Slow/Stuttered scroll in tableview - ios

Question

I'm loading about 150 elements from an array of arrays of dictionaries (tasks) and I can get all of the data into my tableview but when I scroll its stupid slow. When I print out the information of one of my functions to the console, it looks like I am getting all of the data back every time I scroll. Is this something I am not loading well (i.e. asynchronously) or do I need to change my functions?
func querySections() -> [String] {
var sectionsArray = [String]()
for task in tasks {
let dueTimes = task.dueTime
sectionsArray.append(dueTimes)
}
let uniqueSectionsArray = Array(Set(sectionsArray.sort()))
// print(uniqueSectionsArray)
return uniqueSectionsArray
}
func queryDueTimes(section:Int) -> [Task] {
var sectionItems = [Task]()
for task in tasks {
let dueTimes = task.dueTime
if dueTimes == querySections()[section] {
sectionItems.append(task)
}
}
print(sectionItems)
return sectionItems
}
// MARK: - Table view data source
override func tableView(tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
return querySections()[section]
}
override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
return querySections().count
}
override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return queryDueTimes(section).count
}
override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCellWithIdentifier("TaskCell", forIndexPath: indexPath) as! TaskCell
// Configure the cell...
cell.selectionStyle = .None
let times = queryDueTimes(indexPath.section)
let task = times[indexPath.row]
cell.label.text = task.title
if task.done == true {
cell.checkBox.image = UIImage(named: "checkedbox")
cell.detailLabel.text = "Completed By: \(task.completedBy)"
}
else {
cell.checkBox.image = UIImage(named: "uncheckedbox")
cell.detailLabel.text = ""
}
cell.delegate = self
return cell
}
Basically, in querySections, I'm iterating through all of the dueTimes for each task and then changing them into an array of a set so I can filter out all of the duplicates. This is giving me all of my sections. For queryDueTimes, I'm iterating through the tasks and matching them to a section.
I had a thought about calling the functions in viewDidLoad but that isn't working (it keeps giving me an empty array when I try to pass it to another empty array thats more accessible outside of the function) and I can't access section (for queryDueTimes) in viewDidLoad (as far as what I know how to do).
Update 1:
I think the mistake is on my end. I said that I tasks is an array of arrays when its just an array of Tasks (a struct with all of the properties of each task). When I load the app, I append all of the tasks from my backend to a local array ("tasks"). Should I have an array of arrays for this to work or can I amend my code somehow and get it to work?
Update 2:
I'm getting sectionTimes and tasksInSectionArray as empty arrays when I print them.
var sectionTimes = [String]()
var tasksInSectionArray = [[Task]]()
var tasks = [Task]() {
didSet {
tableView?.reloadData()
}
}
func updateTableView() {
sectionTimes = Set(tasks.map{$0.dueTime}).sort()
tasksInSectionArray = sectionTimes.map{section in tasks.filter{$0.dueTime == section}}
print(sectionTimes)
print(tasksInSectionArray)
tableView.reloadData()
}
override func tableView(tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
return sectionTimes[section]
}
override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
return sectionTimes.count
}
override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return tasksInSectionArray[section].count
}
override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCellWithIdentifier("TaskCell", forIndexPath: indexPath) as! TaskCell
// Configure the cell...
cell.selectionStyle = .None
let task = tasksInSectionArray[indexPath.section][indexPath.row]

Answer 1

Like you guessed, the data is being loaded and sorted over and over again, instead of only once. Save the results of querySelections and queryDueTimes and use that inside the table view data source methods.
You can do this in viewDidLoad - call both functions once and assign the results to a variable at the class level, and then call tableView.reloadData() (assuming you have a reference to the table view).

Answer 2

var sections: [String] = []
var data: [[Tasks]] = []
func updateTableView() {
sections = Set(tasks.map { $0.dueTime }).sort()
data = sections.map { section in tasks.filter { $0.dueTime == section } }
tableView.reloadData()
}
override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
return sections.count
}
override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return data[section].count
}
override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let task = data[indexPath.section][indexPath.row]
// Cell configuration
}
This is basically what DMan said, but I've made an example for you.