Long delays displaying UIImageView loaded from local file? - ios

Question

I see questions regarding long delays in displaying UIImageViews after downloading, but my question involves long delays when
reading from local storage.
After archiving my hierarchy of UIImageViews to a local file (as per narohi's answer in
How to output a view hierarchy & contents to file? ),
I find that if I want to reload them, it takes 5 to 20 seconds for the views to actually appear on screen,
despite my setting setNeedsDiplay() on the main view and all the subviews.
I can immediately query the data contained in the
custom subclasses of UIView that get loaded -- showing that NSKeyedUnarchiver and all the NS-decoding and all the init()'s have completed -- however
the images just don't appear on the screen for a long time. Surely the next redraw cycle is shorter than 5-20 seconds...?
It seems odd that images from PhotoLibrary appear instantly, but anything loaded from local file storage using NSKeyedUnarchiver takes "forever."
What's going on here, and how can I speed this up?
.
.
To be explicit, the relevant part of my Swift code looks like this:
let view = NSKeyedUnarchiver.unarchiveObjectWithFile(path) as! UIView!
if (nil == view) {
return
}
myMainView.addSubview(view)
view.setNeedsDisplay()
// now do things with the data in view ...which all works fine
I find that, even if I add something like...
for subview in view.subviews {
subview.setNeedsDisplay()
}
...it doesn't speed up the operations.
We are not talking huge datasets either, it could be just a single imageview that's being reloaded.
Now, I do also notice these delays occurring when downloading from the internet using a downloader like the one shown in
https://stackoverflow.com/a/28221670/4259243
...but I have the downloader print a completion message after not only the download but when the (synchronous operation)
data.writeToFile() is complete (and before I try to load it using NSKeyedUnarchiver), so this indicates that the delay
in UIImageView redraws is NOT because the download is still commencing....and like I say, you can query the properties of the data and it's all in memory, just not displaying on the screen.
UPDATE: As per comments, I have enclosed the needsDisplay code in dispatch_async as per Leo Dabus's advice, and done some Time Profiling as per Paulw11's. Link to Time Profiling results is here: https://i.imgur.com/sa5qfRM.png I stopped the profiling immediately after the image appeared on the screen at around 1:00, but it was actually 'loaded' during the bump around 20s. During that period it seems like nothing's happening...? The code is literally just waiting around for a while?
Just to be clear how I'm implementing the dispatch_async, see here:
func addViewToMainView(path: String) {
let view = NSKeyedUnarchiver.unarchiveObjectWithFile(path) as! UIView!
if (nil == view) {
return
}
dispatch_async(dispatch_get_main_queue(), {
self.myMainView.addSubview(view)
view.setNeedsDisplay()
self.myMainView.setNeedsDisplay()
})
}
...Since posting this I've found a few posts where people are complaining about how slow NSKeyedUnarchiver is. Could it just be that? If so, :-(.
SECOND UPDATE: Ahh, the "let view = " needs to be in the dispatch_async. In fact, if you just put the whole thing in the dispatch_async, it works beautifully! so...
func addViewToMainView(path: String) {
dispatch_async(dispatch_get_main_queue(), {
let view = NSKeyedUnarchiver.unarchiveObjectWithFile(path) as! UIView!
if (nil == view) {
return
}
self.myMainView.addSubview(view)
view.setNeedsDisplay()
self.myMainView.setNeedsDisplay()
})
}
This works instantly. Wow.. Credit to Leo Dabus. Leaving this here for others...