iphone - 核心数据 : does a fetch have to make a trip to persistent store?

Question

假设我这样做:

NSManagedObjectContext *context = #a managed object context";
NSString *entityName = #an entity name#;
NSFetchRequest *requestForAll = [NSFetchRequest requestWithEntityName:entityName];
NSArray *allObj = [context executeFetchRequest:requestForAll];

for (NSString *name in allNamesArray){
    NSFetchRequest *requestForOne = [NSFetchRequest requestWithEntityName:entityName];
    requestForOne.predicate = [NSPredicate predicateWithFormat:@"name == %@",name];
    NSArray *ObjsWithName = [context executeFetchRequest:requestForOne];
    #do some work with the obj#
}

循环中的获取是否每次都会导致访问持久存储？或者这些提取只会在 coredata 的行缓存中执行？

编辑我写了一段测试代码:您需要创建一个名为“Person”的核心数据实体，它应该有一个名为“name”的属性，该属性是字符串类型。

使用此代码填充一些数据:

self.array = @[@"alkjsdfkllaksjdf",@"asldjflkajdklsfjlk;aj",@"aflakjsdl;kfjalksdjfklajkldhkl;aj",@"aljdfkljalksdjfl;j" ,@"flajdl;kfjaklsdjflk;j",@"akldsjfklajdslkf",@"alkdjfkljaklsdjflkaj",@"alsdjflkajsdflj",@"adlkfjlkajsdfkljkla",@"alkdjfklajslkdfj"];

NSString *firstRunKey = @"oh its first run!";
NSString *firstRun = [[NSUserDefaults standardUserDefaults] objectForKey:firstRunKey];
if (!firstRun) {
    for (NSString *name in self.array) {
        Person *p = [NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:self.managedObjectContext];
        p.name = name;
    }
}
[self.managedObjectContext save];
[[NSUserDefaults standardUserDefaults] setObject:firstRunKey forKey:firstRunKey];
[[NSUserDefaults standardUserDefaults] synchronize];

剖析这两种方法，您会发现使用 CoreData 比使用 FilterArray 花费更多的时间!

static int caseCount = 1000;
-(void)usingCoreData
{
    NSLog(@"core data");
    NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Person"];
    NSArray *allPersons = [self.managedObjectContext executeFetchRequest:request error:nil];

    for (int i = 0; i < caseCount; i++){
        for (NSString *name in self.array) {
            request.predicate = [NSPredicate predicateWithFormat:@"name == %@",name];
            NSArray *result = [self.managedObjectContext executeFetchRequest:request error:nil];
        }
    }
}

-(void)usingFilterArray
{
    NSLog(@"filter array");
    NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Person"];
    NSArray *allPersons = [self.managedObjectContext executeFetchRequest:request error:nil];

    for (int i = 0; i < caseCount; i++){
        for (NSString *name in self.array) {
            NSArray *array = [allPersons filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"name == %@",name]];
        }
    }
}

Answer 1

我想我需要自己回答我的问题。

我测试了一下，每次执行fetch的时候，core data都会把你的NSFetchRequest翻译成SQL命令，然后调用数据库查询，查询结果首先是NSManagedObjectIDs，通过缓存从NSManagedObjectID中获取NSManagedObject。

总之，它缓存对象，但不缓存查询结果。

这意味着您执行同一个 NSFetchRequest 10 次，它会查询您的持久存储 10 次，尽管您会得到 10 次相同的结果。所以在这种情况下，在内存中过滤数组会比获取更好。