如何在没有 StoryBoard 的两个 ViewController 之间使用委托？

Question

我想触发从 viewController 到另一个 viewController 的消息。 我编码如下，但 viewController 没有调用委托。 我想在没有 stroyboard 的情况下调用代表。我只是想向另一个 viewController 发送消息。

viewController2.h

@protocol ViewController2Delegate;

@interface ViewController2 : UIViewController
@property (nonatomic, weak) id<ViewController2Delegate> delegate;
@end

@protocol ViewController2Delegate <NSObject>
- (void)showSomethingByDelegate;
@end

viewController2.m

#import "ViewController2.h"

@interface ViewController2 ()

@end

@implementation ViewController2

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
}
- (IBAction)buttonPressed:(id)sender {
    [self.delegate showSomethingByDelegate];
}

@end

viewController.m

#import "ViewController.h"
#import "ViewController2.h"

@interface ViewController ()
<ViewController2Delegate>

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    ViewController2 *vc2 = [[ViewController2 alloc] init];
    vc2.delegate = self;

}
// Below method was not called by delegate.
- (void)showSomethingByDelegate {
    NSLog(@"Button Was Pressed!!!");
}


@end

Answer 1

让我举一个更简单的例子。

文件：firstVC.h

/* This define the protocol object,
 you can write methods required or optional the diference is when
 the protocol is used in a class, xcode show you a yellow warning with required methods
 like UITableViewDelegate, UITextFieldDelegate... */

@protocol firstVCDelegate <NSObject>

@required
- (void)didMessageFromOtherViewController: (NSString *)messageStr;

@optional
- (void)didOtherMessageNotRequired: (NSString *)messageStr;

@end

/* This is the definition of first UIViewController */

@interface firstViewController : UIViewController
@end


/* This is the definition of second UIViewController object with
 a property that is our protocol 'firstVCDelegate' */

@interface secondViewController : UIViewController

@property (nonatomic, weak) id <firstVCDelegate> firstVCDelegate;

- (void)executeDelegateProcess;

@end

文件：firstVC.m

#import "firstVC.h"

#pragma mark - First UIViewController with delegate

@interface firstViewController() <firstVCDelegate>
@end

@implementation firstViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Creating the 'secondViewController' object with delegate on this class
    secondViewController *svc = [secondViewController new];

    // Assign the delegate class
    [svc setFirstVCDelegate: self];

    // Run the delegate logic
    [svc executeDelegateProcess];
}

- (void)didMessageFromOtherViewController:(NSString *)messageStr
{
    // Receiving the message from the instance of our protocol in the 'secondViewController' class
    NSLog(@"MESSAGE #1: %@", messageStr);
}

- (void)didOtherMessageNotRequired:(NSString *)messageStr
{
    // Receiving the message in optional method
    NSLog(@"MESSAGE #2: %@", messageStr);
}

@end

#pragma mark - Second UIViewController

@implementation secondViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
}

- (void)setFirstVCDelegate:(id<firstVCDelegate>)firstVCDelegate
{
    if (firstVCDelegate)
        _firstVCDelegate = firstVCDelegate;
}

- (void)executeDelegateProcess
{
    // This method is only for demo
    // You can execute your delegate in the way you need to use

    if (_firstVCDelegate) {
        [_firstVCDelegate didMessageFromOtherViewController: @"Hello world, using the required method from secondViewController class"];

        [_firstVCDelegate didOtherMessageNotRequired: @"Hello code using the optional method"];
    }
}

@end

在你的 appDelegate.m 方法 didFinishLaunchingWithOptions 中你可以放这个，你需要 #import "firstVC.h"

self.window = [[UIWindow alloc] initWithFrame: [[UIScreen mainScreen] bounds]];
        self.window.autoresizesSubviews = YES;
        [self.window makeKeyAndVisible];

        [_window setRootViewController: [firstViewController new]];

执行并看到两条日志消息，希望对我有所帮助:)

Answer 2

ViewController2 *vc2 = [[ViewController2 alloc] init];

这行代码并没有像你想象的那样做，这是创建ViewController2 的新实例并将其存储在变量vc2 中。绝不会（根据您的代码）将此变量添加到导航堆栈并显示在屏幕上。

编辑
同样正如 Holex 所注意到的，您的 vc2 变量将在 viewDidLoad 完成后从内存中删除，因为您没有保留它的引用。您要么需要创建一个属性来保存它，要么将它推送到导航堆栈上以便导航控制器保存它。

有多种方法可以解决这个问题：

1. 不要创建新的viewController2，而是查找对已显示在屏幕上的引用（如果它在屏幕上）。
2. 如果它不在导航堆栈中，请将 vc2 添加到导航堆栈 [self.navigationController pushViewController:vc2 animated:YES];（假设您有导航控制器）。
3. 如果不在堆栈上，并且不打算使用导航控制器，则以模态方式呈现vc2，例如[self presentViewController:vc2 animated:YES completion:nil];