[Cocoa]_[初級]_[NSNotificationCenter 消息中心在程序中的具體應用]

1.NSNotificationCenter是整個消息操作的首腦。它允許你註冊觀察者對象，發佈通知到通告板上，和註銷觀察對象。

2.消息中心機制流程圖：

主要是對觀察者對象進行註冊，註冊之後NSNotificationCenter對所有觀察者對象進行統一管理，如果觀察者要執行方法，發送一條消息給消息中心，消息中心監聽到這個消息之後，就會把你感興趣的方法顯示到通告板上，以方便用戶預覽。其中［NSNotificationCenter defaultCenter］這個是消息中心對象是全局的，處理消息的方式是同步模式。

3.場景：我們以切換視圖爲例，使用消息中心來切換視圖，可以避免視圖之間的相互引用。所有視圖的切換命令都有消息中心來監聽管理，只需要給消息中心發送一個命令請求，消息中心監聽到這個請求命令之後就會執行我們要操作的方法。

4.具體實現步驟：

1）創建一個消息中心

NSNotificationCenter *nc =[NSNotificationCenter defaultCenter];

2）註冊觀察者

+ (instancetype)notificationWithName:(NSString *)aName object:(id)anObject;

3）發送消息

- (void)postNotificationName:(NSString *)aName object:(NSString *)anObject;

4）註銷觀察者

- (void)removeObserver:(id)observer name:(NSString *)aName object:(id)anObject;

下面看具體例子說明

WindowNotification.h

#import <Foundation/Foundation.h>

extern NSString * const kWindowMessageSwitchToMainView;
extern NSString * const kWindowMessageSwitchToFirstView ;
extern NSString * const kWindowMessageSwitchToSecondView ;

WindowNotification.m

#import "WindowNotification.h"

//定義各個視圖的觀察者名稱
NSString * const kWindowMessageSwitchToMainView = @"kWindowMessageSwitchToMainView";
NSString * const kWindowMessageSwitchToFirstView = @"kWindowMessageSwitchToFirstView";
NSString * const kWindowMessageSwitchToSecondView = @"kWindowMessageSwitchToSecondView";

FirstViewController.m

#import "FirstViewController.h"
#import "WindowNotification.h"

@interface FirstViewController ()

@end

@implementation FirstViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Initialization code here.
    }
    return self;
}
-(IBAction)onBackToHomePager:(id)sender
{
    //返回到主視圖
    NSNotificationCenter *nc =[NSNotificationCenter  defaultCenter];
    [nc postNotificationName:kWindowMessageSwitchToMainView object:nil];
}

@end

SecondViewController.m

#import "SecondViewController.h"
#import "WindowNotification.h"

@interface SecondViewController ()

@end

@implementation SecondViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Initialization code here.
    }
    return self;
}

-(IBAction)onBackToHomePager:(id)sender
{
    //返回到主視圖
    NSNotificationCenter *nc =[NSNotificationCenter  defaultCenter];
    [nc postNotificationName:kWindowMessageSwitchToMainView object:nil];
}

MainDelegate.h

#import <Foundation/Foundation.h>
@class FirstViewController;
@class SecondViewController;

@interface MainDelegate : NSObject
{
    IBOutlet NSView *mainView;
    IBOutlet NSScrollView* parentView;
    
    FirstViewController* firstViewController;//視圖1
    SecondViewController* secondViewController;//視圖2
}

-(IBAction)onfirstView:(id)sender;
-(IBAction)onSecondView:(id)sender;
-(IBAction)onRemoveObserver:(id)sender;

-(void) doSwitchMainWindow;
-(void) doSwitchFirstViewController;
-(void) doSwitchSecondViewController;

@end

MainDelegate.m

#import "MainDelegate.h"
#import "WindowNotification.h"

#import "FirstViewController.h"
#import "SecondViewController.h"

@implementation MainDelegate

-(void)awakeFromNib
{
    firstViewController =[[FirstViewController alloc] initWithNibName:@"FirstViewController" bundle:nil];
    secondViewController =[[SecondViewController alloc] initWithNibName:@"SecondViewController" bundle:nil];
    //創建一個消息中心對象
    NSNotificationCenter *nc =[NSNotificationCenter  defaultCenter];
    //註冊觀察者對象
    [nc addObserver:self selector:@selector(doSwitchMainWindow) name:kWindowMessageSwitchToMainView object:nil];
     [nc addObserver:self selector:@selector(doSwitchFirstViewController) name:kWindowMessageSwitchToFirstView object:nil];
     [nc addObserver:self selector:@selector(doSwitchSecondViewController) name:kWindowMessageSwitchToSecondView object:nil];
}


-(IBAction)onfirstView:(id)sender
{
    //發送消息切換視圖1
     NSNotificationCenter *nc =[NSNotificationCenter  defaultCenter];
    [nc postNotificationName:kWindowMessageSwitchToFirstView object:nil];
}
-(IBAction)onSecondView:(id)sender
{
    //發送消息切換視圖2
    NSNotificationCenter *nc =[NSNotificationCenter  defaultCenter];
    [nc postNotificationName:kWindowMessageSwitchToSecondView object:nil];
}
-(IBAction)onRemoveObserver:(id)sender
{
    //註銷所有觀察者
    NSNotificationCenter *nc =[NSNotificationCenter  defaultCenter];
    [nc removeObserver:self name:kWindowMessageSwitchToFirstView object:nil];
    [nc removeObserver:self name:kWindowMessageSwitchToSecondView object:nil];
    
}

-(void) doSwitchMainWindow
{
    [mainView retain];
    [parentView setDocumentView:mainView];
}
-(void) doSwitchFirstViewController
{
    [[parentView documentView] retain];
    [parentView setDocumentView:firstViewController.view];
}
-(void) doSwitchSecondViewController
{
    [[parentView documentView] retain];
    [parentView setDocumentView:secondViewController.view];
}

@end

運行結果：