Schedulers

What are Schedulers?

A Scheduler in Combine determines where and when work happens.
It controls which thread code runs on, ensuring smooth performance and responsiveness.

Think of it like a delivery system:

• A publisher prepares the package (data).
• The scheduler decides the best route (thread) for delivery.
• The subscriber receives the package on the expected thread (UI/main thread, background, etc.).

Schedulers manage threading and execution timing for Combine operations.

🔗 Official Docs: Apple Documentation

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How Schedulers Work

• Determines which thread work runs on (e.g., UI updates on the main thread, background tasks elsewhere).
• Used to switch between threads for performance optimization.
• Ensures smooth UI updates by keeping heavy work off the main thread.

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Example: Running a Task on a Background Thread

import Combine
 
let publisher = Just("Processing Data")
    .subscribe(on: DispatchQueue.global(qos: .background)) // Work starts in the background
    .receive(on: DispatchQueue.main) // Results are delivered on the main thread
 
let cancellable = publisher
    .sink { print("📢 Received on main thread: \($0)") }
 

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Common Schedulers

• DispatchQueue.main → Runs on the main UI thread (for UI updates).
• DispatchQueue.global(qos: .background) → Runs in the background (for heavy tasks).
• RunLoop.main → For timing-sensitive operations (e.g., animations).
• OperationQueue → Uses a queue of worker threads.
• immediateScheduler → Runs work immediately (mostly for debugging).

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Key Takeaways

• Schedulers control where Combine operations run.
• Use .subscribe(on:)to specify where work should start.
• Use .receive(on:)to specify where results should be handled.
• Helps keep UI responsive by running heavy work in the background.