FlowOn for changing dispatcher in Kotlin - Step-by-Step Execution

Concept Flow - FlowOn for changing dispatcher

Start Flow

↓

Emit values on IO Dispatcher

↓

Apply flowOn(Dispatchers.IO)

↓

Switch context to IO Dispatcher

↓

Collect values on Default Dispatcher

↓

End Flow

flowOn switches the context to the IO dispatcher for upstream operations like emitting values, while collection happens on the Default dispatcher.

Execution Sample

Kotlin

flow {
    emit(1)
    emit(2)
}.flowOn(Dispatchers.IO).collect { value ->
    println("Collected $value on ${Thread.currentThread().name}")
}

This code emits two values in a flow on the IO dispatcher via flowOn, then collects and prints each value with the current thread name on the Default dispatcher.

Execution Table

Step	Action	Dispatcher/Thread	Value Emitted/Collected	Output
1	Start flow builder	Default dispatcher	-	-
2	Emit value 1	IO dispatcher	1	-
3	Switch context with flowOn	IO dispatcher	-	-
4	Collect value 1	Default dispatcher	1	Collected 1 on Default-thread
5	Emit value 2	IO dispatcher	2	-
6	Collect value 2	Default dispatcher	2	Collected 2 on Default-thread
7	Flow completes	Default dispatcher	-	-

💡 Flow completes after all values are emitted and collected with context switched by flowOn.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
value	-	1	1	2	2
dispatcher	Default	IO	Default	Default	Default

Key Moments - 2 Insights

Why does the collection happen on the default dispatcher and not the IO dispatcher?

Because flowOn changes the upstream context to IO dispatcher, so collection runs on the original Default dispatcher as shown in steps 4 and 6 of the execution_table.

Does flowOn change the dispatcher for the emit calls?

Yes, emit calls happen on the IO dispatcher as seen in steps 2 and 5; flowOn changes the upstream dispatcher for emit calls, while downstream operations like collect run on original.

Visual Quiz - 3 Questions

Look at the execution_table, on which dispatcher is the value 1 collected?

A Main dispatcher

B Default dispatcher

C IO dispatcher

D Unconfined dispatcher

💡 Hint

Check step 4 in the execution_table where collection happens on Default dispatcher.

At which step does the flow switch context to the IO dispatcher?

A Step 3

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Action' column in execution_table for flowOn context switch.

If flowOn was removed, where would collection happen?

A On Main dispatcher

B On IO dispatcher

C On Default dispatcher

D On Unconfined dispatcher

💡 Hint

Without flowOn, collection stays on the original dispatcher as shown by variable 'dispatcher' in variable_tracker.

Concept Snapshot

flowOn changes the dispatcher for upstream flow operations.
Emit runs on dispatcher set by flowOn.
Collect runs on original dispatcher.
Useful to switch context for heavy or IO tasks.
Syntax: flow.flowOn(Dispatchers.IO).collect { ... }

Full Transcript

This example shows how Kotlin Flow emits values on the IO dispatcher. When flowOn is applied with Dispatchers.IO, the upstream operations like emit run on the IO dispatcher. The execution table traces each step: starting the flow, emitting values on the IO dispatcher, switching context with flowOn, and collecting values on the Default dispatcher. Variables track the current value and dispatcher context. Key moments clarify that flowOn affects only upstream operations like emit calls, not downstream collect. The visual quiz tests understanding of dispatcher switching and flowOn behavior. The concept snapshot summarizes that flowOn changes the dispatcher for upstream flow operations, allowing context switching for tasks like IO.