I have some question about use dispatch_sync when I read a shared resource.
I have searched several questions on Stack Overflow (such as: GCD dispatch_barrier or dispatch_sync?), but I didn't found an exact answer.
I don't understand that why to use
- (void)addPhoto:(Photo *)photo
{
if (photo) { // 1
dispatch_barrier_async(self.concurrentPhotoQueue, ^{ // 2
[_photosArray addObject:photo]; // 3
dispatch_async(dispatch_get_main_queue(), ^{ // 4
[self postContentAddedNotification];
});
});
}
}
- (NSArray *)photos
{
__block NSArray *array; // 1
dispatch_sync(self.concurrentPhotoQueue, ^{ // 2
array = [NSArray arrayWithArray:_photosArray];
});
return array;
}
I know why to use dispatch_barrier_async,but I don't know why to use dispatch_sync when I read _photosArray, I guess the write operation of _photosArray is in the self.concurrentPhotoQueue, so the read operation of _photosArray also need in the self.concurrentPhotoQueue or else use dispatch_sync in order to achieve multi-read?
What will happen if I don't use dispatch_sync when I do read operation? such as:
- (NSArray *)photos
{
__block NSArray *array;
array = [NSArray arrayWithArray:_photosArray];
return array;
}
Thank you very much!
Probably concurrentPhotoQueue is a serial queue. The main reason for concurrentPhotoQueue is to synchronize the access to the photos array.
Since it is serial all accesses from this queue are serialized, and no race conditions may occur if there are no accesses from other queues / threads in your app.
Writing access may be asynchronous because the writer needs no result of the writing operation in general. But reading must be done synchronously, because the caller has to wait for the result. If your photos method would use dispatch_async it would write the result to array after the photos method has returned. Thus, photos always would return nil.
Your unsynchronized version of photos might produce a race condition: _photosArray could be modified while it copies its contents, such that the number of copied items and the length of the array differ. This could lead to a crash inside of arrayWithArray:.
Related
I have an object that can execute an arbitrary queue of updates. I use blocks to embody the updates. I add an update using my addUpdate: method.
- (void) addUpdate: (void(^)())block {
[self.updates addObject: block];
}
Later, I want to execute all of them. I don't care if they run concurrently or not. The basic primitive way would seem to be something like:
for (NSUInteger index = 0; index < self.updates.count; index++) {
void (^block)() = self.updates[index];
block();
}
or with fast enumeration
for (void (^block)() in self.updates) {
block();
}
Or is there something I should be doing with GCD to make this happen?
The most terse way I can think of to do this would be:
[self.updates makeObjectsPerformSelector: #selector(invoke)];
How "idiomatic" that is will probably be situation-dependent...
EDIT: This depends on the fact that blocks are implemented in the runtime as Objective-C objects, and respond to the selector -invoke. In other words, the expression block(); can also be expressed as [block invoke];. I'm not aware of any more succinct way to execute an array of blocks.
For non-concurrent execution, for-in is the way to go. For concurrent execution, you could use NSArray's -enumerateUsing... methods and pass the concurrent flag, or use dispatch_apply() instead of a loop.
I have this method with a block in it, I want it to send the userID to another method as soon as it exists. userID is a value that is parsed from the internet, so it usually takes about 2 seconds to load up and 'exist'. Is there any way I can do a 'when userID exists, send it to another method?
Here's all my code:
- (void)parseForUserID {
//Get the Data you need to parse for (i.e. user main page returned as a block of NSData.
TClient *client = [[TClient alloc] init];
[client loginToMistarWithPin:#"20014204" password:#"yuiop" success:^{
[client getUserID:^(NSString *result) {
NSString *userIDWithHTML = [self userIDRegex:result];
NSString *userID = [self onlyNumbersRegex:userIDWithHTML];
//if userID exists, send it to another method in a different class
}];
} failure:^{
NSLog(#"login failed from controller");
}];
}
I see that this is the third question you ask related to the same issue, so I guess you're having some trouble understanding blocks.
First you have to understand that the block, in a certain sense, can be seen as a function. The difference is that, unlike a function, the block has no name, and instead of using function's name you just place the code inline where you need it.
Second thing to understand is that a block is usually used as a callback. Other callback mechanisms are function pointers and delegates. When you pass a block as a parameter to a function you're basically telling the function: "Hey, when certain conditions are met, execute this little code for me, please"
Third think to understand is if the block (or any callback) will be called synchronously. Actually this has nothing to do with the block itself, per se, but rather with the function being called. If the function is asynchronous, the function will create another thread and return immediately to execute the next line after the one that invoked the asynchronous function. Meanwhile the new thread will execute some code (the body of the async function) and, eventually execute the block passed as parameter, and finally the thread is killed and doesn't exist any more. (Note: There's no way to know if a function is synchronous or asynchronous other that reading the documentation for such a function).
Now let's go back to your code.
[client loginToMistarWithPin:#"20014204" password:#"yuiop" success:^{
[client getUserID:^(NSString *result) {
NSString *userIDWithHTML = [self userIDRegex:result];
NSString *userID = [self onlyNumbersRegex:userIDWithHTML];
// PLACE HERE THE CODE TO EXECUTE WHEN SUCCESSFULLY LOGGED IN
[anotherClassInstance someMethod:userID];
}];
} failure:^{
NSLog(#"login failed from controller");
}];
Everything that should be executed once the user logged in should be placed inside the block (if the function is synchronous you could place it after the block). To send the userID to another class, just call that class' method as you would in any other part of your code.
In my opinion using a delegate is not necessary (although only you would know, since you're the architect of your app).
As #santhu said, use either the delegate pattern or notification pattern. It's also a common practice to use both of them. Usually a delegate is the correct approach but sometimes you need a notification. Using both covers all your bases.
Look them up before deciding which and for full details on how they work, but basically:
[client getUserID:^(NSString *result) {
NSString *userIDWithHTML = [self userIDRegex:result];
NSString *userID = [self onlyNumbersRegex:userIDWithHTML];
// delegate pattern:
if ([self userIdIsValid:userID]) {
if (self.delegate && [self.delegate respondsToSelector:#selector(foundValidUserID:)]) {
[self.delegate foundValidUserID:userID];
}
} else {
if (self.delegate && [self.delegate respondsToSelector:#selector(foundInvalidUserID:)]) {
[self.delegate foundInvalidUserID:userID];
}
}
// notification pattern:
if ([self userIdIsValid:userID]) {
[[NSNotificationCenter defaultCenter] postNotificationName:MyFoundValidUserIDNotification object:self userInfo:#{#"userID": userID}];
}
} else {
[[NSNotificationCenter defaultCenter] postNotificationName:MyFoundInvalidUserIDNotification object:self userInfo:#{#"userID": userID}];
}
}];
There is a third option, which is you could use a block callback... this is how the new kids on the block do it... there's no well defined pattern here, blocks are brand new and delegates/notifications are 20 years old. But here's how I'd use a block to define a callback:
typedef void (^UserIdCallbackBlock)(NSString *userID);
- (void)parseForUserIDOnSuccess:(UserIdCallbackBlock)successCallback onFailure:(UserIdCallbackBlock)failureCallback {
...
NSString *userID = [self onlyNumbersRegex:userIDWithHTML];
if ([self userIdIsValid:userID]) {
successCallback(userID);
} else {
failureCallback(userID);
}
...
}
I would like to give a hint regarding your comment:
for code readability, it's not that I just have one more task to do, the thing I put inside this block will also have a block and another block and another.
This is a typical asynchronous pattern - called "continuation".
Given, that you should also implement proper error handling and that you should also provide a means to cancel that whole "chain" of asynchronous tasks at any point, the typical solutions with NSOperationQueues and NSOperations, dispatch_queue and blocks, NSNotifications or delegates will inevitable become unduly elaborate, complex and difficult to comprehend by others. (There's already an answer here that demonstrates this grandiose ;) )
So, whenever problems become more complex and the "built-in frameworks" don't provide a comfortable solution, third party libraries come into play to help you.
But first, lets have a non-trivial example, based on your comment:
it's not that I just have one more task to do, the thing I put inside this block will also have a block and another block and another
OK, lets suppose your objective is actually:
Asynchronously perform a Login for a web service.
Then, if that succeeded, asynchronously fetch a list of objects as JSON.
Then, if that succeeded, parse the JSON response.
Then, if that succeeded, insert the objects into a managed object context and asynchronously save the chain of managed object contexts and make it persistent.
When this all above succeeded, update the UI on the main thread
If anything fails, report the error of the task that failed
I will show how a solution utilizing a library implementing "promises" (see wiki Future and promises) may look like:
Without further ado, and without thorough explanation what that "Promise" is about, suppose we have a method defined in your View Controller, which is declared:
- (RXPromise*) loginToMistarWithPin:(NSString*)pin
password:(NSString*)password;
Note: The above method is asynchronous and it is functional equivalent to the form:
typedef void (^completion_t)(id result, NSError*error);
- (void) loginToMistarWithPin:(NSString*)pin
password:(NSString*)password
completion:(completion_t)completion;
then suppose we have another method in your View Controller, fetching objects from a remote server (asynchronous as well):
- (RXPromise*) fetchObjects;
Then, suppose we have a class CoreDataStack which consists of a "root context" saving to the persistent store having a child managed object context, the "main context", which is associated to the main thread.
The class CoreDataStack defines this method, which saves a chain of managed object contexts, which is basically setup: childContext -> main_context -> root_context:
- (RXPromise*) saveWithChildContext:(NSManagedObjectContext*)childContext;
Then, the whole task as stated in the steps 1. through 5. can be expressed as follows:
[client loginToMistarWithPin:#"20014204" password:#"yuiop"]
.then(^id(id result){
// login succeed, ignore result which is #"OK"
// Now fetch the objects with an asynchronous network request,
// returning JSON data as a NSData object when it succeeds:
return [client fetchAllUsers];
}, nil)
.then(^id(NSData* json){
// The network request succeeded, and we obtain the JSON as NSData.
// Parse it and get a Foundation representation:
NSError* error;
id jsonArray = [NSJSONSerialization JSONObjectWithData:json
options:0
error:&error];
if (jsonArray) {
return jsonArray; // handler succeeded
}
else {
return error; // handler failed
}
})
.then(^id(NSArray* objects){
// Parsing succeeded. Parameter objects is an array containing
// NSDictionaries representing a type "object".
// Save into Core Data:
// Create a managed object context, which is a child of the
// "main context" of a Core Data stack:
NSManagedObjectContext* moc = [[NSManagedObjectContext alloc]
initWithConcurrencyType:NSPrivateQueueConcurrencyType];
moc.parentContext = self.coreDataStack.managedObjectContext;
// Create managed objects and initialize them with the given
// NSDictionary:
for (NSDictionary* object in objects) {
// note: `createWithParameters:inManagedObjectContext` executes on
// the context's queue
[Object createWithParameters:object inManagedObjectContext:moc];
}
// Finally, asynchronously save into the persistent store and
// return the result (a RXPromise):
return [self.coreDataStack saveWithChildContext:moc];
}, nil)
.thenOn(dispatch_get_main_queue(), ^id(id result){
// Saving to the backing store succeeded. Now, we possibly want to
// update some UI on the main thread. We are executing on the main
// thread already (see thenOn(dispatch_get_main_queue())
...
[self.tableView reloadData];
return nil;
}, nil)
.then(nil, ^id(NSError* error){
// If something went wrong in any of the above four steps, the error
// will be propagated down and "cought" in this error handler:
NSLog(#"Error: %#", error);
});
Disclaimer: I'm the author of the library RXPromise available at GitHub. There are a few more Objective-C libraries which implement Promises.
I know what #synchronized() does, but...
sometimes we have:
1- #synchronized(self)
2- #synchronized([MyClass class])
3- #synchrinized(myObj)
What is the difference, and what is the parameter I should pass to this block ?
From the documentation:
The object passed to the #synchronized directive is a unique
identifier used to distinguish the protected block. If you execute the
preceding method in two different threads, passing a different object
for the anObj parameter on each thread, each would take its lock and
continue processing without being blocked by the other. If you pass
the same object in both cases, however, one of the threads would
acquire the lock first and the other would block until the first
thread completed the critical section.
So it depends on what you want to protect from being executed simultaneously,
and there are applications for all three cases.
For example, in
-(void)addToMyArray1:(id)obj
{
#synchronized(self) {
[self.myArray1 addObject:obj];
}
}
-(void)addToMyArray2:(id)obj
{
#synchronized(self) {
[self.myArray2 addObject:obj];
}
}
both #synchronized blocks cannot be executed simultaneously by two threads calling
the method on the same instance (self), thus protecting simultaneous access to the
arrays from different threads.
But it also prevents the block from the first method
to be executed simultaneously executed with the block from the second method, because they
use the same lock self. Therefore, for more fine-grained locking, you could use
different locks:
-(void)addToMyArray1:(id)obj
{
#synchronized(self.myArray1) {
[self.myArray1 addObject:obj];
}
}
-(void)addToMyArray2:(id)obj
{
#synchronized(self.myArray2) {
[self.myArray2 addObject:obj];
}
}
Now the simultaneous access to self.myArray1 and self.myArray2 from different threads
is still protected, but independently of each other.
A lock on the class can be used to protect access to a global variable.
This is just a trivial example for demonstration purposes:
static int numberOfInstances = 0;
-(id)init
{
self = [super init];
if (self) {
#synchronized([self class]) {
numberOfInstances++;
}
}
}
#synchronized should have the same object passed each time. So #synchronized(self) would work best.
I am transitioning some thread safety code from #synchronized to NSRecursiveLock.
Consider this code in which myItemsArray is an NSMutableArray:
- (NSUInteger) numberOfItems {
#synchronized(self.myItemsArray) {
return self.myItemsArray.count;
}
}
I believe the following code is incorrect because the lock would never get unlocked:
- (NSUInteger) numberOfItems {
[self.myRecursiveLock lock];
return self.myItemsArray.count;
[self.myRecursiveLock unlock];
}
So I'm using this approach instead:
- (NSUInteger) numberOfItems {
[self.myRecursiveLock lock];
NSUInteger itemCount = self.myItemsArray.count;
[self.myRecursiveLock unlock];
return itemCount;
}
However, I think this approach would break the thread safety, since another thread could add or remove an item after -unlock is called, but before itemCount is returned.
I'm not sure if I'm correct that the last approach isn't thread-safe, because I see this pattern in many widely used third-party libraries (for example, [AFHTTPRequestOperation -responseObject])
What is the correct way to return a value from a method synchronized using NSRecursiveLock?
Protecting numberOfItems can never ensure that the count is up-to-date. The array might be modified by another thread immediately after the method returns. It only prevents that two threads call the count method simultaneously.
I'm having trouble implementing the thread-safe core data concepts outlined in this tutorial. My goal is to have a reusable portion of code that can take arguments in, do core data operations (adds, updates, deletes) and then callback asynchronously when done.
So heres the block that 'safely' modifies core data objects:
+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock
{
NSManagedObjectContext *context = [NSManagedObjectContext context];
[context setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
[defaultContext setMergePolicy:NSMergeObjectByPropertyStoreTrumpMergePolicy];
[defaultContext observeContext:context];
block(context);
if ([context hasChanges])
{
[context save];
}
}
From the way I understand it, this executes a block of code? I don't understand how the 'context' being passed in figures in. Is this part of the block's signature?
So here is the wrapper that does the operation in the background and adds a completion call:
+ (void)saveDataInBackgroundWithContext:(void(^)(NSManagedObjectContext *context))saveBlock completion:(void(^)(void))completion
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
[self saveDataInContext:saveBlock];
dispatch_sync(dispatch_get_main_queue(), ^{
completion();
});
});
}
Here is an example using it:
NSArray *listOfPeople = ...;
[NSManagedObjectHelper saveDataInBackgroundWithContext:^(NSManagedObjectContext *localContext){
for (NSDictionary *personInfo in listOfPeople)
{
PersonEntity *person = [PersonEntity createInContext:localContext];
[person setValuesForKeysWithDictionary:personInfo];
}
} completion:^{
self.people = [PersonEntity findAll];
}];
What is the 'localContext' passed in here? I think most of my issues here revolve around not understanding blocks.
A brief look at that tutorial shows it is talking about magical record. I have never used it, so I can't speak for it.
// This declares a class method that returns void and takes a block as parameter.
// The block returns void and has one parameter, namely, a pointer to an
// NSManagedObjectContext object.
+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock;
You would call that method like this...
[SomeClass saveDataInContext:^(NSManagedObjectContext *context){
// Some code
}];
That means you are passing in a block of code to the function. At some point it will execute the bock of code you gave it. When it does, it's going to pass a managed object context into the block so it can do something with it.
Now, look at the implementation of that method...
+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock
{
// Create a MOC - note there is no concurrency type, so it will get
// NSConfinementConcurrencyType, which means it must be used exclusively
// from the thread in which it was created. Since it is a local variable
// and it gets destroyed after this function is called, that should be cool
// PROVIDED the using block does not do anything untoward with it.
NSManagedObjectContext *context = [NSManagedObjectContext context];
// Set the merge policy
[context setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
// MR must set some default context...
// Some how the above context needs a persistent store to save...
[defaultContext setMergePolicy:NSMergeObjectByPropertyStoreTrumpMergePolicy];
// Probably setting up notification handler for DidSave
[defaultContext observeContext:context];
// Now, this is where the block you passed in gets called.
// Note, that the managed object context has already been setup for you.
// Now that it's setup, the block of code that you passed in is going
// to be called, and it will be given a context that it can use to execute
// code in the calling thread.
block(context);
// If you changed something to the context in your block of code, the save.
if ([context hasChanges])
{
[context save];
}
}
Let's revisit a our code that called this method...
[SomeClass saveDataInContext:^(NSManagedObjectContext *context){
// Now, the saveDataInContext method has been called. However, inside
// that method, a call was made to the block that was passed in.
// That would be this here block of code. So, if you look up in
// the method, where is calls "block(context)" this block of code will
// be executed right there. Mentally, you can cut and paste this code
// in that spot.
// The context parameter is the context that was passed to this block.
// you can use it to do any Core Data stuff...
}];
Now, this code is very similar, but it takes two blocks. One is used to execute some code on the context, and the other is a block that will get executed with the asynchronous save has completed.
saveBlock should be familiar. It's the same concept as in the above example.
completion is a block, that returns void, and takes not parameters. It will get called when all the work has been done.
+ (void)saveDataInBackgroundWithContext:(void(^)(NSManagedObjectContext *context))saveBlock completion:(void(^)(void))completion
{
// Dispatch some work on one of the global concurrent queues. It will
// get done on some thread, nobody knows which one, but it does not matter
// because the code in this block calls saveDataInContext, and passes the
// block it was given that does some modifications to the context.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
[self saveDataInContext:saveBlock];
// Now, when the above work is done, we are still running in some random
// thread. I guess the library wants all callbacks to happen on the main
// thread, so this block is dispatched on the main thread. Note that it
// calls the second bock passed in as the completion block.
// So the <saveBlock> will be run on some random thread, and then, when
// it is done, the <completion> block will be called on the main thread.
dispatch_sync(dispatch_get_main_queue(), ^{
completion();
});
});
}
Like earlier, when you call that method, you can mentally replace the with the first block you pass in, and replace with the second block.
[NSManagedObjectHelper saveDataInBackgroundWithContext:^(NSManagedObjectContext *localContext){
// This is the first block. It gets executed where you see <saveBlock>
// being used in the earlier method. You are being given the already
// prepared MOC, and it's name is <localContext>. Do your managed object
// context stuff with it. Note that it will be running in some unknown thread.
for (NSDictionary *personInfo in listOfPeople)
{
PersonEntity *person = [PersonEntity createInContext:localContext];
[person setValuesForKeysWithDictionary:personInfo];
}
} completion:^{
// Now, this is the second block, which is run when all the core data saving
// has been completed. It will run on the main thread.
self.people = [PersonEntity findAll];
}];
Hopefully, that helps you understand what is happening, even though I don't know what magical record is really doing under the covers.
EDIT
In response to this comment...
I don't think I understand how these blocks work. If a block has this
method signature "+
(void)saveDataInContext:(void(^)(NSManagedObjectContext
*context))saveBlock" why is the block NOT using "context" or "saveBlock"? Which is the return value for a block and which is the
passed in value? – Mike S
First, the block does not have this signature...
+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock;
That is a class method. Let's break it down piece by piece. First, however, let's forget the block parameter, and use something easy, for comparison.
+ (void)foo:(NSInteger)someInteger;
That is a is a class method, foo:, which returns void and takes one argument. The type of that single argument is NSInteger. If I wanted to call it, I would do so like this:
[SomeClass foo:42];
Likewise...
+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock;
is a class method, saveDataInContext:, which returns void and takes one argument. The type of that single argument is void(^)(NSManagedObjectContext *context).
Now, don't let that gobbly-dee-gook fool you. It's just a type (albeit a somewhat confusing one to parse if you don't understand much C) So, what is void(^)(NSManagedObjectContext *context)
First, it is a block. If that (^) after the void were (*) it would be a function pointer.
Basically, it means that the type of that argument is a block that returns void and has one parameter, namely a pointer to a NSManagedObjectContext (with a name context).
So, if we read it out loud...
+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock;
is a class method, with selector saveDataInContext: which returns void and has one parameter, which has a name saveBlock and is of the type "block that returns void and has one parameter of type NSManagedObjectContext *."
Just like we call the first example like this...
[SomeClass foo:42];
we call the latter example like this...
[SomeClass saveDataInContext:^(NSManagedObjectContext *context){
// We are creating a bock of code, so stuff some code in here.
}];
Now, just like you passed the integer 42 to foo: you are passing the block in between the {} as the argument to saveDataInContext:.
Now, note that the signature of thesaveDataInContext: method wants a block that itself has a parameter. So, when you provide your block, you are basically saying, "Hey, here is a block of code for you to call, and when you do so, make sure you give me a pointer to a NSManagedObjectContext object that I can use.
What this means is that when your block is called, the calling code will call your block and provide a NSManagedObjectContext * to you with the variable name context.
Think of it like this, as a trivial example of saveDataInContext:.
+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock {
// Create a context to give the block we are going to call..
NSManagedObjectContext *moc = //
saveBlock(moc);
}
Now, when your code is called, you will get the moc object as your argument. Bascially, that method creates a managed object context, does all the thread safety stuff, then calls your block of code, and gives you a pointer to the managed object context that it has safely created. Your code is executed within the confines of that safe environment, using the MOC passed to it as a function (block) parameter.
I hope that didn't make it worse...