What should I have to return at kotlin Flow first function?

Question 1

I am using first function from kotlin flow. The reason why I'm using this first function is that I don't have to collect after the first time. If I don't return any boolean value, it makes red underline that I have to return a boolean value. What should I return? There aren't any problem when I just return true, but I want to know what it means.

    private fun getGroupNameData() {
        viewModelScope.launch {
            repository.loadGroupsWithFlow()
                .buffer()
                .first { newList ->
                    groupData.clear()
                    newList.forEach { newGroupData ->
                        groupData[newGroupData.id] = newGroupData.name
                    }
                    true // <- what is this boolean value?
                }
        }
    }

first Code.

/**
 * The terminal operator that returns the first element emitted by the flow matching the given [predicate] and then cancels flow's collection.
 * Throws [NoSuchElementException] if the flow has not contained elements matching the [predicate].
 */
public suspend fun <T> Flow<T>.first(predicate: suspend (T) -> Boolean): T {
    var result: Any? = NULL
    collectWhile {
        if (predicate(it)) {
            result = it
            false
        } else {
            true
        }
    }
    if (result === NULL) throw NoSuchElementException("Expected at least one element matching the predicate $predicate")
    return result as T
}

Question 2

This overload of Flow.first() is used to get the first value of flow which matches the given predicate. That's why the lambda expects you to return a boolean at the end. For whichever value the lambda returns true, that value will be returned and flow will be cancelled.

If you only need the first value, you should the other overload which doesn't accept a predicate lambda.

val newList = repository.loadGroupsWithFlow().buffer().first() // Use this first()
groupData.clear()
newList.forEach { newGroupData ->
    groupData[newGroupData.id] = newGroupData.name
}

Btw I don't think buffer is required. You can remove that.

Arpit Shukla · Answer 1 · 2021-11-24T06:26:16

This overload of Flow.first() is used to get the first value of flow which matches the given predicate. That's why the lambda expects you to return a boolean at the end. For whichever value the lambda returns true, that value will be returned and flow will be cancelled.

If you only need the first value, you should the other overload which doesn't accept a predicate lambda.

val newList = repository.loadGroupsWithFlow().buffer().first() // Use this first()
groupData.clear()
newList.forEach { newGroupData ->
    groupData[newGroupData.id] = newGroupData.name
}

Btw I don't think buffer is required. You can remove that.

Thank you, Could you explain why the buffer is not required?
You can take a look at its documentation. In your case you only care about the first value emitted by the flow so you need not buffer anything.
Oh I understand. As I only need the first value, I don't need the buffer. Thank you!

What should I have to return at kotlin Flow first function?

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