Objective-C：我认为UILabel被“卡住”了

Question

I'm working on a tabbed app that (among other things) creates a simple Facebook feed. The Facebook section is a UINavigationController holding a Master Table View and a Detail View. When I click one of the rows in the Master Table View, it brings up the detail view (and the appropriately filled labels) as intended. However, if I go back to the Master View and select another row, something strange happens. If the "detailLab" label (which is sizeToFit 'd) was, say, four lines long on the first occurrence of Detail View, and the "detailLab" label was 2 lines long on the second occurrence of Detail View, then on the second occurrence of Detail View, "detailLab" will display the first two lines properly, but underneath them will be the last two lines from the first occurrence of Detail View.

A picture of this happening .

And some (hopefully) relevant code:

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    CGRect labelFrame = CGRectMake(100, 40, 220, 150);
    detailLab = [[UILabel alloc] initWithFrame:labelFrame];
    [detailLab setTextColor:[UIColor darkGrayColor]];
    detailLab.font = [UIFont systemFontOfSize:14];

    [detailLab setText:detail];
    [detailLab setNumberOfLines:0];
    [detailLab sizeToFit];

    [self.view addSubview:detailLab];

    NSURL *url = [NSURL URLWithString:pictureString];
    NSData *imageData = [[NSData alloc] initWithContentsOfURL:url];
    picture = [UIImage imageWithData:imageData];
    bigImage.image = picture;
    NSLog(@"dateString: %@", dateString);
    dateLabel.text = dateString;
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    bigImage = nil;
    detailLab = nil;
    detailLab.hidden = YES;
    // Release any retained subviews of the main view.
    self.detailDescriptionLabel = nil;
}

If you need any other code or details, just ask; I'd be happy to provide it.

Answer 1

The problem is that each time viewWillAppear is called you are creating a new label without, I assume, removing it in viewWillDisappear . So the labels are stacking up, one on top of the other.

Remember you may get called like this (ignoring irrelevant calls):

viewDidLoad

viewWillAppear
viewWillDisappear

viewWillAppear        <- Whoops! Two labels
viweWillDisappear

viewDidUnload

A neater solution would be to add your label view in viewDidLoad and only alter the position of that already existing view in viewWillAppear . You still need to remember to dispose of the view in viewDidUnLoad .

Here's what I mean (note this was only eyeball compiled so you may need to correct some typos):

- (void)viewDidLoad
{
    [super viewDidLoad];
    detailLab = [[UILabel alloc] initWithFrame:CGRectZero];
    [detailLab setTextColor:[UIColor darkGrayColor]];
    detailLab.font = [UIFont systemFontOfSize:14];
    [self.view addSubview:detailLab];
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    CGRect labelFrame = CGRectMake(100, 40, 220, 150);
    [detailLab setFrame:labelFrame];
    [detailLab setText:detail];
    [detailLab setNumberOfLines:0];
    [detailLab sizeToFit];

    NSURL *url = [NSURL URLWithString:pictureString];
    NSData *imageData = [[NSData alloc] initWithContentsOfURL:url];
    picture = [UIImage imageWithData:imageData];
    bigImage.image = picture;
    NSLog(@"dateString: %@", dateString);
    dateLabel.text = dateString;
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    bigImage = nil;
    detailLab = nil;
    detailLab.hidden = YES;
    // Release any retained subviews of the main view.
    self.detailDescriptionLabel = nil;
}

Answer 2

That is normal with Master View to Detail Views - Frustrating, but Normal. The solution I found when I had the problem was to reset the label at the top of each -ViewDidLoad.

One option would be to remove it in viewDidUnload. Another would be to reset the label each time, using a segment of code like the one below, positioned above the section of your code where it actually loads in the content

detailLab = [NSString stringWithFormat:@""];

The issue is that with the setup of most Master/Detail views, it's not actually creating new detail views each time you add a view, it's just creating another link to the same view in the master view section. This is fine, but you just need to remember to use viewDidUnload lots

Hope it Helps