Created
April 14, 2023 18:49
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A cool way to do derivedState with LiveData
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import androidx.arch.core.executor.testing.InstantTaskExecutorRule | |
import androidx.lifecycle.LiveData | |
import androidx.lifecycle.MutableLiveData | |
import androidx.lifecycle.Observer | |
import org.junit.Assert.assertEquals | |
import org.junit.Rule | |
import org.junit.Test | |
//pattern inspired by https://github.com/ensody/ReactiveState-Kotlin, no code was copied. | |
class DeriveLiveDataTest { | |
@get:Rule | |
val rule = InstantTaskExecutorRule() | |
@Test | |
fun `test that derivedData works`() { | |
val x = MutableLiveData(0) | |
val y = MutableLiveData(0) | |
val list = mutableListOf<Int>() | |
val derived = derive { | |
val x = get(x) ?: 0 | |
val y = get(y) ?: 0 | |
x + y | |
} | |
derived.observeForever { | |
list.add(it) | |
} | |
x.value = 30 | |
assertEquals(30, derived.value) | |
x.value = 20 | |
assertEquals(20, derived.value) | |
x.value = 10 | |
assertEquals(10, derived.value) | |
y.value = 10 | |
assertEquals(20, derived.value) | |
x.value = 10 | |
assertEquals(20, derived.value) | |
assertEquals(listOf(0, 30, 20, 10, 20), list) | |
} | |
} | |
fun <T> derive(block: DeriveScope<T>.() -> T): LiveData<T> { | |
val ret = MutableLiveData<T>() | |
fun update(deriveScope: DeriveScope<T>) { | |
val result = deriveScope.block() | |
println(result) | |
ret.value = result | |
} | |
val d = DeriveScope { deriveScope -> | |
update(deriveScope) | |
} | |
update(d) | |
return ret | |
} | |
class DeriveScope<T>(val onDataChanged: (DeriveScope<T>) -> Unit) { | |
val liveDataToObserver = mutableMapOf<LiveData<in Nothing>, Observer<Any?>>() | |
fun <A : Any> get(data: LiveData<A>): A? { | |
val o = SkipFirstObserver<Any?> {//subscribing to a LiveData always emits immediately, even though nothing's changed. we don't want to listen to that | |
onChanged() | |
} | |
data.observeForever(o) | |
liveDataToObserver[data] = o | |
return data.value | |
} | |
fun onChanged() { | |
liveDataToObserver.forEach { (data, observer) -> | |
data.removeObserver(observer) | |
} | |
liveDataToObserver.clear() | |
onDataChanged(this) | |
} | |
} | |
class SkipFirstObserver<T>(val block: (T) -> Unit) : Observer<T>{ | |
var first = true | |
var currentValue: T? = null | |
override fun onChanged(newValue: T) { | |
if(first) { | |
first = false | |
currentValue = newValue | |
return | |
} else if (currentValue == newValue) { | |
return | |
} else{ | |
println("emitting $newValue") | |
currentValue = newValue | |
block(newValue) | |
} | |
} | |
} |
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Some LiveData does nothing without any Observer. Because of it, we need to add liveData.observeForever(errorsObservable) for tests in the testSetup() function