Event loops (Flutter- Async Programming with Dart) — Part 2

Part of Flutter/Dart Shorts: To the point| Simple.

1. Prerequisites

• Read prev article on isolates.
• Basic understanding of threading.

2. Why is this important?

• Event loops lie at the base of all of the high level asynchronous programming API.

3. Isolates, Event loops, Queues and their relationship.

• App -> Isolate -> Event loop -> Event Queue and Microtask Queue

4. What is an event QUEUE?

• Actions/events/tasks that are queued and waiting to be executed.
• Example 1: User interaction with the app such as scrolling, button presses etc.
• Example 2: Messages between isolates.

5. What is Microtask QUEUE?

• This comes into play when we need to complete a task later and before returning the control to event loop.
• Warning: Use this for super short tasks only since it blocks the event loop.
• Example: When mutation changes are reported asynchronously, Microtask queue helps in reporting these changes before the next event is picked up by the loop.

6. What is an event Loop?

• The event loop job is to pick up items from the queues and execute them one at a time.

7. Okay, Wire-up everything you just said please?

• First items are picked up from Microtask Queue, executed on first come first serve basis, one at a time.
• Then the next item from Event Queue is picked up and executed, again in FIFO.
• Then the cycle repeats. Executes the items in the Microtask Queue and then dequeue next item from the event queue and execute it.

7. What happens when Queues are empty?

• The event loop just sits there, waiting for items to be added in the queues.

That’s all for this one. Next week — Futures :-)

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