Skip to main content

Keeping app alive in Background State

By
Smart phones reinforces multitasking possible with all extends. In such cases the app enters to non-foreground (background & suspended) state from various user interactions such as pressing home button, switching apps or even when it could be simply pressing from power button (or Even it could be from System events). These events can cause a suspended app to be returned to the background, or cause a not running app to be launched directly into the background. System events can cause a suspended app to be returned to the background, or cause a not running app to be launched directly into the background.

Why I am stressing these points ? 

In my recent app I introduced a new feature where I need to:
  • Show a timer that ticks every seconds.
  • A sound file that plays on each seconds.
  • A local notification has to fire on completion of every timer.
All the above has to be run in background state without interrupting timer progress bars when the user switches back to app. The existing logic of UI elements, animations, time triggers are tightly coupled with UI layer. Uhh.. !!

Stepping into solution : 

At first I thought - cache timer values in user defaults when applicationDidEnterBackground(_:) notification fired from UIApplication. Later resume when ‘willEnterForgroundState’ called. Since it was not feasible & gives boiler plate code for resuming UI states and also timer tick sound & notification timings will not be in sync . More over ‘applicationDidEnterBackground(_:)’ method gives at most 5 seconds to perform my tasks and return.

A few moment later after some SO searches, I extended app’s runtime by calling the beginBackgroundTask(withName:expirationHandler:) method. Calling this method gives you extra time to perform important tasks. (We can find out how much time remains using the backgroundTimeRemainingproperty.) When you finish your tasks, call the endBackgroundTask(_:) method right away to let the system know that you are done. If you do not end your tasks in a timely manner, the system terminates your app. 
Along with Apple’s Background mode enabled from app capabilities:

✔️Enabled mode - Audio, AirPlay and Picture in Picture.
Apps may enable multiple background modes, and be launched in response to events for any of them.

Keeping all the above in mind, I wrote below pseudo code that helped me to execute background tasks in a simple block. Entire timer logic was under the same block & along GCD main thread block for UI updates when user switches back to foreground.

 
class BackgroundExecutable {
    var identifier: UIBackgroundTaskIdentifier
    let function: (() -> Void) -> Void
 
    init(function: @escaping (_ completion: () -> Void) -> Void) {
        self.function = function
        self.identifier = UIBackgroundTaskInvalid
    }
 
    func execute() {
        let application = UIApplication.shared
        identifier = application.beginBackgroundTask {
            application.endBackgroundTask(self.identifier)
        }
        function(endBackgroundTask)
    }
 
    func endBackgroundTask() {
        UIApplication.shared.endBackgroundTask(identifier)
    }
}
All I need to write a block of code that will handle my timer logic. 

 let bgTask = BackgroundExecutable { (f) in         
         //Timer Logic goes here...
        DispatchQueue.main.async { [unowned self] in
             //Do UI updates here when app in foreground
         }           
   }
   bgTask.execute()
  // when the timer completed in some point of app.
   bgTask.endBackgroundTask()
Labels:

Popular posts from this blog

Animating label text update - choosing a better way

By
Recently I published a countdown app .  At one point of development - I have to show a timer on a UILabel which ticks on each seconds. As usual I started  setting text to a label object - self .timerLabel.text = someString Easy piece of cake right !?   But wait ... it won't take much user attention when timer ticks on every seconds. So I decided to make use of a simple animation while label gets text update. I found there are dozens of ways to animate a label. In this short article, I listed 3 best way you can animate text on a label. ( Spoiler Alert 👀- I decided to go with 3rd option)  1. Fade In - Fade out animation : CATransition class has got transition type `fade`. With timing function of CATransition - I was able to see the below result. let animation: CATransition = CATransition () animation.timingFunction = CAMediaTimingFunction (name: CAMediaTimingFunctionName .easeInEaseOut) animation.type = CATransitionType .fade animation.subtype = CATransitionSubtype .
Read more

Prevent Navigationbar or Tabbar overlapping Subview - solved for Card view

By
Recently, I started with a Card view added as a subview of UIView in a view-controller. When a view controller created along subviews, it tends to use entire screen bounds and also slips behind Tab bar or Navigation bar. In my current situation, it's second case. Casually new iOS developers will write a patch by additional value for coordinate y and subtracting bar height from its size. A lot of them posted in SO threads too : How to prevent UINavigationBar from covering top of view? View got hidden below UINavigationBar iOS 7 Navigation Bar covers some part of view at Top So, how I got solved ? self.edgesForExtendedLayout = [] This  will avoid all subviews in a view controller get behind any bars. Read full apple  documentation on here. Full Source code below :  //Simple view controller where its view layed-out as a card. class WidgetCardViewController : UIViewController { var containerView = UIView () //MARK:- View Controller Life Cycle ov
Read more

UICollectionViewCell shows with wrong size on First time - Solved

By
We commonly use Collection view where its cell size calculated run time. The flow layout delegate is responsible to return individual cell sizes. BUT in most of the cases, delegate method `collectionView: layout sizeForItem:` expects cell size too early. Before generating actual cell size. extension YourViewController : UICollectionViewDelegateFlowLayout { func collectionView ( _ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize { return CGSize (width: externalWidth, height: externalHeight) } } For instance, if a cell size depends on external view and its frame is not yet ready - results with wrong (or outdated) cell size. Typically happens for the first time view controller laid out all views. You can find similar queries in StackOverflow community : Collection view sizeForItemNotWorking UICollectionViewCell content wrong size on first load How to refresh UICollection
Read more

Implementing autocompletion OTP field in iOS

By
Long waiting is over. !!  iOS 12 brings Autofill for OTP text field which is close to Android provided a decade back. Previously in iOS we used to toggle between OTP text screen and message inbox.  Which was hard to remember and time consuming resulting a bad user experience. Personally, I have been asked from the client/customer couple of times to implement autocompletion for OTP field and took me a lot of time to convey that it is not possible in iOS. Why Autofill was not possible previously?  We all know that Apple gives at most care for user privacy. When we see iOS architecture, each individual app is like a separate island. There is no inter-app bridge between apps (exception for Keychain and URLSchemes APIs which gives very limited scope). Thus we cannot read message content from inbox. Where to start Autofilling? First of all, the target SMS need to have the OTP Code with prefix string "Code" or "Passcode"on its message content. Beware of OTP c
Read more

Printing Staircase Pattern : Swift coding challenge

By
In this post, we can try interesting pattern printing challenge in Swift. The problem statement goes like this " Print a staircase of given size 'n'. Make sure that its base and height are both equal to n, and the image is drawn only using `#` symbols and spaces. The last line is not preceded by any spaces." Expected Output : # ## ### #### ##### ###### Working solution: func makePatternOf ( _ size : Int ) { var str = "" // 1 for index in ( 0 ..< size ) { let stop = size -index-1; // 2 for _ in 0 ..< stop { str. append ( " " ) ; } // 3 for _ in 0 .. . index { str. append ( "#" ) ; } print ( str ) str = "" } } makePatternOf ( 6 ) Loop to visit every row of stair case. Loop for appe
Read more