当我更改 UILabel 的文本时屏幕不更新

Question

我的环境是 Xcode 6.3，目标是 iOS8.3 for iPad。 我正在使用 Xcode 的模板学习 Master-Detail 视图。当我尝试在详细视图中设置标签的文本时出现一些奇怪的问题：
如果我将更改标签文本的语句放在 P1 位置，它会起作用。
但是如果我把语句放在 P2 位置，它就不起作用了。

在这两种情况下，detailDescriptionLabel.text 都发生了变化，但实际上只有 P1 语句更新了屏幕。（我检查过函数 configureView() 被调用了两次）

有人可以帮助我吗？非常感谢。

下面是我的代码sn-ps：

MasterController.m：

#import "MasterViewController.h"
#import "DetailViewController.h"

@interface MasterViewController ()

@property (copy, nonatomic) NSMutableArray *objects;

@end

@implementation MasterViewController

- (void)awakeFromNib {
    [super awakeFromNib];
    self.clearsSelectionOnViewWillAppear = NO;
    self.preferredContentSize = CGSizeMake(320.0, 600.0);
}

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    NSString *path = [[NSBundle mainBundle] pathForResource:@"PresidentList" ofType:@"plist"];
    NSDictionary *presidentInfo = [NSDictionary dictionaryWithContentsOfFile:path];
    self.objects = [NSMutableArray arrayWithArray:[presidentInfo objectForKey:@"presidents"]];

    self.detailViewController = (DetailViewController *)[[self.splitViewController.viewControllers lastObject] topViewController];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - Segues

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    if ([[segue identifier] isEqualToString:@"showDetail"]) {
        DetailViewController *controller = (DetailViewController *)[[segue destinationViewController] topViewController];
        controller.navigationItem.leftBarButtonItem = self.splitViewController.displayModeButtonItem;
        controller.navigationItem.leftItemsSupplementBackButton = YES;

        //P1
        //controller.detailItem = (self.objects[indexPath.row])[@"url"];
    }
}



#pragma mark - Table View

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    return self.objects.count;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"Cell" forIndexPath:indexPath];

    cell.textLabel.text = (self.objects[indexPath.row])[@"name"];
    return cell;
}

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    //P2
    //self.detailViewController.detailItem = (self.objects[indexPath.row])[@"url"];
}
@end

DetailController.m：

#import "DetailViewController.h"

@interface DetailViewController ()
@property (strong, nonatomic) UIPopoverController *masterPopoverController;
@end

@implementation DetailViewController

#pragma mark - Managing the detail item

- (void)setDetailItem:(id)newDetailItem {
    if (_detailItem != newDetailItem) {
        _detailItem = newDetailItem;
        // Update the view.
        [self configureView];
    }
    if (self.masterPopoverController != nil) {
        [self.masterPopoverController dismissPopoverAnimated:YES];
    }
}

- (void)configureView {
    // Update the user interface for the detail item.
    if (self.detailItem) {
        self.detailDescriptionLabel.text = self.detailItem;
    }
}

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    [self configureView];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - Split view

- (void)splitViewController:(UISplitViewController *)svc willHideViewController:(UIViewController *)aViewController withBarButtonItem:(UIBarButtonItem *)barButtonItem forPopoverController:(UIPopoverController *)pc {
    barButtonItem.title = NSLocalizedString(@"Master", @"Master");
    [self.navigationItem setLeftBarButtonItem:barButtonItem animated:YES];
    self.masterPopoverController = pc;
}

- (void)splitViewController:(UISplitViewController *)svc willShowViewController:(UIViewController *)aViewController invalidatingBarButtonItem:(UIBarButtonItem *)barButtonItem {
    [self.navigationItem setLeftBarButtonItem:nil animated:YES];
    self.masterPopoverController = nil;
}
@end

Answer 1

我猜 self.detailViewController 是 nil。因此 P2 将无法工作。

如果没有看到所有代码很难说，但在 prepareForSegue 中，您可能希望将其分配在那里。

Answer 2

我猜你正在使用 segues，这就是使用专门指定的专用函数 (prepareForSegue) 来在创建视图目标视图之后初始化值的重点。

除非您在didSelectRowAtIndexPath 中自己实例化并显示视图，否则为这样的视图控制器设置一些东西是没有意义的。