Goal is to "launch a spinner graphic at start of viewWillAppear that loads data before showing the tableview" so the user doesn't wonder why there's a delay before seeing the table. I.e. a UIActivityIndicatorView has been added to the window, and I just want to set the alpha to hide/show it.
I get this strange error when starting a thread to make sure the "spinning gears" imageview (tag=333) gets shown, before moving on to load/calculate stuff in viewWillAppear.
I don't get it on every call to [appdel addGearz] and [appdel removeGearz], it happens for both these, and it's random. It can happen after 2 viewWillAppears, or after 15. If I comment out the line that sets the alpha, everything works.
A typical viewWillAppear looks something like this,
[super viewWillappear];
self.title=#"Products listing"; //and other simple things
[appdel addGearz];
[self getProducts];
[self getThumbnails];
[myTableView reloadData]; //in case view already loaded and coming back from subview and data changed
And here is the code that crashes if the lines with .alpha are not commented out
-(void)addGearz {
[NSThread detachNewThreadSelector:#selector(gearzOn) toTarget:self withObject:nil];
}
-(void)removeGearz {
[NSThread detachNewThreadSelector:#selector(gearzOff) toTarget:self withObject:nil];
}
- (void)gearzOn {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[window viewWithTag:333].alpha=1.0;
//
// [[window viewWithTag:333] setNeedsDisplay];
[pool drain];
}
- (void) gearzOff {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[window viewWithTag:333].alpha=0.0;
//
// [[window viewWithTag:333] setNeedsDisplay];
[pool drain];
}
I've used someone else's code, so... anything obvious you can see? Surely I must be able to change alpha of UIViews in a thread? Do I need to "embed" the alpha-change in some "stop enumerating while I change this"-code?
I made it not crash by moving that alpha-change-line to above the pool alloc or below the [pool drain], but then I get a lot of "autoreleased with no pool in place - just leaking"-messages.
Apparently, there's something I don't understand about this thread code.
You must not try to modify the UI on a separate thread. UI should only be manipulated on the main thread.
Instead of detaching a new thread, you should use performSelectorOnMainThread:withObject:waitUntilDone:. This will ensure that the method will be called on the proper thread.
Related
Scenario is like this--
In my app there is an Scroll View with many instances of
MyCustomImageDownloaderController(containing imageViews where images are to be assigned after downloading) .
As per our requirement, an image has to be downloaded as we move on to a page.
Scroll View + (MyCustomImageDownloaderController1, MyCustomImageDownloaderController2, MyCustomImageDownloaderController3.... etc)
Let's say i am scrolling on it,
i reached to page 1 --> image for it should start downloading
i reached to page 2 --> image for it should start downloading...so on
and if i am on page 3 and images for previous pages if not been dowloaded, they should stop downloading.
So i tried it with using threads..
on API..
- (void)scrollViewDidEndDecelerating:(UIScrollView *)sender{
Step 1) calculated currentPageNumber
Step 2) started thread for downloading image with url for this currentPage
//startBackGroundThreadForPlaceImage:(NSURL *) url
Step 3)stopped thread for previous page , if that is still running
}
Now My MyCustomImageDownloaderController is as
-(void) startBackGroundThreadForPlaceImage:(NSURL *) url{
if(isImageDownloaded == NO){
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
//[self performSelectorInBackground:#selector(loadImageInBackground:) withObject:imageUrl];
myThread = [[NSThread alloc] initWithTarget:self selector:#selector(loadImageInBackground:) object:imageUrl];
//[NSThread detachNewThreadSelector:#selector(loadImageInBackground:) toTarget:self withObject:imageUrl];
[myThread start];
NSLog(#"The current thread is %# ", [[NSThread currentThread] name]);
[pool release];
}
}
NOW Here selector does the work of loading image and assigning to image view
Now Stopping the thread
-(void) stopBackgroundThread{
[myThread cancel];
//[[NSThread currentThread] cancel];
//if([[NSThread currentThread] isCancelled]) {
//[NSThread exit];
//}
[NSThread exit];
}
-(BOOL) isThreadRunning{
return [myThread isExecuting];
}
So i tried a lot of things, but could not Stop the thread in between..
Basically once instantiated thread using any of three methods
1) perform Selector in BackGround
2) NSThread detach new thread
3) NSThread alloc..init with..
In first 2 methods how to get the instance of the newly created thread, so that i could stoop it,
as NSThread currentThread doest not give that
in Method 3,
myThread = [[NSThread alloc] initWithTarget:self selector:#selector(loadImageInBackground:) object:imageUrl];
when i tried
[myThread cancel];
It did not cancel that thread,,
When i tried
[NSThread exit];
it hangs on current screen,,,,i guess it has stopped the main thread
Please help me
Thanks in Advance*strong text*
It's generally better to ask the thread to stop, rather than forcing it, which should be considered a last resort. You should, therefore, frequently check a 'stop flag' and when this gets set, terminate the current thread (by simply exiting the method, in this case). You just need to provide a property on the class the thread is operating on so callers can ask the thread to stop.
It's no different in C++ or Java.
I'm trying to do something like this:
- (void)sectionChanged:(id)sender {
[self.view addSubview:loadingView];
// Something slow
[loadingView removeFromSuperview];
}
where loadingView is a semi-transparent view with a UIActivityIndicatorView. However, it seems like added subview changes don't take effect until the end of this method, so the view is removed before it becomes visible. If I remove the removeFromSuperview statement, the view shows up properly after the slow processing is done and is never removed. Is there any way to get around this?
Run your slow process in a background thread:
- (void)startBackgroundTask {
[self.view addSubview:loadingView];
[NSThread detachNewThreadSelector:#selector(backgroundTask) toTarget:self withObject:nil];
}
- (void)backgroundTask {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
// do the background task
[self performSelectorOnMainThread:#selector(backgroundTaskDone) withObject:nil waitUntilDone:NO];
[pool release];
}
- (void)backgroundTaskDone {
[loadingView removeFromSuperview];
}
Two potential problems spring to mind, both centred around how you've implemented the 'do something slow here' code.
First off, if it's locking up the main thread then it's possible the application's UI isn't being redrawn in time to display the view, i.e. Add Subview, tight loop/intensive processing tying up the main thread, then immediately after the view is removed.
Secondly if the 'something slow' is being done asynchronously, then the view is being removed while the slow processing is running.
One things for sure, your requirements are as follows:
Add a subview to display some kind of 'loading' view
Invoke a slow running piece of functionality
Once the slow running functionality completes, remove the 'loading' subview.
- (void)beginProcessing {
[self.view addSubview:loadingView];
[NSThread detachNewThreadSelector:#selector(process) toTarget:self withObject:nil];
}
- (void)process {
// Do all your processing here.
[self performSelectorOnMainThread:#selector(processingComplete) withObject:nil waitUntilDone:NO];
}
- (void)processingComplete {
[loadingView removeFromSuperview];
}
You could also achieve something similar with NSOperations.
I'm trying to add a progress meter, or other "I'm busy right now" notification to my view hierarchy right before doing some intense computation that will block the UI. My code looks some thing like:
//create view
[currentTopView addSubView:imBusyView];
//some initialization for the intense computation
[computation startComputing];
Unfortunately, my progress meter doesn't display until after the computation completes. It appears like the views aren't re-drawn until the run loop completes. I'm pretty sure that setNeedsDisplay and setNeedsLayout will also wait until the run loop completes.
How do I get the view to display immediately?
Redrawing only occurs when your code returns control to the run loop. So the easiest way would be for you to schedule the startComputing call with a zero delay. That way, it will be executed during the next run loop iteration (right after redrawing):
[computation performSelector:#selector(startComputing) withObject:nil afterDelay:0.0];
Be aware, though, that unless you do your computation in another thread you will not be able to update the UI during the computation.
If you are doing heavy calculations maybe spawning a new thread is a good idea.
Here I have an activityIndicator displayed and starts a large XMLParse operation in a background thread:
- (void) setSearchParser {
activityIndicator = [[ActivityIndicatorView alloc] initWithActivity];
[self.view addSubview:activityIndicator];
[NSThread detachNewThreadSelector:#selector(getSearchResults:) toTarget:self withObject:[searchParser retain]];
}
then the getSearchResults method:
- (void) getSearchResults: (SearchResultParser *) parser {
NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
[parser startParser];
[self performSelectorOnMainThread:#selector(searchResultsReady:) withObject:[parser data] waitUntilDone:NO];
[pool release];
}
So firstly make a new thread:
[NSThread detachNewThreadSelector:#selector(getSearchResults:) toTarget:self withObject:[searchParser retain]];
this means that all code inside the getSearchResults will be executed on a different thread. getSearchResults also get's passed a parameter of "searchParser" thats a large object that just needs startParse called on it to begin.
This is done in getSearchResults. When the [parser startParser] is done, the results is passed back to the main thread method called "searchResultsReady" and the threads autorelease pool is released.
All the time it took from my parser began to it had finished, a gray view covered the screen an an activityIndicator ran.
You can have the small activityIndicator class I wrote:
-(id) initWithActivity {
[self initWithFrame:[self bounds]];
[self setBackgroundColor:[UIColor blackColor]];
[self setAlpha:0.8];
activityView = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
activityView.center = CGPointMake(160, 240);
[self addSubview:activityView ];
[activityView startAnimating];
return self;
}
- (void) dealloc {
[activityView release];
[super dealloc];
}
Hope it helps you out, even though threads seems a bit confusing, they can help to make the UI not freeze up, which is especially important on the iPhone.
Kinda new to iPhone programming and was experimenting with threads
- (void)viewDidLoad {
[super viewDidLoad];
[NSThread detachNewThreadSelector:#selector(changeMain) toTarget:self withObject:nil];
[NSThread detachNewThreadSelector:#selector(changeThread) toTarget:self withObject:nil];
}
- (void)changeMain{
NSAutoreleasePool* arp = [[NSAutoreleasePool alloc] init];
for (int i = 0; i < 1000000; i++) {
[mainValue setText:[NSString stringWithFormat:#"%d",i]];
[self.view setNeedsDisplay];
}
[arp release];
}
- (void)changeThread{
NSAutoreleasePool* arp = [[NSAutoreleasePool alloc] init];
for (int i = 0; i < 1000000; i++) {
[threadValue setText:[NSString stringWithFormat:#"%d",i]];
[self.view setNeedsDisplay];
}
[arp release];
}
mainValue and threadValue are both just UILabels. I expected this to run and see both labels run up to 999999 but instead it starts at some low number (what it is when the screen initally refreshing i assume), pauses for a bit, then updates to 999999. I'm thinking the screen just isn't refreshing.
Is this correct? Am I doing it wrong?
You have to perform any Cocoa Touch operations in main thread, in other case results are unpredictable.
You don't have to call setNeedsDisplay manually.
So I'd recommend to use the following construction:
[threadValue performSelectorOnMainThread:#selector(setText:) withObject:[NSString stringWithFormat:#"%d",i] waitUntilDone:YES];
Additional notes:
1. 100000 runs can overflow the main thread queue so some values will disappear
2. You can use waitUntilDone:NO too
The setNeedsDisplay message triggers a redraw, but it only happens during the next time the main thread becomes active. So your side threads trigger a million redraws but they are queued. As soon as the main thread continues, it "collapses" all requests into one redraw.
Most likely setNeedsDisplay just sets a flag that is checked once during each run of the main loop, so setting it 1000000 to true doesn't influence anything. Try to let the "worker threads" sleep after each iteration to give the main thread some time to redraw.
Don't use for() for animations. The for will process in the same "frame". Maybe just have an ivar i and in changeMain you can have if (i<10000) { mainValue.text = [NSString stringWithFormat:#"%d",i]; i++;} or something like that. This way the setText only happens once per "frame".
I'm not sure if this will work, but you could try to force the setNeedsDisplay method to be executed on the main thread, using e.g. [self performSelectorOnMainThread:#selector(setNeedsDisplay) withObject:nil waitUntilDone:YES]. This should (hopefully, i didn't test it!) update the view after every increment. You could also try to set waitUntiDone:NO, but I'm unsure what will happen then.
See here
I have a background task that updates a view. That task calls -setNeedsDisplay to have the view drawn.
This works:
- (void) drawChangesTask;
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
if (pixels) {
drawChanges((UInt32 *) origPixels, (UInt32 *) pixels, CGBitmapContextGetBytesPerRow(ctx)/4, CGBitmapContextGetHeight(ctx), count--);
if (count < 0) {
count = 150;
}
else
[self performSelectorInBackground:#selector(drawChangesTask) withObject:nil ];
[self performSelectorOnMainThread:#selector(setNeedsDisplay) withObject:nil waitUntilDone:NO ];
}
[pool release];
}
This does not work:
- (void) drawChangesTask;
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
if (pixels) {
drawChanges((UInt32 *) origPixels, (UInt32 *) pixels, CGBitmapContextGetBytesPerRow(ctx)/4, CGBitmapContextGetHeight(ctx), count--);
if (count < 0) {
count = 150;
}
else
[self performSelectorInBackground:#selector(drawChangesTask) withObject:nil ];
[self setNeedsDisplay];
}
[pool release];
}
Anyone know why? When I say it doesn't work, I mean that it runs tens of iterations, sometimes I see portions of my image shifted up or down, or entirely blank, and then the deugger give me an “EXC_BAD_ACCESS” somewhere in CoreGraphics.
Also, if I don't handle the autorelease pool myself, then I get leaking error messages. Don't understand why that is either. My drawChanges() doesn't create any new objects. Here's the error:
2009-08-17 11:41:42.358 BlurApp[23974:1b30f] *** _NSAutoreleaseNoPool(): Object 0xd78270 of class NSThread autoreleased with no pool in place - just leaking
UIKit simply isn't thread-safe — you need to call methods that update UIKit controls on the main thread.
I think that this line:
[self performSelectorInBackground:#selector(drawChangesTask) withObject:nil];
Is causing trouble. Have you tried simply calling it again on the current thread? If you need the runloop to execute between the calls, use:
[self performSelector:#selector(drawChangesTask) withObject:nil afterDelay:0.0];
This will call the method on the current thread after the method you're in has finished and the runloop has gone round once.
Problem here is that UIKit is not thread safe, if you tell your UI to do something from a background thread nothign is guaranteed, what you want to do is use the performSelectorOnMainThread method to do updates t o your UI elements