ThreadContextElement

interface ThreadContextElement<S> : CoroutineContext.Element

Defines elements in CoroutineContext that are installed into thread context every time the coroutine with this element in the context is resumed on a thread.

Implementations of this interface define a type S of the thread-local state that they need to store on resume of a coroutine and restore later on suspend. The infrastructure provides the corresponding storage.

Example usage looks like this:

// Appends "name" of a coroutine to a current thread name when coroutine is executed
class CoroutineName(val name: String) : ThreadContextElement<String> {
// declare companion object for a key of this element in coroutine context
companion object Key : CoroutineContext.Key<CoroutineName>

// provide the key of the corresponding context element
override val key: CoroutineContext.Key<CoroutineName>
get() = Key

// this is invoked before coroutine is resumed on current thread
override fun updateThreadContext(context: CoroutineContext): String {
val previousName = Thread.currentThread().name
Thread.currentThread().name = "$previousName # $name"
return previousName
}

// this is invoked after coroutine has suspended on current thread
override fun restoreThreadContext(context: CoroutineContext, oldState: String) {
Thread.currentThread().name = oldState
}
}

// Usage
launch(Dispatchers.Main + CoroutineName("Progress bar coroutine")) { ... }

Every time this coroutine is resumed on a thread, UI thread name is updated to "UI thread original name # Progress bar coroutine" and the thread name is restored to the original one when this coroutine suspends.

To use ThreadLocal variable within the coroutine use ThreadLocal.asContextElement function.

Functions

restoreThreadContext
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abstract fun restoreThreadContext(context: CoroutineContext, oldState: S)
Restores context of the current thread.
updateThreadContext
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abstract fun updateThreadContext(context: CoroutineContext): S
Updates context of the current thread.

Sources

jvm source
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