Update a progress bar while JSONDecoder is decoding a huge object, in Swift

问题

I'm decoding several gigabytes of JSON encoded data like this

let decoder = JSONDecoder()
let table = try decoder.decode([LogRow].self, from: content!)

where content is plain text. Now, this operation can take several minutes, depending on the size of content, and I'd like to show some kind of progress. This is a command line program, so even a periodic update on the length of table would be enough. The problem is that I don't see anything like a callback or something like that. I tried with a rather awkward Timer like this

var table: [LogRow]? = []
let timer = Timer(fire: Date(), interval: 1.0, repeats: true) { t in
    print("\(table?.count ?? 0) rows parsed.")
}
timer.fire()
table = try decoder.decode([LogRow].self, from: content!)
timer.invalidate()

but that only runs once -- is it because the decoder blocks the main thread and I'm running the timer in the same thread? I'm a bit of a noob with the GCD so I'm not sure how to go about using a DispatchQueue for this.

Any ideas?

回答1:

Then can declare your timer like:

    let timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(self.updateUI), userInfo: nil, repeats: true)
    DispatchQueue.global(qos: .background).async {
        self.perform(#selector(self.decode), on: Thread.current, with: nil, waitUntilDone: true)
        timer.invalidate()
    }

which means you want to trigger the updateUI action every second. Then you start to decode in a background thread and wait until done before invalidate your timer.

var totalDuration: Int = 0

@objc func updateUI () {
    self.currentDuration += 1
    print("------------> \(self.currentDuration)")
}

@objc func decode () {
    table = try decoder?.decode([LogRow].self, from: content!)
}

I added a currentDuration variable which could be used in your progressBar. But you must know the total duration if you need to show a percentage to your user.

来源：https://stackoverflow.com/questions/46809851/update-a-progress-bar-while-jsondecoder-is-decoding-a-huge-object-in-swift