我对“scrollToRowAtIndexPath”有疑问。我得到这个错误。 （我已经上传了课程）

Question

所以首先你可以在这里下载课程：http://dl.dropbox.com/u/3951219/classes.zip

我收到此错误：*** Terminating app due to uncaught exception 'NSRangeException', reason: '-[UITableView scrollToRowAtIndexPath:atScrollPosition:animated:]: row (-1) beyond bounds (0) for section (0).'

这里有两个类（如果你能下载包含 2 个类和 xib 的 zip 会更好） ChatViewController.h：

#import <UIKit/UIKit.h>


@interface ChatViewController : UIViewController {
    IBOutlet UITableView *tbl;
    IBOutlet UITextField *field;
    IBOutlet UIToolbar *toolbar;
    NSMutableArray *messages;
}
- (IBAction)add;

@property (nonatomic, retain) UITableView *tbl;
@property (nonatomic, retain) UITextField *field;
@property (nonatomic, retain) UIToolbar *toolbar;
@property (nonatomic, retain) NSMutableArray *messages;


@end

ChatViewController.m：

#import "ChatViewController.h"


@implementation ChatViewController
@synthesize tbl, field, toolbar, messages;

- (void)viewDidLoad {
    [super viewDidLoad];    

    messages = [[[NSMutableArray alloc] initWithObjects:@"Hi",nil] retain];
    [tbl reloadData];

}

- (void)viewWillAppear:(BOOL)animated {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWillShow:) 
                                                 name:UIKeyboardWillShowNotification object:self.view.window]; 
}

- (void)viewWillDisappear:(BOOL)animated {
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillShowNotification object:nil]; 
}

- (void)add {
    if(![field.text isEqualToString:@""])
    {
        [messages addObject:field.text];
        [tbl reloadData];
        NSUInteger index = [tbl numberOfRowsInSection:0] - 1;
        [tbl scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:index inSection:0] atScrollPosition:UITableViewScrollPositionBottom animated:NO];

        field.text = @"";
    }
}

- (BOOL)textFieldShouldReturn:(UITextField *)textField {
    [textField resignFirstResponder];
    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDuration:0.3];  
    toolbar.frame = CGRectMake(0, 372, 320, 44);
    tbl.frame = CGRectMake(0, 0, 320, 372); 
    [UIView commitAnimations];

    return YES;
}

- (void)keyboardWillShow:(NSNotification *)notif {
    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDuration:0.3];  
    toolbar.frame = CGRectMake(0, 156, 320, 44);
    tbl.frame = CGRectMake(0, 0, 320, 156); 
    [UIView commitAnimations];

    if([messages count] > 0)
    {
        [tbl reloadData];
        NSUInteger index = [tbl numberOfRowsInSection:0] - 1;
        [tbl scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:index inSection:0] atScrollPosition:UITableViewScrollPositionBottom animated:NO];
    }
}

#pragma mark -
#pragma mark Table view methods

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    return [messages count];
}

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

    static NSString *CellIdentifier = @"Cell";

    UIImageView *balloonView;
    UILabel *label;

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
        cell.selectionStyle = UITableViewCellSelectionStyleNone;
        tableView.separatorStyle = UITableViewCellSeparatorStyleNone;       

        balloonView = [[UIImageView alloc] initWithFrame:CGRectZero];
        balloonView.tag = 1;

        label = [[UILabel alloc] initWithFrame:CGRectZero];
        label.backgroundColor = [UIColor clearColor];
        label.tag = 2;
        label.numberOfLines = 0;
        label.lineBreakMode = UILineBreakModeWordWrap;
        label.font = [UIFont systemFontOfSize:14.0];

        UIView *message = [[UIView alloc] initWithFrame:CGRectMake(0.0, 0.0, cell.frame.size.width, cell.frame.size.height)];
        message.tag = 0;
        [message addSubview:balloonView];
        [message addSubview:label];
        [cell.contentView addSubview:message];

        [balloonView release];
        [label release];
        [message release];
    }
    else
    {
        balloonView = (UIImageView *)[[cell.contentView viewWithTag:0] viewWithTag:1];
        label = (UILabel *)[[cell.contentView viewWithTag:0] viewWithTag:2];
    }

    NSString *text = [messages objectAtIndex:indexPath.row];
    CGSize size = [text sizeWithFont:[UIFont systemFontOfSize:14.0] constrainedToSize:CGSizeMake(240.0f, 480.0f) lineBreakMode:UILineBreakModeWordWrap];

    UIImage *balloon;

    if(indexPath.row % 2 == 0)
    {
        balloonView.frame = CGRectMake(320.0f - (size.width + 28.0f), 2.0f, size.width + 28.0f, size.height + 15.0f);
        balloon = [[UIImage imageNamed:@"green.png"] stretchableImageWithLeftCapWidth:24 topCapHeight:15];
        label.frame = CGRectMake(307.0f - (size.width + 5.0f), 8.0f, size.width + 5.0f, size.height);
    }
    else
    {
        balloonView.frame = CGRectMake(0.0, 2.0, size.width + 28, size.height + 15);
        balloon = [[UIImage imageNamed:@"grey.png"] stretchableImageWithLeftCapWidth:24 topCapHeight:15];
        label.frame = CGRectMake(16, 8, size.width + 5, size.height);
    }

    balloonView.image = balloon;
    label.text = text;

    return cell;
}

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
    NSString *body = [messages objectAtIndex:indexPath.row];
    CGSize size = [body sizeWithFont:[UIFont systemFontOfSize:14.0] constrainedToSize:CGSizeMake(240.0, 480.0) lineBreakMode:UILineBreakModeWordWrap];
    return size.height + 15;
}

#pragma mark -

- (void)didReceiveMemoryWarning {
    // Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];

    // Release any cached data, images, etc that aren't in use.
}

- (void)viewDidUnload {
    self.tbl = nil;
    self.field = nil;
    self.toolbar = nil;
}

/*
 // Override to support conditional rearranging of the table view.
 - (BOOL)tableView:(UITableView *)tableView canMoveRowAtIndexPath:(NSIndexPath *)indexPath {
 // Return NO if you do not want the item to be re-orderable.
 return YES;
 }
 */


- (void)dealloc {
    [tbl release];
    [field release];
    [toolbar release];
    [messages release];
    [super dealloc];
}


@end

Answer 1

- (void)add {
    if(![field.text isEqualToString:@""])
    {
        [messages addObject:field.text];
        [tbl reloadData];
        NSUInteger index = [tbl numberOfRowsInSection:0] - 1;
        [tbl scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:index inSection:0] atScrollPosition:UITableViewScrollPositionBottom animated:NO];
ui      

        field.text = @"";
    }
}

错误可能在这里，更准确

if(![field.text isEqualToString:@""]) - 在这里。

如果 field.text = nil，那么它会通过条件，这不是真的。

你最好试试 if( [field.text length] )