ios - 将数据从子模态 VC 传递到父 View Controller 的最佳方式？

Question

将数据从子模态视图传递到父 View Controller 的最佳方法是什么？

我的 iPad 应用程序上有一个子模式登录屏幕，我想将用户信息传递回父 Split View Controller 。

我正在考虑使用 NSNotification，但我不确定这是否是将数据传回父级的最简单/最有效的方法。

谢谢!
艾伦

Answer 1

我建议，as iPatel did , 使用 delegation来解决你的问题。父 View Controller 和登录 View Controller 之间的关系使这种模式很合适。当一个对象创建另一个对象以履行特定职责时，应将委托(delegate)视为使创建的对象与创建者通信的一种方式。选择委托(delegate)的一个特别令人信服的理由是，如果要完成的任务可能有多个步骤，需要对象之间的高级交互。你可以看看 NSURLConnectionDelegate protocol作为一个例子。连接到 URL 是一项复杂的任务，涉及处理响应、满足身份验证挑战、保存下载的数据和处理错误等阶段，连接和委托(delegate)在连接的整个生命周期内一起处理这些。

您可能已经注意到，在 Objective-C 中，协议(protocol)用于实现委托(delegate)，而无需将创建的对象(在本例中为您的登录 View Controller )与创建它的对象(父 View Controller )紧密耦合。然后，登录 View Controller 可以与任何可以接收其协议(protocol)中定义的消息的对象进行交互，而不是依赖于任何特定的类实现。明天，如果您收到允许任何 View Controller 显示登录 View 的要求，则登录 View Controller 不需要更改。您的其他 View Controller 可以实现其委托(delegate)协议(protocol)，创建和呈现登录 View ，并将自己分配为委托(delegate)，而登录 View Controller 不知道它们的存在。

您在 Stack Overflow 上找到的一些委托(delegate)示例可能非常令人困惑，并且与内置框架中的示例非常不同。必须仔细选择协议(protocol)的名称和接口(interface)，以及分配给每个对象的职责，以便最大限度地重用代码，达到代码的目的。

您应该首先查看内置框架中的许多委托(delegate)协议(protocol)，以了解在代码中表达的关系是什么样的。这是另一个基于您的登录用例的小示例。我希望你会发现委托(delegate)的目的是明确的，所涉及的对象的角色和职责是明确的，并通过它们在代码中的名称来表达。

首先，我们看一下 LoginViewController 的委托(delegate)协议(protocol):

#import <UIKit/UIKit.h>

@protocol LoginViewControllerDelegate;

@interface LoginViewController : UIViewController

// We choose a name here that expresses what object is doing the delegating
@property (nonatomic, weak) id<LoginViewControllerDelegate> delegate;

@end

@protocol LoginViewControllerDelegate <NSObject>

// The methods declared here are all optional
@optional

// We name the methods here in a way that explains what the purpose of each message is
// Each takes a LoginViewController as the first argument, allowing one object to serve
// as the delegate of many LoginViewControllers
- (void)loginViewControllerDidLoginSuccessfully:(LoginViewController *)lvc;
- (void)loginViewController:(LoginViewController *)lvc didFailWithError:(NSError *)error;
- (void)loginViewControllerDidReceivePasswordResetRequest:(LoginViewController *)lvc;
- (void)loginViewControllerDiDReceiveSignupRequest:(LoginViewController *)lvc;
- (BOOL)loginViewControllerShouldAllowAnonymousLogin:(LoginViewController *)lvc;

@end

登录 Controller 可以将许多事件传递给它的委托(delegate)，以及向它的委托(delegate)询问用于定制其行为的信息。它在其实现中将事件传达给委托(delegate)，作为其对用户操作的响应的一部分:

#import "LoginViewController.h"

@interface LoginViewController ()

@property (weak, nonatomic) IBOutlet UIButton *anonSigninButton;

@end

@implementation LoginViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    //  Here we ask the delegate for information used to layout the view
    BOOL anonymousLoginAllowed = NO;
    //  All our protocol methods are @optional, so we must check they are actually implemented before calling.
    if ([self.delegate respondsToSelector:@selector(loginViewControllerShouldAllowAnonymousLogin:)]) {
        // self is passed as the LoginViewController argument to the delegate methods
        // in this way our delegate can serve as the delegate of multiple login view controllers, if needed
        anonymousLoginAllowed = [self.delegate loginViewControllerShouldAllowAnonymousLogin:self];
    }
    self.anonSigninButton.hidden = !anonymousLoginAllowed;
}

- (IBAction)loginButtonAction:(UIButton *)sender
{
    // We're preteneding our password is always bad. So we assume login succeeds when allowed anonmously
    BOOL loginSuccess = [self isAnonymousLoginEnabled];
    NSError *loginError = [self isAnonymousLoginEnabled] ? nil : [NSError errorWithDomain:@"domain" code:0 userInfo:nil];

    //  Fake concurrency
    double delayInSeconds = 1.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        //  Notify delegate of failure or success
        if (loginSuccess) {
            if ([self.delegate respondsToSelector:@selector(loginViewControllerDidLoginSuccessfully:)]) {
                [self.delegate loginViewControllerDidLoginSuccessfully:self];
            }
        }
        else {
            if ([self.delegate respondsToSelector:@selector(loginViewController:didFailWithError:)]) {
                [self.delegate loginViewController:self didFailWithError:loginError];
            }
        }
    });
}

- (IBAction)forgotPasswordButtonAction:(id)sender
{
    //  Notify delegate to handle forgotten password request.
    if ([self.delegate respondsToSelector:@selector(loginViewControllerDidReceivePasswordResetRequest:)]) {
        [self.delegate loginViewControllerDidReceivePasswordResetRequest:self];
    }
}

- (IBAction)signupButtonAction:(id)sender
{
    //  Notify delegate to handle signup request.
    if ([self.delegate respondsToSelector:@selector(loginViewControllerDiDReceiveSignupRequest:)]) {
        [self.delegate loginViewControllerDiDReceiveSignupRequest:self];
    }
}

- (BOOL)isAnonymousLoginEnabled
{
    BOOL anonymousLoginAllowed = NO;

    if ([self.delegate respondsToSelector:@selector(loginViewControllerShouldAllowAnonymousLogin:)]) {
        anonymousLoginAllowed = [self.delegate loginViewControllerShouldAllowAnonymousLogin:self];
    }
    return  anonymousLoginAllowed;
}

@end

主视图 Controller 实例化并呈现一个登录 View Controller ，并处理其委托(delegate)消息:

#import "MainViewController.h"
#import "LoginViewController.h"

#define LOGGED_IN NO

@interface MainViewController () <LoginViewControllerDelegate>

@end

@implementation MainViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    //  Fake loading time to show the modal cleanly
    if (!LOGGED_IN) {
        double delayInSeconds = 1.0;
        dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
        dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
            //  Create a login view controller, assign its delegate, and present it
            LoginViewController *lvc = [[LoginViewController alloc] init];
            lvc.delegate = self;
            [self presentViewController:lvc animated:YES completion:^{
                NSLog(@"modal completion finished.");
            }];
        });
    }
}

#pragma mark - LoginViewControllerDelegate


- (void)loginViewControllerDidLoginSuccessfully:(LoginViewController *)lvc
{
    NSLog(@"Login VC delegate - Login success!");
    [self dismissViewControllerAnimated:YES completion:NULL];
}

- (void)loginViewController:(LoginViewController *)lvc didFailWithError:(NSError *)error
{
    // Maybe show an alert...
    // UIAlertView *alert = ...
}

- (void)loginViewControllerDidReceivePasswordResetRequest:(LoginViewController *)lvc
{
    // Take the user to safari to reset password maybe
     NSLog(@"Login VC delegate - password reset!");
}

- (void)loginViewControllerDiDReceiveSignupRequest:(LoginViewController *)lvc
{
    // Take the user to safari to open signup form maybe
    NSLog(@"Login VC delegate - signup requested!");
}

- (BOOL)loginViewControllerShouldAllowAnonymousLogin:(LoginViewController *)lvc
{
    return YES;
}

@end

在某些方面，登录可能是一个复杂的交互式过程，因此我建议您认真考虑使用委托(delegate)而不是通知。但是，可能有问题的一件事是委托(delegate)必须只是一个对象。如果您需要让多个不同的对象知道登录 View Controller 的进度和状态，那么您可能需要使用通知。尤其是如果登录过程可以被限制为非常简单，以一种除了传递单向消息和数据之外不需要任何交互的方式，那么通知可以成为一种可行的选择。您可以在通知中将任意变量传递回 userInfo 中。属性是 NSDictionary无论你决定往里面塞什么。通知会影响性能，但我知道现在只有当观察者人数达到数百人时才会发生这种情况。尽管如此，这在我看来并不是最自然的，因为您有父对象(或多或少控制子对象的生命周期)向第三方对象询问子对象的更新。