Timer on table view Cell - swift

Question

there are 5 cells in table view, each have countdown on it, but countdown running fast, it decreases more than 1 second in one call.
this is my table view cell class, I have created a labelSatus here
class ActivityCell: UITableViewCell {
#IBOutlet weak var labelStatus: UILabel!
//Variable Declaration
var timer: Timer?
var totalTime:Double = 0
override func awakeFromNib() {
super.awakeFromNib()
}
override func setSelected(_ selected: Bool, animated: Bool) {
super.setSelected(selected, animated: animated)
}
func designCell(data:ActivityModal, removeTimer:Bool){
if removeTimer {
if let timer = self.timer {
timer.invalidate()
self.timer = nil
}
}
else{
let timeStampLimit = (data.date)/1000 + 86400 //for 24 hours
let currentTimeStamp = NSDate().timeIntervalSince1970
if timeStampLimit > currentTimeStamp{
self.totalTime = timeStampLimit - currentTimeStamp
self.timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(updateTimer), userInfo: nil, repeats: true)
}
}
}
#objc func updateTimer() {
self.labelStatus.text = String.getString(CommonUtils.convertTimeStampToHour(unixtimeInterval: self.totalTime, dateFormat:"HH:mm:ss"))
if self.totalTime != 0 {
self.totalTime -= 1 // decrease counter timer
} else {
if let timer = self.timer {
timer.invalidate()
self.timer = nil
}
}
}
}

Answer 1

There are a few issues to take care of when working with timers on table view cells. Note that the UITableView reuses the cell objects, so after a cell disappears from the screen after scrolling, it will be reused to be displayed in another place inside the table view.
Supposing you call designCell() in tableView:(_:cellForRowAt:) method, you might be creating more timers for a single cell (i.e. a lot of timers which will trigger the same cell's updateTimer() method). Note that a Timer will not be deallocated after you drop all your references to it, if the timer is still valid. At this point, you might be creating a new timer for a specific cell without having the chance to invalidate the old one.
This would be handled by stopping the timer in preapreForReuse() method from the table view cell subclass. This method is called when the cell is going to be reused:
class ActivityCell: UITableViewCell {
override func prepareForReuse() {
super.prepareForReuse()
self.timer?.invalidate()
self.timer = nil
}
}
This will solve (one of) the timer problem(s), but there is one more issue: you are going to lose the time tracked by that cell. There is no way to preserve that state into the cell itself. You must do it in the view controller and pass the displayed cell the time it needs to track in tableView(_:cellForRowAt:) method. If you are already doing it, then this is not a problem.
Another problem is that the timers will live forever (or until it's cell stops it). For some of the timers, if the cell object lost the reference to it, nobody will ever stop that timer anymore, so it will just fire every second until the app is killed.
I'm not really sure at this point whether the timer will retain the cell object using the target-selector method for creating a timer, but there is an issue regardless of that:
If the timer retains the cell object, that means that you will have memory leaks: that means that the timers and the cells will never be deallocated and will continuously use memory and processing resources;
If the timer does not retain the cell object, then there might occur a crash once the cell is being deallocated.
You should stop the cell's timer in tableView(_:didEndDisplaying:forRowAt) method. At this point, the cell is being hidden, so the timer makes no sense anymore.
Of course, all of this will lead you to another issue: preserving the time state for the cells. This handling should be done from the view controller presenting the table view, and not from the table view cell.
TL;DR:
You will have a lot of headaches having a timer on each cell, in fact it's almost impossible to handle things that way. You should have your timers corresponding to each cell in the view controller that presents the table view and the things will simplify a lot.