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(It’s not immediately clear which of these is the one to use; put the correct one first. Add Swift.) |
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The modern approach to creating a singleton class is the following: | |||
<source lang="objc"> | |||
+ (instancetype)sharedInstance { | |||
static dispatch_once_t onceToken; | |||
static XXXClass *sharedInstance; | |||
dispatch_once(&onceToken, ^{ | |||
sharedInstance = [[self alloc] init]; | |||
}); | |||
return sharedInstance; | |||
} | |||
</source> | |||
Or in Swift: | |||
<source lang="swift"> | |||
static let shared = XXXClass() | |||
// Hides the init() method from other classes to ensure XXXClass.shared is always used. | |||
private init() {} | |||
</source> | |||
Note that <code>static let</code> are lazy-loaded, so the implementation is identical to the Objective-C example. | |||
== Other Methods (Discouraged) == | |||
The traditional singleton creation | The traditional singleton creation | ||
<source lang="objc"> | <source lang="objc"> | ||
Line 20: | Line 45: | ||
pthread_once(&XXXClass_onceControl, &XXXInitializeOnce); | pthread_once(&XXXClass_onceControl, &XXXInitializeOnce); | ||
return XXXClass_sharedInst; | return XXXClass_sharedInst; | ||
} | } | ||
</source> | </source> |
Latest revision as of 04:05, 29 May 2021
The modern approach to creating a singleton class is the following:
+ (instancetype)sharedInstance {
static dispatch_once_t onceToken;
static XXXClass *sharedInstance;
dispatch_once(&onceToken, ^{
sharedInstance = [[self alloc] init];
});
return sharedInstance;
}
Or in Swift:
static let shared = XXXClass()
// Hides the init() method from other classes to ensure XXXClass.shared is always used.
private init() {}
Note that static let
are lazy-loaded, so the implementation is identical to the Objective-C example.
Other Methods (Discouraged)
The traditional singleton creation
+ (XXXClass *)sharedInstance {
static XXXClass* XXXClass_sharedInst = nil;
@synchronized(self) {
if (XXXClass_sharedInst == nil) {
XXXInitialize(&XXXClass_sharedInst);
}
}
return __sharedInst;
}
is extremely inefficient because @synchronized not only lock a mutex, but also insert an exception handler (try/catch block). If you don't care about exceptions and willing to use C functions, pthread_once() is a better alternative:
static XXXClass *XXXClass_sharedInst = nil;
static pthread_once_t XXXClass_onceControl = PTHREAD_ONCE_INIT;
static void XXXInitializeOnce(void) { XXXInitialize(&XXXClass_sharedInst); }
...
+ (XXXClass *)sharedInstance {
pthread_once(&XXXClass_onceControl, &XXXInitializeOnce);
return XXXClass_sharedInst;
}
pthread_once() is implemented using a spin lock (which leads to a syscall_thread_switch() kernel call in the worst case).
If it is safe to create multiple copies of the singleton and destroy the extra ones, you may even use CAS:
+ (XXXClass *)sharedInstance {
static XXXClass* XXXClass_sharedInst = nil;
if (XXXClass_sharedInst == nil) {
XXXClass* tmp;
XXXInitialize(&tmp);
if (!OSAtomicCompareAndSwapPtrBarrier(nil, tmp, (void*volatile*)&XXXClass_sharedInst)))
XXXDestroy(tmp);
}
return XXXClass_sharedInst;
}
Unfortunately, OSAtomicCompareAndSwapPtrBarrier() on ARMv4 is implemented using a global spin lock, so in principle this is nowhere faster than pthread_once(), and is even more error-prone. On ARMv6 it is implemented as a LDREX/STREX pair[1]:
ENTRY_POINT(_OSAtomicCompareAndSwapPtrBarrier)
1: ldrex r3, [r2] // load existing value and tag memory
teq r3, r0 // is it the same as oldValue?
movne r0, #0 // if not, return 0 immediately
bxne lr
strex r3, r1, [r2] // otherwise, try to store new value
cmp r3, #0 // check if the store succeeded
bne 1b // if not, try again
mov r0, #1 // return true
bx lr