I am trying to create a method that will return me a ALAsset for a given asset url. (I need upload the asset later and want to do it outside the result block with the result.)
+ (ALAsset*) assetForPhoto:(Photo*)photo
{
ALAssetsLibrary* library = [[[ALAssetsLibrary alloc] init] autorelease];
__block ALAsset* assetToReturn = nil;
NSURL* url = [NSURL URLWithString:photo.assetUrl];
NSLog(#"assetForPhoto: %#[", url);
[library assetForURL:url resultBlock:^(ALAsset *asset)
{
NSLog(#"asset: %#", asset);
assetToReturn = asset;
NSLog(#"asset: %# %d", assetToReturn, [assetToReturn retainCount]);
} failureBlock:^(NSError *error)
{
assetToReturn = nil;
}];
NSLog(#"assetForPhoto: %#]", url);
NSLog(#"assetToReturn: %#", assetToReturn); // Invalid access exception coming here.
return assetToReturn;
}
The problem is assetToReturn gives an EXC_BAD_ACCESS.
Is there some problem if I try to assign pointers from inside the block? I saw some examples of blocks but they are always with simple types like integers etc.
A few things:
You must keep the ALAssetsLibrary instance around that created the ALAsset for as long as you use the asset.
You must register an observer for the ALAssetsLibraryChangedNotification, when that is received any ALAssets you have and any other AssetsLibrary objects will need to be refetched as they will no longer be valid. This can happen at any time.
You shouldn't expect the -assetForURL:resultBlock:failureBlock:, or any of the AssetsLibrary methods with a failureBlock: to be synchronous. They may need to prompt the user for access to the library and will not always have their blocks executed immediately. It's better to put actions that need to happen on success in the success block itself.
Only if you absolutely must make this method synchronous in your app (which I'd advise you to not do), you'll need to wait on a semaphore after calling assetForURL:resultBlock:failureBlock: and optionally spin the runloop if you end up blocking the main thread.
The following implementation should satisfy as a synchronous call under all situations, but really, you should try very hard to make your code asynchronous instead.
- (ALAsset *)assetForURL:(NSURL *)url {
__block ALAsset *result = nil;
__block NSError *assetError = nil;
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
[[self assetsLibrary] assetForURL:url resultBlock:^(ALAsset *asset) {
result = [asset retain];
dispatch_semaphore_signal(sema);
} failureBlock:^(NSError *error) {
assetError = [error retain];
dispatch_semaphore_signal(sema);
}];
if ([NSThread isMainThread]) {
while (!result && !assetError) {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
}
}
else {
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
}
dispatch_release(sema);
[assetError release];
return [result autorelease];
}
You should retain and autorelease the asset:
// ...
assetToReturn = [asset retain];
// ...
return [assetToReturn autorelease];
Related
Is it something wrong with the requestPanoramaNearCoordinate Google maps SDK method? cause it get´s stuck in the while loop. I´v written the loop cause I want to wait with executing the rest of the method until the asynchronous callback method has completed. But the the while loop loops infinitely. Is it my code that´s simply wrong?
__block GMSPanorama *panPhoto = nil;
__block BOOL finished = NO;
[self.panoService requestPanoramaNearCoordinate:ranLatLng callback:^(GMSPanorama *panorama, NSError *error) {
NSLog(#"panorama: %# error: %#", panorama, error);
panPhoto = panorama;
finished = YES;
}];
while (!finished) {
// Do nothing);
}
if (!panPhoto) return [self randomLatitudeLongitude];
return ranLatLng;
}
Why you launch async method and then doing loop? You must add block (with GMSPanorama argument) as parameter to your method and call this block inside callback:^(GMSPanorama *panorama, NSError *error){
Smth like that:
- (void) methodNameWithBlock:(BlockName)block;
__block GMSPanorama *panPhoto = nil;
__block BOOL finished = NO;
[self.panoService requestPanoramaNearCoordinate:ranLatLng callback:^(GMSPanorama *panorama, NSError *error) {
NSLog(#"panorama: %# error: %#", panorama, error);
panPhoto = panorama;
finished = YES;
BlockName handler = [block copy];
if (!ranLatLng){
handler([self randomLatitudeLongitude])
} else {
handler(ranLatLng)
}
}];
}
I've got this parsing operation that currently works fine, but I've started to notice that it is freezing up my UI slightly so I'm trying to refactor and get this done asynchronously. I'm having some issues however and was hoping someone could point me in the right direction. Here's my current (synchronous) code:
- (NSArray *)eventsFromJSON:(NSString *)objectNotation
{
NSParameterAssert(objectNotation != nil);
NSData *unicodeNotation = [objectNotation dataUsingEncoding:NSUTF8StringEncoding];
NSError *error = nil;
NSDictionary *eventsData = [NSJSONSerialization JSONObjectWithData:unicodeNotation options:0 error:&error];
if (eventsData == nil) {
//invalid JSON
return nil;
}
NSArray *events = [eventsData valueForKeyPath:#"resultsPage.results"];
if (events == nil) {
//parsing error
return nil;
}
NSLog(#"events looks like %#", events);
NSMutableArray *formattedEvents = [NSMutableArray arrayWithCapacity:events.count];
for (id object in [events valueForKeyPath:#"event"]) {
Event *event = [[Event alloc] init];
event.latitude = [object valueForKeyPath:#"location.lat"];
event.longitude = [object valueForKeyPath:#"location.lng"];
event.title = [object valueForKeyPath:#"displayName"];
event.venue = [object valueForKeyPath:#"venue.displayName"];
event.ticketsLink = [NSURL URLWithString:[object valueForKeyPath:#"uri"]];
event.artist = [object valueForKeyPath:#"performance.artist.displayName"];
event.date = [object valueForKeyPath:#"start.datetime"];
[formattedEvents addObject:event];
}
return [NSArray arrayWithArray:formattedEvents];
}
I've been looking into NSOperationQueue's and I'm struggling to find a solution as I'd like to return an array from this method and operation queues are not meant to have return values. I'm also looking at GCD and i've got somethinbg like this:
- (NSArray *)eventsFromJSON:(NSString *)objectNotation
{
dispatch_queue_t backgroundQueue =
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
__block NSMutableArray *mutable = [NSMutableArray array];
dispatch_async(backgroundQueue, ^{
NSParameterAssert(objectNotation != nil);
NSData *unicodeNotation = [objectNotation dataUsingEncoding:NSUTF8StringEncoding];
NSError *error = nil;
NSDictionary *eventsData = [NSJSONSerialization JSONObjectWithData:unicodeNotation options:0 error:&error];
if (eventsData == nil) {
//invalid JSON
mutable = nil;
}
NSArray *events = [eventsData valueForKeyPath:#"resultsPage.results"];
if (events == nil) {
//parsing error
mutable = nil;
}
NSLog(#"events looks like %#", events);
NSMutableArray *formattedEvents = [NSMutableArray arrayWithCapacity:events.count];
for (id object in [events valueForKeyPath:#"event"]) {
Event *event = [[Event alloc] init];
event.latitude = [object valueForKeyPath:#"location.lat"];
event.longitude = [object valueForKeyPath:#"location.lng"];
event.title = [object valueForKeyPath:#"displayName"];
event.venue = [object valueForKeyPath:#"venue.displayName"];
event.ticketsLink = [NSURL URLWithString:[object valueForKeyPath:#"uri"]];
event.artist = [object valueForKeyPath:#"performance.artist.displayName"];
event.date = [object valueForKeyPath:#"start.datetime"];
[formattedEvents addObject:event];
}
mutable = [NSMutableArray arrayWithArray:formattedEvents];
});
return [mutable copy];
}
For some reason, this seems to be returning the object before the parsing has finished however, as I'm gettting no data out of that mutable object, but I'm noticing that the parsing is indeed occurring (i'm logging out the results). can anyone give me an idea about how to get this asynch stuff going?
Thanks!!
You primary problem is that by their very nature asynchronous operations can't synchronously return a result. Instead of returning an array from -eventsFromJSON:, you should provide a way for the caller to receive a callback when the results are finished. There are two common approaches to this in Cocoa.
You can create a delegate with an associated delegate protocol including a method like -parser:(Parser *)parser didFinishParsingEvents:(NSArray *)events, then have your parser call this method on its delegate when parsing is finished.
Another solution is to allow the caller to provide a completion block to be executed when parsing is complete. So, you might do something like this:
- (void)eventsFromJSON:(NSString *)objectNotation completionHandler:(void (^)(NSArray *events))completionHandler)
{
dispatch_queue_t backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(backgroundQueue, ^{
NSMutableArray *mutable = [NSMutableArray array];
NSParameterAssert(objectNotation != nil);
NSData *unicodeNotation = [objectNotation dataUsingEncoding:NSUTF8StringEncoding];
NSError *error = nil;
// Snip...
mutable = [NSMutableArray arrayWithArray:formattedEvents];
dispatch_async(dispatch_get_main_queue(), ^{
completionHandler([mutable copy]);
});
});
}
Then you can call this code some thing like this:
- (void)parseJSONAndUpdateUI // Or whatever you're doing
{
NSString *jsonString = ...;
Parser *parser = [[Parser alloc] init];
[parser parseEventsFromJSON:jsonString completionHandler:^(NSArray *events){
// Update UI with parsed events here
}];
}
I like the second, block-based approach better. It makes for less code in most cases. The code also reads closer to the synchronous approach where the method just returns an array, since the code that uses the resultant array simply follows the method call (albeit indented since it's in the completion block's scope).
I would recommend using a completion block that you pass into your parse method. This way you don't have to return a value, but can do what you need to with the information once it is parsed. You just have to make sure you use GCD again to put the completion block on the main thread.
You could also post a notification on the main thread once the operation is complete that contains the array in userInfo.
Returning a value will not work however for asynchronous operations.
You are getting a returned object before the parsing has finished because your return [mutable copy] is outside of the dispatch_async block. Since dispatch_async functions asynchronously, it will return immediately, and then calls your return [mutable copy] (which is empty because it's not done parsing).
Source
So, I have a QTCaptureSession set up thusly:
//Setup Camera
cameraSession = [[QTCaptureSession alloc] init];
QTCaptureDevice *camera = [QTCaptureDevice deviceWithUniqueID: cameraID];
BOOL success = [camera open: &error];
if (!success || error)
{
NSLog(#"Could not open device %#.", cameraID);
NSLog(#"Error: %#", [error localizedDescription]);
return nil;
}
//Setup Input Session
QTCaptureDeviceInput *cameraInput = [[QTCaptureDeviceInput alloc] initWithDevice: camera];
success = [cameraSession addInput: cameraInput error: &error];
if (!success || error)
{
NSLog(#"Could not initialize input session.");
NSLog(#"Error: %#", [error localizedDescription]);
return nil;
}
//Setup Output
QTCaptureDecompressedVideoOutput *cameraOutput = [[QTCaptureDecompressedVideoOutput alloc] init];
[cameraOutput setDelegate: self];
success = [cameraSession addOutput: cameraOutput error: &error];
if (!success || error)
{
NSLog(#"Could not initialize output session.");
NSLog(#"Error: %#", [error localizedDescription]);
return nil;
}
And the QTCaptureDecompressedVideoOutput delegate's captureOutput:didOutputVideoFrame:WithSampleBuffer:fromConnection: thusly:
- (void)captureOutput:(QTCaptureOutput *)captureOutput didOutputVideoFrame:(CVImageBufferRef)videoFrame withSampleBuffer:(QTSampleBuffer *)sampleBuffer fromConnection:(QTCaptureConnection *)connection
{
NSLog(#"starting convert\n");
}
I then start the capture processing using:
[cameraSession startRunning];
All of the variables initialize fine, and the session starts fine, but captureOutput:didOutputVideoFrame:withSampleBuffer:fromConnection: never gets called.
Context
This is a command-line app, compiled with the GCC. It's linked against the following frameworks:
Foundation
Cocoa
QTKit
QuartzCore
Relevant Miscellany
The frame is not likely dropping because captureOutput:didDropVideoFrameWithSampleBuffer:fromConnection: is also not getting called.
So, with some help from Mike Ash, I managed to figure out that my program was terminating immediately and not waiting for the delegate callback (which, according to Apple's QTKit docs, might occur on a separate thread).
My solution was to add a BOOL properties to my object named captureIsFinished, then add this to the main() function:
//Wait Until Capture is Finished
while (![snap captureIsFinished])
{
[[NSRunLoop currentRunLoop] runUntilDate: [NSDate dateWithTimeIntervalSinceNow: 1]];
}
Which effectively perpetuates the run-loop of the app for 1 second, checks to see if the capture is finished, then runs for another second.
I have a set of asynchronous calls being spawned using NSInvocationOperation:
- (void)listRequestQueue:(StoreDataListRequest *)request {
[openListRequests addObject:request];
NSInvocationOperation *requestOp = [[NSInvocationOperation alloc]
initWithTarget:self
selector:#selector(listRequestStart:)
object:request];
[opQueue addOperation:requestOp];
[requestOp release];
}
- (void)listRequestStart:(StoreDataListRequest *)request {
if(self.resourceData == nil) {
//TODO fail appropriately...
return;
}
StoreDataListResponse *response = [self newListResponseForProductID:request.productID];
[self performSelectorOnMainThread:#selector(listRequestFinish:)
withObject:response waitUntilDone:NO];
[response release];
[self performSelectorOnMainThread:#selector(cleanUpListRequest:)
withObject:request waitUntilDone:NO];
}
- (void)listRequestFinish:(StoreDataListResponse *)response {
[self.delegate storeData:self didReceiveListResponse:response];
}
- (StoreDataListResponse *)newListResponseForProductID:(NSString *)productID {
CollectionData *data = [self.resourceData dataForProduct:productID];
if(data == nil) {
//TODO do something
}
StoreDataListResponse *response = [[StoreDataListResponse alloc] init];
response.productID = productID;
if(productID != data.productID) {
//TODO fail; remove product from list
}
response.name = NSLocalizedString(#"Loading...", #"Loading message");
response.blurb = NSLocalizedString(#"Waiting for response from server", #"Waiting for website to respond");
return response;
}
For each of the TODOs in the above code, I should resolve the issue and let any handlers know that things have failed and why. Looking at the NSError class and documentation, it appears to be the appropriate answer but I am having trouble figuring out how to get to work with NSInvocationOperation and performSelectorOnMainThread:withObject:waitUntilDone:. I can certainly get the NSErrors out of the newListResponseForProductID: method by changing it to something like this:
- (StoreDataListResponse *)newListResponseForProductID:(NSString *)productID error:(NSError **)error;
How do I get the error generated back into the main thread so I can deal with the failed request?
The easiest way to run any code on the main thread is to use GCD and blocks on iOS 4 and above. Like this:
dispatch_async(dispatch_get_main_queue(), ^(void) {
// Your code to run on the main thread here,
// any variables in scope like NSError is captured.
});
Or use dispatch_sync() if you know you are on a background thread and want the block to complete on the main thread before you continue.
I am trying to download a file from a server. My code is following. In didFinishLaunchingWithOptions method, I create a new thread using detachNewThreadSelector which runs the following code.
NSString *destPath = [self.home_dir_path stringByAppendingPathComponent:[NSString stringWithFormat:#"_%#",content_data_file_name]];
[ContentBO downloadFile:destPath content_name:content_data_file_name];
if([self updatesAvailable]){
//update content
}else{
//launch app
}
My code for downloadFile is:
#try{
NSString *url = [NSString stringWithFormat:#"%#/%#",ServerURL,content_name];
NSLog(#"downloading URL is: %#",url);
self.request = [ASIHTTPRequest requestWithURL:[NSURL URLWithString:[url stringByAddingPercentEscapesUsingEncoding:NSASCIIStringEncoding]]];
[self.request setRequestMethod:#"GET"];
[self.request setDownloadDestinationPath:destFilePath];
NSLog(#"destination path is: %#",destFilePath);
[self.request setTimeOutSeconds:30];
[self.request setDelegate:self];
[self.request startSynchronous];
NSError *error = [self.request error];
NSData *receivedData = nil;
if (!error) {
isSuccess = YES;
self.responseStr = [request responseString];
receivedData = [NSData dataWithData:[self.request responseData]];
}
else {
isSuccess = NO;
NSLog(#"The following error occurred: %#", error);
}
}#catch(NSException *e){
NSLog(#"exception occured.");
}
What my understanding about synchronous call is that this is a blocking call and control should not go below
[ContentBO downloadFile:destPath content_name:content_data_file_name];
until control is out of requestFinished method of ASIHTTPRequestDelegate. In my case what happening is that the control is simultaneously executing code in requestFinished and below
[ContentBO downloadFile:destPath content_name:content_data_file_name];
But I don't want the control to go below [ContentBO downloadFile...] before coming out of requestFinished method.
The requestFinished delegate call is run on the main thread asynchronously, and your code is not running on the main thread, so it is expected that both would run at the same time.
However, as you are using synchronous requests why not remove the contents of requestFinished and put the code after the 'startSyncronous' line? You are guaranteed the request has finished when startSynchronous returns.
In one of my projects the app had to do heavy server side data syncing. In that process one operation should had start after the successful execution of it's previous process and I was using ASIHttp synchronous requests in that. I was facing the same issue you mentioned, so to tackle it I used NSCondiiton. All it requires that you lock the thread after you call:
[self.request startSynchronous];. When the requests delegate method is called after the exection of the request, issue a signal command, and the next line of the code after the thread lock statement will be executed. Here is a rough example:
//declare a pointer to NSCondition in header file:
NSCondition *threadlock;
-(id) init
{
threadlock = [[NSCondition alloc] init]; //release it in dealloc
}
-(void)downLoadFile
{
[thread lock];
//your request code
[self.request setDidFinishSelector:#selector(downLoadFileRequestDone:)];
[self.request setDidFailSelector:#selector(downLoadFileRequestWentWrong:)];
[self.request startSynchronous];
[thread wait];
//the lines here will be executed after you issue a signal command in the request delegate method
[thread unlock];
}
-(void) downLoadFileRequestDone:(ASIHTTPRequest *)request
{
[thread lock];
//perform desire functionality
//when you are done call:
[thread signal];
[thread unlock];
}
It worked perfect for me... hope it will help.