I have a read-only text field that I use as a log display. I have a operation that removes all the files in app's document directory. I want to insert a line of log when I remove each file. But the text field is only updated when the whole operation got finished. How do I fix this?
Here is my code:
NSFileManager *fm = [[[NSFileManager alloc] init] autorelease];
NSError *error = nil;
for (NSString *fileName in array) {
NSString *filePath = [DOCUMENT_PATH_VALUE stringByAppendingFormat:#"/%#", fileName];
[fm removeItemAtPath:filePath error:&error];
if (!error) {
NSString *log = [NSString stringWithFormat:#"removed success: %#", fileName];
[self logThis:log];
}else{
NSString *log = [NSString stringWithFormat:#"remove failed: %#, %#", fileName, [error localizedDescription] ];
[self logThis:log];
-(void)logThis:(NSString*) text{
NSRange range = NSMakeRange([updateLogTextView.text length], [text length]);
updateLogTextView.text = [updateLogTextView.text stringByAppendingFormat:#"%#\n", text];
[updateLogTextView scrollRangeToVisible:range];
}
You need to move your long-running operation into a background thread/queue and make sure that your UI-updating code is always executed on the main thread.
Example:
- (void)processFiles:(NSArray *)array
{
//You need to create your own autorelease pool in background threads:
NSAutoreleasePool *pool = [NSAutoreleasePool new];
//...
NSString *log = ...;
[self performSelectorOnMainThread:#selector(logThis:) withObject:log waitUntilDone:NO];
//...
[pool release];
}
You would then start your operation using:
[self performSelectorInBackground:#selector(processFiles:) withObject:files];
Using Grand Central Dispatch with blocks would be another option.
Refer to the Threading Programming Guide and the Concurrency Programming Guide for more in-depth information.
You need to look into Threading and making that call Asynchronous. You are currently blocking your main thread (assuming it's called from there). You need to separate the two so a separate thread does the heavy operation while your main thread is updated with the new text in the UI.
I've just started ios dev but sounds like you need to redraw that UIText I'm not sure how though I'd assume that when your log writes that you could dealloc your original UIText object the realloc it and it should redraw with the new log message. Only problem is that it will most likely update very very fast so redrawing wouldnt be worth it unless ios has a sleep function?
Something like:
Alloc and init text
Then in for loop after each write or new message in log dealloc your text and realloc and init your text
This should redraw the UIText object and update the text, if it doesnt expect a memory write error/app crash
Related
I would like to update a property of my ViewController self.matchedUsers, which takes data from a query that I run asynchronously through a block.
Then somewhere later When I retrieve the count via [self.matchedUsers count], I still get 0, despite knowing that multiple objects was added to my property. My question is, how do I ensure that my property gets updated even when I am retrieving data asynchronously through a block? Thanks!
Update:
For context, here is the block:
//Way earlier in an initializer:
self.matchedUsers = [[NSMutableArray alloc] init];
//In a method much later
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
if (!error){
//Code that updates self.matchedUsers with the NSArray objects
dispatch_async(dispatch_get_main_queue(), ^{
[self.matchedUsers addObjectsFromArray: objects];
});
//Question relates to ensure how property could be updated
}
}];
This should work provided you didn't forget to initialize matchedUsers, you check for its value after it's been changed and array does not lose its elements between the time you schedule and execute the block.
However, I would prefer to write a method that can be called from any thread, say
- (void)addUser ...
#synchronized(self.usersToAdd) {
[self.usersToAdd addObjectsFromArray: array];
Enqueue on main thread {
NSArray *addingNow;
#synchronized(self.usersToAdd) {
addingNow = [self.usersToAdd copy];
[self.usersToAdd removeObjects...
}
if (addingNow.count) {
[self.users addObjectsFromArray: addingNow;
[self.tableView insertRowsWithIndexPaths...
}
}
}
}
As others have written the problem could be missing initialization of matchedUsers but...
...the problem could also be due to your main thread being blocked. You write that you "somewhere later retrieve the count". If that is within the same method as the one that made the first dispatch you will be in trouble. Consider this code
NSMutableArray *collection = [[NSMutableArray alloc] init];
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSArray *array = [[NSArray alloc] initWithObjects:#"1", nil];
dispatch_async(dispatch_get_main_queue(), ^{
for (NSString *item in array){
[collection addObject:item];
}
NSLog(#"A");
});
});
[NSThread sleepForTimeInterval:5];
NSLog(#"B");
If this is running on the main thread it will output first B on then A (no matter the sleep time), because the block is not run until the method finishes executing. If you on the other hand dispatch to another global queue instead of the main queue it will be A and then B.
I want to perform a image upload while the running application at the background. I am able to upload the image to the server using the code on this link.
How can I upload a photo to a server with the iPhone?
I heard the NSUrlConnection can be asynchronous and it was used in the EPUploader. In my code, I add some extra method that will create a file in the application directory used for the EPUploader. During this creation of the file, I don't want it to create on the main thread of the application so I wrap all the code including the EPUploader itself with the
dispatch_async on the global queue. That way I won't block the main thread while the file are creating.
It has no problem if I use dispatch_sync but dispatch_async I find something weird when I placed the breakpoint at NSUrlConnection connection :
- (void)upload
{
NSData *data = [NSData dataWithContentsOfFile:filePath];
//ASSERT(data);
if (!data) {
[self uploadSucceeded:NO];
return;
}
if ([data length] == 0) {
// There's no data, treat this the same as no file.
[self uploadSucceeded:YES];
return;
} /* blah blah */
NSURLConnection * connection = [[NSURLConnection alloc] initWithRequest:urlRequest delegate:self];
if (!connection) {
[self uploadSucceeded:NO];
return;
}
else
return;
I went to debug at the breakpoint and instead of going to if statement, the debugger jumps to the first return statement of this method. After that, the selectors I passed on to this class never gets called. This happen only on dispatch_async and it work on dispatch_sync on the global queue.
Does anybody know how to solve this issue?
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);
dispatch_async(queue, ^{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
self.uploadIndex = 0;
ALAsset *asset = [self.assets objectAtIndex:0];
[[FileUploader alloc] initWithAsset:[NSURL URLWithString:#"http://192.168.0.3:4159/default.aspx"]
asset:asset
delegate:self
doneSelector:#selector(onUploadDone:)
errorSelector:#selector(onUploadError:)];
//[self singleUpload:self.uploadIndex];
[pool release];
});
There are a couple of things that should be changed.
Remove the NSAutoreleasePool, it is not needed.
Copy the block to the heap because it's life will probably exceed that of the calling code.
Example:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);
dispatch_async(queue, [[^{
self.uploadIndex = 0;
ALAsset *asset = [self.assets objectAtIndex:0];
[[FileUploader alloc] initWithAsset:[NSURL URLWithString:#"http://192.168.0.3:4159/default.aspx"]
asset:asset
delegate:self
doneSelector:#selector(onUploadDone:)
errorSelector:#selector(onUploadError:)];
} copy] autorelease]);
If you are using ARC (which you certainly are since you should be) there is no need for the copy or autorelease.
I'm attempting to download an MP3 file from my server when a user selects a row using blocks and a dispatch_queue. Things seem to work great about 80% of the time.
Here is my thought process:
When the user selects the row, update the UI to show that a download has begun (spinner)
Add a block to a background thread to begin the download in the background (I use the thread blocking dataWithContentsOfURL method. I believe this is ok because it is on an async dispatch queue.
on the main_queue, When the download is completed, create an AVAudioPlayer with the data from the dispatch queue.
on the main_queue, Play sound file.
on the main_queue, update UI to reflect that the sound is playing and the download is complete.
Like I said, this works perfectly 80% of the time. When something works 'most' of the time, it screams memory management issues.
So here is the snippet:
//download url
dispatch_queue_t downloadQueue = dispatch_queue_create("com.mycompany.audiodownload", NULL);
dispatch_retain(downloadQueue);
JAAudioMessageEvent *event = [self.cheers objectAtIndex:indexPath.row];
dispatch_async(downloadQueue, ^{
NSURL *sampleURL = [NSURL URLWithString:event.sample];
NSLog(#"start downloading file %#", sampleURL);
NSData *data = [NSData dataWithContentsOfURL:sampleURL];
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"play file %#", (data == nil) ? #"data file is nil":#"");
NSError *error;
self.eventPlayer = [[AVAudioPlayer alloc] initWithData:data error:&error];
if (error) {
NSLog(#"there was an error when playing %#", error);
}
[self.eventPlayer play];
self.eventPlayer.delegate = self;
[activityIndicator removeFromSuperview];
[activityIndicator release];
self.eventInProgress = NO;
});
dispatch_release(downloadQueue);
});
So the issue is that during this 20% of the time when things go wrong, it would seem as though the event is being released and I'm given a null url. I fetch a null url and I get an error when attempting to play said null url.
So the question becomes, how do I manage the event object so that it persists over the 2 async threads?
Thanks in advance
I have a small iPhone app which has a button on the first view. When I select this button I load up my new view which has an image on it. I'm currently using the following code to load the image from an online source on a separate thread, whilst allowing the user to continue controlling the application:
- (void) loadImageTest
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSURL *url = [[NSURL alloc] init];
url = [NSURL URLWithString:logoPath];
NSData *data = [[NSData alloc] init];
data = [NSData dataWithContentsOfURL:url];
loadingImage = [UIImage imageWithData:data];
titleLogoImage.image = loadingImage;
//[pool drain];
[pool release];
}
This is called from this line of code in the new view's init:
[NSThread detachNewThreadSelector:#selector(loadImageTest) toTarget:self withObject:nil];
Now this works fine (ish), but if I close the new view and then load a new one up again in quick succession (or just after-wards in some cases) it will bomb out with the classic EXC_BAD_ACCESS. I'm sure that this is due to the code within the thread pool, but can anyone see why this is happening?
Thanks
Yours:
// This is ok, you might try using NSURLConnections asynchronous methods instead of making
// your own thread.
[NSThread detachNewThreadSelector:#selector(loadImageTest) toTarget:self withObject:nil];
- (void)loadImageTest
{
// This is fine
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
// you're making and then abandoning this url object so it will leak
NSURL *url = [[NSURL alloc] init];
// this is fine
url = [NSURL URLWithString:logoPath];
// again making and abandoning an object
NSData *data = [[NSData alloc] init];
data = [NSData dataWithContentsOfURL:url];
// this works, but is not thread safe,
// can't operate on UIKit objects off the
// main thread
loadingImage = [UIImage imageWithData:data];
titleLogoImage.image = loadingImage;
// this is fine
//[pool drain];
[pool release];
}
Cleaned up to make things thread safe, etc. Should help:
// I'm assuming you have a iVar for logoPath but we don't want to
// make threaded calls to an iVar (it's not mutable, so you could do it, but it's just bad form)
// If i'm wrong about logoPath being an iVar don't sweat copying it.
- (void)whateverMethodYouWant
{
NSString *aLogoPath = [[logoPath copy] autorelease];
[NSThread detachNewThreadSelector:#selector(loadImageForPath:) toTarget:self withObject:aLogoPath];
}
- (void)loadImageForPath:(NSString *)aLogoPath
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:aLogoPath]];
// the performSelector will retain the data until the method can be performed
[self performSelectorOnMainThread:#selector(setImageForTitleLogo:) withObject:imgData waitUntilDone:NO];
[pool release];
}
- (void)setImageForTitleLogo:(NSData *)imgData
{
// This part is not strictly necessary
// but it's a nice way to wrap a method that needs to happen on the main thread.
if ([NSThread isMainThread])
{
// make the image (i'm assuming you meant loadingImage to be a local scope variable)
UIImage *loadingImage = [UIImage imageWithData:imgData];
// make sure the button still exists
// also make sure you're setting any references to this button to nil when you're releasing and making new views
// so that you're not addressing a button that's been released
// (assigning the image to the button will cause the button to retain it for you)
if (titleLogoImage != nil)
titleLogoImage.image = loadingImage;
}
else
{
[self performSelectorOnMainThread:#selector(setImageForTitleLogo:) withObject:imgData waitUntilDone:NO];
}
}
You're doing weird stuff.
NSURL *url = [[NSURL alloc] init];
means you create an NSURL object, which you own.
url = [NSURL URLWithString:logoPath];
This means you create another NSURL object, which is autoreleased. The NSURL you just created, just leaks. The same goes for the NSData here.
The loadingImage must be retained by titleLogoImage, or it will be deallocated on the drain of your NSAutoReleasePool. What is titleLogoImage and does it retain image?
edit ps: what also disturbes me, is that loadingImage is not confined to the scope of the function. To cut things short:
NSURL *url = [NSURL URLWithString:logoPath];
NSData *data = [NSData dataWithContentsOfURL:url];
titleLogoImage.image = [UIImage imageWithData:data];
may save some headaches, at least.
There is nothing in the posted code that would trigger a crash. Depending on how titleLogoImage is defined, it might be affects by the autorelease.
However, beyond the problems outlined by mvds, you have no pressing needs for a localized autorelease pool. It will do nothing here but cause you trouble.
Autorelease pools are dangerous and not for beginners. They will kill any autoreleased objects in them. You generally only create your own pool when you are rapidly creating a large number of memory intensive objects. That does not appear to be the case here.
Despite the attention given them, custom pools are seldom used. After over a decade Apple API work, I can probably count on my fingers the number of times I have used my own custom pool.
I have a strange issue, when it comes to parsing XML with NSXMLParser on the iPhone. When starting the app, I want to preload 4 table-views, that are populated by RSS-Feeds in the background.
When I init the table-views one-by-one, than loading, parsing and displaying all works like a charm. But when I try to init all view at once (at the same time), than it seems, that the XML-parser-instances are disturbing each other. Somehow data from one XML-Feed are "broadcasted" into other xml-parser instances, where they do not belong. Example: there is a "teammember" item, with "This is my name". When this bug occurs, there is a string from another xml-feed added, i.e. resulting in: "This is my name58", where 58 is the chart-position of something from the other view. "58" seems to miss then on the other instance.
It looks to me, that this bug occurs because of the NSXMLParser-delegate method:
- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string {
if (!currentStringValue) {
currentStringValue = [[NSMutableString alloc] initWithCapacity:50];
}
[currentStringValue appendString:string];
}
In this case "by coincidence" bytes are appended to strings, where they do not belong to.
The strange thing is, that every instance of NSXMLParser is unique, got its own unique delegates, that are attached to their own ViewController. Every parsing-requests spawns it own background-task, with its own (also also unique named) Autorelease-pool.
I am calling the NSXMLParser like this in the ViewController:
// prepare XML saving and parsing
currentStringValue = [[[NSMutableString alloc] initWithCapacity:50] retain];
charts = [[NSMutableArray alloc] init];
NSURL *url = [[NSURL alloc] initWithString:#"http://(SOME XML URL)"];
xmlParser = [[[NSXMLParser alloc] initWithContentsOfURL:url] retain];
//Set delegate
[xmlParser setDelegate:self];
//loading indicator
progressWheel = [[[UIActivityIndicatorView alloc] initWithFrame:CGRectMake(150.0,170.0,20.0,20.0)] autorelease];
progressWheel.activityIndicatorViewStyle = UIActivityIndicatorViewStyleGray;
[self.view addSubview:progressWheel];
[progressWheel startAnimating];
// start loading and parsing the xml-feed in the background
//[self performSelectorInBackground:#selector(parse:) withObject:xmlParser]; -> I also tried this
[NSThread detachNewThreadSelector:#selector(parse:) toTarget:self withObject:xmlParser];
And this is one of the background-tasks, parsing the feed:
-(void)parse:(NSXMLParser*)myParser {
NSAutoreleasePool *schedulePool = [[NSAutoreleasePool alloc] init];
BOOL success = [myParser parse];
if(success) {
NSLog(#"No Errors. xmlParser got: %#", myParser);
(POST-PROCESSING DETAILS OF THE DATA RETURNED)
[self.tableView reloadData];
} else {
NSLog(#"Couldn't initalize XMLparser");
}
[progressWheel stopAnimating];
[schedulePool drain];
[myParser release];
}
What could cause this issue? Am I calling the background-task in the right way? Why is this bug approaching, since every XML-Parser got its own, unique instance?
You should not be updating UI elements (like progressWheel) from inside a background thread. UI updates should be done on the main thread.
Use -performSelectorOnMainThread:withObject:waitUntilDone: to update UI elements from within a background thread.
I've released an open source RSS/Atom Parser for iPhone and it makes reading and parsing web feeds extremely easy.
You can set it to download the data asynchronously, or you could run it in a background thread synchronously to collect the feed data.
Hope this helps!