Alternative to closure-style to execute code in UI thread

Question

From background thread, when UI needs to be updated, it needs to post to DispatchQueue.main using the async(execute:) function, as shown below:

static func executeInUIThread(_ uiThreadFunc: @escaping (Any?) -> Void, _ parms: Any?) {
  DispatchQueue.main.async {
    // Update UI 
    uiThreadFunc(parms)
  }
}

It is possible to access uiThreadFunc and parms inside the closure because closures capture variables from their 'surrounding context'.

But let's say, I don't like the lambda-style (called closures in swift) of programming. How can I do this without closures?

I attempted the following:

static func executeInUIThread(_ uiThreadFunc: @escaping (Any?) -> Void, _ parms: Any?) {
  let workItem = DispatchWorkItem(block: EventLoopMgr.InternalExecuteInUIThread)

  DispatchQueue.main.async(execute: workItem)
}

private static func InternalExecuteInUIThread() {
  // How to execute the uiThreadfunc? This block doesn't take any parameters.
}

It doesn't work because, the block when initialising the DispatchWorkItem takes no parameters. Therefore, I can't pass uiThreadFunc and parms to this block.

I can store the uiThreadFunc and parms as static variables, but then it needs to be made multi-thread friendly.

Is there an easier way to execute in UIThread using DispatchQueue.main but without using closures?

Answer 1

In the project I'm working on, it's a convention not to have anonymous blocks of code in the middle of the function. It doesn't depict the natural flow of execution

Okay, so you can make this not anonymous:

// I made this generic for you :)
func executeInUIThread<T>(_ uiThreadFunc: @escaping (T) -> Void, _ parms: T) {
    func helper() {
        uiThreadFunc(parms)
    }
    DispatchQueue.main.async(execute: helper)
}

Here I gave the name helper to the block of code that will be run on the main queue.

Technically, a local function still counts as a closure according to the Swift Guide, but if all you want to avoid is an "anonymous" block of code, using a local function definitely achieves that.

If the local function is too much noise, you can move it out of executeInUIThread like this:

func partiallyApply<T>(_ arg: T, to function: (T) -> Void) -> () -> Void {
    func helper() { function(arg) }
    return helper
}

Then you can just do:

DispatchQueue.main.async(execute: partiallyApply(parms, to: uiThreadFunc))

which arguably is more readable. You

Answer 2

Will this be helpful for you?

test to trigger it and updateView is one example of your uiThreadFunc

func test() {
    let aFunc = #selector(updateView)
    let success = executeInUIThread(caller: self, uiThreadFunc: aFunc, "xxx")
}

func executeInUIThread(caller: NSObject, uiThreadFunc: Selector, _ parms: Any?) -> Bool {
    if caller.responds(to: uiThreadFunc) {
        caller.performSelector(onMainThread: uiThreadFunc, with: parms, waitUntilDone: false)
        return true
    } else {
        return false
    }
}

@objc func updateView(param: String) { }

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For more Info, please have a check GCD vs performSelector