Tips for saving memory on older devices while using images

Question

in my app I need to display several images that I fetch from Parse and everything was great until I tested it on iPad mini and iPad 2...

To give you a better idea, my Home page is a tableview with 4 cells. The first cell must display a "big" image (50-55KB), the second cell shows a table view of thumbnail images(5-8KB) - but no more than 5 collection view cells! - and the other 2 cells are used for text, buttons etc.

I'm using Parse as a backend and I started with its tools to work with images like PFImageView: this way to load an image from the server I was using these two methods:

theImageView.file = thePFObject["<PFFileEntity>"] as? PFFile
theImageView..loadInBackground()

Once the app finished to load the home page , used memory device was at 100-110Mb, really close to 125-130Mb at witch the app is closed by the system.

I replaced the lines above with:

let imageData = NSData(contentsOfURL: NSURL(string: (stringContainingTheURL))!)
dispatch_async(dispatch_get_main_queue()) {
self.recipeImageView.image = UIImage(data: imageData!) }

and the memory usage is now at 83-85Mb. Better but as I go into another view containing only an YTPlayerView (for embedding youTube video) and a collection view of 5 thumbnail images, it easily shut down.

My strategies to reduce memory impact are: thumbnails images where possible; avoid UIImage (named:) method; avoid to work with PFFiles: this way then I store PFObjects into array, I work with strings instead of files; call didReciveMemoryWarnings() where possible to release memory

I've checked that there aren't memory leaks but I don't know how to improve the behavior. Any idea?

I'll leve you my code for the first two cells in order to compare with you.

class HomeTableViewController: UITableViewController, UICollectionViewDelegate, UICollectionViewDataSource {

//MARK: -outlets

var collectionView: UICollectionView! { didSet {collectionView.reloadData() } }

// MARK: - Model
let indexPathforcell0 = NSIndexPath(forRow: 0, inSection: 0)
let indexPathforcell1 = NSIndexPath(forRow: 1, inSection: 0)

var firstCellObject:  PFObject? { didSet{ tableView.reloadRowsAtIndexPaths([indexPathforcell0], withRowAnimation: .Fade) } }
var secondCellArrayOfObjects = [PFObject]() { didSet { tableView.reloadRowsAtIndexPaths([indexPathforcell1], withRowAnimation: .Fade) } }

// MARK: - life cycle

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
}

override func viewDidLoad() {

    super.viewDidLoad()
    //fetch
    fetchFirstCellObject()
    fetchSecondCellArrayOfObjects()
}

 private func fetchFirstCellObject() {
//At the end...
// self.firstCellObject = something
}

private func fetchSecondCellArrayOfObjects() {
//self.secondCellArrayOfObjects = something

}

// MARK: - Table view data source

override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
    return 1
}

override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return 4
}

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {

    var mainCell = UITableViewCell()
    switch indexPath.row {

    case 0:
        let cell = tableView.dequeueReusableCellWithIdentifier("Cell1", forIndexPath: indexPath) as! CustomTableViewCell
        cell.object = firstCellObject
        mainCell = cell

    case 1:
        var cell = self.tableView.dequeueReusableCellWithIdentifier("Cell2") as! UITableViewCell
        collectionView = cell.viewWithTag(1) as! UICollectionView
        collectionView.delegate = self
        collectionView.dataSource = self
        mainCell = cell


    return mainCell
}

// MARK: - UICollectionViewDataSource

func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
    return 1
}

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 5
}

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    var cell = collectionView.dequeueReusableCellWithReuseIdentifier("Cell", forIndexPath: indexPath) as! CommentedRecipeCollectionViewCell

    cell.object = secondCellArrayOfObjects[indexPath.row]

    return cell
}
}

And here how I implement the UITableViewCell:

class CustomTableViewCell: UITableViewCell {

// MARK: - Model

var object:  PFObject? { didSet { updateUI() } }

@IBOutlet weak var objectImageView: PFImageView!
@IBOutlet weak var objectLabel: UILabel!


private func updateUI() {
    let qos = Int(QOS_CLASS_USER_INITIATED.value)
    dispatch_async(dispatch_get_global_queue(qos, 0)) {
        let URLToUse = // ....
        if let stringToUse = theString {
            let imageData = NSData(contentsOfURL: NSURL(string: (URLToUse))!)
            dispatch_async(dispatch_get_main_queue()) {
                self.homeImageView.image = UIImage(data: imageData!)
            }
         }
    }

    objectLabel.text = // ....
}
}