向 UITableViewCells 添加子视图会导致滚动时出现问题

Question

我完全不知道如何解决这个问题。 所以我有一个 UITableView 并且在委托方法 cellForRowAtIndex: 如果单元格为 nil （表格视图的初始构建），我将向每个单元格添加几个子视图。一切正常，表格视图已构建，但是，当我在应用程序中向下滚动一点时，应用程序突然与 SIGBART 崩溃并给我错误 * 由于未捕获的异常“NSInvalidArgumentException”而终止应用程序，原因：“+[NSIndexPath setImage:]: unrecognized selector sent to class 0x3c361e68”** 这很奇怪，因为我是甚至没有在我的代码中的任何地方调用 setImage 方法。 这是委托方法的代码。

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


static NSString *CellIdentifier = @"Cell";
UIImageView* imageView;
UILabel* ttitle;
UILabel* ttitle2;
UILabel* ttitle3;
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];

    // Configure cell:
    // *** This section should configure the cell to a state independent of
    //  whatever row or section the cell is in, since it is only executed
    //  once when the cell is first created.


    imageView=[[UIImageView alloc]initWithFrame:CGRectMake(10.0, 11.0, 50.0, 50.0)];
    [imageView setContentMode:UIViewContentModeScaleAspectFill];
    imageView.layer.masksToBounds=YES;
    imageView.layer.cornerRadius=5.0;
    [cell.contentView addSubview:imageView];

    ttitle = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, 7.0, 200, 20)] autorelease];
    ttitle.textColor= [UIColor blackColor];
    ttitle.numberOfLines=1;
    ttitle.backgroundColor=[UIColor clearColor];
    ttitle.font=[UIFont fontWithName:@"Arial Bold" size:15.0];
    [cell.contentView addSubview:ttitle];

    if (indexPath.row==0) {

        CGSize size=[[[data objectAtIndex:indexPath.row] valueForKey:@"content"] sizeWithFont:[UIFont systemFontOfSize:14.0f] constrainedToSize:CGSizeMake(265.0f, CGFLOAT_MAX) lineBreakMode:NSLineBreakByWordWrapping];
        ttitle2 = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, 27.5, 200, size.height)] autorelease];
        ttitle2.textColor= [UIColor darkGrayColor];
        ttitle2.backgroundColor=[UIColor clearColor];
        ttitle2.numberOfLines=0;
        ttitle2.textAlignment = NSTextAlignmentLeft;
        ttitle2.lineBreakMode=NSLineBreakByWordWrapping;
        ttitle2.font=[UIFont fontWithName:@"Arial" size:14.0];
        [cell.contentView addSubview:ttitle2];

        ttitle3 = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, ttitle2.frame.origin.y+ttitle2.frame.size.height-8.0, 210, 40)] autorelease];
        ttitle3.textColor= [UIColor darkGrayColor];
        ttitle3.backgroundColor=[UIColor clearColor];
        ttitle3.numberOfLines=1;
        ttitle3.textAlignment = NSTextAlignmentLeft;
        ttitle3.lineBreakMode=NSLineBreakByWordWrapping;
        ttitle3.font=[UIFont fontWithName:@"Arial" size:11.0];
        [cell.contentView addSubview:ttitle3];


    }
    else{

        CGSize size=[[[data objectAtIndex:indexPath.row] valueForKey:@"content"] sizeWithFont:[UIFont systemFontOfSize:14.0f] constrainedToSize:CGSizeMake(265.0f, CGFLOAT_MAX) lineBreakMode:NSLineBreakByWordWrapping];
        ttitle2 = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, 27.0, 200, size.height)] autorelease];
        ttitle2.textColor= [UIColor darkGrayColor];
        ttitle2.backgroundColor=[UIColor clearColor];
        ttitle2.numberOfLines=0;
        ttitle2.textAlignment = NSTextAlignmentLeft;
        ttitle2.lineBreakMode=NSLineBreakByWordWrapping;
        ttitle2.font=[UIFont fontWithName:@"Arial" size:14.0];
        [cell.contentView addSubview:ttitle2];


        ttitle3 = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, ttitle2.frame.origin.y+ttitle2.frame.size.height-9.0, 210, 40)] autorelease];
        ttitle3.textColor= [UIColor darkGrayColor];
        ttitle3.backgroundColor=[UIColor clearColor];
        ttitle3.numberOfLines=1;
        ttitle3.textAlignment = NSTextAlignmentLeft;
        ttitle3.lineBreakMode=NSLineBreakByWordWrapping;
        ttitle3.font=[UIFont fontWithName:@"Arial" size:11.0];
        [cell.contentView addSubview:ttitle3];

    }

}

// Customize cell:
// *** This section should customize the cell depending on what row or section
//  is passed in indexPath, since this is executed every time this delegate method
//  is called.
imageView.image=[UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[[data objectAtIndex:indexPath.row] valueForKey:@"thumbnail"]]]];

[ttitle setText:[[data objectAtIndex:indexPath.row] valueForKey:@"name"]];
[ttitle2 setText:[[data objectAtIndex:indexPath.row] valueForKey:@"content"]];

NSString* first=[[[data objectAtIndex:indexPath.row] valueForKey:@"hashtag"] stringByAppendingString:@"     "];
NSString* second =[first stringByAppendingString:[[data objectAtIndex:indexPath.row] valueForKey:@"place"]];
NSString* third=[second stringByAppendingString:@"        "];
NSString* fourth=[third stringByAppendingString:@"¤ "];
NSString* conversion=[[[data objectAtIndex:indexPath.row] valueForKey:@"counter"] stringValue];
NSString* fifth=[fourth stringByAppendingString:conversion];
[ttitle3 setText:fifth];





return cell;

}

感谢帮助的家伙！

*更新代码

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


static NSString *CellIdentifier = @"Cell";
UIImageView* imageView;
UILabel* ttitle;
UILabel* ttitle2;
UILabel* ttitle3;
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];

    // Configure cell:
    // *** This section should configure the cell to a state independent of
    //  whatever row or section the cell is in, since it is only executed
    //  once when the cell is first created.


    imageView=[[UIImageView alloc]initWithFrame:CGRectMake(10.0, 11.0, 50.0, 50.0)];
    [imageView setContentMode:UIViewContentModeScaleAspectFill];
    imageView.layer.masksToBounds=YES;
    imageView.layer.cornerRadius=5.0;
    imageView.tag=1;
    [cell.contentView addSubview:imageView];

    ttitle = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, 7.0, 200, 20)] autorelease];
    ttitle.textColor= [UIColor blackColor];
    ttitle.numberOfLines=1;
    ttitle.tag=69;
    ttitle.backgroundColor=[UIColor clearColor];
    ttitle.font=[UIFont fontWithName:@"Arial Bold" size:15.0];
    [cell.contentView addSubview:ttitle];

    if (indexPath.row==0) {

        CGSize size=[[[data objectAtIndex:indexPath.row] valueForKey:@"content"] sizeWithFont:[UIFont systemFontOfSize:14.0f] constrainedToSize:CGSizeMake(265.0f, CGFLOAT_MAX) lineBreakMode:NSLineBreakByWordWrapping];
        ttitle2 = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, 27.5, 200, size.height)] autorelease];
        ttitle2.textColor= [UIColor darkGrayColor];
        ttitle2.backgroundColor=[UIColor clearColor];
        ttitle2.numberOfLines=0;
        ttitle2.tag=70;
        ttitle2.textAlignment = NSTextAlignmentLeft;
        ttitle2.lineBreakMode=NSLineBreakByWordWrapping;
        ttitle2.font=[UIFont fontWithName:@"Arial" size:14.0];
        [cell.contentView addSubview:ttitle2];

        ttitle3 = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, ttitle2.frame.origin.y+ttitle2.frame.size.height-8.0, 210, 40)] autorelease];
        ttitle3.textColor= [UIColor darkGrayColor];
        ttitle3.backgroundColor=[UIColor clearColor];
        ttitle3.numberOfLines=1;
        ttitle3.tag=71;
        ttitle3.textAlignment = NSTextAlignmentLeft;
        ttitle3.lineBreakMode=NSLineBreakByWordWrapping;
        ttitle3.font=[UIFont fontWithName:@"Arial" size:11.0];
        [cell.contentView addSubview:ttitle3];


    }
    else{

        CGSize size=[[[data objectAtIndex:indexPath.row] valueForKey:@"content"] sizeWithFont:[UIFont systemFontOfSize:14.0f] constrainedToSize:CGSizeMake(265.0f, CGFLOAT_MAX) lineBreakMode:NSLineBreakByWordWrapping];
        ttitle2 = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, 27.0, 200, size.height)] autorelease];
        ttitle2.textColor= [UIColor darkGrayColor];
        ttitle2.backgroundColor=[UIColor clearColor];
        ttitle2.numberOfLines=0;
        ttitle2.tag=70;
        ttitle2.textAlignment = NSTextAlignmentLeft;
        ttitle2.lineBreakMode=NSLineBreakByWordWrapping;
        ttitle2.font=[UIFont fontWithName:@"Arial" size:14.0];
        [cell.contentView addSubview:ttitle2];


        ttitle3 = [[[UILabel alloc] initWithFrame:CGRectMake(70.0, ttitle2.frame.origin.y+ttitle2.frame.size.height-9.0, 210, 40)] autorelease];
        ttitle3.textColor= [UIColor darkGrayColor];
        ttitle3.backgroundColor=[UIColor clearColor];
        ttitle3.numberOfLines=1;
        ttitle3.tag=71;
        ttitle3.textAlignment = NSTextAlignmentLeft;
        ttitle3.lineBreakMode=NSLineBreakByWordWrapping;
        ttitle3.font=[UIFont fontWithName:@"Arial" size:11.0];
        [cell.contentView addSubview:ttitle3];

    }

    imageView.image=[UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[[data objectAtIndex:indexPath.row] valueForKey:@"thumbnail"]]]];

    [ttitle setText:[[data objectAtIndex:indexPath.row] valueForKey:@"name"]];
    [ttitle2 setText:[[data objectAtIndex:indexPath.row] valueForKey:@"content"]];

    NSString* first=[[[data objectAtIndex:indexPath.row] valueForKey:@"hashtag"] stringByAppendingString:@"     "];
    NSString* second =[first stringByAppendingString:[[data objectAtIndex:indexPath.row] valueForKey:@"place"]];
    NSString* third=[second stringByAppendingString:@"        "];
    NSString* fourth=[third stringByAppendingString:@"¤ "];
    NSString* conversion=[[[data objectAtIndex:indexPath.row] valueForKey:@"counter"] stringValue];
    NSString* fifth=[fourth stringByAppendingString:conversion];
    [ttitle3 setText:fifth];


}
else {
    imageView =[cell viewWithTag:1];
    ttitle=[cell viewWithTag:69];
    ttitle2=[cell viewWithTag:70];
    ttitle3=[cell viewWithTag:71];
}

//STUFFOUTSIDE

// Customize cell:
// *** This section should customize the cell depending on what row or section
//  is passed in indexPath, since this is executed every time this delegate method
//  is called.



return cell;

}

Answer 1

问题是当重新使用单元格时，您的局部变量没有被初始化。这是 imageView 的当前流程：

UIImageView* imageView;

if (cell == nil)
{
    // Create imageView
    imageView=...
}

// If cell is being reused (ie cell is not nil) then imageView is nil at this point.
imageView.image=...

当您重用一个表格视图单元格时，tableView:dequeueReusableCellWithIdentifier: 返回一个实际的单元格而不是 nil，并跳过 imageView 的初始化。

您需要“找到”重用单元格中的 imageView 才能对其进行更改。

因此，我建议你这样修改你的逻辑：

UIImageView* imageView;

if (cell == nil)
{
    // Create imageView
    imageView=...
}
else
{
    imageView = // get a reference to the imageView
}

imageView.image=...

所以现在，当然，问题是“如何？”。

一种很常见的方法是在创建视图时设置视图的tag，以便以后轻松检索。您可以像这样使用这种技术：

// Use a unique tag number for each subview.
#define MY_IMAGEVIEW_TAG 1000

UIImageView* imageView;

if (cell == nil)
{
    // Create imageView
    imageView=... // Same as before
    imageView.tag = MY_IMAGEVIEW_TAG; 
}
else
{
    // This is a cell that is being re-used and was previously created.
    // Retrieve a reference to the existing image view that is already in the cell
    imageView = [cell viewWithTag:MY_IMAGEVIEW_TAG];
}

// Now imageView is "safe" to use whether it is a new cell or one that is reused!
imageView.image=...

注意：如果您经常这样做，创建一个具有每个子视图属性的 UITableViewCell 子类将使标签和 viewWithTag 的使用变得不必要，并且使您的代码更易于阅读。

Answer 2

@Inafziger 已经发布了这个问题的正确答案，我只想更详细地解释一下为什么你会看到这个“奇怪”的崩溃。
不过，我不建议过度使用标签。创建UITableViewCell 的子类可能是一个更好的主意。

您没有初始化您的 imageView 和 ttitle 变量：

UIImageView *imageView; // imageView can point to anything now!
UILabel* ttitle;

通常，当您声明局部变量以避免此类悬空指针时，您会将它们初始化为 nil 或 0（任何有意义的）。

因为您正在重用您的表格视图单元格 cell 并不总是 nil 并且您的 if 子句不会被执行：

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) { // Won't be executed if the cell could be dequeued!
    ...
    imageView = ...;
}

因此，如果cell 可以出队，那么您的imageView 和ttitle 变量在您使用它们时仍然没有被分配给任何东西！

然后您将设置视图的属性：

imageView.image=[UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[[data objectAtIndex:indexPath.row] valueForKey:@"thumbnail"]]]];
[ttitle setText:[[data objectAtIndex:indexPath.row] valueForKey:@"name"]];
[ttitle2 setText:[[data objectAtIndex:indexPath.row] valueForKey:@"content"]]

imageView.image = ...; 与调用[imageView setImage:...]; 相同。你可以在这里阅读更多信息：http://www.cocoacast.com/cocoacast/?q=node/103

这时候所有的东西都在一起了：imageView 没有初始化，你正在调用-setImage:。轰隆隆，崩溃！

在您的情况下，imageView 指向 NSIndexPath 类。不过，这可以是任何东西。
因此，您实际上是在 NSIndexPath 类上调用 -setImage:（相当于：+[NSIndexPath setImage:]）并且应用程序崩溃并显示 +[NSIndexPath setImage:]: unrecognized selector sent to class 0x3c361e68 错误消息。