把模型鏈接到視圖（Connecting the Model to Views）

在macOS上，Core Data與用戶界面被設(shè)計(jì)為通過Cocoa綁定工作蛤高。然后，Cocoa綁定在iOS上并不是用戶界面的一部分。在IOS中通過NSFetchedResultsController將模型（Core Data）與視圖（storyboards）鏈接在一起钓株。

創(chuàng)建一個(gè)結(jié)果控制器（Creating a Fetched Results Controller）

當(dāng)你使用一個(gè)基于表格視圖（UITableView）布局的Core Data時(shí)，NSFetchedResultsController為你提供的數(shù)據(jù)通常由UItableViewController的實(shí)例將要使用時(shí)初始化陌僵。這個(gè)初始化可以在viewDidLoad或viewWillAppear：方法里轴合，再或者在視圖控制器的生命周期的另一個(gè)邏輯起點(diǎn)。

這是個(gè)NSFetchedResultsController初始化的例子：

Objective-C:

@property (nonatomic, strong) NSFetchedResultsController *fetchedResultsController;
 
- (void)initializeFetchedResultsController
{
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Person"];
 
    NSSortDescriptor *lastNameSort = [NSSortDescriptor sortDescriptorWithKey:@"lastName" ascending:YES];
 
    [request setSortDescriptors:@[lastNameSort]];
 
    NSManagedObjectContext *moc = …; //Retrieve the main queue NSManagedObjectContext
 
    [self setFetchedResultsController:[[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:moc sectionNameKeyPath:nil cacheName:nil]];
    [[self fetchedResultsController] setDelegate:self];
 
    NSError *error = nil;
    if (![[self fetchedResultsController] performFetch:&error]) {
        NSLog(@"Failed to initialize FetchedResultsController: %@\n%@", [error localizedDescription], [error userInfo]);
        abort();
    }
}

Swift

var fetchedResultsController: NSFetchedResultsController!
 
func initializeFetchedResultsController() {
    let request = NSFetchRequest(entityName: "Person")
    let departmentSort = NSSortDescriptor(key: "department.name", ascending: true)
    let lastNameSort = NSSortDescriptor(key: "lastName", ascending: true)
    request.sortDescriptors = [departmentSort, lastNameSort]
    
    let moc = dataController.managedObjectContext
    fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: moc, sectionNameKeyPath: nil, cacheName: nil)
    fetchedResultsController.delegate = self

上面所示的初始化（initiallizeFetchedResultsController）方法在UITableViewController的實(shí)例里面拾弃，首先構(gòu)建一個(gè)讀取請(qǐng)求（NSFetchRequest）值桩，這是NSFetchRequestController的核心。注意豪椿，讀取請(qǐng)求含有一種特殊的描述符（NSSortDescriptor）奔坟。NSFetchRequestController至少需要一種描述符來控制數(shù)據(jù)顯示的順序。

在初始化讀取請(qǐng)求之后搭盾，你可以初始化一個(gè)NSFetchedResultsController的實(shí)例咳秉。獲取結(jié)果控制器（fetched results controller）在運(yùn)行之前，你需要一個(gè)NSFetchRequest的實(shí)例和一個(gè)托管對(duì)象上下文（NSManagedObjectContext）的參考鸯隅。sectionNameKeyPaht和cacheName屬性都是可選的澜建。

在獲取結(jié)果控制器（fetched results controller）初始化之后，將其分派給委托蝌以。當(dāng)數(shù)據(jù)發(fā)生變化時(shí)代理通知表格視圖控制器（UITableViewController）炕舵。通常情況下，表格視圖控制器（UITableViewController）也通過代理告知獲取結(jié)果控制器（fetched results controller）跟畅，以便任何時(shí)候當(dāng)數(shù)據(jù)發(fā)生改變時(shí)接收到回調(diào)咽筋。

接下來，開始通過調(diào)用NSFetchedRrsuletsController的performFetfch:方法徊件，這個(gè)調(diào)用檢索要顯示的初始數(shù)據(jù)奸攻，并且作為NSFetchedResultsController實(shí)例開始監(jiān)測(cè)托管上下文（managed object context）開始（原因）蒜危。

將獲取到的結(jié)果與表格視圖的數(shù)據(jù)源集成（Integrating the Fetched Results Controller with the Table View Data Source）

當(dāng)你在表格視圖里初始化獲取結(jié)果控制器（initialized fetched results controller）并且有了可以用于顯示的數(shù)據(jù)之后，你就可以把獲取結(jié)果的控制器（fetched results controller）和表格視圖的數(shù)據(jù)源相互整合（UITabaleViewDataSource）睹耐。

Objective-C

#pragma mark - UITableViewDataSource
 
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    id cell = [tableView dequeueReusableCellWithIdentifier:CellReuseIdentifier];
 
    NSManagedObject *object = [self.fetchedResultsController objectAtIndexPath:indexPath];
    // Configure the cell from the object
    return cell;
}
 
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return [[[self fetchedResultsController] sections] count];
}
 
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    id< NSFetchedResultsSectionInfo> sectionInfo = [[self fetchedResultsController] sections][section];
    return [sectionInfo numberOfObjects];
}

Swift

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    guard let cell = tableView.dequeueReusableCell(withIdentifier: "cellIdentifier", for: indexPath) else {
        fatalError("Wrong cell type dequeued")
    }
    // Set up the cell
    guard let object = self.fetchedResultsController?.object(at: indexPath) else {
        fatalError("Attempt to configure cell without a managed object")
    }
    //Populate the cell from the object
    return cell
}
 
override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
    return fetchedResultsController.sections!.count
}
 
override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    guard let sections = fetchedResultsController.sections else {
        fatalError("No sections in fetchedResultsController")
    }
    let sectionInfo = sections[section]
    return sectionInfo.numberOfObjects
}

每個(gè)UITableViewDataSource的方法如上圖所示辐赞，與獲取結(jié)果控制器（fetched results controller）的集成是減少到一個(gè)單一的方法調(diào)用，是專門設(shè)計(jì)的表格視圖數(shù)據(jù)源整合硝训。

將數(shù)據(jù)更改與表格視圖通信（Communicationg Data Chagnes to the Table View）

除了更容易的整合Core Date和表格視圖數(shù)據(jù)源（UITableViewDataSource）响委，NSFetchResultsController在數(shù)據(jù)發(fā)生變化是還處理和表格視圖控制器（UITableViewController）的通信。通過實(shí)現(xiàn)NSFetchedResultsControllerDelegae協(xié)議來實(shí)施捎迫。

Objective-C

#pragma mark - NSFetchedResultsControllerDelegate
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    [[self tableView] beginUpdates];
}
- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type
{
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [[self tableView] insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeDelete:
            [[self tableView] deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeMove:
        case NSFetchedResultsChangeUpdate:
            break;
    }
}
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
{
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [[self tableView] insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeDelete:
            [[self tableView] deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeUpdate:
            [self.tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeMove:
            [[self tableView] deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [[self tableView] insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    [[self tableView] endUpdates];
}

Swift

func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.beginUpdates()
}
 
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange sectionInfo: NSFetchedResultsSectionInfo, atSectionIndex sectionIndex: Int, for type: NSFetchedResultsChangeType) {
    switch type {
    case .insert:
        tableView.insertSections(IndexSet(integer: sectionIndex), with: .fade)
    case .delete:
        tableView.deleteSections(IndexSet(integer: sectionIndex), with: .fade)
    case .move:
        break
    case .update:
        break
    }
}
 
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
    switch type {
    case .insert:
        tableView.insertRows(at: [newIndexPath!], with: .fade)
    case .delete:
        tableView.deleteRows(at: [indexPath!], with: .fade)
    case .update:
        tableView.reloadRows(at: [indexPath!], with: .fade)
    case .move:
        tableView.moveRow(at: indexPath!, to: newIndexPath!)
    }
}
 
func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.endUpdates()
}

上面的4個(gè)協(xié)議提供了當(dāng)?shù)讓訑?shù)據(jù)變化時(shí)自動(dòng)更新相關(guān)表格的方法晃酒。

添加的部分（Adding Sections）

到目前位置，一直使用的表格視圖都是只有一個(gè)seciton的單一表格窄绒，只有一個(gè)section的表格需要所有的數(shù)據(jù)用以展示（或者說只能展示一個(gè)真題的數(shù)據(jù)）贝次。（接著前面的例子）如果你正在使用大量的員工對(duì)象數(shù)據(jù)，那么將表格分成多個(gè)部分是非常有利的彰导。通過部門對(duì)員工進(jìn)行分組使員工名單更加易于管理蛔翅。如果沒有Core Data，一個(gè)具有多個(gè)部分的表格視圖將涉及數(shù)組嵌套數(shù)組的結(jié)構(gòu)位谋，或者更為復(fù)雜的數(shù)據(jù)結(jié)構(gòu)山析。使用Core Data可以簡(jiǎn)單方便的更改獲取結(jié)果控制器（fetched results controller）以達(dá)到目的。

Objective-C

- (void)initializeFetchedResultsController
{
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Person"];
    NSSortDescriptor *departmentSort = [NSSortDescriptor sortDescriptorWithKey:@"department.name" ascending:YES];
    NSSortDescriptor *lastNameSort = [NSSortDescriptor sortDescriptorWithKey:@"lastName" ascending:YES];
    [request setSortDescriptors:@[departmentSort, lastNameSort]];
    NSManagedObjectContext *moc = [[self dataController] managedObjectContext];
    [self setFetchedResultsController:[[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:moc sectionNameKeyPath:@"department.name" cacheName:nil]];
    [[self fetchedResultsController] setDelegate:self];
}

Swift

func initializeFetchedResultsController() {
    let request = NSFetchRequest(entityName: "Person")
    let departmentSort = NSSortDescriptor(key: "department.name", ascending: true)
    let lastNameSort = NSSortDescriptor(key: "lastName", ascending: true)
    request.sortDescriptors = [departmentSort, lastNameSort]
    let moc = dataController.managedObjectContext
    fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: moc, sectionNameKeyPath: "department.name", cacheName: nil)
    fetchedResultsController.delegate = self
    do {
        try fetchedResultsController.performFetch()
    } catch {
        fatalError("Failed to initialize FetchedResultsController: \(error)")
    }
}

在這個(gè)例子中添加一個(gè)NSSortDescriptor實(shí)例到NSFetchRequest實(shí)例掏父。把新的描述符SectionNameKeyPath作為相同的密匙來初始化NSFetchedResultsController笋轨。獲取結(jié)果控制器（fetched results controller）用這些將控制器（controller）初始化用來打斷并且分類排序成多個(gè)sections，因此要求請(qǐng)求密匙必須匹配赊淑。

這個(gè)更改會(huì)讓獲取結(jié)果控制器（fetched results controller）根據(jù)每個(gè)實(shí)例關(guān)聯(lián)的部門名稱將返回的人員實(shí)例分成多個(gè)部分爵政。使用此功能的條件是：

• sectionNameKeyPath 的屬性也必須是一個(gè)NSSortDescriptor的實(shí)例。（The sectionNameKeyPath property must also be an NSSortDescriptor instance）
• NSSortDescriptor在請(qǐng)求數(shù)組中必須是第一個(gè)描述符陶缺。（The NSSortDescriptor must be the first descriptor in the array passed to the fetch request）

為性能添加緩存（Adding Caching for Performance）

在許多情況下钾挟，表格視圖（tableView）表示相對(duì)靜態(tài)的數(shù)據(jù)類型。在表格視圖控制器中定義了一個(gè)獲取請(qǐng)求饱岸，它在整個(gè)應(yīng)用程序的生命周期中不會(huì)改變掺出。在這種情況下，它有利于增加緩存（cache）的NSFetchedResultsController實(shí)例苫费，以便于在數(shù)據(jù)沒有改變的時(shí)候應(yīng)用程序一遍又一遍的啟動(dòng)所引起的表格視圖的突然初始化汤锨。緩存對(duì)于顯示異常大的數(shù)據(jù)時(shí)非常有用。

Objective-C

[self setFetchedResultsController:[[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:moc sectionNameKeyPath:@"department.name" cacheName:@"rootCache"]];

Swift

fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: moc, sectionNameKeyPath: "department.name", cacheName: "rootCache")

如上圖所示百框，當(dāng)NSFetchedResultsController實(shí)例初始化設(shè)置cacheName屬性的時(shí)候泥畅，獲取結(jié)果的控制器（fetched results controller）自動(dòng)獲得緩存增益，隨后加載的數(shù)據(jù)幾乎瞬間完成。

注
如果發(fā)生請(qǐng)求伴隨著獲取結(jié)果控制器（fetched results controller）需要改變的情況位仁，那么在更改獲取控制器（fetched results controller）之前使緩存失效是至關(guān)重要的。你可以通過調(diào)用deleteCacheWithName:的方法使緩存（cache）失效方椎，這是一個(gè)類級(jí)別的方法在NSFetchedResultsController里聂抢。
原文：If a situation occurs where the fetch request associated with a fetched results controller needs to change, then it is vital that the cache be invalidated prior to changing the fetched results controller. You invalidate the cache by calling deleteCacheWithName:, which is a class level method on NSFetchedResultsController.