ios - 如果显示 2 秒，则将显示的 UITableView 单元格存储在 NSString 中

Question

我有一个 UITableView，里面有很多单元格。我想跟踪滚动了哪些单元格并将关联的 ID 存储在 NSUserDefaults 中。

这一切都非常好，但现在我想添加一个计时器，以防止将 ID 附加到字符串，除非它已在屏幕上显示整整两秒或更长时间。

这是我将每个新的滚动 ID 附加到 NSString 的行:

NSString *scrolledIdPre = [NSString stringWithFormat:@"-%@-", locationId];

这是我的完整方法:

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

    NSDictionary *item;

    if (tableView == self.searchDisplayController.searchResultsTableView){
        item = [[NSDictionary alloc] initWithDictionary:[filteredListItems 
        objectAtIndex:indexPath.row]];
    }else {
        item = [[NSDictionary alloc] initWithDictionary:[listItems 
        objectAtIndex:indexPath.row]];
    }

    if(cell == nil){
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle 
        reuseIdentifier:@"cell"];
    }

    NSString *locationId  = [item objectForKey:@"location_id"];

    NSString *scrolledIdPre = [NSString stringWithFormat:@"-%@-", locationId];

    [scrolledIds appendString:scrolledIdPre];

    [[NSUserDefaults standardUserDefaults] setObject:scrolledIds forKey:@"impressions"];
    [[NSUserDefaults standardUserDefaults] synchronize];

    return cell;
}

任何想法都会很棒。谢谢!

Answer 1

您可以使用延迟调度并检查单元格是否仍然可见

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

    NSDictionary *item;

    if (tableView == self.searchDisplayController.searchResultsTableView){
        item = [[NSDictionary alloc] initWithDictionary:[filteredListItems 
        objectAtIndex:indexPath.row]];
    }else {
        item = [[NSDictionary alloc] initWithDictionary:[listItems 
        objectAtIndex:indexPath.row]];
    }

    if(cell == nil){
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle 
        reuseIdentifier:@"cell"];
    }

    double delayInSeconds = 2.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){

        // nil means that cell is no longer visible so it didn't meet the requirement for 2s displaying
        if ([tableView cellForRowAtIndexPath:indexPath] == nil) {
            return;
        }

        NSString *locationId  = [item objectForKey:@"location_id"];

        NSString *scrolledIdPre = [NSString stringWithFormat:@"-%@-", locationId];

        [scrolledIds appendString:scrolledIdPre];

        [[NSUserDefaults standardUserDefaults] setObject:scrolledIds forKey:@"impressions"];
        [[NSUserDefaults standardUserDefaults] synchronize];
    });

    return cell;
}