DefaultDispatcherProvider

[RBusarow]

DefaultDispatcherProvider is currently a class which implements DispatcherProvider, just returning the corresponding values from the Dispatchers singleton.

The idea is to change DefaultDispatcherProvider into a singleton object, with atomic get and set functions.

In the library, every DispatcherProvider reference goes through this extension property:

public val CoroutineContext.dispatcherProvider: DispatcherProvider
  get() = get(DispatcherProvider) ?: DefaultDispatcherProvider()

Which can be converted to:

public val CoroutineContext.dispatcherProvider: DispatcherProvider
  get() = get(DispatcherProvider) ?: DefaultDispatcherProvider.get()

This allows every "miss" when referencing the DispatcherProvider to return a global singleton instance. In other words, developers don't need to inject or service-locate CoroutineScopes or CoroutineContexts in order to have control. All that's needed is to set the singleton on app launch, or to set/reset the instance in tests.

@Test
fun `my test`() = runBlocking {

  DefaultDispatcherProvider.set(TestDispatcherProvider())
  
  withMain {
    // this would normally crash without using Dispatchers.setMain
    // but "main" here comes from the TestDispatcherProvider created above -- not Dispatchers.Main
  }
  
  DefaultDispatcherProvider.reset() // from dispatch-test
} 

This would be a breaking change in that the DefaultDispatcherProvider type would be re-purposed, and would no longer implement DispatcherProvider. The existing DefaultDispatcherProvider constructor can be recreated as a companion object operator fun invoke() factory which is marked as deprecated, so it will continue to compile. This deprecated function will be removed before the 1.0 release.