单例模式简单实现

@interface HMMusicTool : NSObject
+ (instancetype)sharedMusicTool;
@end

@implementation HMMusicTool
static id _instance;

/**
 *  alloc方法内部会调用这个方法
 */
+ (id)allocWithZone:(struct _NSZone *)zone
{
    if (_instance == nil) { // 防止频繁加锁
        @synchronized(self) {
            if (_instance == nil) { // 防止创建多次
                _instance = [super allocWithZone:zone];
            }
        }
    }
    return _instance;
}

+ (instancetype)sharedMusicTool
{
    if (_instance == nil) { // 防止频繁加锁
        @synchronized(self) {
            if (_instance == nil) { // 防止创建多次
                _instance = [[self alloc] init];
            }
        }
    }
    return _instance;
}

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