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Description
async/await for Android built upon coroutines introduced in Kotlin 1.1
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Async/Await
A Kotlin library for Android to write asynchronous code in a simpler and more reliable way using
async/
awaitapproach, like:
async {
progressBar.visibility = View.VISIBLE
// Release main thread and wait until text is loaded in background thread
val loadedText = await { loadFromServer() }
// Loaded successfully, come back in UI thread and show the result
txtResult.text = loadedText
progressBar.visibility = View.INVISIBLE
}
As you see in the example above, you can write asynchronous code in a imperative style, step by step. Calling
awaitto run code in background doesn't lock the UI thread. And execution continues in UI thread after background work is finished. There is no magic, see how it works.
Dependency
compile 'co.metalab.asyncawait:asyncawait:1.0.0'
Usage
async
Coroutine code has to be passed as a lambda in
asyncfunction
Kotlin
async {
// Coroutine body
}
await
Long running code has to be passed as a lambda in
awaitfunction
Kotlin
async {
val result = await {
//Long running code
}
// Use result
}
You may have many awaitcalls inside
asyncblock, or have
awaitin a loop
async {
val repos = await { github.getRepos() }
showList(repos)
repos.forEach { repo ->
val stats = await { github.getStats(repo.name) }
showStats(repo, stats)
}
}
awaitWithProgress
Use it to show loading progress, its second parameter is a progress handler.
Kotlin
val loadedText = awaitWithProgress(::loadTextWithProgress) {
// Called in UI thread
progressBar.progress = it
progressBar.max = 100
}
A data loading function (like the loadTextWithProgressabove) should have a functional parameter of type
(P) -> Unitwhich can be called in order to push progress value. For example, it could be like:
Kotlin
private fun loadTextWithProgress(handleProgress: (Int) -> Unit): String {
for (i in 1..10) {
handleProgress(i * 100 / 10) // in %
Thread.sleep(300)
}
return "Loaded Text"
}
Handle exceptions using try/catch
async {
try {
val loadedText = await {
// throw exception in background thread
}
// Process loaded text
} catch (e: Exception) {
// Handle exception in UI thread
}
}
Handle exceptions in onError
block
Could be more convenient, as resulting code has fewer indents.
onErrorcalled only if exception hasn't been handled in
try/catch.
Kotlin
async {
val loadedText = await {
// throw exception in background thread
}
// Process loaded text
}.onError {
// Handle exception in UI thread
}
Unhandled exceptions and exception delivered in
onErrorwrapped by
AsyncExceptionwith convenient stack trace to the place where
awaitbeen called originally in UI thread
finally
execution
finallyalways executed after calling
onErroror when the coroutine finished successfully.
Kotlin
async {
// Show progress
await { }
}.onError {
// Handle exception
}.finally {
// Hide progress
}
Safe execution
The library has
Activity.asyncand
Fragment.asyncextension functions to produce more safe code. So when using
asyncinside Activity/Fragment, coroutine won't be resumed if
Activityis in finishing state or
Fragmentis detached.
Avoid memory leaks
Long running background code referencing any view/context may produce memory leaks. To avoid such memory leaks, call
async.cancelAll()when all running coroutines referencing current object should be interrupted, like
Kotlin
override fun onDestroy() {
super.onDestroy()
async.cancelAll()
}
The asyncis an extension property for
Anytype. So calling
[this.]async.cancelAllintrerrupts only coroutines started by
[this.]async {} function.
Common extensions
The library has a convenient API to work with Retrofit and rxJava.
Retorift
awaitSuccessful(retrofit2.Call)
Returns
Response.body()if successful, or throws
RetrofitHttpErrorwith error response otherwise.
Kotlin
async {
reposList = awaitSuccessful(github.listRepos(userName))
}
rxJava
- await(Observable)
Waits until
observableemits first value.
Kotlin
async {
val observable = Observable.just("O")
result = await(observable)
}
How to create custom extensions
You can create your own
awaitimplementations. Here is example of rxJava extension to give you idea. Just return the result of calling
AsyncController.awaitwith your own lambda implementation. The code inside
awaitblock will be run on a background thread.
Kotlin
suspend fun AsyncController.await(observable: Observable): V = this.await {
observable.toBlocking().first()
}
How it works
The library is built upon coroutines introduced in Kotlin 1.1.
The Kotlin compiler responsibility is to convert coroutine (everything inside
asyncblock) into a state machine, where every
awaitcall is a non-blocking suspension point. The library is responsible for thread handling, error handling and managing state machine. When background computation is done the library delivers result back into UI thread and resumes coroutine execution.