ios - 为什么我的自定义委托(delegate)在 Objective-C 中不起作用？

Question

这是我的代码。 AddItemViewController.h:

#import <UIKit/UIKit.h>

@class AddItemViewController, ChecklistItem;
@protocol AddItemViewControllerDelegate <NSObject>
@optional
- (void)addItemViewControllerDidCancel:(AddItemViewController *)controller;
- (void)addItemViewController:(AddItemViewController *)controller didFinishAddingItem:(ChecklistItem *)item;
@end

@interface AddItemViewController : UITableViewController
@property (weak, nonatomic) id<AddItemViewControllerDelegate> delegate;
@end

AddItemViewController.m:

#import "AddItemViewController.h"
#import "ChecklistItem.h"

@interface AddItemViewController () <UITextFieldDelegate>
- (IBAction)Done:(id)sender;
- (IBAction)Cancel:(id)sender;

@end
...
- (IBAction)Done:(id)sender {
    ChecklistItem *item = [[ChecklistItem alloc]init];
    item.text = self.textField.text;
    item.checked = false;

    if ([self.delegate respondsToSelector:@selector(addItemViewController:didFinishAddingItem:)]) {
        [self.delegate addItemViewController:self didFinishAddingItem:item];
    }
}

- (IBAction)Cancel:(id)sender {
    [self.delegate addItemViewControllerDidCancel:self];
}

...

ChecklistViewController 调用委托(delegate)。 ChecklistViewController.m:

#import "ChecklistViewController.h"
#import "ChecklistItem.h"
#import "AddItemViewController.h"

@interface ChecklistViewController () <UITableViewDelegate, UITableViewDataSource, AddItemViewControllerDelegate>
@end

@implementation ChecklistViewController
...
- (void)viewDidLoad {
    [super viewDidLoad];
    AddItemViewController *addItem = [[AddItemViewController alloc]init];
    addItem.delegate = self;    
}
- (void)addItemViewControllerDidCancel:(AddItemViewController *)controller{
    [self dismissViewControllerAnimated:YES completion:nil];
}
- (void)addItemViewController:(AddItemViewController *)controller didFinishAddingItem:(ChecklistItem *)item{
    [self dismissViewControllerAnimated:YES completion:nil];
}
...
@end

我已经调试了应用程序。当 AddItemViewController 运行时，'self.delegate' 为空。任何人都可以告诉我我的代码有什么问题。非常感谢!

Answer 1

代表被弱持有。您已经使用

正确设置了它

@property (weak, nonatomic) id<AddItemViewControllerDelegate> delegate;

但是，由于它是弱持有的，因此必须有其他东西持有对该委托(delegate)对象的引用。

此外，我在您发布的分配委托(delegate)对象的代码中没有看到任何地方。所以，有几件事可以尝试:

1. 确保分配委托(delegate)(AddItemViewController instance.delegate = ChecklistViewController instance)
2. 确保其他东西引用了您的 ChecklistViewController，这样它就不会在您需要时消失。