Does forEach Run Asynchronously?

Using Swift
1

My app got a memory issue. The code was like

func foo() {
  var bar = [String:[String]]()
  ....
  #if DEBUG
  bar.keys.sorted().forEach { key in
    print(key, "\t", bar[key]!)
  }
  #endif

Since there was a memory issue, I decided to move the bar to be a class instance variable and free it every time.

class some {
  lazy private var bar = [String:[String]]()

  func foo() {
    defer {
      bar..removeAll(keepingCapacity: true)
    }  
    ....
    #if DEBUG
    bar.keys.sorted().forEach { key in
      print(key, "\t", bar[key]!)
    }
    #endif

However, the second code crashed immediately after the app ran on print(key, "\t", bar[key]!).

The reason of the crash was because the bar was already empty, which was not expected by me. So the forEach must be ran asyncronously. And the removeAll ran first.

I could fix the app by changing forEach to for-loop, but I wanted to make sure my thinking was right or not.

2

Have you tried placing a breakpoint inside the defer and inside the forEach and seeing which gets triggered first? I would be very surprised if forEach ran async.

3

It turned out more complex than my think. The issue was not on the forEach, as for loop also crashed.

The issue was because I used 2 Tasks in my function, so the app was not run in the order as I wanted. As I didn't know Task was async.

I changed the code and tried to use less and simple Tasks. Everything went fine now.