Question

这对NSFetchedResultsController的getter方法是否正确，使用最现代的Object-C和LLVM编译器，不需要合成属性？

- (NSFetchedResultsController *)fetchedResultsController {
    if (_fetchedResultsController) {
        return _fetchedResultsController;
    }

    if (!self.managedObjectContext) {
        self.managedObjectContext = [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Session" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    [fetchRequest setPredicate: self.predicate];
    [fetchRequest setFetchBatchSize:20];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"timeStamp" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];

    NSFetchedResultsController *aFetchedResultsController= [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];
    aFetchedResultsController.delegate = self;

    NSError *error = nil;
    if (![aFetchedResultsController performFetch:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return aFetchedResultsController;
}

在我的代码的其他部分，我改为：

- (NSFetchedResultsController *)fetchedResultsController {
    if (_fetchedResultsController) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Set" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    [fetchRequest setPredicate:[NSPredicate predicateWithFormat: @"sets == %@", self.exercise]];
    [fetchRequest setFetchBatchSize:20];
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"timeStamp" ascending:YES];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    [fetchRequest setSortDescriptors:sortDescriptors];
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc]initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    NSError *error = nil;
    if (![self.fetchedResultsController performFetch:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return self.fetchedResultsController;
}

Answer 1

在第一个示例中，您从不为属性或ivar分配任何内容，因此每次访问该属性时都会发生延迟加载。这不对。

在你的第二个例子中，你分配给属性，（从而触发属性设置器的任何副作用），并返回属性的值（这不会带来很多错误，让你无限循环）。它可能现在可以工作，但很容易打破一个粗心的更新。

分配到内部的ivar并返回它会更清晰。

所以，在你的第一个例子结束时：

_fetchedResultsController = aFetchedResultsController;
 return _fetchedResultsController;

这是NSFetchedResultsController的正确使用吗？

1 个答案: