调用委托方法时无法识别选择器

Question

我正在尝试在master和detail中实现一个带有NavigationController的SplitViewController。我一直在学习本教程，但我仍然遇到了一个相当奇怪的问题。当我尝试调用委托方法时，我得到-[UINavigationController selectedStudent:]: unrecognized selector sent to instance...

任何帮助都将不胜感激。

这是代码：

学生选拔代表.h

#import <Foundation/Foundation.h>
@class Student;
@protocol StudentSelectionDelegate <NSObject>
@required
-(void)selectedStudent:(Student *)newStudent;
@end

StudentDetail表示拆分视图中的细节。在StudentDetail.h我有

#import "StudentSelectionDelegate.h"
@interface StudentDetail : UITableViewController <StudentSelectionDelegate>
...

StudentDetail.m

@synthesize SentStudent;
...
-(void)selectedStudent:(Student *)newStudent
{
    [self setStudent:newStudent];
}

StudentList表示拆分视图的主视图。在StudentList.h中我得到了：

#import "StudentSelectionDelegate.h"
...
@property (nonatomic,strong) id<StudentSelectionDelegate> delegate;

在didSelectRowAtIndexPath 中的StudentList.m中

[self.delegate selectedStudent:SelectedStudent];

并且没有"SelectedStudent"不是空

最后是AppDelegate.m

#import "AppDelegate.h"
#import "StudentDetail.h"
#import "StudentListNew.h"
...
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    [[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleLightContent];
    UISplitViewController *splitViewController = (UISplitViewController *)self.window.rootViewController;
    UINavigationController *leftNavController = [splitViewController.viewControllers objectAtIndex:0];
    StudentListNew  *leftViewController = (StudentListNew *)[leftNavController topViewController];
    StudentDetail  *rightViewController = [splitViewController.viewControllers objectAtIndex:1];
    leftViewController.delegate = rightViewController;
    return YES;
}

附言：几个小时以来，我一直在寻找解决方案。

Answer 1

[splitViewController.viewControllers objectAtIndex:1]是UINavigationController，而不是StudentDetail。

错误消息告诉您UINavigationController没有selectedStudent属性。

您的代理指向的不是StudentDetail，而是一个导航控制器，它甚至没有实现< StudentSelectionDelegate>。然而，由于您指定了类型转换，Objective C不能警告您所转换的对象实际上并不是您所转换为的类

你应该考虑像苹果的代码那样对对象进行类型检查，以确保对象是你期望的类

这是更正后的代码：

UINavigationController *rightNavController = [splitViewController.viewControllers objectAtIndex:1];
StudentDetail  *rightViewController = (StudentDetail *)[rightNavController topViewController];
leftViewController.delegate = rightViewController;

至于确保您的代表实施该方法，

if ([self.delegate respondsToSelector:@selector(selectedStudent:)]) {
    [self.delegate selectedStudent:SelectedStudent];
}

尽管您必须使用调试器才能意识到self.delegate不是StudentDetail，但本可以避免出现异常。