【原】ios下比较完美的单例模式，已验证

static PRAutoLoginView *s_sharedInstance = nil;
+ (PRAutoLoginView *)shareInstance
{
    @synchronized(self)
    {
        if (s_sharedInstance == nil) {
            s_sharedInstance = [[[self class] hideAlloc] init];
        }
    }
    return s_sharedInstance;
}

#pragma mark --
#pragma mark singleton apis
+ (id)hideAlloc
{
    return [super alloc];
}

+ (id)alloc//彻底屏蔽掉alloc函数
{
    NSAssert(1 == 0, @"[PRAutoLoginView]please use +shareInstance instead of alloc!");
    return nil;
}

+ (id)new
{
    return [self alloc];
}

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

- (id)copyWithZone:(NSZone *)zone
{
    NSAssert(1 == 0, @"[PRAutoLoginView]copy is not permitted!");
    [self retain];
    return self;
}

- (id)mutableCopyWithZone:(NSZone *)zone
{
    return [self copyWithZone:zone];
}