In Objective-c we have declared a completion block that acts as a callback for web requests.
If the request fails then I would like to be able to call the function again, from within the completion block. When I do this I get EXC_BAD_ACCESS due to the block not being fully defined.
This is annoying because in Java I can do this with listeners.
Is it possible to call a completion block from within?
Thanks.
The following code throws the error, how could I fix this please?
CustomErrorBlock errorBlock = ^(NSError *error) {
_processing = NO;
loadingView.hidden = YES;
self.infoText.hidden = NO;
self.infoText.text = #"A network connection error has occurred.";
};
CustomCompletionBlock completionBlock = ^(NSData *data){
if(![self processGetTransactions:data withDict:updateDict]) {
[webClient getTransactions:updateDict WithCompletionBlock:completionBlock andErrorBlock:errorBlock];
}
};
[webClient getTransactions:updateDict WithCompletionBlock:completionBlock andErrorBlock:errorBlock];
Should be able to stop the EXC_BAD_ACCESS crash by using __block which tells compiler it must be treated in a special way and also __weak which makes the object not retain strong.
__block CustomErrorBlock errorBlock = ^(NSError *error) {
_processing = NO;
loadingView.hidden = YES;
self.infoText.hidden = NO;
self.infoText.text = #"A network connection error has occurred.";
};
__block CustomCompletionBlock completionBlock = ^(NSData *data) {
if(![self processGetTransactions:data withDict:updateDict]) {
__weak CustomCompletionBlock weakCompletionBlock = completionBlock; // Note seems to always have retain cycle
[webClient getTransactions:updateDict WithCompletionBlock:weakCompletionBlock andErrorBlock:errorBlock];
}
};
[webClient getTransactions:updateDict WithCompletionBlock:completionBlock andErrorBlock:errorBlock];
Note: weak object still displays a message about object being strong for some reason but it sill works without any problems.
Related
I'm trying to make an equivalent to the .NET recognize() call, which is synchronous, for ios in objective-c. I found code to recognize speech but the string that was recognized is only inside a block.
I've tried making the block not a block (it seems to be part of the API that it be a block), making __block variables and returning their values, also out parameters in the caller/declarer of the block; finally I wrote a file while in the block and read the file outside. It still didn't work like I want because of being asynchronous although I at least got some data out. I also tried writing to a global variable from inside the block and reading it outside.
I'm using code from here: How to implement speech-to-text via Speech framework, which is (before I mangled it):
/*!
* #brief Starts listening and recognizing user input through the
* phone's microphone
*/
- (void)startListening {
// Initialize the AVAudioEngine
audioEngine = [[AVAudioEngine alloc] init];
// Make sure there's not a recognition task already running
if (recognitionTask) {
[recognitionTask cancel];
recognitionTask = nil;
}
// Starts an AVAudio Session
NSError *error;
AVAudioSession *audioSession = [AVAudioSession sharedInstance];
[audioSession setCategory:AVAudioSessionCategoryRecord error:&error];
[audioSession setActive:YES withOptions:AVAudioSessionSetActiveOptionNotifyOthersOnDeactivation error:&error];
// Starts a recognition process, in the block it logs the input or stops the audio
// process if there's an error.
recognitionRequest = [[SFSpeechAudioBufferRecognitionRequest alloc] init];
AVAudioInputNode *inputNode = audioEngine.inputNode;
recognitionRequest.shouldReportPartialResults = YES;
recognitionTask = [speechRecognizer recognitionTaskWithRequest:recognitionRequest resultHandler:^(SFSpeechRecognitionResult * _Nullable result, NSError * _Nullable error) {
BOOL isFinal = NO;
if (result) {
// Whatever you say in the microphone after pressing the button should be being logged
// in the console.
NSLog(#"RESULT:%#",result.bestTranscription.formattedString);
isFinal = !result.isFinal;
}
if (error) {
[audioEngine stop];
[inputNode removeTapOnBus:0];
recognitionRequest = nil;
recognitionTask = nil;
}
}];
// Sets the recording format
AVAudioFormat *recordingFormat = [inputNode outputFormatForBus:0];
[inputNode installTapOnBus:0 bufferSize:1024 format:recordingFormat block:^(AVAudioPCMBuffer * _Nonnull buffer, AVAudioTime * _Nonnull when) {
[recognitionRequest appendAudioPCMBuffer:buffer];
}];
// Starts the audio engine, i.e. it starts listening.
[audioEngine prepare];
[audioEngine startAndReturnError:&error];
NSLog(#"Say Something, I'm listening");
}
I want to call Listen(), (like startListening() above), have it block execution until done, and have it return the string that was said. But actually I would be thrilled just to get result.bestTranscription.formattedString somehow to the caller of startListening().
I'd recommend you to take another approach. In Objective-C having a function that blocks for a long period of time is an anti-pattern.
In this language there's no async/await, nor cooperative multitasking, so blocking for long-ish periods of time might lead to resource leaks and deadlocks. Moreover if done on the main thread (where the app UI runs), the app might be forcefully killed by the system due to being non-responsive.
You should use some asynchronous patterns such as delegates or callbacks.
You might also try using some promises library to linearize your code a bit, and make it look "sequential".
The easiest approach with callbacks would be to pass a completion block to your "recognize" function and call it with the result string when it finishes:
- (void)recognizeWithCompletion:(void (^)(NSString *resultString, NSError *error))completion {
...
recognitionTask = [speechRecognizer recognitionTaskWithRequest:recognitionRequest
resultHandler:^(SFSpeechRecognitionResult *result, NSError *error)
{
...
dispatch_async(dispatch_get_main_queue(), ^{
completion(result.bestTranscription.formattedString, error);
});
...
}];
...
}
Note that the 2nd parameter (NSError) - is an error in case the caller wants to react on that too.
Caller side of this:
// client side - add this to your UI code somewhere
__weak typeof(self) weakSelf = self;
[self recognizeWithCompletion:^(NSString *resultString, NSError *error) {
if (!error) {
[weakSelf processCommand:resultString];
}
}];
// separate method
- (void)processCommand:(NSString *command) {
// can do your processing here based on the text
...
}
I'm having trouble returning an asynchronous response/error pair captured in a block back to caller. Here is the code:
- (NSData *)sendSynchronousRequest:(NSURLRequest *)request returningResponse:(NSURLResponse **)response error:(NSError **)error
{
__block NSData* b_data = nil;
__block NSURLResponse* b_response = nil;
__block NSError* b_error = nil;
dispatch_semaphore_t wait = dispatch_semaphore_create(0);
[self sendRequest:request completion:^(NSHTTPURLResponse* c_response, NSData* c_data, NSError* c_error) {
b_data = c_data;
b_response = c_response;
b_error = c_error;
dispatch_semaphore_signal(wait);
}];
dispatch_semaphore_wait(wait, DISPATCH_TIME_FOREVER);
response = &b_response; // ERROR: Assigning NSURLResponse *__strong * to NSURLResponse *__autoreleasing * changes retain/release properties of pointer
error = &b_error; // ERROR: Assigning NSError *__strong * to NSError *__autoreleasing * changes retain/release properties of pointer
return b_data;
}
Basically, I'm converting an async request to a synchronous one. I need to return the response/error pair back to the caller through a pointer-to-a-pointer. However, I'm running into some problems with ARC and blocks.
I would like to copy the response/error out of the block and return it to the caller without changing the method signature (there is a slew of legacy code that calls this method - I'm trying to gingerly replace NSURLConnection with NSURLSession).
Is there a nice way I can extract the results and return them to the caller?
Your two errors have nothing to do with the block. Those lines should be:
if (response) {
*response = b_response;
}
if (error) {
*error = b_error;
}
The if checks are required incase the caller passes nil for the parameter. You will get a crash trying to dereference a nil pointer.
It's also unnecessary to copy the response/error pair to local __block variables. You can write them directly to the parameter after checking for nil.
I have developed a leaderboard display class for my iPhone game. The class has the following instance method.
-(void)displayScoresWithRequest:(CBLeaderboard*)request completionHandler:(void(^)())completionHandler
{
if (request_ != nil)
return;
request_ = [[CBLeaderboard alloc] init];
[request_ setCategory:[request category]];
[request_ setPlayerScope:[request playerScope]];
[request_ setTimeScope:[request timeScope]];
[request_ setRange:[request range]];
__block CBLeaderboardDisplay* blockSelf = self;
[request_ loadScoresWithCompletionHandler:^(NSArray* scores, NSError* error)
{
blockSelf->request_ = nil;
NSUInteger scoresCount = [scores count];
if (scoresCount == 0 && error != nil)
return;
NSMutableArray* playerIDs = [NSMutableArray array];
for (GKScore* score in scores)
[playerIDs addObject:[score playerID]];
[GKPlayer loadPlayersForIdentifiers:playerIDs withCompletionHandler:^(NSArray* players, NSError* error)
{
if (scoresCount > [players count] && error != nil)
return;
[blockSelf displayScores:scores players:players];
completionHandler();
}];
}];
[request_ release];
}
As you can see, the method copies a leaderboard request, executes it, and calls the supplied completion handler. A layer in my game calls this method as follows.
-(void)refreshDisplay
{
CBLeaderboard* request = [[CBLeaderboard alloc] init];
[request setCategory:[[sharedGameCenterManager_ classicLeaderboard] category]];
[request setPlayerScope:GKLeaderboardPlayerScopeFriendsOnly];
[request setTimeScope:GKLeaderboardTimeScopeAllTime];
static NSRange kRequestRange = NSMakeRange(1, 3);
[request setRange:kRequestRange];
__block GJGameOver* blockSelf = self;
[display_ displayScoresWithRequest:request completionHandler:^
{
CGSize displayContentSize = [blockSelf->display_ contentSize];
displayContentSize.width = width(blockSelf) - 2.0 * kGJLabelPadding;
[blockSelf->display_ setContentSize:displayContentSize];
CGFloat displayHeight =
bottomEdge(blockSelf->multiplierLabel_) - topEdge(blockSelf->menu_) - 2.0 * kGJLabelPadding;
CGFloat displayScoreDisplaysCount = [blockSelf->display_ scoreDisplaysCount];
CGFloat displayLabelPadding =
(displayHeight - [blockSelf->display_ minContentSize].height) / displayScoreDisplaysCount;
[blockSelf->display_ setLabelPadding:MIN(floor(displayLabelPadding), kGJLabelPadding)];
static CGFloat kFadeInDuration = 2.0;
if ([blockSelf->display_ opacity] == 0)
[blockSelf->display_ runAction:[CCFadeIn actionWithDuration:kFadeInDuration]];
}];
[request release];
}
My game crashes when both the layer and hence display are deallocated and the request has not completed. When the request completes, it attempts to send a message to a deallocated instance and the crash ensues. Is it possible to cancel a leaderboard request? If not, is there any way I can avoid the crash without causing a memory leak?
In both of your blocks, you use __block to allow the block to reference self without retaining it. This is the problem, because you are doing an asynchronous operation, and if the block is executed after self has been deallocated, it is using a dangling pointer. The whole point of blocks retaining objects they capture is to keep them alive so the block can use it.
Not retaining self when making blocks is commonly done to avoid retain cycles. However, I don't see any retain cycles here:
The request_ in displayScoresWithRequest probably retains the block in displayScoresWithRequest
The block in displayScoresWithRequest retains self, the CBLeaderboardDisplay object
The block in displayScoresWithRequest retains the block from refreshDisplay
The block in refreshDisplay retains self, the GJGameOver object
The GJGameOver object retains display_, the CBLeaderboardDisplay object
However, the CBLeaderboardDisplay object does not retain its instance variable request_. (This code is extremely poorly written, as request_ is released at the end of the method but not set to nil. It should probably be made a local variable or something. And a boolean flag should be used if you want to check whether the code has run once or not.)
I'm using what seems to be a simple invocation of the NSFileVersion class method removeOtherVersionsOfItemAtURL: inside a coordinated writing block for some iCloud conflict resolution.
When my devices go into 'spaz mode', which is a technical term for repeatedly opening and closing the application on a few devices, an EXC_BAD_ACCESS exception is thrown internally. Code snippet:
- (void)compareVersionChanges:(NSFileVersion *)version {
if (![DataLoader iCloudPreferenceEnabled]) {
NSLog(#"Ignoring iCloud changes (version comparison) based on user preference");
return;
}
NSLog(#"compareVersionChanges");
dispatch_queue_t aQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(aQueue, ^(void) {
NSError *readError = nil;
NSFileCoordinator *coordinator = [[NSFileCoordinator alloc] initWithFilePresenter:(id)self];
[coordinator coordinateReadingItemAtURL:[version URL] options:0 error:&readError byAccessor:^(NSURL *newURL) {
DataContext *loadedContext = nil;
NSData *data = [NSData dataWithContentsOfURL:newURL];
NSError *e = nil;
loadedContext = [self convertXmlDataToContext:data error:&e];
if (e) {
NSLog(#"Done loading, error: %#", e);
[[DataLoader applicationDelegate] displayError:e];
loadedContext = nil;
}
if (!loadedContext) {
return;
}
id appDelegate = [DataLoader applicationDelegate];
DataContext *inMemoryContext = nil;
if (appDelegate != nil && [appDelegate respondsToSelector:#selector(context)]) {
inMemoryContext = [appDelegate performSelector:#selector(context)];
}
if (inMemoryContext) {
NSLog(#"Performing iCloud context synchronizating...");
DataContextSynchronizer *synchronizer = [[DataContextSynchronizer alloc] init];
ChangeSet *changes = [synchronizer compareLocalContext:inMemoryContext andRemoteContext:loadedContext];
if ([[changes changes] count] > 0) {
[SelectionManager disable];
#synchronized(appDelegate) {
NSLog(#"Applying synchronization changes...");
[synchronizer applyChangeSet:changes toDataContext:inMemoryContext];
NSLog(#"Synchronization changes applied");
}
[SelectionManager enable];
if ([appDelegate respondsToSelector:#selector(setSkipRefreshSave:)]) {
[appDelegate performSelector:#selector(setSkipRefreshSave:) withObject:[NSNumber numberWithBool:YES]];
}
dispatch_queue_t mainQueue = dispatch_get_main_queue();
dispatch_async(mainQueue, ^(void) {
[SelectionManager notifyListeners];
});
if ([appDelegate respondsToSelector:#selector(setSkipRefreshSave:)]) {
[appDelegate performSelector:#selector(setSkipRefreshSave:) withObject:[NSNumber numberWithBool:NO]];
}
[self save:[[DataLoader applicationDelegate] context]];
} else {
NSLog(#"No sync changes applicable.");
}
NSError *coordinateWriteRemoveError = nil;
[coordinator coordinateWritingItemAtURL:newURL options:NSFileCoordinatorWritingForDeleting error:&coordinateWriteRemoveError byAccessor:^(NSURL *theURL) {
theURL = [theURL copy];
NSError *removeOtherVersionsError = nil;
[NSFileVersion removeOtherVersionsOfItemAtURL:theURL error:&removeOtherVersionsError];
if (removeOtherVersionsError) {
NSLog(#"Error removing other versions: %#", removeOtherVersionsError);
}
}];
if (coordinateWriteRemoveError) {
NSLog(#"Error occurred coordinating write for deletion of other file versions: %#", coordinateWriteRemoveError);
}
}
}];
if (readError) {
NSLog(#"Done loading (outside block) error: %#", readError);
}
});
}
I thought a little syntax highlighting might make this easier to examine:
Link to image of code snippet and failure stack in Xcode
The error actually occurs on line 1404, and as you can see from the below screenshot, it's deep in Apple code territory.
Link to image of debugger
Before submitting a radar, I thought I'd check here to see if there's something I'm doing wrong? The extra [... copy] on line 1402 was just a quick check to make sure I'm not losing the reference to the block-provided argument, and will be removed.
Edit: An important note! I'm using ARC.
Edit 2: I've noticed that when calling:
[NSFileVersion otherVersionsOfItemAtURL:theURL]
The return value is nil, which indicates (via the documentation):
...or nil if there is no such file. The array does not contain the version object returned by the currentVersionOfItemAtURL: method.
So by checking the return value of this method before I make the call to removeOtherVersionsOfItemAtURL:, it has alleviated the issue. But I still find it strange that an EXC_BAD_ACCESS is thrown, rather than that method handling it properly.
I've noticed that when calling:
[NSFileVersion otherVersionsOfItemAtURL:theURL]
immediately prior to the call to removeOtherVersionsOfItemAtURL:, the return value is nil, which indicates (via the documentation):
Returns: An array of file version objects or nil if there is no such
file. The array does not contain the version object returned by the
currentVersionOfItemAtURL: method.
So by checking the return value of this method before I make the call to removeOtherVersionsOfItemAtURL:, it has alleviated the issue. But I still find it strange that an EXC_BAD_ACCESS is thrown by removeOtherVersionsOfItemAtURL:, rather than that method simply returning NO, or simply populating the provided NSError object.
I'll be filing a Radar and will update here when I hear back.
Having a heck of a time with this one.
I've got a super-simple Cocoa app containing one WebView, a WebScripting API defined in the page, and a single NSObject defined on that API. When I turn on the debugger tools (in the embedded WebView), I can see the API on the JavaScript window object, and I can see my "api" property defined on that -- but when I call the API's "get" method, the arguments aren't being serialized -- when the Obj-C method gets called, the arguments are missing. See below, which hopefully illustrates:
I've combed through the docs, I've (apparently) set the appropriate methods to expose everything that needs to be exposed, and I can see the method being called. There has to be something stupid I'm missing, but as a relative newbie to this environment, I'm not seeing it.
Thanks in advance for your help!
Have you set WebKitDeveloperExtras to YES in your default user defaults when you send -[NSUserDefaults registerDefaults:]?
Depending on what version of Xcode you're using you could be getting a known error. If you're using LLDB on anything but the most recent version, it might not be giving you the right variables in the debugger. The solution has been to use GDB instead of LLDB until Apple fixes the problem. But I think they fixed the problem in the latest version. I'd change the debugger to use GDB and see if you're getting the right variables in Xcode. (Product-> Edit Scheme...-> Run -> Debugger). I came across this problem in iOS, though, so I don't know its applicability to OSX. Worth a try anyway.
I originally came across the problem here: https://stackoverflow.com/a/9485349/1147934
I process javascript in the main thread of my app from a local file stored in the apps directory. I check for beginning and ending tokens for the js functions I am executing and whether the function contains a variable.
Hopefully this can give you some good ideas for your issue. You could also do alerts in the js to see if the values post correctly as you run the app (I am sure you thought of that already, but it's worth mentioning.) Happy coding! I hope this helps!
in the .h file define:
NSMutableString *processedCommand;
NSArray *commandArguments;
In the .m file:
// tokens
#define kOpenToken #"<%%"
#define kCloseToken #"%%>"
// this will throw
-(void)executeJScriptCommand:(NSString *)aCommand {
[self performSelectorOnMainThread:#selector(executeThisCommand:) withObject:aCommand waitUntilDone:YES];
}
// this will throw
-(NSString *)executeCommand:(NSString *)command {
NSString *aCommand = [[[command stringByReplacingOccurrencesOfString:kOpenToken withString:#""]
stringByReplacingOccurrencesOfString:kCloseToken withString:#""]
stringByTrimmingLeadingAndTrailingWhitespaces];
if ([aCommand hasPrefix:#"="])
{
// variable. get value
[self getVariableFromCommand:aCommand];
}
else {
[self executeThisCommand:aCommand];
}
NSString *returnValue = [NSString stringWithString:processedCommand];
self.processedCommand = nil;
self.commandArguments = nil;
return returnValue;
}
-(void)executeThisCommand:(NSString *)aCommand {
BOOL hasError = NO;
// clear result
self.processedCommand = nil;
self.commandArguments = nil;
BOOL isFromJS = NO;
NSString *function = nil;
NSMutableArray *commandParts = nil;
#try {
// first, break the command into its parts and extract the function that needs to be called, and the (optional) arguments
commandParts = [[NSMutableArray alloc] initWithArray:[aCommand componentsSeparatedByString:#":"]];
if ([[[commandParts objectAtIndex:0] lowercaseString] isEqualToString:#"js-call"]) {
isFromJS = YES;
[commandParts removeObjectAtIndex:0];
}
// get our function, arguments
function = [[commandParts objectAtIndex:0] retain];
[commandParts removeObjectAtIndex:0];
if ([commandParts count] > 0){
if (isFromJS == YES) {
NSString *arguments = [[commandParts objectAtIndex:0] stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
if ([arguments length] > 0) {
self.commandArguments = [arguments JSONValue];
}
}
else {
self.commandArguments = [NSArray arrayWithArray:commandParts];
}
}
// build invoke
SEL sel = NSSelectorFromString(function);
if ([self respondsToSelector:sel]) {
[self performSelectorOnMainThread:sel withObject:nil waitUntilDone:YES];
// using invocation causes a SIGABORT because the try/catch block was not catching the exception.
// using perform selector fixed the problem (i.e., the try/catch block now correctly catches the exception, as expected)
}
else {
[appDelegate buildNewExceptionWithName:#"" andMessage:[NSString stringWithFormat:#"Object does not respond to selector %#", function]];
}
}
#catch (NSException * e) {
hasError = YES;
[self updateErrorMessage:[NSString stringWithFormat:#"Error processing command %#: %#", aCommand, [e reason]]];
}
#finally {
[function release];
[commandParts release];
}
if (hasError == YES) {
[appDelegate buildNewExceptionWithName:#"executeThisCommand" andMessage:self.errorMessage];
}
}
// this can return nil
-(NSString *)getQueryStringValue:(NSString *)name {
NSString *returnValue = nil;
if (queryString != nil) {
returnValue = [queryString objectForKey:[name lowercaseString]];
}
return returnValue;
}