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Diffstat (limited to 'docs/topics/select-expression.md')
| -rw-r--r-- | docs/topics/select-expression.md | 69 |
1 files changed, 37 insertions, 32 deletions
diff --git a/docs/topics/select-expression.md b/docs/topics/select-expression.md index 082a50d6..3d20ff39 100644 --- a/docs/topics/select-expression.md +++ b/docs/topics/select-expression.md @@ -120,31 +120,32 @@ buzz -> 'Buzz!' ## Selecting on close The [onReceive][ReceiveChannel.onReceive] clause in `select` fails when the channel is closed causing the corresponding -`select` to throw an exception. We can use [onReceiveOrNull][onReceiveOrNull] clause to perform a +`select` to throw an exception. We can use [onReceiveCatching][ReceiveChannel.onReceiveCatching] clause to perform a specific action when the channel is closed. The following example also shows that `select` is an expression that returns the result of its selected clause: ```kotlin suspend fun selectAorB(a: ReceiveChannel<String>, b: ReceiveChannel<String>): String = select<String> { - a.onReceiveOrNull { value -> - if (value == null) - "Channel 'a' is closed" - else + a.onReceiveCatching { it -> + val value = it.getOrNull() + if (value != null) { "a -> '$value'" + } else { + "Channel 'a' is closed" + } } - b.onReceiveOrNull { value -> - if (value == null) - "Channel 'b' is closed" - else + b.onReceiveCatching { it -> + val value = it.getOrNull() + if (value != null) { "b -> '$value'" + } else { + "Channel 'b' is closed" + } } } ``` -Note that [onReceiveOrNull][onReceiveOrNull] is an extension function defined only -for channels with non-nullable elements so that there is no accidental confusion between a closed channel -and a null value. Let's use it with channel `a` that produces "Hello" string four times and channel `b` that produces "World" four times: @@ -158,17 +159,21 @@ import kotlinx.coroutines.selects.* suspend fun selectAorB(a: ReceiveChannel<String>, b: ReceiveChannel<String>): String = select<String> { - a.onReceiveOrNull { value -> - if (value == null) - "Channel 'a' is closed" - else + a.onReceiveCatching { it -> + val value = it.getOrNull() + if (value != null) { "a -> '$value'" + } else { + "Channel 'a' is closed" + } } - b.onReceiveOrNull { value -> - if (value == null) - "Channel 'b' is closed" - else + b.onReceiveCatching { it -> + val value = it.getOrNull() + if (value != null) { "b -> '$value'" + } else { + "Channel 'b' is closed" + } } } @@ -215,7 +220,7 @@ the first one among them gets selected. Here, both channels are constantly produ being the first clause in select, wins. However, because we are using unbuffered channel, the `a` gets suspended from time to time on its [send][SendChannel.send] invocation and gives a chance for `b` to send, too. -The second observation, is that [onReceiveOrNull][onReceiveOrNull] gets immediately selected when the +The second observation, is that [onReceiveCatching][ReceiveChannel.onReceiveCatching] gets immediately selected when the channel is already closed. ## Selecting to send @@ -375,19 +380,19 @@ Deferred 4 produced answer 'Waited for 128 ms' Let us write a channel producer function that consumes a channel of deferred string values, waits for each received deferred value, but only until the next deferred value comes over or the channel is closed. This example puts together -[onReceiveOrNull][onReceiveOrNull] and [onAwait][Deferred.onAwait] clauses in the same `select`: +[onReceiveCatching][ReceiveChannel.onReceiveCatching] and [onAwait][Deferred.onAwait] clauses in the same `select`: ```kotlin fun CoroutineScope.switchMapDeferreds(input: ReceiveChannel<Deferred<String>>) = produce<String> { var current = input.receive() // start with first received deferred value while (isActive) { // loop while not cancelled/closed val next = select<Deferred<String>?> { // return next deferred value from this select or null - input.onReceiveOrNull { update -> - update // replaces next value to wait + input.onReceiveCatching { update -> + update.getOrNull() } - current.onAwait { value -> + current.onAwait { value -> send(value) // send value that current deferred has produced - input.receiveOrNull() // and use the next deferred from the input channel + input.receiveCatching().getOrNull() // and use the next deferred from the input channel } } if (next == null) { @@ -423,12 +428,12 @@ fun CoroutineScope.switchMapDeferreds(input: ReceiveChannel<Deferred<String>>) = var current = input.receive() // start with first received deferred value while (isActive) { // loop while not cancelled/closed val next = select<Deferred<String>?> { // return next deferred value from this select or null - input.onReceiveOrNull { update -> - update // replaces next value to wait + input.onReceiveCatching { update -> + update.getOrNull() } - current.onAwait { value -> + current.onAwait { value -> send(value) // send value that current deferred has produced - input.receiveOrNull() // and use the next deferred from the input channel + input.receiveCatching().getOrNull() // and use the next deferred from the input channel } } if (next == null) { @@ -491,7 +496,7 @@ Channel was closed [ReceiveChannel.receive]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/-receive-channel/receive.html [ReceiveChannel.onReceive]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/-receive-channel/on-receive.html -[onReceiveOrNull]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/on-receive-or-null.html +[ReceiveChannel.onReceiveCatching]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/-receive-channel/on-receive-catching.html [SendChannel.send]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/-send-channel/send.html [SendChannel.onSend]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/-send-channel/on-send.html @@ -499,4 +504,4 @@ Channel was closed [select]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.selects/select.html -<!--- END -->
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