//或者不加参数，默认就是这个
NSConfinementConcurrencyType

//绑定到一个后台线程
NSPrivateQueueConcurrencyType

//绑定到一个主线程
NSMainQueueConcurrencyType

//可以将如下代码，封装到NSManagedObjectContext得一个分类里面去

static NSManagedObjectContext *_foregroudContext;
static NSManagedObjectContext *_backgroudContext;

+ (NSManagedObjectContext *)foregroudContext {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{

        //1. 创建一个单例主线程context
        _foregoundContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];

        //2. 默认设置一个版本自动迁移的存储器
        id persistentStoreCoordinator = [NSPersistentStoreCoordinator coordinatorUseAutoMigration];//使用一个分类封装的
        [_foregoundContext setPersistentStoreCoordinator:persistentStoreCoordinator];

    };
    return     _foregroudContext;
}

+ (NSManagedObjectContext *)backgroudContext {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{

        //1. 创建一个新的后台线程context
        id context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSConfinementConcurrencyType];

        //2. 设置为后台线程context
        [self setBackgroudContext:context];

        //3.将_foregroudContext 设置为 子线程context 的parantContext
        context.parantContext = [self foregroundContext];

    };
    return     _backgroudContext;
}

//可以创建一个新的后台线程context作为 _backgroudContext
+ (void)setBackgroudContext:(NSManagedObjectContext *)context {
    if (context == _backgoundContext)
        return;
    
    _backgoundContext = nil;
    _backgoundContext = context;
}

//获取context的统一入口
+ (instancetype)managedObjectContext {
    if ([NSThread isMainThread]) {
        return [self foregroundContext];
    } else {
        return [self backgroundContext];
    }
}