ios - 为什么 UITableView 在同时滚动和更新时会丢失部分标题？

Question

我遇到了一个错误，其中 TableView 可能会丢失对部分标题的跟踪，从而在其位置创建一个新的部分标题。 (见下图；右边的屏幕截图显示了丢失的 header 。)

虽然我不知道确切的原因，但当我以编程方式滚动 TableView 并通过调用“beginUpdates”和“endUpdates”同时更新它时，这似乎会发生。谁能解释为什么表格 View 丢失了部分标题？这究竟发生在什么时候？

下面，我提供了演示此问题的 View Controller 代码。该代码显示一个 TableView ，其数据模型是一个字符串数组。当您按下添加按钮时，一个新行将添加到表中并滚动到 View 中。添加片刻后，新行会比其他单元格高，以表明它是新添加的单元格。要重现错误，请多次按“添加”按钮，然后滚动到顶部。 (我在 4 英寸 64 位 iPhone 模拟器中对此进行了测试。)

可能导致该错误的代码在“pressedAddButton:”方法中，因为我在该方法中同时滚动和更新 TableView 。 (编辑:许多其他代码只是用于设置数据模型并为 TableView 提供数据源和委托(delegate)方法的样板。)

注意:要让代码运行，请将 View Controller 放在 UINavigationController 中并显示导航 Controller :

ABCTestViewController * testViewController = [[ABCTestViewController alloc] initWithNibName:nil bundle:nil];
UINavigationController * navigationController = [[UINavigationController alloc] initWithRootViewController:testViewController];
// Now show the navigation controller.

在ABCTestViewController.h中:

#import <UIKit/UIKit.h>

@interface ABCTestViewController : UIViewController <UITableViewDataSource, UITableViewDelegate>

@property (weak, nonatomic) UITableView * tableView;

// The data model is an array of arrays of strings.
@property (strong, nonatomic) NSMutableArray * dataModel;

// The special row will be shown as being taller than the other rows.
@property (strong, nonatomic) NSIndexPath * indexPathOfSpecialRow;

@end

在ABCTestViewController.m中:

#import "ABCTestViewController.h"

#import <QuartzCore/QuartzCore.h>

@interface ABCTestViewController ()

@end

@implementation ABCTestViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        [self initializeNavigationItem];
        [self initializeDataModel];
    }
    return self;
}

- (void)initializeNavigationItem
{
    self.title = @"Test";

    self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"Add" style:UIBarButtonItemStylePlain target:self action:@selector(pressedAddButton:)];
}

- (void)initializeDataModel
{
    self->_dataModel = [[NSMutableArray alloc] init];

    // Add strings to section 0.
    NSMutableArray * section0 = [[self class] stringsForSection:0 numberOfStrings:2];
    [self.dataModel addObject:section0];

    // Add strings to section 1.
    NSMutableArray * section1 = [[self class] stringsForSection:1 numberOfStrings:10];
    [self.dataModel addObject:section1];
}

// Utility.
+ (NSMutableArray *)stringsForSection:(NSUInteger)section numberOfStrings:(NSUInteger)numberOfStrings
{
    NSMutableArray * strings = [[NSMutableArray alloc] initWithCapacity:numberOfStrings];
    for (NSUInteger i = 0; i < numberOfStrings; ++i) {
        NSString * string = [NSString stringWithFormat:@"%d-%d", section, i];
        [strings addObject:string];
    }
    return strings;
}

- (void)loadView
{
    [super loadView];
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Add a table view.
    UITableView * tableView = [[UITableView alloc] initWithFrame:self.view.bounds];
    self.tableView = tableView;
    self.tableView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
    [self.view addSubview:self.tableView];

    // Configure the table view.
    self.tableView.sectionHeaderHeight = 40;
    self.tableView.allowsSelection = NO;

    self.tableView.dataSource = self;
    self.tableView.delegate = self;
}

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

// ---------------------

// Event Handers

- (void)pressedAddButton:(id)sender
{
    // Update data model: add a new string to the second section.
    NSUInteger section = 1;
    NSMutableArray * strings = [self.dataModel objectAtIndex:section];
    NSUInteger newStringIndex = strings.count;
    NSString * newString = [NSString stringWithFormat:@"1-%d (New String)", newStringIndex];
    [strings insertObject:newString atIndex:newStringIndex];

    // Update display: add a new row in the table for the newly added string.
    NSIndexPath * indexPath = [NSIndexPath indexPathForRow:newStringIndex inSection:section];
    [self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];

    // Scroll to the newly added row.
    [self.tableView scrollToRowAtIndexPath:indexPath atScrollPosition:UITableViewScrollPositionNone animated:YES];

    // After a brief pause, show that the newly added row is "special" by making its height taller than the other rows.
    double delayInSeconds = 0.1;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        // Update display model: set the special row to be the newly added row.
        self.indexPathOfSpecialRow = indexPath;

        // Update display: tell the table view to update the heights of the rows
        // (i.e., tableView:heightForRowAtIndexPath: will be called for each row).
        [self.tableView beginUpdates];
        [self.tableView endUpdates];
    });
}

// ---------------------

// Table View Data Source and Delegate Methods

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return self.dataModel.count;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    NSArray * strings = [self.dataModel objectAtIndex:section];

    return strings.count;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    // Data from the data model.
    NSArray * strings = [self.dataModel objectAtIndex:indexPath.section];
    NSString * string = [strings objectAtIndex:indexPath.row];

    static NSString * const kCellIdentifier = @"Cell";
    UITableViewCell * cell = [tableView dequeueReusableCellWithIdentifier:kCellIdentifier];
    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:kCellIdentifier];
    }

    cell.textLabel.text = string;

    return cell;
}

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (self.indexPathOfSpecialRow != nil && [indexPath isEqual:self.indexPathOfSpecialRow]) {
        return tableView.rowHeight * 1.5;
    }
    else {
        return tableView.rowHeight;
    }
}

- (UIView *)tableView:(UITableView *)tableView viewForHeaderInSection:(NSInteger)section
{
    static NSString * const kHeaderIdentifier = @"Header";
    UITableViewHeaderFooterView * headerView = [tableView dequeueReusableHeaderFooterViewWithIdentifier:kHeaderIdentifier];
    if (headerView == nil) {
        headerView = [[UITableViewHeaderFooterView alloc] initWithReuseIdentifier:kHeaderIdentifier];

        // Give the header view a border (need to link to QuartzCore.framework to access layer property of UIView).
        headerView.contentView.layer.borderColor = [UIColor colorWithWhite:0 alpha:0.5].CGColor;
        headerView.contentView.layer.borderWidth = 1;
    }

    headerView.textLabel.text = [NSString stringWithFormat:@"Section %d", section];

    return headerView;
}

@end

Answer 1

我知道我的回答晚了，但我遇到了同样的问题。IOS 的所有系统动画都是由 CATransaction 制作的。我的解决方案是:

[CATransaction begin];

[CATransaction setCompletionBlock:^{
    //scroll view here
}];

[self.tableView beginUpdates];
[self.tableView endUpdates];

[CATransaction commit];