为什么将“viewWithTag”与“dequeueReusableCellWithIdentifier”一起使用？

Question

can someone please explain why you should use viewWithTag to get subviews (eg UILabel etc) from a cell in dequeueReusableCellWithIdentifier ?

Some background info: I've got a custom UITableViewCell with a couple of UILabel s in it (I've reproduced a simple version of this below). These labels are defined in the associated NIB file and are declared with IBOutlet s and linked back to the custom cell's controller class. In the tableview's dequeueReusableCellWithIdentifier , I'm doing this:

CustomCell *customCell = (CustomCell *)[tableView dequeueReusableCellWithIdentifier:@"CustomCellId"];
if (customCell == nil) {
    NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@"customCell" owner:self options:nil];
    for (id oneObject in nib)
        if ([oneObject isKindOfClass:[CustomCell class]])
            customCell = (CustomCell *)oneObject;
}

customCell.firstLabel.text = @"Hello";
customCell.secondLabel.text = @"World!";

return customCell;

Everything works fine. However from the tutorials I've seen, it looks like when changing the labels' values I should be doing this instead:

UILabel *firstLabel = (UILabel *)[customCell.contentView viewWithTag:555];
firstLabel.text = @"Hello";

UILabel *secondLabel = (UILabel *)[customCell.contentView viewWithTag:556];
secondLabel.text = @"World!";

(The labels' tag values have been set in the NIB).

Can someone tell me which method is preferred and why?

Thanks!

Answer 1

viewWithTag: is just a quick and dirty way to pull out child views without having to set up IBOutlet properties on the parent, or even without having to create a UITableViewCell subclass.

For very simple cases this is an acceptable solution, that's what viewWithTag: was intended for. However if you are going to reuse that cell a lot or you want it to have a more developer-friendly interface then you will want to subclass and use real properties as in your first example.

So use viewWithTag: if it's a very simple cell you designed in IB with no subclass and with just a couple of labels. Use a cell subclass with real properties for anything more substantial.

Answer 2

我已经意识到，如果元素以编程方式添加到单元格中（即未在NIB中定义并通过IBOutlets连接），则使用“viewWithTag”检索元素很有用 - 这样可以防止为每个实例创建多个标签等细胞

Answer 3

For me , viewWithTag is a God given. First of all : treating all views in a loop like taskinoor said is really easy. Also , I personally prefer this way because if I take a look on the code and want to see what happens with a view , I simply search for the tag. It's used everywhere the view is handled. Opposed to the xib approach where you have to look in the code and xib too. Also , if you have an offscreen view in a xib , you might oversee it. I found a lot of xibs made by other programmers that were FULL with lots and lots of views. Some hidden , some offscreen , couldn't tell which is which since there were all overlapping. In those cases , I think xibs are bad. They are not easy to read anymore. I prefer everything made in code.

But if you decide to work with tags, remember to avoid hard-coding any tag. Instead make a list of #define definitions to keep the code clean and readable.

Answer 4

I always hook subviews to properties of my UITableViewCell subclass via IBOutlets, as you have done. I can't think of any good reason to use viewWithTag.

Answer 5

From UITableViewCell Class Reference: "The table view's delegate in tableView:cellForRowAtIndexPath: should always reset all content when reusing a cell." Keep it simple, clear out the content view. This makes no assumptions about custom cell classes, no casts, no class inspection:

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellId];
if (cell != nil)
{
    NSArray* contentSubViews = [cell.contentView subviews];
    for (UIView* viewToRemove in contentSubViews)
    {
        [viewToRemove removeFromSuperview];
    }
}

Answer 6

viewWithTag: allows styling without creating a custom subclass of UITableViewCell .

You can assign a tag and reuse identifier to a prototype UITableViewCell in Interface Builder, then dequeue and modify the view with that tag within the implementation of your UITableViewController , without creating a custom class for that cell or creating IBOutlets for the cell's subviews.

In some cases, the simplicity of a cell makes a custom class feel like overkill. viewWithTag: allows you to add custom text and image to a cell in the Storyboard, then set those customizations via code, without adding extra class files to your Xcode project.