tableview上的textfield.text显示值null

Question

I use the codes below to add textfield to each tableview cell,

- (UITableViewCell *)tableView:(UITableView *)tableView
         cellForRowAtIndexPath:(NSIndexPath *)indexPath
{

    NSInteger row=[indexPath row];
    static NSString *SimpleTableIdentifier1 = @"CellTableIdentifier";
    //if I change the code to [NSString *SimpleTableIdentifier1 =NSString stringWithFormat:@"CellTableIdentifier%d",row]; everything is fine         


    UITableViewCell *cell=[tableView dequeueReusableCellWithIdentifier: SimpleTableIdentifier1 ];

    if  (cell == nil){

        CGRect cellframe=CGRectMake(0, 0, 200, 60);
        cell=[[[UITableViewCell alloc] initWithFrame: cellframe  reuseIdentifier:SimpleTableIdentifier1] autorelease];
        UITextField * textfieldCell =[[UITextField alloc]init];
        textfieldCell.frame = CGRectMake(100.0f,20.0f,60.0f,26.0f) ;
        [textfieldCell setDelegate:self];
        [textfieldCell setTag:40000+row];//add row value here for later use, 
        [cell.contentView addSubview:textfieldCell];
        [textfieldCell release];

    }
    UITextField *textfieldCell ;

    textfieldCell =(UITextField*)[cell.contentView viewWithTag:40000+row];
    textfieldCell.text=[ NSMutableString  stringWithString:@"aaa1"];
    DebugLog(@"---------%@",textfieldCell.text);

    return cell;


}

textfieldCell.text sometimes displays null rather than my expectation value of 'aaa1'

this means that the code line at:

textfieldCell =(UITextField*)[cell.contentView viewWithTag:40000+row];

sometimes returns nil, try to fix this confused result but failed

Your comment welcome

Answer 1

This dequeueReusableCellWithIdentifier

 UITableViewCell *cell=[tableView dequeueReusableCellWithIdentifier: SimpleTableIdentifier1 ];

returns a non nil cell Docs

This method dequeues an existing cell if one is available or creates a new one using the class or nib file you previously registered. If no cell is available for reuse and you did not register a class or nib file, this method returns nil

that may has a textField with different tag resulting in nil for

 UITextField *textfieldCell ;

as these 2 lines

// here rhs may be nil
textfieldCell =(UITextField*)[cell.contentView viewWithTag:40000+row];
textfieldCell.text=[ NSMutableString  stringWithString:@"aaa1"];

has no effect with nil textfield , also it's shocking how you still work with MRC ( Manual reference counting ) here

[textfieldCell release];

please update to ARC ( Automatic reference counting ) which will removes your worries about memory management issues

Answer 2

UITableView uses a dynamic pool of cells for performance reasons. The only cells which are actually active are those which are currently displayed on the screen. Any cells which are scrolled out of view are removed from the view and returned to a pool identified by, in your case, "SimpleTableIdentifier1"

The method dequeueReusableCell retrieves a cell from the pool, if one is available, or creates one if a class or XIB has been registered, otherwise returns nil.

The upshot of this is that your code may return a previously created cell which has been used for some other row earlier. The tag which exists for that cell will most likely relate to some other row. This will all depend on the scrolling of rows on and off screen.

Changing the identifier to "CellTableIdentifier"%row causes each cell to be allocated from its own pool and avoids this conflict. However that will completely defeat the purpose of reusing cells.

Since you are using tag just to locate the text field within the cell you do not need to make the tag unique across all cells of the table. Just use a simple constant ie. 4000 not 4000 + row. viewWithTag will return the view within the hierarchy which matches, and you are starting at the content view anyway.

Having said all that, the structure of your code is really not ideal. The better approach is to define your cell as a class, the layout in the storyboard, and access the text field directly as a member of the class.