重新加载tableView时应用程序崩溃

Question

Normally if I reload the table it does not crash. But when I get some data in background and then reload table to display that data and at same time if user is scrolling the table then app crashes. The reason is array of objects chatData is empty. I don't get how it is empty. Because just before reloading the table I set object into chatData . Note that it crashes only if user is scrolling at the same time.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    // Here app crashes when chatData is empty. Don't get why it is ever empty, because reloadData is called only after setting objects.
    if ([user.userId isEqualToString:[[chatData objectAtIndex:row] objectForKey:SET_SENDER]])
    {

    }
}

- (void)refreshTable
{
    .
    .
    .
    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error)
    {
        self.chatData = [objects mutableCopy];
        [chatTable reloadData];
    }
}

Answer 1

[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error)
    {
        self.chatData = [objects mutableCopy];
        [chatTable reloadData];
    }];

I assume this is doing its work on a background thread?

If so, you should move the assignment of the chatData and the reloadData call to the main thread, using dispatch_async , as any UI calls and any data that the UI touches should be done and assigned on the main thread.

Like so:

[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error)
    {
        dispatch_async(dispatch_get_main_queue(), ^{
            self.chatData = [objects mutableCopy];
            [chatTable reloadData];
        });            
    }];

Answer 2

问题是我在代码中的某处清空了chatData ，之后如果重新加载表，那么[chatData objectAtIndex:row]会导致应用程序崩溃。