
val number = phoneNumber.takeUnless { it.matches("[0-9]+".toRegex()) }
number will have a string of phoneNumber only if not matches the regex. Otherwise, it will be null.
You can view similar answers which is usefull here difference between kotlin also, apply, let, use, takeIf and takeUnless in Kotlin
According to my experience, since such functions are inline syntactic sugar with no performance difference, you should always choose the one that requires writing the least amount of code in the lamda.
To do this, first determine whether you want the lambda to return its result (choose run/let) or the object itself (choose apply/also); then in most cases when the lambda is a single expression, choose the ones with the same block function type as that expression, because when it's a receiver expression, this can be omitted, when it's a parameter expression, it is shorter than this:
val a: Type = ...
fun Type.receiverFunction(...): ReturnType { ... }
a.run/*apply*/ { receiverFunction(...) } // shorter because "this" can be omitted
a.let/*also*/ { it.receiverFunction(...) } // longer
fun parameterFunction(parameter: Type, ...): ReturnType { ... }
a.run/*apply*/ { parameterFunction(this, ...) } // longer
a.let/*also*/ { parameterFunction(it, ...) } // shorter because "it" is shorter than "this"
However, when the lambda consists of a mix of them, it's up to you then to choose the one that fits better into the context or you feel more comfortable with.
Also, use the ones with parameter block function when deconstruction is needed:
val pair: Pair<TypeA, TypeB> = ...
pair.run/*apply*/ {
val (first, second) = this
...
} // longer
pair.let/*also*/ { (first, second) -> ... } // shorter
Here is a brief comparison among all these functions from JetBrains's official Kotlin course on Coursera Kotlin for Java Developers:
I must admit that the difference is not so obvious at first glance, among other things because these 5 functions are often interchangeable. Here is my understanding :
APPLY -> Initialize an object with theses properties and wait for the object
val paint = Paint().apply {
this.style = Paint.Style.FILL
this.color = Color.WHITE
}
LET -> Isolate a piece of code and wait for the result
val result = let {
val b = 3
val c = 2
b + c
}
or
val a = 1
val result = a.let {
val b = 3
val c = 2
it + b + c
}
or
val paint: Paint? = Paint()
paint?.let {
// here, paint is always NOT NULL
// paint is "Paint", not "Paint?"
}
ALSO -> Execute 2 operations at the same time and wait for the result
var a = 1
var b = 3
a = b.also { b = a }
WITH -> Do something with this variable/object and don't wait for a result (chaining NOT allowed )
with(canvas) {
this.draw(x)
this.draw(y)
}
RUN -> Do something with this variable/object and don't wait for a result (chaining allowed)
canvas.run {
this.draw(x)
this.draw(y)
}
or
canvas.run {this.draw(x)}.run {this.draw(x)}