核心数据初始化

Question

我对coredata很陌生，所以我有一个非常基本的问题--我有提供者实体，在viewDidLoad的控制器中，我实现了这个方法：

-(NSDictionary *) allProviders {
    Providers *orangeFR = [NSEntityDescription insertNewObjectForEntityForName:@"Providers" inManagedObjectContext:self.managedObjectContext];
    orangeFR.name = [NSString stringWithFormat:@"Orange"];
    orangeFR.adress = [NSString stringWithFormat:@"Champ De Mars"];
    Providers *sfrFR = [NSEntityDescription insertNewObjectForEntityForName:@"Providers" inManagedObjectContext:self.managedObjectContext];
    sfrFR.name = [NSString stringWithFormat:@"SFR"];
    sfrFR.adress = [NSString stringWithFormat:@"SFR Adress"];
    NSArray *FRProviders = @[orangeFR, sfrFR];

    Providers *ATT = [NSEntityDescription insertNewObjectForEntityForName:@"Providers" inManagedObjectContext:self.managedObjectContext];
    BeelineRU.name = [NSString stringWithFormat:@"ATT"];
    BeelineRU.adress = [NSString stringWithFormat:@"ATT adress"];
    Providers *Verizon = [NSEntityDescription insertNewObjectForEntityForName:@"Providers" inManagedObjectContext:self.managedObjectContext];
    MTSRU.name = [NSString stringWithFormat:@"Verizon"];
    MTSRU.adress = [NSString stringWithFormat:@"Verizon Adress"];
    NSArray *USProviders = @[ATT, Verizon];


    _allProviders = @{@"France": FRProviders, @"USA":USProviders};
    return _allProviders;
}

所以一切都很好，但是在应用程序的第二次启动时，我需要检查是否已经填充了我的提供者实体。如何做到这一点，这样我就可以在这两种情况下获得我的_allProviders了？

Answer 1

创建一个NSMutableDictionary

NSMutableDictionary *providersByCountry = [NSMutableDictionary dictionary]

获取所有实体：

NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = [NSEntityDescription entityForName:@"Providers" inManagedObjectContext:self.managedObjectContext];

NSArray *providers = [self.managedObjectContext executeFetchRequest:request error:NULL];

将提供程序插入字典(我假设您的Providers实体上有一个Providers属性.有吗？)：

for (Providers *provider in providers) {
    NSArray *providersForThisCountry = [providersByCountry[provider.country]
    if (!providersForThisCountry) {
        providersForThisCountry = [NSArray array];
    }

    providersForThisCountry = [providersForThisCountry arrayByAddingObject:provider];

    providersByCountry[provider.country] = providersForThisCountry;
}

最后，创建NSMutableDictionary的不可变副本，并返回它：

return providersByCountry.copy;

由于这可能是一个相当慢的操作，因此您可能希望在私有@property - but中缓存结果，因此无论何时修改提供程序列表，都需要清除缓存。

Answer 2

NSArray* allProviders = [managedObjectContext executeFetchRequest:[[NSFetchRequest alloc] initWithEntityName:@"Providers"] error:nil];