首先，协议中定义了一个方法，不通过block来进行加锁操作
/// Try to take lock without blocking. Returns whether the lock was taken.
- (BOOL)tryLock;
关键数据结构，一个最多包含kLockSetCapacity把锁的数组，这里用的是普通结构体类型，需要手动释放内存。
/**
* A set of locks acquired during ASLockSequence.
*/
typedef struct {
unsigned count;
CFTypeRef _Nullable locks[kLockSetCapacity];
} ASLockSet;
这里通过__unused定义，避免了警告，在不使用时不会编译
通过__attribute__((cleanup(ASUnlockSet))，可以在其离开作用域之后自动执行ASUnlockSet这个方法
/**
* Declare a lock set that is automatically unlocked at the end of scope.
*
* We use this instead of a scope-locking macro because we want to be able
* to step through the lock sequence block in the debugger.
*/
#define ASScopedLockSet __unused ASLockSet __attribute__((cleanup(ASUnlockSet)))
向集合中添加一个锁的方法，返回添加成功与否
/**
* A block that attempts to add a lock to a lock sequence.
* Such a block is provided to the caller of ASLockSequence.
*
* Returns whether the lock was added. You should return
* NO from your lock sequence body if it returns NO.
*
* For instance, you might write `return addLock(l1) && addLock(l2)`.
*
* @param lock The lock to attempt to add.
* @return YES if the lock was added, NO otherwise.
*/
typedef BOOL(^ASAddLockBlock)(id lock);
用于添加多个锁进入锁队列，例如将一个队列操作的锁的block，逐个调用方法，当且仅当所有锁添加成功，才返回YES。
此处用了NS_NOESCAPE逃逸标志，代表这个ASAddLockBlock会在ASLockSequenceBlock返回之前就执行完毕
/**
* A block that attempts to lock multiple locks in sequence.
* Such a block is provided by the caller of ASLockSequence.
*
* The block may be run multiple times, if not all locks are immediately
* available. Therefore the block should be idempotent.
*
* The block should attempt to invoke addLock multiple times with
* different locks. It should return NO as soon as any addLock
* operation fails.
*
* For instance, you might write `return addLock(l1) && addLock(l2)`.
*
* @param addLock A block you can call to attempt to add a lock.
* @return YES if all locks were added, NO otherwise.
*/
typedef BOOL(^ASLockSequenceBlock)(NS_NOESCAPE ASAddLockBlock addLock);
解除并且释放队列所有的锁，此处用了NS_INLINE，内联块，在别处调用此方法时会直接拷贝代码到相应位置，可以加快运行速度，但是换来的是内存上的增长。
/**
* Unlock and release all of the locks in this lock set.
*/
NS_INLINE void ASUnlockSet(ASLockSet *lockSet) {
for (unsigned i = 0; i < lockSet->count; i++) {
CFTypeRef lock = lockSet->locks[i];
[(__bridge id)lock unlock];
CFRelease(lock);
}
}
用于加锁的函数，需要提供一个包含一系列锁的body，超过数量限制会失败，并且撤回加锁操作，执行sched_yield()让出资源给其他等于或大于当前优先级的线程执行任务，继续while，直到成功加锁。
/**
* Take multiple locks "simultaneously," avoiding deadlocks
* caused by lock ordering.
*
* The block you provide should attempt to take a series of locks,
* using the provided `addLock` block. As soon as any addLock fails,
* you should return NO.
*
* For example:
* ASLockSequence(^(ASAddLockBlock addLock) ^{
* return addLock(l0) && addLock(l1);
* });
*
* Note: This function doesn't protect from lock ordering deadlocks if
* one of the locks is already locked (recursive.) Only locks taken
* inside this function are guaranteed not to cause a deadlock.
*/
NS_INLINE ASLockSet ASLockSequence(NS_NOESCAPE ASLockSequenceBlock body)
{
__block ASLockSet locks = (ASLockSet){0, {}};
BOOL (^addLock)(id) = ^(id obj) {
// nil lock = ignore.
if (!obj) {
return YES;
}
// If they go over capacity, assert and return YES.
// If we return NO, they will enter an infinite loop.
if (locks.count == kLockSetCapacity) {
ASDisplayNodeCFailAssert(@"Locking more than %d locks at once is not supported.", kLockSetCapacity);
return YES;
}
if ([obj tryLock]) {
locks.locks[locks.count++] = (__bridge_retained CFTypeRef)obj;
return YES;
}
return NO;
};
/**
* Repeatedly try running their block, passing in our `addLock`
* until it succeeds. If it fails, unlock all and yield the thread
* to reduce spinning.
*/
while (true) {
if (body(addLock)) {
// Success
return locks;
} else {
ASUnlockSet(&locks);
locks.count = 0;
sched_yield();
}
}
}
/**
* These Foundation classes already implement -tryLock.
*/
@interface NSLock (ASLocking)
@end
@interface NSRecursiveLock (ASLocking)
@end
@interface NSConditionLock (ASLocking)
@end