Coroutine chaining in Unity - Step-by-Step Execution

Concept Flow - Coroutine chaining

Start First Coroutine

↓

Yield Until First Coroutine Completes

↓

Start Second Coroutine

↓

Yield Until Second Coroutine Completes

↓

Done

Start one coroutine, wait for it to finish, then start the next coroutine, chaining them in sequence.

Execution Sample

Unity

IEnumerator First() {
  Debug.Log("First start");
  yield return new WaitForSeconds(1);
  Debug.Log("First end");
}

IEnumerator Second() {
  Debug.Log("Second start");
  yield return new WaitForSeconds(1);
  Debug.Log("Second end");
}

IEnumerator Chain() {
  yield return StartCoroutine(First());
  yield return StartCoroutine(Second());
  Debug.Log("Chain complete");
}

Runs First coroutine, waits 1 second, then runs Second coroutine, waits 1 second, then prints completion.

Execution Table

Step	Action	Coroutine Running	Yield/Wait	Output
1	Start Chain coroutine	Chain	Start First coroutine	No output
2	Start First coroutine	First	WaitForSeconds(1)	Print: First start
3	Wait 1 second	First	Waiting	No output
4	First coroutine resumes	First	Complete	Print: First end
5	Return to Chain coroutine	Chain	Start Second coroutine	No output
6	Start Second coroutine	Second	WaitForSeconds(1)	Print: Second start
7	Wait 1 second	Second	Waiting	No output
8	Second coroutine resumes	Second	Complete	Print: Second end
9	Return to Chain coroutine	Chain	Complete	Print: Chain complete
10	Chain coroutine ends	None	None	No output

💡 Chain coroutine finishes after Second coroutine completes and prints final message.

Variable Tracker

Coroutine	Start	After Step 2	After Step 4	After Step 6	After Step 8	Final
Chain	Running	Waiting for First	Waiting for Second	Waiting for Second	Completed	Completed
First	Not started	Running	Completed	Completed	Completed	Completed
Second	Not started	Not started	Not started	Running	Completed	Completed

Key Moments - 3 Insights

Why does the Chain coroutine wait at 'yield return StartCoroutine(First())'?

Because 'yield return StartCoroutine(First())' pauses Chain until First finishes, as shown in execution_table rows 2 to 4.

What happens if we don't yield return the StartCoroutine call?

Chain would not wait for the coroutine to finish and would start the next immediately, losing the chaining effect (not shown in this trace).

Why do we see 'Chain complete' only after Second coroutine ends?

Because Chain waits for Second to finish before continuing, as shown in execution_table rows 7 to 9.

Visual Quiz - 3 Questions

Look at the execution_table, what coroutine is running at Step 6?

A First coroutine

B Second coroutine

C Chain coroutine

D No coroutine running

💡 Hint

Check the 'Coroutine Running' column at Step 6 in the execution_table.

At which step does the Chain coroutine resume after First coroutine completes?

A Step 3

B Step 4

C Step 5

D Step 7

💡 Hint

Look for when Chain starts Second coroutine after First ends in the execution_table.

If we remove 'yield return' before StartCoroutine(First()), what changes in variable_tracker?

A Second coroutine would start before First ends

B Chain coroutine would be 'Running' throughout without waiting

C First coroutine would never start

D Chain coroutine would end immediately

💡 Hint

Think about how 'yield return' controls waiting shown in variable_tracker and execution_table.

Concept Snapshot

Coroutine chaining in Unity:
Use 'yield return StartCoroutine(OtherCoroutine())' to run one coroutine after another.
The calling coroutine pauses until the called coroutine finishes.
This creates a sequence of asynchronous steps.
Without 'yield return', coroutines run concurrently, not sequentially.

Full Transcript

This example shows how coroutine chaining works in Unity. The Chain coroutine starts and yields to First coroutine. First coroutine runs, waits 1 second, then ends. Chain waits until First finishes before starting Second coroutine. Second coroutine runs, waits 1 second, then ends. Finally, Chain prints 'Chain complete' and ends. The execution table tracks each step, showing which coroutine runs and what output is printed. Variable tracker shows coroutine states changing from running to waiting to completed. Key moments clarify why 'yield return' is needed to pause and wait for coroutines. The visual quiz tests understanding of coroutine states and chaining behavior.