How does performSelectorInBackground and detachNewThreadSelector work? - ios

I need to perform an asynchronous function execution because it is blocking the main thread and hence the UI is not available.
After looking at the questions in stackoverflow, I know there are three ways to do asynchronous function.
An example:
[NSThread detachNewThreadSelector:#selector(showSpinner:) toTarget:self withObject:self.view];
// or
[self performSelectorInBackground:#selector(showSpinner:) withObject:self.view];
// or
NSInvocationOperation *invOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:#selector(showSpinner:) object:self.view];
NSOperationQueue *opQueue = [[NSOperationQueue alloc] init];
[opQueue addOperation:invOperation];
// or
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
dispatch_async(dispatch_get_main_queue(), ^{
[self showSpinner:self.view];
});
});
My question is how does performSelectorInBackground and detachNewThreadSelector return back to main thread? how do you know that they are done?

A couple of thoughts:
You might want to check Migrating Away From Threads in the Concurrency Programming Guide, which makes a compelling argument for dispatch queues and operation queues, which are discussed earlier in that same guide.
Also, as you delve into various asynchronous operations, remember, do time consuming stuff in background queues/threads, but always dispatch UI stuff back to the main queue. I only mention that because your task, showSpinner sounds a lot like a UI task, which you would never want to do in a background queue/thread. If it has some "expensive" non-UI related tasks, then fine, do that in the background, but make sure the UI stuff gets dispatched back to the main queue.
There are, as an aside, other renditions of the operations queues, e.g., block operations:
NSOperationQueue *opQueue = [[NSOperationQueue alloc] init];
[opQueue addOperationWithBlock:^{
// do some slow stuff in the background here
// ok, now do UI stuff in the main queue
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
[self showSpinner:self.view];
}];
}];
This is roughly equivalent to the GCD (dispatch queue) rendition:
dispatch_queue_t dispatchQueue = dispatch_queue_create("com.ramshad.app", 0);
dispatch_async(dispatchQueue, ^{
// do some slow stuff in the background here
// ok, now do UI stuff in the main queue
dispatch_async(dispatch_get_main_queue(), ^{
[self showSpinner:self.view];
});
});
There are tons of subtle pros and cons between the operation queues and dispatch queues (which we should not get into here because it's been discussed hundreds of times elsewhere on Stack Overflow), but both let you do surprisingly rich asynchronous operations with less complexity than traditional thread programming.
If you decide to stick with threads versus operation and/or dispatch queues (which I wouldn't necessarily recommend), you might want to check out the Threading Programming Guide.

To identify performSelectorInBackground & detachNewThreadSelector end of execution,call a method at the end of the thread method on main thread.
Additionaly NSThread provides an propery as isFinished which returns a Boolean value that indicates whether the receiver has finished execution.
Example:
[self performSelectorOnMainThread:#selector(threadMethod)
withObject:nil
waitUntilDone:NO];
or
[NSThread detachNewThreadSelector:#selector(threadMethod)
toTarget:self
withObject:nil];
-(void)threadMethod{
//here your implementation code
//here call the end notification method.
[self performSelectorOnMainThread:#selector(ThreadExecutionDone)
withObject:nil
waitUntilDone:YES];
}
-(void)ThreadExecutionDone{
//end of the performSelectorInBackground or detachNewThreadSelector.
}

Related

Wait for value to change in thread Objective C

I have a thread call in objective C and i want once this thread ends i want to return a value ;the value will be changed inside the thread
So the method must not return the value unless the tread ends
Here is the code i use:
[NSThread detachNewThreadSelector:#selector(CheckBeforePrint2) toTarget:self withObject:nil];
This is My Full Code
- (NSString *) getStatusPrinter:(NSString *) printerSerialNumber
{
self.printerSN = printerSerialNumber;
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSString *Result = #"-1";
[NSThread sleepForTimeInterval:2.0f];
[NSThread detachNewThreadSelector:#selector(CheckBeforePrint) toTarget:self withObject:Result];
[pool release];
return Result;
}
Now i want to wait for the value of Result and return it i am using
cocoa
And i am returning the value to another app
Can anyone help in that.
Thanks
What you are doing here requires the use of a semaphore for example. If there is nothing more to it than you are providing here then a completion block to a background running method is the best way to do it. See option 2 below
Either way, why do you want the parent thread (the thread dispatching a new thread) to wait for another thread? If the parent thread is waiting, it is locked until the dispatched thread is done (according to your requirement). This situation is redundant because the whole point of dispatching another thread is so that the parent thread can continue with other things. Unless of course the parent thread needs to wait for multiple threads, then it makes sense to lock it.
Having said that, its best to just let the dispatching thread / parent thread do the processing that you are dispatching on to another thread. Im only saying this given the details you have provided.
OPTION 1 use a semaphore
Use a semaphore to lock and unlock parent thread
-(void)getStatusPrinter()
{
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
[NSThread detachNewThreadSelector:#selector(checkBeforePrint2) toTarget:self withObject: semaphore];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
[self print]; // this will run after semaphore is unlocked
}
-(void)checkBeforePrint2:(dispatch_semaphore_t)sem
{
//this is within child thread
//do some processing,
dispatch_semaphore_signal(sem);//unlock semaphore
}
But, as I mentioned before, this situation seems redundant because the parent thread waits (therefore unusable) for child thread; why can't it just do the work itself...
OPTION 2 use completion block (PREFERABLE)
Use a completion block that you pass to the child thread. This allows the parent thread to continue. If it is the main thread it remains free for UI stuff.
-(void)getStatusPrinter()
{
[self checkBeforePrint2WithCompletion: ^{
[self print];
}];
//continue with more stuff
}
-(void)checkBeforePrint2WithCompletion:(void (^ __nullable)(void))completion
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//do something before executing completion code
if(completion){
completion();
}
});
}
Disclaimer: there may be typos in this code as it was not written in an editor/ IDE. Please comment if any.
UPDATE in response to added details.
Okay, for the fact that you said you need to return the result to another application, that means the entry thread at getStatusPrinter can not be allowed to return after dispatching a new thread. If you really need to create a new thread for CheckBeforePrint then the entry thread has to wait. That to me is pointless. You can simply run everything on the entry thread.
If you are using openURL:options:completionHandler: then the entry thread doesn't need to wait. The value of result can be passed back within the completion block.
Please refer to Apple's documentation on openURL with a completion handle

IOS thread pool

I've got this method
-(void)addObjectToProcess(NSObject*)object;
and i want this method to add the object to process queue which can process up to 4 objects in parallel.
i've created my own dispatch_queue and semhphore
_concurrentQueue = dispatch_queue_create([queue_id UTF8String],DISPATCH_QUEUE_CONCURRENT);
_processSema = dispatch_semaphore_create(4);
and the implementation of the method is:
-(void)addObjectToProcess(NSObject*)object {
dispatch_semaphore_wait(self.processSema, DISPATCH_TIME_FOREVER);
__weak MyViewController* weakSelf = self;
dispatch_async(self.concurrentQueue, ^{
// PROCESS...........
// ..................
dispatch_semaphore_signal(self.processSema);
dispatch_async(dispatch_get_main_queue(), ^{
// call delegate from UI thread
});
});
}
it seems the caller sometimes gets blocked cause of the semaphore barrier.
is there any other/easier option to implement what i'm trying to make here ?
Thanks
The problem is that you're calling dispatch_semaphore_wait on whatever thread you called addObjectToProcess on (presumably the main thread). Thus, if you already have four tasks running, when you schedule this fifth process, it will wait on the main thread.
You don't, though, just want to move the waiting for the semaphore into the block dispatched to self.concurrentQueue, because while that will successfully constrain the "PROCESS" tasks to four at a time, you will consume another worker thread for each one of these backlogged dispatched tasks, and there are a finite number of those worker threads. And when you exhaust those, you could adversely affect other processes.
One way to address this would be to create a serial scheduling queue in addition to your concurrent processing queue, and then dispatch this whole scheduling task asynchronously to this scheduling queue. Thus you enjoy the maximum concurrency on the process queue, while neither blocking the main thread nor using up worker threads for backlogged tasks. For example:
#property (nonatomic, strong) dispatch_queue_t schedulingQueue;
And
self.schedulingQueue = dispatch_queue_create("com.domain.scheduler", 0);
And
- (void)addObjectToProcess(NSObject*)object {
dispatch_async(self.schedulingQueue, ^{
dispatch_semaphore_wait(self.processSema, DISPATCH_TIME_FOREVER);
typeof(self) __weak weakSelf = self;
dispatch_async(self.concurrentQueue, ^{
// PROCESS...........
// ..................
typeof(self) __strong strongSelf = weakSelf;
if (strongSelf) {
dispatch_semaphore_signal(strongSelf.processSema);
dispatch_async(dispatch_get_main_queue(), ^{
// call delegate from UI thread
});
}
});
});
}
Another good approach (especially if the "PROCESS" is synchronous) is to use NSOperationQueue that has a maxConcurrentOperationCount, which controls the degree of concurrency for you. For example:
#property (nonatomic, strong) NSOperationQueue *processQueue;
And initialize it:
self.processQueue = [[NSOperationQueue alloc] init];
self.processQueue.maxConcurrentOperationCount = 4;
And then:
- (void)addObjectToProcess(NSObject*)object {
[self.processQueue addOperationWithBlock:^{
// PROCESS...........
// ..................
dispatch_async(dispatch_get_main_queue(), ^{
// call delegate from UI thread
});
}];
}
The only trick is if the "PROCESS", itself, is asynchronous. If you do that, then you can't just use addOperationWithBlock, but rather have to write your own custom, asynchronous NSOperation subclass, and then use addOperation to the NSOperationQueue. It's not hard to write asynchronous NSOperation subclass, but there are a few little details associated with that. See Configuring Operations for Concurrent Execution in the Concurrency Programming Guide.

Executing Methods One After Another in iOS [duplicate]

This question already has answers here:
How do I perform several methods in sequence?
(2 answers)
Closed 8 years ago.
In my application I have four methods like
- (void)Method1;
- (void)Method2;
- (void)Method3;
- (void)Method4;
I want to execute these methods one after another. I search for this one in some sources they are using "dispatch_Time" in some sources they are using "NSThread sleepForTimeInterval:" But in my application I don't want to execute those methods using time. I want to execute them if the previous method execution is completed. How can I do this?
Solution Overview
The right way of doing this is to enqueue the methods onto a serial queue.
A serial queue executes one task at a time and thus ensures that a task is only executed once all it's predecessors were executed.
There are several ways of accomplishing what you wish. I will describe 2 of them, one using Grand Central Dispatch and one using NSOperationQueue.
Grand Central Dispatch
Create a queue onto you will enqueue your task. A good practice would be to keep the queue as an instance variable so that you could access it from instance methods (unlike NSOperationQueue, by default custom dispatch queues are serial by default, i.e. they execute one task at a time):
dispatch_queue_t my_queue = dispatch_queue_create("com.suresh.methodsqueue", NULL);
self.methods_queue = my_queue;
Enqueue your methods onto the designated queue one after the other:
dispatch_async(self.methods_queue, ^{ [someObject method1] });
dispatch_async(self.methods_queue, ^{ [someObject method2] });
dispatch_async(self.methods_queue, ^{ [someObject method3] });
dispatch_async(self.methods_queue, ^{ [someObject method4] });
Further re GCD in Apple's developer guides.
Operation Queue
Initialize a queue:
NSOperationQueue* aQueue = [[NSOperationQueue alloc] init];
self.methods_queue = aQueue;
Make sure the queue is serial:
[self.methods_queue setMaxConcurrentOperationCount:1]
Enqueue the methods onto the queue, there's several ways of doing so, the following requires the least amount of code:
[self.methods_queue addOperationWithBlock:^{ [someObject method1] }];
[self.methods_queue addOperationWithBlock:^{ [someObject method2] }];
[self.methods_queue addOperationWithBlock:^{ [someObject method3] }];
[self.methods_queue addOperationWithBlock:^{ [someObject method4] }];
Further re Operation Queues in Apple's developer guides.
If the methods don't return until they've finished their work, just call them in order:
[self Method1];
[self Method2];
[self Method3];
[self Method4];
Otherwise, you need to use something like dispatch groups.
call first method somewhere like
[self (void)Method1];
and implement like
(void)Method1{
// logic here
[self (void)Method2];
}
(void)Method2{
// logic here
[self (void)Method3];
}
and so on ...............

How to dispatch code blocks to the same thread in iOS?

Main aspect of the question: It's about iOS. Can I somehow dispatch code blocks in a way, that they will all (a) run in background and (b) on the same thread? I want to run some time-consuming operations in background, but these have to be run on the same thread, because they involve resources, that mustn't be shared among threads.
Further technical details, if required: It's about implementing an sqlite plugin for Apache Cordova, a framework for HTML5 apps on mobile platforms. This plugin should be an implementation of WebSQL in the means of the Cordova's plugin API. (That means, it's not possible to wrap entire transactions within single blocks, what could make everything easier.)
Here is some code from Cordova's Docs:
- (void)myPluginMethod:(CDVInvokedUrlCommand*)command
{
// Check command.arguments here.
[self.commandDelegate runInBackground:^{
NSString* payload = nil;
// Some blocking logic...
CDVPluginResult* pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_OK messageAsString:payload];
// The sendPluginResult method is thread-safe.
[self.commandDelegate sendPluginResult:pluginResult callbackId:command.callbackId];
}];
}
But as far as I know, there is no guarantee, that those dispatched code blocks (see runInBackground) will run on the same thread.
GCD makes no guarantee that two blocks run on the same thread, even if they belong to the same queue (with the exception of the main queue, of course). However, if you're using a serial queue (DISPATCH_QUEUE_SERIAL) this isn't a problem as you then know that there is no concurrent access to your data.
The man page for dispatch_queue_create says:
Queues are not bound to any specific thread of execution and blocks submitted to independent queues may execute concurrently.
I'm not aware of any way to bind a queue to a specific thread (after all, not needing to care about threads is a major point of GCD). The reason why you can use a serial queue without worrying about the actual thread is this promise:
All memory writes performed by a block dispatched to a serial queue are guaranteed to be visible to subsequent blocks dispatched to the same queue.
That is, a memory barrier seems to be used.
When dealing with threading issues, your main concern is usually to avoid that two threads access something concurrently. If you're using a serial queue you do not have this problem. It usually doesn't really matter which thread is accessing your resources. For example, we're using serial queues to manage Core Data access without a problem.
Edit:
It seems you really found a rare case where you need to be working on the same thread. You could implement your own worker thread:
Prerequisites:
A NSMutableArray (let's call it blockQueue).
A NSCondition (let's call it queueCondition).
Create a new NSThread.
The thread's method has an endless loop in which it locks the condition, waits for it if the queue is empty (and the "quit" bool is false), dequeues a block and executes it.
A method that locks the condition an enqueues the block.
Due to the condition, the thread will simply sleep while there's no work to do.
So, roughly (untested, assuming ARC):
- (void)startWorkerThread
{
workerThread = [[NSThread alloc]
initWithTarget:self
selector:#selector(threadMain)
object:nil
];
[workerThread start];
}
- (void)threadMain
{
void (^block)();
NSThread *currentThread;
currentThread = [NSThread currentThread];
while (1) {
[queueCondition lock];
{
while ([blockQueue count] == 0 && ![currentThread isCancelled]) {
[queueCondition wait];
}
if ([currentThread isCancelled]) {
[queueCondition unlock];
return;
}
block = [blockQueue objectAtIndex:0];
[blockQueue removeObjectAtIndex:0];
}
[queueCondition unlock];
// Execute block outside the condition, since it's also a lock!
// We want to give other threads the possibility to enqueue
// a new block while we're executing a block.
block();
}
}
- (void)enqueue:(void(^)())block
{
[queueCondition lock];
{
// Copy the block! IIRC you'll get strange things or
// even crashes if you don't.
[blockQueue addObject:[block copy]];
[queueCondition signal];
}
[queueCondition unlock];
}
- (void)stopThread
{
[queueCondition lock];
{
[workerThread cancel];
[queueCondition signal];
}
[queueCondition unlock];
}
Untested Swift 5 port:
var workerThread: Thread?
var blockQueue = [() -> Void]()
let queueCondition = NSCondition()
func startWorkerThread() {
workerThread = Thread() {
let currentThread = Thread.current
while true {
self.queueCondition.lock()
while self.blockQueue.isEmpty && !currentThread.isCancelled {
self.queueCondition.wait()
}
if currentThread.isCancelled {
self.queueCondition.unlock()
return
}
let block = self.blockQueue.remove(at: 0)
self.queueCondition.unlock()
// Execute block outside the condition, since it's also a lock!
// We want to give other threads the possibility to enqueue
// a new block while we're executing a block.
block()
}
}
workerThread?.start()
}
func enqueue(_ block: #escaping () -> Void) {
queueCondition.lock()
blockQueue.append(block)
queueCondition.signal()
queueCondition.unlock()
}
func stopThread() {
queueCondition.lock()
workerThread?.cancel()
queueCondition.signal()
queueCondition.unlock()
}
In GCD: no, that's not possible with the current lib dispatch.
Blocks can be executed by dispatch lib on whatever thread which is available, no matter to which queue they have been dispatched.
One exception is the main queue, which always executes its blocks on the main thread.
Please file a feature request to Apple, since it seems justified and sound. But I fear it's not feasible, otherwise it would already exist ;)
You can use NSOperationQueue. You can make it use just one thread by using method - (void)setMaxConcurrentOperationCount:(NSInteger)count. Set it to 1
Create a serial dispatch queue, and dispatch all the calls to that serial dispatch queue. All the calls will be performed in the background, but sequentially on the same thread.
If you want to perform a selector in Main Thread, you can use
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait
and if you want to it to perform in background thread
- (void)performSelectorInBackground:(SEL)aSelector withObject:(id)object
and if you want to perform in any other thread use GCD(Grand Central Dispatch)
double delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
//code to be executed on the main queue after delay
});
You can either use NSOperationQueue with a MaxConcurrentOperationCount of 1 or go the manual way down the road by using NSThread model (rather than Grand Central Dispatch).
Using the latter I would recommend you to implement a worker-method which runs in a thread and pulls work-packages (or commands) from a queue or a pool that is being feed from outside of the thread. Just make sure you use Locks / Mutex / Synchronisation.
Never tried this but this might do the trick. Use separate properties of atomic dispatch queues for each operation.
#property (strong, atomic) dispatch_queue_t downloadQueue;
Queue/Thread 1 for first operation
downloadQueue = dispatch_queue_create("operation1", NULL);
etc.
Since atomic is thread safe, downloadQueue should not be accessed by other threads. So it makes sure that there will be only single thread per operation and other threads will not access it.
Just like this,
dispatch_asyn(dispatch_get_current_queue, ^ {
});

Parsing JSON data from WebService on a background thread

I have a method that builds a package, sends it to a web service, gets a package back, opens it and returns me a nsdictionary. How can I call it on a background queue in order to display a HUD while it requests the data?
You could detach a new thread like following
- (void) fetchData
{
//Show Hud
//Start thread
[NSThread detachNewThreadSelector:#selector(getDataThreaded)
toTarget:self
withObject:nil];
}
- (void) getDataThreaded
{
//Start Fetching data
//Hide hud from main UI thread
dispatch_async(dispatch_get_main_queue(), ^{
//Update UI if you have to
//Hide Hud
});
}
Grand central dispatch (gcd) provides great support for doing what you ask. Running something in the background using gcd is simple:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_NORMAL, 0) ^{
NSDictionary* data = [self fetchAndParseData];
dispatch_async(dispatch_get_main_queue(), ^{
[self dataRetrieved:data];
});
});
This call will return immediately (so your UI will continue to be responsive) and dataRetrieved will be called when the data is ready.
Now, depending on how fetchAndParse data works it may need to be more complicated. If you NSURLConnection or something similar, you might need to create an NSRunLoop to process data callbacks on the gcd thread. NSURLConnection for the most part is asynchronous anyway (though callbacks like didReceiveData will be routed through the UI thread) so you can use gcd only to do the parsing of the data when all the data has been retrieved. It depends on how asynchronous you want to be.
In addition to previous replies, why don't you use NSOperation and NSOperationQueue classes? These classes are abstractions under GCD and they are very simple to use.
I like NSOperation class since it allows to modularize code in apps I usually develop.
To set up a NSOperation you could just subclass it like
//.h
#interface MyOperation : NSOperation
#end
//.m
#implementation MyOperation()
// override the main method to perform the operation in a different thread...
- (void)main
{
// long running operation here...
}
Now in the main thread you can provide that operation to a queue like the following:
MyOperation *op = [[MyOperation alloc] initWithDocument:[self document]];
[[self someQueue] addOperation:op];
P.S. You cannot start an async operation in the main method of a NSOperation. When the main finishes, delegates linked with that operations will not be called. To say the the truth you can but this involves to deal with run loop or concurrent behaviour.
Here some links on how to use them.
http://www.cimgf.com/2008/02/16/cocoa-tutorial-nsoperation-and-nsoperationqueue/
https://developer.apple.com/cocoa/managingconcurrency.html
and obviously the class reference for NSOperation

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