I'm writing a mac app that runs its own web server, using the GCDWebServer library (https://github.com/swisspol/GCDWebServer). My app delegate handles GET requests like so:
__weak typeof(self) weakSelf = self;
[webServer addDefaultHandlerForMethod:#"GET"
requestClass:[GCDWebServerRequest class]
processBlock:^GCDWebServerResponse *(GCDWebServerRequest* request) {
return [weakSelf handleRequest:request];
}];
And then the handleRequest method returns the response data, something like:
return [GCDWebServerDataResponse responseWithHTML:#"<html><body><p>Hello World!</p></body></html>"];
So far so good. Except now I want the handleRequest method to use NSSpeechSynthesizer to create an audio file with some spoken text in it, and then wait for the speechSynthesizer:didFinishSpeaking method to be called before returning to the processBlock.
// NSSpeechSynthesizerDelegate method:
- (void)speechSynthesizer:(NSSpeechSynthesizer *)sender didFinishSpeaking:(BOOL)success
{
NSLog(#"did finish speaking, success: %d", success);
// return to processBlock...
}
Problem is, I have no idea how to do this. Is there a way to return from the speechSynthesizer:didFinishSpeaking method into the processBlock defined above?
You need to run the speech synthesizer on a separate thread with its own run loop, and use a lock to allow your request thread to wait for the operation to complete on the speech thread.
Assuming the web server maintains its own thread(s) and runloop, you can use your app's main thread to run the speech synthesizer, and you can use NSCondition to signal completion to the web response thread.
A basic (untested) example (without error handling):
#interface SynchroSpeaker : NSObject<NSSpeechSynthesizerDelegate>
- (id)initWithText:(NSString*)text outputUrl:(NSURL*)url;
- (void)run;
#end
#implementation SynchroSpeaker
{
NSCondition* _lock;
NSString* _text;
NSURL* _url;
NSSpeechSynthesizer* _synth;
}
- (id)initWithText:(NSString*)text outputUrl:(NSURL*)url
{
if (self = [super init])
{
_text = text;
_url = url;
_lock = [NSCondition new];
}
return self;
}
- (void)run
{
NSAssert(![NSThread isMainThread], #"This method cannot execute on the main thread.");
[_lock lock];
[self performSelectorOnMainThread:#selector(startOnMainThread) withObject:nil waitUntilDone:NO];
[_lock wait];
[_lock unlock];
}
- (void)startOnMainThread
{
NSAssert([NSThread isMainThread], #"This method must execute on the main thread.");
[_lock lock];
//
// Set up your speech synethsizer and start speaking
//
}
- (void)speechSynthesizer:(NSSpeechSynthesizer *)sender didFinishSpeaking:(BOOL)success
{
//
// Signal waiting thread that speaking has completed
//
[_lock signal];
[_lock unlock];
}
#end
It's used like so:
- (id)handleRequest:(id)request
{
SynchroSpeaker* speaker = [[SynchroSpeaker alloc] initWithText:#"Hello World" outputUrl:[NSURL fileURLWithPath:#"/tmp/foo.dat"]];
[speaker run];
////
return response;
}
GCDWebServer does run into its own threads (I guess 2 of them) - not in the main one. My solution needed to run code in Main Thread when calling the ProcessBlock.
I found this way that suits my needs:
First declare a weak storage for my AppDelegate: __weak AppDelegate *weakSelf = self;. Doing so I can access all my properties within the block.
Declare a strong reference to AppDelegate from within the block like so: __strong AppDelegate* strongSelf = weakSelf;
Use NSOperationQueue to align the operation on mainThread:
[[NSOperationQueue mainQueue] addOperationWithBlock:^ {
//Your code goes in here
NSLog(#"Main Thread Code");
[strongSelf myMethodOnMainThread];
}];
In this way myMethodOnMainThread surely will run where it's supposed to.
For sake of clarity I quote my relevant code section:
webServer = [[GCDWebServer alloc] init];
webServer.delegate = self;
__weak AppDelegate *weakSelf = self;
// Add a handler to respond to GET requests
[webServer addDefaultHandlerForMethod:#"GET"
requestClass:[GCDWebServerRequest class]
asyncProcessBlock:^(GCDWebServerRequest* request, GCDWebServerCompletionBlock completionBlock) {
__strong AppDelegate* strongSelf = weakSelf;
[[NSOperationQueue mainQueue] addOperationWithBlock:^ {
//Your code goes in here
NSLog(#"Main Thread Code");
[strongSelf myMethodOnMainThread];
}];
GCDWebServerDataResponse* response = [GCDWebServerDataResponse responseWithJSONObject:packet];
completionBlock(response);
}];
GCWebServer supports fully asynchronous responses as of version 3.0 and later [1].
[webServer addDefaultHandlerForMethod:#"GET"
requestClass:[GCDWebServerRequest class]
asyncProcessBlock:^(GCDWebServerRequest* request, GCDWebServerCompletionBlock completionBlock) {
// 1. Trigger speech synthesizer on main thread (or whatever thread it has to run on) and save "completionBlock"
// 2. Have the delegate from the speech synthesizer call "completionBlock" when done passing an appropriate response
}];
[1] https://github.com/swisspol/GCDWebServer#asynchronous-http-responses
Related
I am using the below code to wait for async tasks to be completed. It works a couple of times and crashes. The updateFromTable always invokes callback() so that the group calls are balanced, but it still crashes.
- (void)updateFromTable:(Table *)table env:(Env *)env callback:(void (^)(void))callback {
[someasync usingBlock:^{
callback()
}];
}
- (NSString * _Nullable)process {
JSL * __weak weakSelf = self;
NSString __block *ret = nil;
dispatch_group_enter(_dispatchGroup);
dispatch_async(_repQueue, ^{
JSL *this = weakSelf;
[this updateFromTable:[this->_env table] env:this->_env callback:^{
ret = [some op .. ];
dispatch_group_leave(this->_dispatchGroup);
}];
});
dispatch_group_wait(_dispatchGroup, dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC));
info(#"%#", #"done");
return ret;
}
Any idea why it crashes randomly and how to fix this? Basically, what I am trying to achieve is invoke couple of async tasks, wait for all of them to complete and then proceed with the rest.
Referring: How to wait past dispatch_async before proceeding?
You cannot dereference ivars with -> if this is nil. So, the typical solution is to create strong reference that can’t be deallocated while the closure runs, and return if it’s nil:
- (NSString * _Nullable)process {
typeof(self) __weak weakSelf = self;
[self asynchronousMethodWithCompletion:^{
typeof(self) strongSelf = weakSelf;
if (!strongSelf) { return; }
// can now safely use `strongSelf` here
});
...
}
This is “weakSelf-strongSelf dance”. You use it in situations where you need to make sure that self isn’t nil when you use it, e.g. dereferencing ivars (strongSelf->ivar) .
Thus:
- (NSString * _Nullable)process {
typeof(self) __weak weakSelf = self;
NSString __block *ret = nil;
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
dispatch_async(_repQueue, ^{
typeof(self) strongSelf = weakSelf;
if (!strongSelf) { return; }
[strongSelf updateFromTable:[strongSelf->_env table] env:strongSelf->_env callback:^{
ret = [some op .. ];
dispatch_group_leave(group);
}];
});
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
info(#"%#", #"done");
return ret;
}
A few other observations:
The dispatch group should be a local variable of the method rather than an ivar. There’s no need for anything else in your code referencing this group.
Make sure that your dispatch_group_leave calls don’t exceed the number of dispatch_group_enter calls (i.e. that this completion handler block isn’t called multiple times).
I’d suggest waiting for DISPATCH_TIME_FOREVER (assuming you want it to really wait for it to finish).
Also, if these are properties (which I’m guessing they are on the basis of the underscores), then using self.env rather than self->_env is safer, as it won’t crash if self is nil, but rather will just return nil.
I must confess that this still doesn’t look right (e.g. if updateFromTable is asynchronous already, why bother dispatching this asynchronously to _repQueue; if it is synchronous, then again, why dispatch this asynchronously only to wait for it). But it’s impossible to comment further without seeing the updateFromTable implementation.
Or, better, make the method asynchronous:
- (void)processWithCompletion:(void (^)(NSString *))callback {
typeof(self) __weak weakSelf = self;
dispatch_async(_repQueue, ^{
typeof(self) strongSelf = weakSelf;
if (!strongSelf) { return; }
[strongSelf updateFromTable:[strongSelf->_env table] env:strongSelf->_env callback:^{
NSString *ret = [some op .. ];
callback(ret);
}];
});
}
I'd like to init a model, let the model do some async stuff and present a new viewcontroller once completed. But how do i wait for the two async methods to be completed and how do I setup the callback method?
Pseudocode
In my StartViewController.m:
-(void)openArticle
{
article = [Article initWithObject:someObject];
article.callback = changeView;
}
-(void)changeView
{
[self presentViewController:someController];
}
In my ArticleModel.m:
-(void)initWithObject:someObject
{
[self loadImage]
[self geoCode]
}
-(void)loadImage
{
runAsyncMethod: success:^() // This one is actually a AFNetworking setImageWithURLRequest
}
-(void)geoCode
{
runAnotherAsyncMethod: success:^() // This one is actually a geocodeAddressString operation
}
You can achieve this using dispatch_groups
- (void)initWithObject:(id)someObject
{
self = [super init];
if (self) {
self.dispatch_group = dispatch_group_create();
[self loadImage]
[self geoCode]
dispatch_group_notify(self.dispatch_group, dispatch_get_main_queue(), ^{
NSLog(#"Push new view controller");
});
}
return self;
}
- (void)loadImage
{
dispatch_group_enter(self.dispatch_group);
__weak __typeof(self) weakSelf = self;
runAsyncMethod: success:^{
__typeof(weakSelf) strongSelf = weakSelf;
if (strongSelf.dispatch_group) {
dispatch_group_leave(strongSelf.dispatch_group); // You need to ensure that this is called in both success and failure
}
}
}
- (void)geoCode
{
dispatch_group_enter(self.dispatch_group);
__weak __typeof(self) weakSelf = self;
runAnotherAsyncMethod: success:^{
__typeof(weakSelf) strongSelf = weakSelf;
if (strongSelf.dispatch_group) {
dispatch_group_leave(strongSelf.dispatch_group);
}
}
}
You do not wait. If you wait, it isn't asynchronous! You would be losing the entire point of asynchronous if you were to wait.
What you do is, when your success handler is called, you step out to the main thread (just in case you got called back on a background thread) and now do whatever you need to do. In other words, you just let your success handler get called whenever it happens to get called.
In your case, you might like to chain the things you want to do:
Call loadImage
In its callback, call geoCode
In its callback, step out to the main thread and present the new view controller.
You can use dispatch_group so that when a method is over, it just leaves the group. I use a similar code myself and it works like a charm.
- (void)initWithObject:someObject {
// Create a dispatch group
dispatch_group_t group = dispatch_group_create();
[self loadImageWithDispatchGroup:group];
[self geoCodeWithDispatchGroup:group];
// Here we wait for all the requests to finish
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
// Do whatever you need to do when all requests are finished
});
}
- (void)loadImageWithDispatchGroup:(dispatch_group_t)group {
dispatch_group_enter(group);
runAsyncMethod: success:^() // This one is actually a AFNetworking setImageWithURLRequest
// In your success or failure AFNetworking method, call this as soon as the request ended
dispatch_group_leave(group);
}
- (void)geoCodeWithDispatchGroup:(dispatch_group_t)group {
dispatch_group_enter(group);
runAnotherAsyncMethod: success:^() // This one is actually a geocodeAddressString operation
// In your success async geocode callback method, call this as soon as the request ended
dispatch_group_leave(group);
}
I do not known your needs but native GCD way to wait several asynch tasks is
void dispatch_barrier_async(dispatch_queue_t queue, dispatch_block_t block);
https://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html#//apple_ref/c/func/dispatch_barrier_async
Q1: Can I call a method and have it execute on a background thread from inside another method that is currently executing on the main thread?
Q2: As an extension of the above, can I call a method and have it execute on a background thread from inside another method that is currently executing on some other background thread itself?
Q3: And one final question given the above : if I initialize an instance of some object X on some thread (main/background) and then have a method Y, of that object X, executing on some other background thread, can this method Y send messages and update an int property (e.g. of that Object X, or is such communication not possible ?
The reason I'm asking this last question is because I've been going over and over it again and I can't figure what is wrong here:
The following code returns zero acceleration and zero degrees values :
MotionHandler.m
#implementation MotionHandler
#synthesize currentAccelerationOnYaxis; // this is a double
-(void)startCompassUpdates
{
locationManager=[[CLLocationManager alloc] init];
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
locationManager.delegate=self;
[locationManager startUpdatingHeading];
NSLog(#"compass updates initialized");
}
-(int) currentDegrees
{
return (int)locationManager.heading.magneticHeading;
}
-(void) startAccelerationUpdates
{
CMMotionManager *motionManager = [[CMMotionManager alloc] init];
motionManager.deviceMotionUpdateInterval = 0.01;
[motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue currentQueue]
withHandler:^(CMDeviceMotion *motion, NSError *error)
{
self.currentAccelerationOnYaxis = motion.userAcceleration.y;
}
];
}
#end
Tester.m
#implementation Tester
-(void)test
{
MotionHandler *currentMotionHandler = [[MotionHandler alloc] init];
[currentMotionHandler performSelectorInBackground:#selector(startCompassUpdates) withObject:nil];
[currentMotionHandler performSelectorInBackground:#selector(startAccelerationUpdates) withObject:nil];
while(1==1)
{
NSLog(#"current acceleration is %f", currentMotionHandler.currentAccelerationOnYaxis);
NSLog(#"current degrees are %i", [currentMotionHandler currentDegrees]);
}
SomeViewController.m
#implementation SomeViewController
-(void) viewDidLoad
{
[myTester performSelectorInBackground:#selector(test) withObject:nil];
}
#end
However, the following code returns those values normally :
Tester.m
#interface Tester()
{
CLLocationManager *locationManager;
double accelerationOnYaxis;
// more code..
}
#end
#implementation Tester
- (id) init
{
locationManager=[[CLLocationManager alloc] init];
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
locationManager.delegate=self;
[locationManager startUpdatingHeading];
// more code..
}
-(void) test
{
CMMotionManager *motionManager = [[CMMotionManager alloc] init];
motionManager.deviceMotionUpdateInterval = 0.01;
[motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue mainQueue]
withHandler:^(CMDeviceMotion *motion, NSError *error)
{
accelerationOnYaxis = motion.userAcceleration.y;
}
];
while(1==1)
{
NSLog(#"current acceleration is %f", accelerationOnYaxis);
NSLog(#"current degrees are %i", locationManager.heading.magneticHeading);
}
}
SomeViewController.m
#implementation SomeViewController
-(void) viewDidLoad
{
[myTester performSelectorInBackground:#selector(test) withObject:nil];
}
What's wrong with the first version? I really want to use that first one because it seems much better design-wise.. Thank you for any help!
Calling performSelectorInBackground:withObject: is the same as if you called the detachNewThreadSelector:toTarget:withObject: method of NSThread with the current object, selector, and parameter object as parameters (Threading Programming Guide). No matter where you call it, a new thread will be created to perform that selector. So to answer your first two questions: yes and yes.
For your final question, as long as this Object X is the same object in both methods, any of X's properties can be updated. But, beware that this can yield unexpected results (ie. see Concurrency Programming Guide). If multiple methods are updating X's property, values can be overwritten or disregarded. But, if you are only updating it from method Y and reading it from all other methods, such problems shouldn't occur.
You should take a look at the Grand Central Dispatch documentation from Apple. It allows you to use multiple threads in a block-based structure.
2 importants function are dispatch_sync() and dispatch_async().
Some examples:
To execute a certain block of code on a background thread and wait until it is finished:
__block id someVariable = nil;
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
// do some heavy work in the background
someVariable = [[NSObject alloc] init];
});
NSLog(#"Variable: %#", someVariable);
This function modifies the variable someVariable which you can use later on. Please note that the main thread will be paused to wait for the background thread. If that is not what you want, you can use dispatch_async() as follows:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
// do some heavy work in the background
NSObject *someVariable = [[NSObject alloc] init];
// notify main thread that the work is done
dispatch_async(dispatch_get_main_queue(), ^{
// call some function and pass someVariable to it, it will be called on the main thread
NSLog(#"Variable: %#", someVariable);
});
});
I'm playing around with blocks in Objective-C, trying to come up with a reusable mechanism that will take an arbitrary block of code and a lock object and then execute the block of code on a new thread, synchronized on the provided lock. The idea is to come up with a simple way to move all synchronization overhead/waiting off of the main thread so that an app's UI will always be responsive.
The code I've come up with is pretty straightforward, it goes like:
- (void) executeBlock: (void (^)(void))block {
block();
}
- (void) runAsyncBlock: (void (^)(void))block withLock:(id)lock {
void(^syncBlock)() = ^{
#synchronized(lock) {
block();
}
};
[self performSelectorInBackground:#selector(executeBlock:) withObject:syncBlock];
}
So for example, you might have some methods that go like:
- (void) addObjectToSharedArray:(id) theObj {
#synchronized(array) {
[array addObject: theObj];
}
}
- (void) removeObjectFromSharedArray:(id) theObj {
#synchronized(array) {
[array removeObject: theObj];
}
}
Which works fine, but blocks the calling thread while waiting for the lock. These could be rewritten as:
- (void) addObjectToSharedArray:(id) theObj {
[self runAsyncBlock:^{
[array addObject: theObj];
} withLock: array];
}
- (void) removeObjectFromSharedArray:(id) theObj {
[self runAsyncBlock: ^{
[array removeObject: theObj];
} withLock:array];
}
Which should always return immediately, since only the background threads will compete over the lock.
The problem is, this code crashes after executeBlock: without producing any output, error message, crash log, or any other useful thing. Is there something fundamentally flawed in my approach? If not, any suggestions with respect to why this might be crashing?
Edit:
Interestingly, it works without crashing if I simply do:
- (void) runAsyncBlock: (void (^)(void))block withLock:(id)lock {
void(^syncBlock)() = ^{
#synchronized(lock) {
block();
}
};
syncBlock();
}
But of course this will block the calling thread, which largely defeats the purpose. Is it possible that blocks do not cross thread boundaries? I would think not, since that would largely defeat the purpose of having them in the first place.
remember to call [block copy] otherwise it is not correctly retained because block are created on stack and destroyed when exit scope and unless you call copy it will not move to heap even retain is called.
- (void) runAsyncBlock: (void (^)(void))block withLock:(id)lock {
block = [[block copy] autorelease];
void(^syncBlock)() = ^{
#synchronized(lock) {
block();
}
};
syncBlock = [[syncBlock copy] autorelease];
[self performSelectorInBackground:#selector(executeBlock:) withObject:syncBlock];
}
I am using the AWS SDK for iOS to upload and download files to and from local hard drive to Amazon S3 storage. I am capable of making this work but I am unable to get the S3 delegate to respond properly to alert me when operations have finished or resulted in an error.
I have an array of files that I want to upload. For each file I create a NSOperation where the main routine consist mostly of:
AmazonCredentials * credentials = [[AmazonCredentials alloc] initWithAccessKey:ACCESS_KEY_ID withSecretKey:SECRET_KEY];
putObjectRequest = [[S3PutObjectRequest alloc] initWithKey:pathCopy inBucket:[self bucket]];
putObjectRequest.filename = pathSource;
putObjectRequest.credentials=credentials;
[putObjectRequest setDelegate:s3Delegate];
Here, the delegate (s3Delegate) is created as a regular AmazonServiceRequestDelegate which should be able to fire off responses when an operation has finished. Each of my NSOperations are added to my NSOperationQueue which executes operations non-concurrently. If I use the delegate [putObjectRequest setDelegate:s3Delegate] the operations are not working. If I remove the use of the delegate the operations are performed correctly but I am unable to receive any responses to the operations as I do not have a delegate.
If I remove the use of the NSOperationQueue completely and use the [putObjectRequest setDelegate:s3Delegate] the delegate works perfectly.
My question is what am I doing wrong with using a delegate in a queue? Since the delegate is perfectly capable of performing while not in a queue could this be related to not performing on the main thread? I really want to be able to use the queue to limit the number of non-concurrent operations, however I am unable to figure this out. I hope someone has an idea of what is going on here and any example code would be greatly appreciated. Thanks!
Cheers, Trond
It seems that the aws sdk behaves asynchronously after the time you set your delegate.
So in order to have your asynchronous aws stuff work in a (asynchronous) NSOperation, you got to put some magic to wait for AWS to complete:
In your .h NSOperation file, add a boolean:
#interface UploadOperation : NSOperation <AmazonServiceRequestDelegate> {
#private
BOOL _doneUploadingToS3;
}
and in your .m file, your main method will look like this:
- (void) main
{
.... do your stuff …..
_doneUploadingToS3 = NO;
S3PutObjectRequest *por = nil;
AmazonS3Client *s3Client = [[AmazonS3Client alloc] initWithAccessKey:ACCESS_KEY withSecretKey:SECRET_KEY];
s3Client.endpoint = endpoint;
#try {
por = [[[S3PutObjectRequest alloc] initWithKey:KEY inBucket:BUCKET] autorelease];
por.delegate = self;
por.contentType = #"image/jpeg";
por.data = _imageData;
[s3Client putObject:por];
}
#catch (AmazonClientException *exception) {
_doneUploadingToS3 = YES;
}
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
} while (!_doneUploadingToS3);
por.delegate = nil;
.... continue with your stuff ….
}
do not forget to implement your delegate methods
-(void)request:(AmazonServiceRequest *)request didCompleteWithResponse:(AmazonServiceResponse *)response
{
_doneUploadingToS3 = YES;
}
-(void)request:(AmazonServiceRequest *)request didFailWithError:(NSError *)error
{
_doneUploadingToS3 = YES;
}
-(void)request:(AmazonServiceRequest *)request didFailWithServiceException:(NSException *)exception
{
_doneUploadingToS3 = YES;
}
- (void) request:(AmazonServiceRequest *)request didSendData:(NSInteger)bytesWritten totalBytesWritten:(NSInteger)totalBytesWritten totalBytesExpectedToWrite:(NSInteger)totalBytesExpectedToWrite
{
// Do what you want
}
-(void)request:(AmazonServiceRequest *)request didReceiveResponse:(NSURLResponse *)response
{
// Do what you want
}
-(void)request:(AmazonServiceRequest *)request didReceiveData:(NSData *)data
{
// Do what you want
}
Note: this magic can work for any stuff that performs asynchronously but have to be implemented in a NSOperation.