user38931 user38931 - 6 months ago 38
Swift Question

How to release memory for UIImageView (Swift)

I got a problem with UIImage. I've manually added UIImageView objects to a scroll view. The problem is : when I have more than 50 images, memory will increase to around 200MB, and the app will be crash on iphone 4 or 4s. I want to release memory for images that is not in visible part when I receive Memory Warning to prevent crashing but I don't know how to release them with Swift. Help me please.

override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()

// Dispose of any resources that can be recreated.
}

func loadImage(index:Int){
if self.imgPaths.count == 0 {
println("Has not data")
actInd.stopAnimating()
return
}
var imgURL: NSURL = NSURL(string: self.imgPaths[index].stringByTrimmingCharactersInSet(.whitespaceAndNewlineCharacterSet()))!
let width:CGFloat = self.view.bounds.width
let height:CGFloat = self.view.bounds.height
var view:UIView = UIView(frame: CGRectMake(0, 0, width, height));

if let imgObj = self.dicData[index] {

}
else
{
println("imgURL \(imgURL)")
let request: NSURLRequest = NSURLRequest(URL: imgURL)
NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue.mainQueue(), completionHandler: {(response: NSURLResponse!,data: NSData!,error: NSError!) -> Void in
if error == nil {
let imgItem = UIImage(data: data)!

var te :Float = self.imgPaths.count > 0 ? Float(index + 1) / Float(self.imgPaths.count) : 1
self.progressView.setProgress(te, animated: true)

if let imgObj = self.dicData[index] {

if index < self.imgPaths.count - 1
{
var nextIndex:Int = index + 1
self.loadImage(nextIndex)
}
if(index == self.imgPaths.count - 1)
{
if self.currentImageIndex > 0
{
self.isAddedFirstImg = true
}
if !self.isAddedFirstImg
{
self.scrollViews[0].zoomScale = self.zoomScales[0]
self.view.insertSubview(self.scrollViews[0], belowSubview: self.tabBar.viewWithTag(77)!)
self.isAddedFirstImg = true
}

self.actInd.stopAnimating()
println("loaded image")
}
}
else
{
self.dicData[index] = UIImageView(image: imgItem)

self.dicData[index]?.frame = CGRect(origin: CGPointMake(0.0, 0.0), size:imgItem.size)

// 2
self.scrollViews[index].addSubview(self.dicData[index]!)
self.scrollViews[index].contentSize = imgItem.size

// 3
var doubleTapRecognizer = UITapGestureRecognizer(target: self, action: "scrollViewDoubleTapped:")
doubleTapRecognizer.numberOfTapsRequired = 2
doubleTapRecognizer.numberOfTouchesRequired = 1
self.scrollViews[index].addGestureRecognizer(doubleTapRecognizer)

var singleTapRecognizer = UITapGestureRecognizer(target: self, action: "scrollViewSingleTapped:")
singleTapRecognizer.numberOfTapsRequired = 1
singleTapRecognizer.numberOfTouchesRequired = 1
self.scrollViews[index].addGestureRecognizer(singleTapRecognizer)

var swipeRight = UISwipeGestureRecognizer(target: self, action: "respondToSwipeGesture:")
swipeRight.direction = UISwipeGestureRecognizerDirection.Right
self.scrollViews[index].addGestureRecognizer(swipeRight)

var swipeLeft = UISwipeGestureRecognizer(target: self, action: "respondToSwipeGesture:")
swipeLeft.direction = UISwipeGestureRecognizerDirection.Left
self.scrollViews[index].addGestureRecognizer(swipeLeft)

// 4
var scrollViewFrame = self.scrollViews[index].frame
var scaleWidth = scrollViewFrame.size.width / self.scrollViews[index].contentSize.width
var scaleHeight = scrollViewFrame.size.height / self.scrollViews[index].contentSize.height
var minScale = min(scaleWidth, scaleHeight)
self.zoomScales[index] = minScale
self.scrollViews[index].minimumZoomScale = minScale

// 5
self.scrollViews[index].maximumZoomScale = 1.0
self.scrollViews[index].delegate = self

// 6
self.centerScrollViewContents(index)

dispatch_async(dispatch_get_main_queue(), {
println("downloaded image index: \(index) CH.\(self.chapterID)")
if(index == 0)
{
self.scrollViews[0].zoomScale = self.zoomScales[0]
self.view.insertSubview(self.scrollViews[0], belowSubview: self.tabBar.viewWithTag(77)!)
self.actInd.stopAnimating()
}

if index < self.imgPaths.count - 1 && !self.stopDownload
{
var nextIndex:Int = index + 1
self.loadImage(nextIndex)
}
if(index == self.imgPaths.count - 1)
{
if self.currentImageIndex > 0
{
self.isAddedFirstImg = true
}
if !self.isAddedFirstImg
{
self.scrollViews[0].zoomScale = self.zoomScales[0]
self.view.insertSubview(self.scrollViews[0], belowSubview: self.tabBar.viewWithTag(77)!)
self.isAddedFirstImg = true
}

self.actInd.stopAnimating()
println("loaded image")
}
})
}
}
else {
println("Error: \(error.localizedDescription)")
}
})
}


}

Answer

Swift uses ARC (Automatic Reference Counting) to manage memory. To free something from memory, you must remove all references to that object. ARC maintains a count of the number of places that hold a reference to the object, and when that count reaches 0, the memory is freed.

In your case, your imageViews are stored in self.dicData[index] and they are added as subviews of self.scrollViews[index]. You will need to remove the imageView from its superview and from self.dicData. Once you do that, the memory will be freed.

If you have the index of the imageView you want to free:

self.dicData[index]?.removeFromSuperview()
self.dicData[index] = nil