StateFlow for observable state in Kotlin - Step-by-Step Execution

Concept Flow - StateFlow for observable state

Create MutableStateFlow

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Emit new value

↓

StateFlow holds latest value

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Collector observes value changes

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Collector reacts to updates

↩Back to Emit new value

StateFlow holds a value that can change over time. When the value changes, collectors see the new value and react.

Execution Sample

Kotlin

val state = MutableStateFlow(0)

state.value = 1

runBlocking {
    state.collect { println(it) }
}

This code creates a StateFlow with initial value 0, updates it to 1, then collects and prints values.

Execution Table

Step	Action	StateFlow Value	Collector Output
1	Create MutableStateFlow with 0	0
2	Update state.value to 1	1
3	Collector starts collecting	1	1
4	No more updates	1	No new output

💡 No more updates to state.value, collection ends or waits for new values

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
state.value	0	1	1	1

Key Moments - 2 Insights

Why does the collector print '1' immediately when it starts?

Because StateFlow always holds the latest value (1 at step 2), the collector receives and prints it right away (step 3).

What happens if we update state.value again after the collector starts?

The collector will receive the new value and print it, showing StateFlow emits updates to active collectors.

Visual Quiz - 3 Questions

Look at the execution table, what is the StateFlow value at Step 2?

A 1

B null

C 0

D No value yet

💡 Hint

Check the 'StateFlow Value' column at Step 2 in the execution_table

At which step does the collector first output a value?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Collector Output' column in the execution_table

If we set state.value = 2 after Step 3, what will the collector output next?

A It will output 1 again

B It will output 2

C No output because collector stopped

D It will output 0

💡 Hint

StateFlow emits new values to collectors as shown in the key_moments explanation

Concept Snapshot

StateFlow holds a current value and emits updates.
Create with MutableStateFlow(initialValue).
Update with state.value = newValue.
Collectors receive latest and future values.
Useful for observable state in Kotlin coroutines.

Full Transcript

StateFlow is a Kotlin tool to hold and observe a value that changes over time. We create a MutableStateFlow with an initial value. When we update its value, StateFlow keeps the latest value. Collectors watch StateFlow and get the current value immediately when they start, then get notified of any changes. This makes StateFlow great for observable state in apps. In the example, we start with 0, update to 1, then collect and print the value 1. If we update again, collectors will see the new value too.