mapLatest(), at its core, works like map(): you give mapLatest() a lambda expression or other function type, and mapLatest() invokes that lambda for every upstream item and emits whatever the lambda returns.

map(), however, is designed for fast operations. That is not always your use case. For example, suppose that you have a search field, and searching involves a database query or a Web service call. You want to show search results as the user types, so you apply a debounce() operator to a Flow representing the user input. But the user might type faster than the database or Web service handles the search. In that case, once you get the updated search expression, you want to cancel any outstanding search operation and start a new one.

That is what mapLatest() is for.

The lambda expression can call suspend functions, such as for I/O. If the lambda expression evaluates to a value before the upstream Flow emits something new, whatever the lambda evaluates to is emitted downstream. However, if the upstream Flow does emit something while the lambda is still doing work, the coroutine invoking the lambda expression is cancelled, and the lambda is re-invoked with the new value.

In this sample, you will only see Result for 4 in the output. While the debounce(250) operator will emit values for both 3 and 4, 4 will be emitted before the mapLatest() lambda completes its delay(), so the 3 invocation of the lambda is cancelled. If you remove or comment out that delay(500) call, though, you will see Result for 3 and Result for 4.

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A Conflated Channel A Fixed-Buffer Channel A Parallel Coroutine Sample A Sequential Coroutine Sample A Simple Channel A Simple Coroutine Sample A Simple Flow A Simple SharedFlow A Simple StateFlow A Zero-Buffer Channel async() BroadcastChannel cancelAndJoin() Canceling a Job Canceling a Parent Cancels Its Children Canceling and Cooperation catch() Channel and receive() Channels and Fan-out combine() Operator Contexts and Jobs Cooperation by Checking isActive Cooperation by Yielding debounce() Operator distinctUntilChanged() Operator distinctUntilChangedBy() Operator drop() Operator dropWhile() Operator filter() Operator filterIsInstance() Operator filterNot() Operator filterNotNull() Operator first() and Flow flattenConcat() Operator flattenMerge() Operator Flow Actions Flow onStart() flowOf() flowOn() Flows and Exceptions fold() Operator Hot Channels Via offer() invokeOnCompletion() Jobs and Parents Jobs Cancel When They Crash join() Lazy Coroutines map() Operator mapLatest() Operator mapNotNull() Operator Massively Parallel Coroutines MutableSharedFlow Replay Option Non-Cancellable Coroutines Parent Jobs Cancel When a Child Crashes promise() retry() retryWhen() runBlocking() sample() Operator scan() Operator single() and singleOrNull() Starting Lazy Coroutines StateFlow and Content Equality suspend Functions and Flows Suspending main() take() Operator takeWhile() Operator The Channel() Function TimeoutCancellationException Timeouts toList() and toSet() toList() for Channel withIndex() Operator withTimeoutOrNull() zip() Operator
A Simple Flow A Simple SharedFlow A Simple StateFlow catch() combine() Operator debounce() Operator distinctUntilChanged() Operator distinctUntilChangedBy() Operator drop() Operator dropWhile() Operator filter() Operator filterIsInstance() Operator filterNot() Operator filterNotNull() Operator first() and Flow flattenConcat() Operator flattenMerge() Operator Flow Actions Flow onStart() flowOf() flowOn() Flows and Exceptions fold() Operator map() Operator mapLatest() Operator mapNotNull() Operator MutableSharedFlow Replay Option retry() retryWhen() sample() Operator scan() Operator single() and singleOrNull() StateFlow and Content Equality suspend Functions and Flows take() Operator takeWhile() Operator toList() and toSet() withIndex() Operator zip() Operator
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