ios - UICollectionView 第一次加载时明显滞后

Question

我有一个简单的应用程序，其中包含应用程序内设置 Table View Controller。当用户选择“App Themes”单元格时，它会segues(推送)到嵌入在 UIViewController 中的 UICollectionView。

这是 12 个 3x4 格式的单元格，其中每个单元格都有一个图像、一个标签和一个复选标记图像(如果已选择该主题)。

单元格在此处的类中动态加载，但我注意到在第一次转到此 UICollectionView 时出现一些重大延迟。随后的每一次，它都运行良好，但在 iPhone 5s 上，有一些明显的延迟，我对 iOS 开发还很陌生，我觉得这与第一次创建 View 的方式有关，等等，但我不确定。

在我的 viewDidLoad 中，我有:

- (void)viewDidLoad
{
    self.title = @"Themes";
    self.navigationController.navigationBar.titleTextAttributes = @{NSForegroundColorAttributeName : [UIColor whiteColor]};
  [super viewDidLoad];
    self.cView.dataSource = self;
    self.cView.delegate = self;


    self.themeLabels = [[NSArray alloc] initWithObjects:@"Original", @"Peacock", @"Mystical", @"Zebra", @"Simplicity", @"Rainbow", @"Prosperity", @"Leopard", @"Hypnotic", @"Dunes", @"Twirl", @"Oceanic", nil];

    self.themeImages = @[[UIImage imageNamed:@"Newiphonebackground.png"], [UIImage imageNamed:@"peacock.png"], [UIImage imageNamed:@"Purplepink.png"], [UIImage imageNamed:@"PinkZebra.png"], [UIImage imageNamed:@"Greenish.png"], [UIImage imageNamed:@"MarblePrint.png"], [UIImage imageNamed:@"Prosperity.png"], [UIImage imageNamed:@"leopard.png"], [UIImage imageNamed:@"CircleEffect.png"], [UIImage imageNamed:@"Orange3.png"], [UIImage imageNamed:@"ReddishBlack.png"], [UIImage imageNamed:@"bluey.png"]];

    [self changeAppThemes];

}

changeAppThemes 方法本质上是循环遍历 NSUserDefaults 并查看要应用哪个主题。

- (void)changeAppThemes
{
    self.selectedTheme = [[NSUserDefaults standardUserDefaults] objectForKey:@"Theme"];

    if ([self.selectedTheme isEqualToString:@"Mystical"])
    {
        self.view.backgroundColor = [UIColor colorWithPatternImage:[UIImage imageNamed:@"Purplepink.png"]];
        UIImage *navBackgroundImage = [[UIImage imageNamed: @"Purplepinknav"] resizableImageWithCapInsets:UIEdgeInsetsMake(0, 0, 0, 0)];
        [self.navigationController.navigationBar setBackgroundImage:navBackgroundImage forBarMetrics:UIBarMetricsDefault];

        UIImage *tabBackground = [[UIImage imageNamed:@"SolidPurple.png"]
                                  resizableImageWithCapInsets:UIEdgeInsetsMake(0, 0, 0, 0)];
        [self.tabBarController.tabBar setBackgroundImage:tabBackground];
    }

etc

viewWillAppear 是:

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    [self changeAppThemes];

    self.selectedThemeString = [[NSUserDefaults standardUserDefaults] objectForKey:@"Selection"];

    if ([self.selectedThemeString isEqualToString:@"Original"])
    {
        self.checkedIndexPath = [NSIndexPath indexPathForRow:0 inSection:0];
    }

    else if ([self.selectedThemeString isEqualToString:@"Oceanic"])
    {
        self.checkedIndexPath = [NSIndexPath indexPathForRow:11 inSection:0];
    }

    [self.cView reloadData];

}

今天之前的 viewWillAppear 只调用了 changeAppThemes 方法，没有其他代码，但是在转到 UICollectionView 时仍然非常没有响应> 第一次。

cellForItemAtIndexPath 是:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{

    ThemeCell *themeCell = (ThemeCell *)[collectionView dequeueReusableCellWithReuseIdentifier:@"Theme Cell" forIndexPath:indexPath];

    NSString *cellData = [self.themeLabels objectAtIndex:indexPath.row];

    themeCell.cellLabel.text = cellData;
    themeCell.cellImages.image = self.themeImages[indexPath.row];
    UIImageView *dot = [[UIImageView alloc]initWithFrame:CGRectMake(5, 0, 1, 2)];
    dot.image=[UIImage imageNamed:@"check-white-hi.png"];

    if([self.checkedIndexPath isEqual:indexPath])
    {
        NSLog(@"Loaded");
        themeCell.backgroundView = dot;
        [themeCell addSubview:dot];

    }
    else
    {
        NSLog(@"Not Loaded");

        themeCell.backgroundView = nil;
    }

我是否应该更改调用 changeAppThemes 的位置？我已经从 viewWillAppear 中删除了调用并将其留在 viewDidLoad 中，反之亦然，在这两种情况下，打开 UICollectionView 时响应速度都非常糟糕 第一次。

如有任何想法，我们将不胜感激。

Answer 1

异步加载您的图片。考虑下面的变体。

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);

    dispatch_async(queue, ^{
        //Do THIS
      UIImage *localImage  [self createImageObjectsforIndex:indexPath.row];
        dispatch_sync(dispatch_get_main_queue(), ^{
            //Do this when done
            cell.image = localImage;
        });

    });