无StoryBoard环境下两个ViewController间代理未触发的解决求助

Hey there! Let's fix that delegate method not firing issue for you—this is super common when working without Storyboards, and it usually boils down to a few easy-to-miss steps. I'll walk you through the corrected code for all three files, plus point out the key mistakes that might have been blocking your proxy call.

First, we need a properly defined protocol with a weak delegate property (critical to avoid retain cycles):

#import <UIKit/UIKit.h>

// Protocol naming follows iOS conventions: ClassName + Delegate
@protocol ViewController2Delegate <NSObject>
// Define your message-passing method here
- (void)viewController2:(ViewController2 *)sender didSendMessage:(NSString *)message;
@end

@interface ViewController2 : UIViewController

// Weak modifier prevents circular references between VCs
@property (nonatomic, weak) id<ViewController2Delegate> delegate;

// A sample method to trigger message sending (you can tie this to a button action)
- (void)triggerMessageSend;

@end

In the second view controller, we'll add logic to check if the delegate exists and implements the method before calling it:

#import "ViewController2.h"

@implementation ViewController2

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = UIColor.systemBlueColor;

    // Add a test button to trigger message sending
    UIButton *sendBtn = [UIButton buttonWithType:UIButtonTypeSystem];
    sendBtn.frame = CGRectMake(100, 200, 200, 50);
    [sendBtn setTitle:@"Send Message Back" forState:UIControlStateNormal];
    [sendBtn addTarget:self action:@selector(triggerMessageSend) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:sendBtn];
}

- (void)triggerMessageSend {
    NSString *message = @"Hey, this is from ViewController2!";
    
    // Critical check: Only call the delegate method if it's implemented
    if ([self.delegate respondsToSelector:@selector(viewController2:didSendMessage:)]) {
        [self.delegate viewController2:self didSendMessage:message];
    }

    // Pop back to the first VC after sending
    [self.navigationController popViewControllerAnimated:YES];
}

@end

Now we need to make the first VC conform to the protocol and set itself as the delegate when pushing VC2:

First, update ViewController.h (or use a class extension in .m) #

#import <UIKit/UIKit.h>
#import "ViewController2.h"

@interface ViewController : UIViewController <ViewController2Delegate>

@end

Then implement the logic in ViewController.m #

#import "ViewController.h"

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = UIColor.systemBackgroundColor;

    // Add a button to navigate to VC2
    UIButton *navBtn = [UIButton buttonWithType:UIButtonTypeSystem];
    navBtn.frame = CGRectMake(100, 200, 200, 50);
    [navBtn setTitle:@"Go to VC2" forState:UIControlStateNormal];
    [navBtn addTarget:self action:@selector(pushToVC2) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:navBtn];
}

- (void)pushToVC2 {
    ViewController2 *vc2 = [[ViewController2 alloc] init];
    // THIS IS THE MOST COMMON MISTAKE: Forgetting to set the delegate!
    vc2.delegate = self;
    [self.navigationController pushViewController:vc2 animated:YES];
}

// Implement the delegate method to receive messages
- (void)viewController2:(ViewController2 *)sender didSendMessage:(NSString *)message {
    NSLog(@"Received message: %@", message);
    // Add your UI update or logic here
    UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"Message Received" message:message preferredStyle:UIAlertControllerStyleAlert];
    [alert addAction:[UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:nil]];
    [self presentViewController:alert animated:YES completion:nil];
}

@end

1. Forgot to set the delegate: Without Storyboard segues, you have to manually set vc2.delegate = self when initializing VC2—this is the #1 culprit.
2. Used strong instead of weak for the delegate property: A strong delegate creates a retain cycle, leading to VC2 not being released properly and the delegate becoming invalid.
3. Didn't conform to the protocol: If your first VC doesn't declare it follows ViewController2Delegate, the compiler might warn you, and runtime calls will fail.
4. No check for method implementation: While this doesn't break the call, it can cause crashes if the delegate doesn't implement the method—always add that respondsToSelector check.

内容的提问来源于stack exchange，提问作者JinseonKim