I'm quite new to multi-threading in iOS, and have some difficulties with the implementation of my task. I'd really appreciate any help from more experienced programmers.
I have one URL address, as a starting point, to which I make a request, check some info, and retrieve all the URL addresses contained on that web-page. After this, I need to do the same with new URL addresses again and again, until a specific number of URLs is checked.
I decided to go with Operations and OperationQueues, because I also need to be able to choose a maximum number of concurrent operations.
I created a custom async operation LoadOperation and applied it to each URL stored in an array. In the completion block, I add new URLs to the array, and call the same function, kind of recursively.
Also I created two queues:
- A concurrent one for requests,
and
- A serial one for accessing shared properties as follows:
class Queue<Item: Equatable> {
// concurrent queue for URL-requests
let loadOperationQueue = OperationQueue() // qos: .utilities
// serial operation queue for accessing shared properties
let serialOperationQueue = OperationQueue() // qos: .utilities
// maximum number of URLs that need to be checked ultimately
var maxNumberOfItems: Int = 1
var pendingItems: [Item] = [] // newly added URLs
var loadingItems: [Item] = [] // loading URLs
var processedItems: [Item] = [] // URLs that were checked
// all URLs for tableView dataSource
var allItems: [Item] {
return processedItems + loadingItems + pendingItems
}
func addToPending(_ item: Item) {
guard allItems.count < maxNumberOfItems else {return}
pendingItems.append(item)
}
func removeFromPending() -> Item? {
guard !self.pendingItems.isEmpty else { return nil }
let item = pendingItems.removeFirst()
loadingItems.append(item)
return item
}
func addToProcessed(_ item: Item) {
loadingItems = loadingItems.filter{$0 != item}
processedItems.append(item)
}
func isFull() -> Bool {
return allItems.count >= maxNumberOfItems
}
}
This is my searching function:
func startSearching() {
// iterate over array of URLs creating async operation
while let currentWebPage = queue.removeFromPending() {
let loadOperation = LoadOperation(currentWebPage, searchString: searchString)
loadOperation.completionBlock = {
let webPage = loadOperation.output!
self.queue.serialOperationQueue.addOperation {
// add checked URL to processed
self.queue.addToProcessed(webPage)
// add new urls to an array of pending URLs
for urlString in webPage.containedLinks {
//check if the limit of checked urls is not exceeded
guard !self.queue.isFull() else { return }
//check if this url is already in queue
guard !self.queue.allItems.contains(where: {$0.url == urlString}) else { continue }
self.queue.addToPending(WebPage(url: urlString, containedLinks: [], status: .pending))
}
// update UI
OperationQueue.main.addOperation {
self.viewDelegate?.reloadTable()
}
// repeat searching process with new urls
self.startSearching()
}
}
queue.loadOperationQueue.addOperation(loadOperation)
}
}
I can't figure out why:
When I run the app, the screen freezes. Even though all my queues are in the background (qos: utilities).
Sometimes when I try to scroll UITableView, I get
sigabortdue to index out of range, even though I try to access all properties in serial queue.
This is a data source code:
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return presenter.getNumberOfRows()
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "WebPageTableViewCell", for: indexPath) as! WebPageTableViewCell
let (url, status) = presenter.getCellContent(at: indexPath.row)
cell.addressLabel.text = url
cell.statusLabel.text = status
return cell
}
And functions from presenter:
func getNumberOfRows() -> Int {
return queue.allItems.count
}
func getCellContent(at index: Int) -> (url: String, status: String) {
return (url: queue.allItems[index].url, status: queue.allItems[index].status.description)
}
