This is perhaps more existential than a concrete question, but I'm struggling with a bit of a user experience issue in my app.
In my app, I have a task that converts a UIView to an UIImage, does some off-screen processing, and updates the UI for the user. All of this happens on the Main Queue, as is required for such UIKit tasks.
I want to have an activity indicator in my app (I'm using a custom designed one, but a regular UIActivityIndicator demonstrates the same issue), which I also have running on the Main Queue, prior to the aforementioned task.
My issue is that once the UIView processing kicks in, my activity indicator freezes until the task completes. This is obviously due to the main queue handling another, more intensive task, hereby hanging the spinner. My curiosity is; how can I ensure the spinner continues, regardless of the UI work happening on the main queue?
Thanks!
I'm afraid this is impossible unless you do the heavyweight operation on the background thread.
You can try to give the main thread a little bit air to breathe by chunking the operation to smaller parts, if that can be done. That would at least allow some updates of the spinner.
While I think that you should keep taking an image and updating the UI on the main thread, considering putting processing the image at the background thread, if that is possible.
I agree with Milan. I'd suggest a slightly different flow:
Start your activity indicator spinning.
Grab your view and convert it to an image.
Pass the image processing to a GCD background queue for processing, and pass in a completion handler.
When the background processing is complete, invoke the completion handler on the main thread. In the body of the completion handler, stop/hide the activity indicator.
I would suggest something like this.
-(void)render {
startSpinner();
backgroundRenderQueue = dispatch_queue_create("backgroundQueue",DISPATCH_QUEUE_CONCURRENT);
dispatch_async(backgroundQueue, ^{
//Render image here
UIImage *image = [UIImage imageWithCGImage:CGBitmapContextCreateImage(context)];
dispatch_async(dispatch_get_main_queue(), ^{
self.imageview.image = image;
stopeSpinner()
});
});
}
Related
I have code that triggers an activity indicator:
activityIndicator.startAnimating()
Once it is triggered I continue to launch some background tasks (e.g. go to server, fetch data, callback when done).
The problem I have is that the behaviour of the activity indicator is inconsistent in the sense that from displaying on UI it sometimes visible/gets rendered and animating and sometimes does not (especially if the app was in the background and returned to foreground)
I am not sure what I need to do to ensure that it consistently rendered on the screen when I call startAnimating() before I proceed to launch background tasks.
Any guidance would be greatly appreciated.
Thanks.
I think it's because you may work with UI outside main thread. Try wrap code in
dispatch_async(dispatch_get_main_queue(), ^{
// Your code to run on the main queue/thread
self.activityIndicator.startAnimating()
});
When using a UIActivityIndicatorView, it is possible to add this view to your view hierarchy and start animating it.
After that you are completely save to block the main thread (not that this is a good approach), there is basically nothing that I know of, that could stop the activity indicator from spinning.
My question is:
How is this done? It seems like UIActivityIndicatorView uses its own thread for rendering.
Is this something that can be achieved with my own views?
I have a CoreAnimation animation that I want to keep playing while the main thread might be blocking for a couple of milliseconds.
How to do that? Thanks for any help or ideas!
//EDIT: To clarify my question: I want to know what Apple does to get UIActivityIndicatorView animating even when you block the main thread. When I trigger my own CoreAnimation and I block the main thread, the animation itself stops. Furthermore to the question what Apple does to achieve that under the hood, I want to know, if I can achieve this myself, with public API.
You best solution is to use a background thread for long processing while keeping the main thread only for display purpose. That way, you will be able to display your view without blocking and run your long precessing at the same time.
You can use Grand Central Dispatch (GCD) dispatch_async to run your long processing code on another thread and then call dispatch_async on the main thread to refresh your UI.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
^{
// Your long processing code
dispatch_async(dispatch_get_main_queue(),
^{
// Refresh UI here
});
});
I have two UILabels that require updating on millisecond basis. I ran my app and noticed that one of the UILabels ("The Second UILabel") updates slower than the first one. It's not a large difference but it's perceptible.
The Second UILabel does a short computation before displaying its results on the UILabel. I suspect I may need to push this to a background thread.
I have used threads in Java but would like to explore GCD, operations queue, dispatch queue, etc. I've read a lot on them (especially from Apple website) but just can't make sense of them.
So I need to check with the gurus here:
Q1: Is The Second UILabel perception problem due to updating of the UI Thread that lead to thread contention?
Q2: Does my app updating The Second UILabel require a background thread or will GCD and the like suffice?
The answer.
dispatch_queue_t background_queue = dispatch_queue_create("label", NULL);
dispatch_async(background_queue, ^{
// do some stuff that takes a long time here...
// follow up with some stuff on the main queue
dispatch_async(dispatch_get_main_queue(), ^{
// Typically updating the UI on the main thread.
});
});
I just wanted to confirm why this is needed.
I added this code to the KIImagePager (a cocoapod) to load images that are local to the app (the default code loads images from a url).
Here's my working code based off what a coworker suggested:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
dispatch_sync(dispatch_get_main_queue(), ^{
[imageView setImage:[UIImage imageNamed:[aImageUrls objectAtIndex:i]]];;
});
});
I noticed that if I take out the inner dispatch_sync, it works but not in the way I want (some of the images on the image pager scrollview aren't loaded yet when I start scrolling). But they do eventually load.
My question is this, does the sync call on the main queue get the image back to the UI (which is on the main queue)? Because it does work with the second async removed.
The internal dispatch executes its code block on the main thread. This is required because all UI operations must be performed on the main thread. And you're image downloading code (context in which this snippet is executed) may be on a background thread.
The external dispatch executes its block on a background thread. The block its given is the one that executes on the main thread. Thus, the external block can be safely removed.
Hrs an outline of the idiom you're using.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
// do blocking work here outside the main thread.
// ...
// call back with result to update UI on main thread
//
// what is dispatch_sync? Sync will cause the calling thread to wait
// until the bloc is executed. It is not usually needed unless the background
// background thread wants to wait for a side effect from the main thread block
dispatch_sync(dispatch_get_main_queue(), ^{
// always update UI on main thread
});
});
You should only work with UI objects on the main thread. If you don't, you will run into a couple of problems. The first, as you saw, is that UI objects will be delayed in updating. The second is that the app could crash if you try to change UI objects simultaneously from multiple threads. You should only work with UI objects on the main thread.
A UIViewController takes about half a second to load its contents and appear on screen. How can I make them all load in the background and appear when they're ready?
There is a LazyTableImages sample on the Apple developer site.
It shows how to perform the heavy lifting in a background thread and update the UI on the main thread.
PerformSelectorInBackground:withObject: is a possible solution, although a more modern method would be to use asynchronous blocks. You can run code on the main thread from within these blocks to update the UI Safely.
The Concurrency Programming Guide is a good place to find more information and examples of this.
A Background Thread cant update the UI,you can perform all the processing logic in background thread and call the main thread for UI update
Example to load a tableView with Data ,use the background thread to process everything and load the Data, call [tableView reloadData] using the main thread, see Grand central Dispatching to know how to Work with Threads in IOS..
Hope it Helps
Create a GCD queue to process your work in a background thread (read the docs, because my "create" label and options may not be what you want).
You send it to the queue asynchronously, meaning that the call to dispatch_async will make appropriate arrangements for the block of code you give it to run in another thread and it will then return back to you "immediately."
All the work in the block you give it will be executed on a separate thread. Note, at the end, you do another call, this time with the well know main queue. This arranges for that code to run in the main thread, which is mandatory for any UI work.
Also, you really should read the documentation on GCD and especially blocks, because there are memory and cycle considerations. Good luck.
dispatch_queue_t workQ = dispatch_queue_create("bgWorkQ", 0);
dispatch_async(workQ, ^{
// This code is now running in a background thread.
// Do all your loading here...
// When ready to touch the UI, you must do that in the main thread...
disptach_async(dispatch_get_main_queue(), ^{
// Now, this code is running in the main thread.
// Update your UI...
});
});
dispatch_release(workQ);
The easiest way is to use NSObject's - (void)performSelectorInBackground:(SEL)aSelector withObject:(id)arg You just pass it a selector and it will happy in the background and not block your UI. Be aware however that there are rules to background threads.
You shouldn't update your UI from the background thread. And you need to make sure the methods you're calling are thread safe.