如何实现与ARC兼容的Objective-C单例？

Question

在Xcode 4.2中使用自动引用计数(ARC)时,如何转换(或创建)编译和行为正确的单例类？

Answer 1

完全一样,你(应该)已经这样做了:

+ (instancetype)sharedInstance
{
    static MyClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[MyClass alloc] init];
        // Do any other initialisation stuff here
    });
    return sharedInstance;
}

Answer 2

如果你想根据需要创建其他实例.这个:

+ (MyClass *)sharedInstance
{
    static MyClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[MyClass alloc] init];
        // Do any other initialisation stuff here
    });
    return sharedInstance;
}

否则,你应该这样做:

+ (id)allocWithZone:(NSZone *)zone
{
    static MyClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [super allocWithZone:zone];
    });
    return sharedInstance;
}

Answer 3

这是ARC和非ARC的版本

如何使用:

MySingletonClass.h

@interface MySingletonClass : NSObject

+(MySingletonClass *)sharedInstance;

@end

MySingletonClass.m

#import "MySingletonClass.h"
#import "SynthesizeSingleton.h"
@implementation MySingletonClass
SYNTHESIZE_SINGLETON_FOR_CLASS(MySingletonClass)
@end