UITableView smooth scrolling - ios

Question

I have UITableView with images in each cell and I want my scrolling be smooth. So I read some post on stackerflow and now I am loading my images in background thread:
override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
var cell: BuildingStatusCell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as! BuildingStatusCell
cell.selectionStyle = UITableViewCellSelectionStyle.None
var node = nodesArray[indexPath.row] as! NSMutableDictionary
if !checkIfImagesLoaded(node[Api.pictures] as! NSMutableArray) {
cell.id = node[Api.buildingStatusId] as! Int
cell.date.text = node[Api.date] as? String
cell.count.text = String((node[Api.pictures] as! NSMutableArray).count)
cell.indicator.hidesWhenStopped = true
cell.indicator.startAnimating()
dbHelper.getBuildingStatusNode(node, callback: self)
} else {
cell.id = node[Api.buildingStatusId] as! Int
cell.date.text = node[Api.date] as? String
cell.count.text = String((node[Api.pictures] as! NSMutableArray).count)
dispatch_async(dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.value), 0)) {
var image = WorkWithImage.loadImageFromSD((node[Api.pictures] as! NSMutableArray)[0]["image"] as! String)! // Bad
dispatch_async(dispatch_get_main_queue()) {
cell.imgView.image = image
cell.indicator.stopAnimating()
}
}
}
return cell
}
dbHelper.getBuildingStatusNode(node, callback: self) method executes in background thread also. But for some reasons when I scroll I still get some delay. I read that it is good to fill my cell with data in tableView:willDisplayCell method instead tableView:cellForRowAtIndexPath and I should return cell as faster as I can in tableView:cellForRowAtIndexPath method. The question is should I now use the code like this:
override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
var cell: BuildingStatusCell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as! BuildingStatusCell
cell.selectionStyle = UITableViewCellSelectionStyle.None
return cell
}
override func tableView(tableView: UITableView, willDisplayCell cell: UITableViewCell, forRowAtIndexPath indexPath: NSIndexPath) {
var cell: BuildingStatusCell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as! BuildingStatusCell
var node = nodesArray[indexPath.row] as! NSMutableDictionary
if !checkIfImagesLoaded(node[Api.pictures] as! NSMutableArray) {
cell.id = node[Api.buildingStatusId] as! Int
cell.date.text = node[Api.date] as? String
cell.count.text = String((node[Api.pictures] as! NSMutableArray).count)
cell.indicator.hidesWhenStopped = true
cell.indicator.startAnimating()
dbHelper.getBuildingStatusNode(node, callback: self)
} else {
cell.id = node[Api.buildingStatusId] as! Int
cell.date.text = node[Api.date] as? String
cell.count.text = String((node[Api.pictures] as! NSMutableArray).count)
dispatch_async(dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.value), 0)) {
var image = WorkWithImage.loadImageFromSD((node[Api.pictures] as! NSMutableArray)[0]["image"] as! String)!
dispatch_async(dispatch_get_main_queue()) {
cell.imgView.image = image
cell.indicator.stopAnimating()
}
}
}
}
And what else I can do to make my scrolling more smooth? BCS I still have lags even when I use willDisplayCell method.
P.S. Image size in my UITableViewCells is fixed.

Answer 1

Try the following
Try removing any shadows.
Make the cell and its subviews opaque. Don't use alpha/transparency.
Try decoding the images on a background thread :
Decode images in background thread?

Answer 2

First of all it is better to subclass UITableViewCell and just pass your Api object to cell and make this mapping inside cell.
Also it is better to use some library like: AFNetworking's extension or AsyncImageView - it is possible to use in Swift.
Try to remove any border rounding, shadow, transparencies - they can cause delays. In this case you need rasterization:
Related question:
Sluggish scrolling experience when using QuartzCore to round corners on UIImageView's within a UITableViewCell

Answer 3

When you load image from URL it takes time to download image and that cause block in scrolling UITableView.
You are doing so much work simply do
Use this class SDWebImage
and in your bridging header file :
#import "UIImageView+WebCache.h"
Here is a code example that should work :
let block: SDWebImageCompletionBlock! = {(image: UIImage!, error: NSError!, cacheType: SDImageCacheType!, imageURL: NSURL!) -> Void in
println(self)
}
let url = NSURL(string: node[Api.pictures] as! NSMutableArray)[0]["image"] as! String)
cell.imgView.sd_setImageWithURL(url, completed: block)