Android LiveData: Handle UI Event With Event Wrapper (SingleLiveEvent Replacement)
August 19, 2019Why not to use LiveData to handle UI event? Because LiveData will trigger again upon configuation change / screen rotation. We want to handle UI event such as click once only.
The above problem could be solved using SingleLiveEvent.
Alternatively, you can use Event wrapper.
open class Event<out T>(private val value: T) {
private val pending = AtomicBoolean(true)
fun getIfPending(): T? {
return if (pending.compareAndSet(true, false)) {
value
}
else {
null
}
}
fun peek(): T = value
fun isPending(): Boolean = pending.get()
}
NOTE: Based on Event by JoseAlcerreca
NOTE: Solution for multiple observers (I didn’t try this)
Usage
val selectItemEvent = MutableLiveData<Event<Int>>()
// trigger ui event on item selected
selectItemEvent.value = Event(10)
selectItemEvent.observe(lifecycleOwner, Observer { event ->
val position = event.getIfPending()
if (position != null) {
// do something
}
})
To reduce boilerplate code, you can use an EventObserver
.
class EventObserver<T>(private val observe: (T) -> Unit) : Observer<Event<T>> {
override fun onChanged(event: Event<T>?) {
event?.getIfPending()?.let { value ->
observe(value)
}
}
}
NOTE: Based on EventObserver by JoseAlcerreca
selectItemEvent.observe(lifecycleOwner, EventObserver { position ->
// do something
})
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