Assume we have one UIViewController, call it A, in the viewDidLoad of that VC we add to it two UIViewControllers( B,C ). now to make the UI smooth in the viewDidLoad of A we do some GCD work
dispatch_queue_t queue = dispatch_queue_create("myqueue", NULL);
dispatch_async(queue, ^{
// Create webviews, do some setup here, etc etc
// Perform on main thread/queue
dispatch_async(dispatch_get_main_queue(), ^{
// this always has to happen on the main thread
[self.view addSubview:webView];
});
});
So the ParentViewController is somewhat better in UI rendenring.
My question is: Is this enough GCD work? or should I do the same thing in thew viewDidLoad of the child viewcontrollers? just because I created those child VC's on a background thread does that mean I need not do any GCD wokr on them? I am trying to make my UI as responsive as possible, but not clutter the code. I guess another way of wording this would be are GCD threads reentrant? is there a concept of reentrancy in iOS?
I don't think that adding a subview is a significant performance hit. Also, playing with views (or UIKit in general) should be done on the main UI thread. As far as I know it's considered bad practice to do such things in the background.
Try to save GCD/Async stuff to processor-intensive work or tasks of unknown durations such as downloading something from the internet.
Source and more info: Warning: UIKit should not be called from a secondary thread
Related
I'm building a scrolling menu that generates new rows of buttons on the fly, and must generate each button from a large number of sprites. Because this is processor intensive, the menu sticks for about a quarter second each time it needs to load a new row of buttons. I realized I needed to add multi-threading so the button load could be handled in a different thread than the scroll animation, but when I do it crashes when it tries to load new buttons. Here is the code I'm using:
-(void)addRowBelow{
_rowIndex--;
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSMutableArray *row = [self addRow:_rowIndex];
[_buttonGrid addObject:row];
[self removeRow:[_buttonGrid objectAtIndex:0]];
});
_nextRowBelowPos += _rowHeight;
_nextRowAbovePos += _rowHeight;
}
Each time I test it I get a different error, sometimes it's a memory error or an assertion failure. I suspect it has to do with calling cocos2d functions asynchronously?
You are probably getting crashing issues because you are multithreading access to the cocos managed objects (sprites, layers, nodes, etc). Since the engine expects to use the internals of these objects for display, GPU operations, etc., and is NOT thread safe, you are probably not going to have good outcomes with multi-threading. You may be changing stuff right in the middle of when it is using it.
Creating/destroying sprites on the fly is probably the reason for your slow down. Cocos2d can display lots (I think it is on the order of 2k) objects on the screen at 60 fps...as long as you don't throttle it down by doing a lot of creation/destruction or AI.
I suggest you preload all your sprites before your scene goes on the stage. You can do this in an intro scene or in the init of the scene itself and let the sprites be owned by the scene. Then you can iterate over them during the update() call and change their positions, make the visible/invisible, etc.
For reference, I usually create different "sprite layers" that load up all their sprites on addition to the scene. If I am going to have dynamic objects, I try to allocate some up front and recycle them when possible. This also allows me to control the order of "what is in front of what" on the screen (see example here). Each layer also draws elements of specific "entity types", giving a nice "MVC" character to a lot of the display.
This is analogous to the way iPhone Apps recycle table cells.
Only create them the first time you need them and have a stash on hand before you need them at all.
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The pattern you probably want to use is
Dispatch work to a background thread. (Note that the work must be safe to execute on a background thread.)
Dispatch back to the main thread to update your UI.
Here's an example of what that looks like in code:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Do work that is safe to execute in the background.
// For example, reading images from disk.
dispatch_async(dispatch_get_main_queue(), ^{
// Do work here that must execute on the main thread.
// For example, calling Cocos2D objects' methods.
NSMutableArray *row = [self addRow:_rowIndex];
[_buttonGrid addObject:row];
[self removeRow:[_buttonGrid objectAtIndex:0]];
});
});
I have a seemingly simple problem that I cannot for the life of me seem to figure out. In my iOS App, I have a UICollectionView that triggers network operation upon tapping it that can take a few seconds to complete. While the information is being downloaded, I want to display a UIView that fills the cell with a UIActivityIndicatorView that sits in the square until the loading is done, and the segue triggered. The problem is that it never appears. Right now my code looks like:
myLoadView.hidden = NO;
//Network Operation
myLoadView.hidden = YES;
The App simply stops for a couple seconds, and then moves on the the next view. I'd imagine Grand Central Dispatch has somthing to do with the solution, however please keep in mind that this code takes place in prepareForSegue, and the network info needs to be passed to the next View. For this reason not finishing the download before switching scenes has an obvious problem. Any help would be VASTLY appreciated. Thanks!
iOS commits changes in the interfaces after working out a routine. Hence you should perform your network operation in a background thread and then get back back on the main and perform the "show my view now thing". Have a look the below code for reference.
myLoadView.hidden = NO;
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
//Network Operation
dispatch_async(dispatch_get_main_queue(), ^{
myLoadView.hidden = YES;
});
});
Your network operation seems to be carried out on the main thread, aka UI thread. This blocks all further UI calls, including the call to unhide a view, until completion.
To resolve this, make your call asynchronous.
You should read this in full, if you haven't already.
As mentioned by other answers, the problem is that the UIView change doesn't happen until the current method finishes running, which is where you are blocking. Before GCD was available I would split methods in two and use performSelector:withObject:afterDelay (to run the second part also on the UI loop) or performSelectorInBackground:withObject: at the end of the first method. This would commit all the waiting animaations first, then do the actual tasks in the second method.
Well the better option for this type of indication is by using the custom HUD libraries like SVProgressHUD or MBProgressHUD
I am new to objective C, coming from .NET and java background.
So I need to create some UIwebviews asynchronously, I am doing this on my own queue using
dispatch_queue_t queue = dispatch_queue_create("myqueue", NULL);
dispatch_async(queue, ^{
// create UIwebview, other things too
[self.view addSubview:webView];
});
as you owuld imagine this throws an error :
bool _WebTryThreadLock(bool), 0xa1b8d70: Tried to obtain the web lock from a thread other
than the main thread or the web thread. This may be a result of calling to UIKit from a
secondary thread. Crashing now...
So how can I add the subview on the main thread?
Since you are already using dispatch queues. I wouldn't use performSelectorOnMainThread:withObject:waitUntilDone:, but rather perform the subview addition on the main queue.
dispatch_queue_t queue = dispatch_queue_create("myqueue", NULL);
dispatch_async(queue, ^{
// create UIwebview, other things too
// Perform on main thread/queue
dispatch_async(dispatch_get_main_queue(), ^{
[self.view addSubview:webView];
});
});
It is fine to instantiate the UIWebView on a background queue. But to add it as a subview you must be on the main thread/queue. From the UIView documentation:
Threading Considerations
Manipulations to your application’s user interface must occur on the main thread. Thus, you should always call the methods of the UIView class from code running in the main thread of your application. The only time this may not be strictly necessary is when creating the view object itself but all other manipulations should occur on the main thread.
Most UIKit objects, including instances of UIView, must be manipulated only from the main thread/queue. You cannot send messages to a UIView on any other thread or queue. This also means you cannot create them on any other thread or queue.
As rob said the UI changes should be done only on main thread.You are trying to add from a secondary thread.Change your code [self.view addSubview:webView]; to
[self.view performSelectorOnMainThread:#selector(addSubview:) withObject:webView waitUntilDone:YES];
I have a table view, and when the user selects a row, i push them to a new ViewController. At first, I initialized all my view objects in the ViewDidLoad method (involving web service calls) but I saw that it made the transition from my tableview to my new viewcontroller very long.
Instead, I moved most of my UI initialization in the ViewDidAppear method, and I like that it sped up my transition from tableview to new viewcontroller.
However, I cannot press any buttons in my NavigationBar at the top of the screen (like the back button) until my ViewDidAppear method completes and the UI is loaded.
What's the solution for this? Is there another way for me to load my UI without it preventing the user from interacting with the buttons in my NavigationBar?
Thanks!!
you do too much on the main thread. off load your longer operations like IO or longer computations BUT take care to not mess with the UI in the background thread.
Only touch the UI on the main thread. (Note sometimes it might seem safe, but in the long run it always end up producing weird issues)
one easy way is to use GCD:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^{
//insert web service requests / computations / IO here
dispatch_async(dispatch_get_main_queue(),^{
//back to the main thread for UI Work
});
});
You could use grand central dispatch to make your web service calls asynchronously, which will keep the UI on the main thread responsive.
//create new queue
dispatch_queue_t backgroundQueue = dispatch_queue_create("com.siteName.projectName.bgqueue", NULL);
//run requests in background on new queue
dispatch_async(backgroundQueue, ^{
//insert web service requests here
});
Here's a more in-depth tutorial:
http://www.raywenderlich.com/4295/multithreading-and-grand-central-dispatch-on-ios-for-beginners-tutorial
Try to initialize your UI in the background by using the following method
[self performSelectorInBackground:#selector(initYourUI) withObject:yourObj];
You can call this in the ViewDidLoad
Can you confirm that setting CALayer.contents property with CGImageRef in background thread still makes the core animation draw the contents image in main thread loop(which is UI Thread) rather than core animation thread or custom background thread which sets the contents property?
The reason I am asking this is that the core animation runs its own thread but however, it appears that when you set the CALayer.contents property the UI thread does the drawing to layer?
My experience is that if you have a visible layer and set the contents in a background thread, it is likely that will not draw immediately. The solution I used was to set the contents property asynchronously using a call to dispatch_async() on the main thread:
dispatch_async(dispatch_get_main_queue(), ^(void) {
layer.contents = (id)myCGImage;
});
In particular, my example here is for working with threads using GCD, where the thread itself may long outlive the operation that you are running. In this case, placing the assignment on the main thread, or forcing a [CATransaction flush] are two ways to indicate to the OS that you want that data to be presented to the user.
Otherwise, in the case of a GCD background thread, you are at the mercy of the thread's run loop which may not execute the [CATransaction flush] for a long time (in my experience, seconds).