Swift - How to wait for something without making the app hanging

Question

I have a code like this:

    print("Migration Execution: Successfully uninstalled MCAfee")
    migrationInfoPicture.image = NSImage(named: "Unroll")
    migrationInfoText.stringValue = NSLocalizedString("Unrolling from old server... Please wait!", comment: "Unrolling")
    while(!readFile(path:logfilePath)!.contains("result: 2 OK")) {
        searchLogForError(scriptPath: scriptOnePath)
    }
    print("Migration Execution: Successfully unrolled from old server")
    migrationInfoText.stringValue = NSLocalizedString("Setting up MDM profile... Please wait!", comment: "Setting up MDM")
    while(!readFile(path:logfilePath)!.contains("result: 3 OK")) {
        searchLogForError(scriptPath: scriptOnePath)
    }

It actually works in the background, reading from the file works and logging works but since the GUI will be hanging executing a while loop with a quickly completed task, the image and the text changes will not be visible.

Code for searchForLogError is:

func searchLogForError(scriptPath:String) {
    if((readFile(path:logfilePath)!.filter { $0.contains("ERROR") }).contains("ERROR")) {
        print("Migration abborted")
        migrationInfoPicture.image = NSImage(named: "FatalError")
        migrationInfoText.stringValue = NSLocalizedString("An error occured: \n", comment: "Error occurence") + readFile(path:logfilePath)!.filter { $0.contains("ERROR") }[0]
        migrationWarningText.stringValue = NSLocalizedString("In order to get further help, please contact: [email protected]", comment: "Error support information")
        self.view.window?.level = .normal
        btnExitApplicationOutlet.isHidden = false
        getScriptProcess(path:scriptPath).terminate()
        return
    }
}

How can I achieve a visible change of NSImage and NSLocalizedString while constantly looking for log file change without a hanging GUI (or even with a hanging GUI, but with enough time to change the visible elements between the while-loops)?

Answer 1

Polling file system resources is a horrible practice. Don't do that. There are dedicated APIs to observe file system resources for example DispatchSourceFileSystemObject

Create a property

var fileSystemObject : DispatchSourceFileSystemObject?

and two methods to start and stop the observer. In the closure of setEventHandler insert the code to read the file

func startObserver(at url: URL)
{
    if fileSystemObject != nil { return }

    let fileDescriptor : CInt = open(url.path, O_EVTONLY);
    if fileDescriptor < 0  {
        print("Could not open file descriptor"))
        return
    }
    fileSystemObject = DispatchSource.makeFileSystemObjectSource(fileDescriptor: fileDescriptor, eventMask: [.write, .rename], queue: .global())

    if fileSystemObject == nil {
        close(fileDescriptor)
        print"Could not create Dispatch Source"))
        return
    }
    fileSystemObject!.setEventHandler {
        if self.fileSystemObject!.mask.contains(.write) {
           // the file has been modified, do something

        }
    }
    fileSystemObject!.setCancelHandler {
        close(fileDescriptor)
    }
    fileSystemObject!.resume()
}

func stopObserver()
{
   fileSystemObject?.cancel()
   fileSystemObject = nil
}