Multi-Threading

Creating a simple thread

The most simple way to create a thread is by calling a selector "in the background". This means a new thread is created to execute the selector. The receiving object can be any object, not just self, but it needs to respond to the given selector.

- (void)createThread {
    [self performSelectorInBackground:@selector(threadMainWithOptionalArgument:)
                           withObject:someObject];
}

- (void)threadMainWithOptionalArgument:(id)argument {
    // To avoid memory leaks, the first thing a thread method needs to do is
    // create a new autorelease pool, either manually or via "@autoreleasepool".
    @autoreleasepool {
        // The thread code should be here.
    }
}

Create more complex thread

Using a subclass of NSThread allows implementation of more complex threads (for example, to allow passing more arguments or to encapsulate all related helper methods in one class). Additionally, the NSThread instance can be saved in a property or variable and can be queried about its current state (whether it's still running).

The NSThread class supports a method called cancel that can be called from any thread, which then sets the cancelled property to YES in a thread-safe way. The thread implementation can query (and/or observe) the cancelled property and exit its main method. This can be used to gracefully shut down a worker thread.

// Create a new NSThread subclass
@interface MyThread : NSThread

// Add properties for values that need to be passed from the caller to the new
// thread. Caller must not modify these once the thread is started to avoid
// threading issues (or the properties must be made thread-safe using locks).
@property NSInteger someProperty;

@end

@implementation MyThread

- (void)main
{
    @autoreleasepool {
        // The main thread method goes here
        NSLog(@"New thread. Some property: %ld", (long)self.someProperty);
    }
}

@end


MyThread *thread = [[MyThread alloc] init];
thread.someProperty = 42;
[thread start];

Thread-local storage

Every thread has access to a mutable dictionary that is local to the current thread. This allows to cache informations in an easy way without the need for locking, as each thread has its own dedicated mutable dictionary:

NSMutableDictionary *localStorage = [NSThread currentThread].threadDictionary;
localStorage[someKey] = someValue;

The dictionary is automatically released when the thread terminates.