iOS单例模式

#import <Foundation/Foundation.h>

@interface Instance : NSObject

+ (instancetype)sharedInstance;

@end

@implementation Instance

static Instance *instance;

+ (instancetype)allocWithZone:(struct _NSZone *)zone
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [super allocWithZone:zone];
    });
    return instance;
}

+ (instancetype)sharedInstance
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[self alloc] init];
    });
    return instance;
}

- (id)copyWithZone:(NSZone *)zone
{
    return instance;
}
@end

@implementation Instance

static Instance *instance;

+ (instancetype)allocWithZone:(struct _NSZone *)zone
{
    @synchronized(self) {
        if (instance == nil) {
            instance = [super allocWithZone:zone];
        }
    }
    return instance;
}

+ (instancetype)sharedInstance
{
    @synchronized(self) {
        if (instance == nil) {
            instance = [[self alloc] init];
        }
    }
    return instance;
}

- (id)copyWithZone:(NSZone *)zone
{
    return instance;
}
@end