Serenity: 掌控 Objective-C 锁的奥秘（27）—— 锁定的底层

<script type="text/template" id="template">
  
  
  
  <markdown>
**Chapter 27: The Underpinnings of Locks** 

In this chapter, we delve into the deepest recesses of the Objective-C locking system, laying bare the foundations upon which the complex dance of concurrency is orchestrated. Brace yourself for a journey into the intricate web of primitives that govern thread safety, where the slightest missteps can lead to a cascade of cryptic errors.

Let us start with the enigmatic entity known as the "lock," a gate- keeper that stands resolute at the heart of every synchronized block. Its purpose is to prevent unruly code from wantonly trampling over shared resources, thus preserving the sanctity of your multithreaded masterpiece.

**The Binary Bastion of Locks** 

Every lock is backed by a simple, yet unyielding, boolean flag. This flag, in its unwavering vigilance, maintains a state of either "locked" or "unlocked," serving as the arbiter of access to the code it guards. As long as the lock remains in its steadfast "locked" position, no thread shall pass.

When a thread desires to cross the guarded  threshold, it dutifully requests the lock's favor. If the lock is in its benevolent "unlocked" state, it graciously bestows its blessing, allowing the thread to proceed. However, if the lock is found to be in a state of lock down, the thread has no choice but to abide, patiently awaiting its turn.

**The Busy Loop of Self-Spinning** 

While patiently awaiting the lock's change of heart, the thread engages in a seemingly mundane, yet critical, dance. In what could be likened to a form of digital limbo, it ceaselessly spins in a loop,  checking and rechecking the lock's status, like a restless child yearning for the playground.

This seemingly frivolous exercise, known as "self-spinning," has a purpose far greater than its simplicity would suggest. By tirelessly monitoring the lock's state, the thread ensures that it will be the first to know when the lock's ironclad  grip finally relents.

With each iteration of its self-spinning waltz, the thread consumes a sliver of the system's resources. This seemingly insignificant  drainage can accumulate over time, impacting the overall performance of your application. Thus, it is imperative to strike a balance between vigilance and efficiency.

**A Delicate Balancing Act** 

The true mastery of Objective-C locks lies in finding the  delicate balance between thread safety and performance. It is a  tightrope act, where every step must be carefully considered. Spin too little, and you risk deadlocks and data corruptions. Spin too much, and you squander  precious resources.

To help you tread this precarious path, the Objective-C masters have crafted a set of time-honored techniques. These techniques, passed down through  generations of thread-weavers, will be revealed in the coming installments of our odyssey into the arcane arts of concurrency.

**A Glimpse into the Source** 

To further illuminate the inner sanctum of locks, let us cast our  gaze upon the Objective-C source code itself. Behold the incantation that brings a lock into existence:

• (instancetype)init
  {
  if ((self = [super init]) {
  _lock = [[NSLock alloc] init];
  }
  return self;
  }

In this snippet, the `init` method of the `NSObject`  subclass weaves its magic, summoning a new lock named `_lock`  from the primordial depths of the Objective-C  framework. This lock will serve as the gate- keeper for the  sensitive data and operations within the class.

But what if, by some mischievous design, the lock itself were to  become entangled? What happens when the lock loses its way in the labyrinth of thread contention? Fear not, for Objective-C has a fail- safe in store.

• (void)dealloc
  {
  [_lock deallo _c];
  [super dealloc];
  }

In the somber twilight of the `dealloc` method, the lock's  lifecycle draws to a close. With gentle hands, the lock is  released from its post, allowing the  threads that relied on it to finally find their peace.

**Conclusion** 

As we delve ever  more  into the intricate  workings of Objective-C locks, remember  that mastery lies not only in knowing their powers, but also in understanding their limits. May your multithreaded creations dance in perfect  harmony, free from the shackles of deadlock and the  tyranny of performance bottlenecks.

May your journey into the depths of concurrency bear  abundant fruit, and may your code forever echo with the  serenity of well-managed locks.</markdown>
</script>