ios单例的创建的注意事项

单例是一个类有且只有一个实例，并且自行实例化向整个系统提供

static XSSingletonManager * instance = nil;
@implementation XSSingletonManager
+(instancetype)XSShareInstance{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[self alloc]init];
    });
    return instance;
}

• 做如下测试：

  XSSingletonManager * objc1 = [XSSingletonManager XSShareInstance];
    NSLog(@"%@",objc1);
    
    XSSingletonManager * objc2 = [XSSingletonManager new];
    NSLog(@"%@",objc2);

    XSSingletonManager * objc3 = [[XSSingletonManager alloc]init];
    NSLog(@"%@",objc3);

测试结果打印出来

2021-06-10 11:54:25.420181+0800 runLoop[5676:2178711] <XSSingletonManager: 0x2821e4730>
2021-06-10 11:54:25.420274+0800 runLoop[5676:2178711] <XSSingletonManager: 0x2821e4720>
2021-06-10 11:54:25.420356+0800 runLoop[5676:2178711] <XSSingletonManager: 0x2821e4710>

很明显，通过三种方式创建出来的三个实例是不同的，这个就违背单例的原则（有且仅有一个实例）
为了防止别人使用alloc/init的方式创建对象，我们要保证自己的类不管用什么方式创建，只能创建同一个实例对象，需要重写一个方法

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

再次调用上面的测试代码，结果如下

2021-06-10 12:52:43.563003+0800 runLoop[5722:2196911] <XSSingletonManager: 0x282978670>
2021-06-10 12:52:43.563049+0800 runLoop[5722:2196911] <XSSingletonManager: 0x282978670>
2021-06-10 12:52:43.563085+0800 runLoop[5722:2196911] <XSSingletonManager: 0x282978670>

再次给单例类里面加几个属性

@property (nonatomic,strong) NSObject * object;
@property (nonatomic,strong) NSMutableArray * array;
@property (nonatomic,assign) NSInteger num;

而一些对象类的初始化，或者是基础类型的默认值设置，都是在init方法里面

-(instancetype)init{
    self = [super init];
    if (self) {
        _num = 10;
        _object = [[NSObject alloc]init];
        _array = [NSMutableArray array];
    }
    return self;
}

重写了description方法

-(NSString *)description{
    NSString * result = @"";
    result = [result stringByAppendingFormat:@"<%@:%p>",[self class],self];
    result = [result stringByAppendingFormat:@" num = %ld",(long)self.num];
    result = [result stringByAppendingFormat:@" array = %p",self.array];
    result = [result stringByAppendingFormat:@" object = %p",self.object];

    return result;
}

调用上面的测试代码

2021-06-10 13:55:46.330450+0800 runLoop[5772:2214526] <XSSingletonManager:0x28199fd60> num = 10 array = 0x2817da4c0 object = 0x281bbc5d0
2021-06-10 13:55:46.330543+0800 runLoop[5772:2214526] <XSSingletonManager:0x28199fd60> num = 10 array = 0x2817da370 object = 0x281bbc5c0
2021-06-10 13:55:46.330615+0800 runLoop[5772:2214526] <XSSingletonManager:0x28199fd60> num = 10 array = 0x2817dac10 object = 0x281bbc5e0

可以看到尽管使用的是同一个实例，可是他的property的值确不一样，因为尽管没有为实例重新分配内存空间，但是又因为执行了init方法，会导致property被重新初始化

所以我们需要修改单例的实现。

第1种方法

将property的初始化或者默认值设置到dispatch_once的block内部

+(instancetype)XSShareInstance{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[self alloc]init];
        instance.num = 10;
        instance.object = [[NSObject alloc]init];
        instance.array = [NSMutableArray array];
    });
    return instance;
}


-(NSString *)description{
    NSString * result = @"";
    result = [result stringByAppendingFormat:@"<%@:%p>",[self class],self];
    result = [result stringByAppendingFormat:@" num = %ld",(long)self.num];
    result = [result stringByAppendingFormat:@" array = %p",self.array];
    result = [result stringByAppendingFormat:@" object = %p",self.object];

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

测试下看下结果

2021-06-10 15:06:08.246233+0800 runLoop[5790:2237632] 此设备未越狱
2021-06-10 15:06:08.246398+0800 runLoop[5790:2237632] <XSSingletonManager:0x283b7ed20> num = 10 array = 0x28353a370 object = 0x283954620
2021-06-10 15:06:08.246467+0800 runLoop[5790:2237632] <XSSingletonManager:0x283b7ed20> num = 10 array = 0x28353a370 object = 0x283954620
2021-06-10 15:06:08.246533+0800 runLoop[5790:2237632] <XSSingletonManager:0x283b7ed20> num = 10 array = 0x28353a370 object = 0x283954620

第2种

+(instancetype)XSShareInstance{
    return [[self alloc] init];
}

-(instancetype)init{
    if (self = [super init]) {
        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{
            instance = [super init];
            instance.num = 10;
            instance.object = [[NSObject alloc]init];
            instance.array = [NSMutableArray array];
        });
        return instance;
    }
    return self;
}
-(NSString *)description{
    NSString * result = @"";
    result = [result stringByAppendingFormat:@"<%@:%p>",[self class],self];
    result = [result stringByAppendingFormat:@" num = %ld",(long)self.num];
    result = [result stringByAppendingFormat:@" array = %p",self.array];
    result = [result stringByAppendingFormat:@" object = %p",self.object];

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

测试下看下结果：

2021-06-10 15:11:30.544370+0800 runLoop[5797:2239565] <XSSingletonManager:0x283b7f900> num = 10 array = 0x283535ad0 object = 0x283958940
2021-06-10 15:11:30.544449+0800 runLoop[5797:2239565] <XSSingletonManager:0x283b7f900> num = 10 array = 0x283535ad0 object = 0x283958940
2021-06-10 15:11:30.544504+0800 runLoop[5797:2239565] <XSSingletonManager:0x283b7f900> num = 10 array = 0x283535ad0 object = 0x283958940