Question

我想从其他类 FilterViewController 调用位于 ProductListViewController 中的方法FromFilter:，

当我尝试调用方法时，我获得了委托的nil值，

这是我的代码段：

ProductListViewController.h

@protocol FirstControllerDelegate<NSObject>
@required
-(void)FromFilter:(NSString *)StrApi;

@end

@interface ProductListViewController : UIViewController<FirstControllerDelegate>{
   ....
}
@property (nonatomic, assign) id <FirstControllerDelegate> delegate;

ProductListViewController.m

-(void)FromFilter:(NSString *)StrApi{
    NSLog(@"------- this is not getting called ---------");
}

FilterViewController.h

@interface FilterViewController : UIViewController{
    ...
}
@property (nonatomic, assign) id <FirstControllerDelegate> delegate;

FilterViewController.m

    - (IBAction)ApplyClicked:(id)sender {

    // -----  Here my _delegate is nil ------

    if (_delegate == nil) {
        [_delegate FromFilter:@"fuyds"];
    }

    [self dismissViewControllerAnimated:YES completion:nil];
}

请帮忙，我在哪里做错了？

Answer 1

似乎你没有分配“FilterViewController”的委托属性。

无论何时创建FilterViewController

示例：

FilterViewController *filterViewController = [[FilterViewController alloc]init];

filterViewController.delegate  = self;//important

如果您要从ProductListViewController呈现FilterViewController，那么您应该将委托指定为self

Answer 2

if (_delegate == nil) {
        [_delegate FromFilter:@"fuyds"];
    }

当然，您的FromFilter方法并未被调用。仅当_delegate为零时，您的代码才会调用_delegate对象的此方法。基本上，您的代码是这样的：

[nil FromFilter];

在Objective C中，您可以向nil发送消息，它们什么都不做，返回0（nil，NULL）。

Answer 3

您是否尝试将代码更改为：

- (IBAction)ApplyClicked:(id)sender {

// -----  Here my _delegate is nil ------

if (_delegate != nil) { // != NOT == --> you want to enter the if clause if the delegate is NOT nil
    [_delegate FromFilter:@"fuyds"];
}

    [self dismissViewControllerAnimated:YES completion:nil];
}

使用协议时委派获得nil

3 个答案: