ios - Core-Data NSFetchedResultsController 对多关系

Question

这是我的数据模型:


我用数据填充了数据库。

现在我想在表格 View 中显示它。
首先，我想选择具有“天”属性的一天，然后学习它的类(class)。
我想在标题中显示属性“end”和“start”(-(NSString *) tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section)

然后我想在右侧部分显示类(class)数据。如何使用 NSFetchedResultsController 做到这一点？

我已经设置了一个基本代码:

NSFetchRequest* fetch = [NSFetchRequest fetchRequestWithEntityName:NSStringFromClass([Lessons class])];

NSSortDescriptor* sortGroup = [NSSortDescriptor sortDescriptorWithKey:@"lesson" ascending:YES];
fetch.sortDescriptors = @[sortGroup];

NSFetchedResultsController *controller = [[NSFetchedResultsController alloc] initWithFetchRequest:fetch managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];

NSError* error;
[controller performFetch:&error];
if (error) {
    NSLog(@"Error: %@", error);
    abort();
}

编辑:

这是 Storyboard:



更新

这是我的代码:

TableView.m(重要方法)

-(id)init
{
    self = [super init];
    if (self) {
        NSLog(@"%s",__PRETTY_FUNCTION__);
        self.del = [[UIApplication sharedApplication] delegate];
        self.managedObjectContext = self.del.managedObjectContext;

        NSFetchRequest* fetch = [NSFetchRequest fetchRequestWithEntityName:@"Lessons"];

        NSString *theSelectedDay = @"Mi";

        NSPredicate *pred = [NSPredicate predicateWithFormat:@"lessonToDay.day == %@", theSelectedDay];
        fetch.predicate = pred;
        NSSortDescriptor *sortTime = [NSSortDescriptor sortDescriptorWithKey:@"lessonToTime.start" ascending:YES];
        //NSSortDescriptor *sortGroup = [NSSortDescriptor sortDescriptorWithKey:@"lesson" ascending:YES];
        fetch.sortDescriptors = @[sortTime];

        self.controller = [[NSFetchedResultsController alloc] initWithFetchRequest:fetch
                                                              managedObjectContext:self.managedObjectContext
                                                                sectionNameKeyPath:@"lessonToTime.start"
                                                                         cacheName:nil];
    }
    return self;
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    // Return the number of sections.
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    // Return the number of rows in the section.
    return 1;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{

    static NSString *CellIdentifier = @"Cell";


    // Configure the cell...
    //UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (!cell) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
    }


    Lessons *lesson = [self.controller objectAtIndexPath:indexPath];
    cell.textLabel.text = lesson.lesson;


    return cell;
}

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section {

    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.controller sections] objectAtIndex:section];
    // This is the first lesson in this section:
    Lessons *lesson = [sectionInfo objects][0];

    //For testing I changed the type of 'start' and 'end' to string
    NSLog(@"start: %@",lesson.lessonToTime.start);
    NSLog(@"end: %@", lesson.lessonToTime.end );

    NSString *title = [NSString stringWithFormat:@"%@-%@",
                       lesson.lessonToTime.start,
                       lesson.lessonToTime.end];
    return title;
}

结果:

Answer 1

首先，您必须添加一个谓词，仅显示所选日期的类(class):

NSPredicate *pred = [NSPredicate predicateWithFormat:@"lessonToDay.day == %@", theSelectedDay];
fetch.predicate = pred;

要将表格 View 分组为多个部分，您必须设置 sectionNameKeyPath:范围
获取的结果 Controller ，例如到 @"lessonToTime.start" .这将组
所有具有相同开始时间的类(class)都归为一个部分。
(据我了解您的评论，具有相同开始时间的类(class)也有相同的结束时间，
所以这应该足够了。)
必须使用相同的键路径作为第一个排序描述符:

NSSortDescriptor *sortTime = [NSSortDescriptor sortDescriptorWithKey:@"lessonToTime.start" ascending:YES];
NSSortDescriptor *sortGroup = [NSSortDescriptor sortDescriptorWithKey:@"lesson" ascending:YES];
fetch.sortDescriptors = @[sortTime, sortGroup];

NSFetchedResultsController *controller = [[NSFetchedResultsController alloc] initWithFetchRequest:fetch
            managedObjectContext:self.managedObjectContext
            sectionNameKeyPath:@"lessonToTime.start"
            cacheName:nil];

要根据您的需要显示节标题，您必须覆盖 titleForHeaderInSection: :

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section { 
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.controller sections] objectAtIndex:section];
    // This is the first lesson in this section:
    Lesson *lesson = [sectionInfo objects][0];
    // Now build some title from lesson.lessonToTime.start and lesson.lessonToTime.end:
    NSString *title = ...;
    return title;
}