When to Semaphore vs Dispatch Group! Careful Multithreaded Shared Resource

Shared resources are often used across many different tasks in a multithreaded iOS application. If you want to make sure that a share resource is not accessed at the same time, you can guarantee the behavior through a Semaphore. However if your tasks are independent of one another, then using a Dispatch Group is probably the way to go.

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[TheJhuerta]

Great video. I like that you're posting more advanced concepts in your channel.

[romanromanenko]

Hey Brain, thanks for the video! You do good job and help people to learn iOS! I have one comment at 9:23 - it's not confusing, it's because you've set DispatchSemafore value to 0. The Semafore value represents how many treads can be executed at one point of time. When you call semaphore.wait(), you decreased value to -1. If semaphore's value is negative integer it is block execution of task, and system will wait when you release tread/treads which are in an awaiting state. What you did by replacing the lines of code, is at first you increased semaphore value to 1 (which is allow to run tasks on 1 tread at one point of time ), then fetchImage() was executing, then you decreased semaphore value back to 0 by calling semaphore.wait(). 

So, if you set DispatchSemafore value to 1 and return the order of lines of code, when you call at first semaphore.wait() and after fetchImage() semaphore.signal() - it will work.

[luizfad]

Hi Brian, I can't count the number of times I clarified some concept by watching your videos. Thank you so much for all the work you're doing!

[gushiciaihaozhe]

Thanks Brian. Please continue videos like this for advanced stuff. Now too many resources only for beginners.

[masonictemple]

Thank you so much Brian! This video was incredible, I was able to use it to improve the user experience in our app when loading multiple images into a slideshow!

[neklasoctan]

Very detail and clear, Im 3 years iOS developer btw. Thank for give me more detail about this.

[onurisk]

Advanced topics, interesting problems and great solutions. Thank you Brain

[GoingWalkabout]

Extremely useful, especially when I was struggling with 2 async fetches that need to complete before continuing.

[srikanthburra]

Helpful :) Expecting to share a single video which covers all advanced concepts

[MrBlaq]

Props. I appreciate the advanced tutorials.

[ragnacode]

Thank you, Brian! Nice video and good explanation of what is going on. Can you please make more videos about multithreading?

[leopapilaya]

Nice! Say, in a home page, if I want to display data section from top to bottom orderly, semaphore is the only way, right?

[philipplamenov]

You are awesome. Many thanks from Bulgaria :)

[alexfrank]

10:31 that's a horrible concurrency code. At that point, you should just create a serial queue...

[mohanraj]

Hi I am a very big fan of your tutorial, In semaphore how to handle error cases, for example, consider there are 3 steps [API Call]
if step 1 got error then step 2 and step 3 should not run. How to do this?

[hasanirshad]

Hello Brian!
Why did u set value 0?
What if I set value 1?

[wassmd]

For shared resource example dispatchGroup.wait() can be used

[bezeltipis]

Is this semaphores technic similar to async await concept?

[comedycentral]

I have a question what will happen if we call the above code inside any completion handler in swift instead of viewDidLoad?

[pass]

The whole async for semaphores is in one block. Of course it will be executed in order, no semaphore would be needed there. Am I missing something?