ios - 将数据传递给下一个 ViewController 时出错

Question

我正在尝试在 ViewController 之间传递数据。数据字符串是从 UIPickerView 中获取的。但是，我收到一个错误。首先，这是我填充 UIPickerView 的地方。

- (IBAction)btnClick:(id)sender
{
    NSDictionary *courseNames;
    if(![_txtBox.text isEqual:@""]) //if not empty
    {
        courseNames = [self retrieveCourseNamesForSemester:_txtBox.text];
        for (NSString *key in courseNames)
        {
            NSString *val = [NSString stringWithFormat:@"%@: %@",key,[courseNames objectForKey:key]];

            if([courseArray count]==0)
            {
                courseArray = [NSMutableArray arrayWithObject:val];
            }
            else
            {
                [courseArray addObject:val];
            }
        }
        [_coursePicker reloadAllComponents];
        _coursePicker.hidden=false;
    }

    [_txtBox resignFirstResponder];
}

这是我尝试将数据传递给第二个 ViewController (GradesViewController) 的方式:

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    GradesViewController *controller = segue.destinationViewController;
    @try
    {
        [controller setCourseToSearch:[NSString stringWithString:selectedCourse]];
    }
    @catch (NSException *exception)
    {
        NSLog(@"%@ --- %@",exception, selectedCourse);
        if([selectedCourse isKindOfClass:[NSString class]])
        {
            NSLog(@"it IS a string");
        }
        else
        {
            NSLog(@"it is NOT a string");
        }
    }
}

最后，这是我设置 selectedCourse 的地方:

- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component
{
    selectedCourse = [NSString stringWithString:[courseArray objectAtIndex:[pickerView selectedRowInComponent:0]]];
}

GradesViewController 中的属性定义如下:

@property (assign) NSString *courseToSearch;

prepareForSegue 中的异常处理输出如下:

2014-03-29 11:00:08.292 WebServiceTest[46154:60b] -[UIViewController setCourseToSearch:]: unrecognized selector sent to instance 0xe29ac60 --- 43-214: Course Name
2014-03-29 11:00:08.292 WebServiceTest[46154:60b] it IS a string

为什么在发送 NSString 时提示我发送了“无法识别的选择器”？赋值 selectedCourse = [NSString stringWithString:[courseArray objectAtIndex:[pickerView selectedRowInComponent:0]]]; 是否应该确保它是一个 NSString？

Answer 1

问题不在于字符串，而在于你的 Storyboard，因为你没有设置正确的 View Controller 类。这就是为什么你在 -[UIViewController setCourseToSearch:]: unrecognized ...

中得到 UIViewController

更正 Storyboard中的类名称。

然后，更改字符串管理，这样就不会重复创建新字符串(即删除 stringWithString: 用法)并更改此属性:

@property (assign) NSString *courseToSearch;

至

@property (strong) NSString *courseToSearch;

因为您应该保留(或复制)对象。