Thread Synchronization in Objective-C

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Concurrent access to objects from two or more independent threads presents a consistent set of problems across almost all programming languages. Java and Objective-C both provide language support in the form of a @synchronized (synchronized) directive. Any block of code marked as @synchronized is protected against being executed by more than one thread at a time. This section will review the synchronization support in Objective-C. In addition to the @synchronized directive, the Cocoa frameworks provide several utility classes that implement different kinds of mutual exclusion semaphores, a set of classes for organizing concurrent tasks, and timers.
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CHAPTER 15 THREADS
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The @synchronize Directive
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Objective-C s @synchronized directive is almost identical to Java s synchronized keyword, with only one minor exception: @synchronized can t be used as a method modifier. To accomplish the equivalent, use @synchronized(self) as the outermost block of your method, as shown in Listing 15-6.
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Listin g 15-6. Synchronized Method
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Java public class Timing { public synchronized void safe( ) { // thread safe code } } Objective-C @interface Timing : NSObject - (void)safe; @end @implementation Timing - (void)safe { @synchronized(self) { // thread safe code } } @end Beyond that minor limitation, you can use @synchronized exactly as you would use synchronized in Java.
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Java 5.0 has introduced a wealth of new concurrent process control classes, including various mutual exclusion semaphores, queues, resource counters, and so on. Objective-C has always had the NSLock, NSRecursiveLock, and NSConditionLock classes. Recent versions of the operating system added NSOperation for managing complex sets of tasks. NSOperation is discussed in the next section. Each of the three NS Lock classes offers a different kind of mutual exclusion semaphore, or mutex for short. They all work by obtaining a lock, approximately equivalent to entering a @synchronized block of code. All other threads attempting to acquire a lock on the same object will be suspended until
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CHAPTER 15 THREADS
the original lock is released. Once the semaphore is unlocked, one of the other threads will be granted the lock and resume execution. All of the mutex semaphore classes have several things in common: They all conform to the NSLocking protocol, which defines the fundamental -lock and -unlock methods. They all implement -lockBeforeDate: and -tryLock methods. They all implement -name and -setName: methods. The -tryLock and -lockBeforeDate: messages attempt to acquire the lock and return YES if successful. -tryLock returns immediately, while -lockBeforeDate: will suspend until either the lock is acquired or the time in the NSDate object occurs. This is one of the significant advantages of using NS Lock objects over @synchronized directives. Finally, you can name your locks for whatever reason.
NSRecursiveLock
NSRecursiveLock objects behave pretty much like @synchronized blocks. An NSRecursiveLock is a mutual exclusion semaphore between threads, but within a single thread it can be locked as many times as desired. The lock is acquired once during the initial -lock message, equivalent to entering a @synchronized block. Subsequent -lock messages sent on the same thread increment a counter. Once the lock has received one -unlock message for every -lock message, the lock is released. The one significant difference is that a @synchronized block will automatically catch an exception and release the lock before rethrowing the exception. If you use NSRecursiveLock objects around code that could throw an exception, make sure you catch the exception and clean up your locks.
NSLock
NSLock is the simplest mutex semaphore and only implements the basic -lock, -tryLock, -lockBeforeDate:, and -unlock methods. Objective-C code written before the @synchronized directive was added would most likely use NSLock objects instead of @synchronized blocks, even though the behavior of @synchronized is closer to NSRecursiveLock.
Caution An NSLock must always be unlocked from the same thread that locked it. Some programmers, in a
misguided attempt to implement cross-thread synchronization, will lock an NSLock object twice on the same thread causing a deadlock then release it by unlocking the object from another thread. This is not guaranteed to work and may cause fatal program errors. The correct solution is to use an NSConditionLock, described later.
The primary advantages of NSLock are speed and simplicity. Unlike an NSRecursiveLock, an NSLock can only be acquired once before being unlocked again. Consider the code in Listing 15-7.
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