ios - 如何使用 KVO 传递值？

Question

我正在学习如何使用 KVO。我创建了两个类，卡车和司机，如下所示。

Truck 类有一个文本字段和一个按钮，文本应包含当前卡车速度，按下按钮时，应调用 prepareForSegue，它包含下面发布的代码。

Driver 类包含一个应填充卡车速度的文本字段。 Truck 类中的卡车速度将通过 KVO 传递到 Driver 类中的文本字段，如代码所示。

我遇到的问题或我正在尝试做的是，当用户在 Truck 类中输入卡车的速度并按下按钮时，我想通过以下方式显示在 Driver 类的文本字段中输入的卡车速度KVO

我根据下面发布的代码得到的结果是，Driver 类中的一个空文本字段

请告诉我为什么驱动程序类中的文本字段为空。以及我应该如何通过 KVO 将卡车速度的值从卡车类传递到司机类

卡车.m

#import "TruckViewController.h"
#import "DriverViewController.h"
#import "ServerViewController.h"

@interface TruckViewController ()

@property (strong, nonatomic) IBOutlet UITextField *textFieldCurrenSpeed;
@property (strong, nonatomic) IBOutlet UIButton *buttonBroadcast;
@property (strong, nonatomic) IBOutlet UIButton 
*buttonToDriverViewController;
@property (strong, nonatomic) IBOutlet UIButton   
*buttonToServerViewController;
@property (strong, nonatomic) TruckViewController *truck;
@property (strong, nonatomic) DriverViewController *driver;
@property (strong, nonatomic) ServerViewController *server;

@end


@implementation TruckViewController


- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view.
}


 - (void)didReceiveMemoryWarning {
 [super didReceiveMemoryWarning];
 // Dispose of any resources that can be recreated.
 }


-(void) prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender 
{
 self.truck = [[TruckViewController alloc] init];
 self.driver = [[DriverViewController alloc] init];
 self.server = [[ServerViewController alloc] init];

if ([segue.identifier isEqualToString:@"segueToDriver"]) {

    [self.truck addObserver:self.driver
            forKeyPath:@"currentSpeedOfTheTruck"
               options:NSKeyValueObservingOptionNew
               context:NULL];

    self.truck.currentSpeedOfTheTruck = [self.textFieldCurrenSpeed 
text];
    NSLog(@"prepareForSegue: %@", self.truck.currentSpeedOfTheTruck);
}

if ([segue.identifier isEqualToString:@"segueToServer"]) {

    [self.truck addObserver:self.server
            forKeyPath:@"currentSpeedOfTheTruck"
               options:NSKeyValueObservingOptionNew
               context:NULL];

    self.truck.currentSpeedOfTheTruck = [self.textFieldCurrenSpeed 
 text];
    NSLog(@"text entered: %@", self.truck.currentSpeedOfTheTruck);

 }

}

- (void)dealloc
{
[self.truck removeObserver:self.driver 
forKeyPath:@"currentSpeedOfTheTruck"];
[self.truck removeObserver:self.server    
forKeyPath:@"currentSpeedOfTheTruck"];
}

Driver.m

  #import "DriverViewController.h"

   @interface DriverViewController ()
   @property (strong, nonatomic) IBOutlet UITextField   
   *textFieldCurrentSpeed;

 @end


 -(void) observeValueForKeyPath:(NSString *)keyPath
                  ofObject:(id)object
                  change:(NSDictionary<NSKeyValueChangeKey,id> 
 *)change
                   context:(void *)context {

 if ([keyPath isEqualToString:@"currentSpeedOfTheTruck"]) {
    NSLog(@"DriverViewController->currentSpeedOfTheTruck: %@",
          [object valueForKey:keyPath]);

    NSLog(@"DriverViewController->currentSpeedOfTheTruck: %@",
          [change objectForKey:NSKeyValueChangeNewKey]);

    self.receivedCurrentSpeed = [change   
 objectForKey:NSKeyValueChangeNewKey];
    [self.textFieldCurrentSpeed setText:self.receivedCurrentSpeed];

 }
 }



  - (void)viewDidLoad {
   [super viewDidLoad];
  // Do any additional setup after loading the view.

   NSLog(@"DriverViewController->viewDidLoad");

   [self.textFieldCurrentSpeed setText:self.receivedCurrentSpeed];
   }

  - (void)didReceiveMemoryWarning {
   [super didReceiveMemoryWarning];
   // Dispose of any resources that can be recreated.
 }

日志输出:

2017-08-13 19:53:55.293 KVC-1[91529:2346883] DriverViewController-  
>currentSpeedOfTheTruck: 45

2017-08-13 19:53:55.294 KVC-1[91529:2346883] DriverViewController- 
>currentSpeedOfTheTruck: 45
2017-08-13 19:53:55.294 KVC-1[91529:2346883] prepareForSegue: 45

Answer 1

问题出在这几行代码上

 self.truck = [[TruckViewController alloc] init];
 self.driver = [[DriverViewController alloc] init];
 self.server = [[ServerViewController alloc] init];

由于您在准备 segue 之前创建了另一个对象而不是一个对象，因此您应该使用 segue.destinationController 来传输值

把上面这行代码改成

 self.truck = (TruckViewController *) segue.destinationViewController;

根据所有 View Controller 。