[Cocoa]_[初级]_[使用NSNotificationCenter作为目标观察者实现时需要注意的事项]

场景

1. 在开发Cocoa程序时，由于界面是用Objective-C写的。无法使用C++的目标观察者^[1]类。如果是使用第二种方案2^[2],那么也需要增加一个代理类。那么有没有更省事的办法？

说明

1. 开发界面的时候，经常是需要在子界面里传递数据给主界面，主界面获得数据后跟业务逻辑交互。目前子界面和主界面的数据交互目前使用发布订阅（观察者模式）的设计模式是比较好的，因为它可以让两个界面解耦合。

2. 之前说过在Cocoa程序里，可以使用NSNotificationCenter通知中心发布接收消息注意事项来作为消息传递的观察者实现。这里要强调一个问题，就是NSNotificationCenter注册的观察者必须要设置接收对象，也就是以下的object参数。如果参数设置为nil，当有多个地方注册同样的观察者时，目标如果发送MyNotificationChangeBackgroundColor消息，所有的观察者都有接收到这个消息，会导致该消息执行多次。特别是以下代码 self对象是多个实例时。

 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(recvNotificationMessage:) name:@(MyNotificationChangeBackgroundColor) object:subject];

3. 发送MyNotificationChangeBackgroundColor消息, 注意要指定object参数值。

-(IBAction)onChange:(id)sender
{static int mod = 1;if(mod++ % 2){NSColor* color = [NSColor colorWithSRGBRed:255/255.0 green:255/255.0 blue:255/255.0 alpha:1];[[NSNotificationCenter defaultCenter] postNotificationName:@(MyNotificationChangeBackgroundColor) object:self userInfo:@{@"color":color}];}else{NSColor* color = [NSColor colorWithSRGBRed:240/255.0 green:122/255.0 blue:55/255.0 alpha:1];[[NSNotificationCenter defaultCenter] postNotificationName:@(MyNotificationChangeBackgroundColor) object:view1_ userInfo:@{@"color":color}];}}

例子

1. 以下例子说明如何创建响应指定目标的观察者。点击两次Change按钮会依次更改子视图1和2的背景色，而它们发送的是同样的消息。

AppDelegate.h

#import <Cocoa/Cocoa.h>
#import "MyView.h"@interface AppDelegate : NSObject <NSApplicationDelegate>
{IBOutlet MyView* view1_;IBOutlet MyView* view2_;
}-(IBAction)onChange:(id)sender;-(IBAction)onReset:(id)sender;@end

AppDelegate.mm

#import "AppDelegate.h"
#include "platform.h"@interface AppDelegate ()@property (weak) IBOutlet NSWindow *window;
@end@implementation AppDelegate-(void)changeBackgroundColor:(NSView*)view withColor:(NSColor*)color1
{[view setWantsLayer:YES];CGColorSpaceRef space = [[color1 colorSpace] CGColorSpace];CGFloat components[] ={color1.redComponent,color1.greenComponent,color1.blueComponent,color1.alphaComponent};CGColorRef color_ref = CGColorCreate(space,components);[[view layer] setBackgroundColor:color_ref];CGColorRelease(color_ref);
}- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {// Insert code here to initialize your application[self onReset:nil];[view1_ registerObserver:self]; // 只接收来自self的消息[view2_ registerObserver:view1_]; // 只接收来自view1的消息
}- (void)applicationWillTerminate:(NSNotification *)aNotification {// Insert code here to tear down your application
}-(IBAction)onChange:(id)sender
{static int mod = 1;if(mod++ % 2){NSColor* color = [NSColor colorWithSRGBRed:255/255.0 green:255/255.0 blue:255/255.0 alpha:1];[[NSNotificationCenter defaultCenter] postNotificationName:@(MyNotificationChangeBackgroundColor) object:self userInfo:@{@"color":color}];}else{NSColor* color = [NSColor colorWithSRGBRed:240/255.0 green:122/255.0 blue:55/255.0 alpha:1];[[NSNotificationCenter defaultCenter] postNotificationName:@(MyNotificationChangeBackgroundColor) object:view1_ userInfo:@{@"color":color}];}}-(IBAction)onReset:(id)sender
{[self changeBackgroundColor:view1_ withColor:[NSColor redColor]];[self changeBackgroundColor:view2_ withColor:[NSColor blueColor]];
}@end

MyView.h

#import <Cocoa/Cocoa.h>NS_ASSUME_NONNULL_BEGIN@interface MyView : NSView-(void)registerObserver:(id __nullable)subject;-(void)removeObserver:(id)subject;@endNS_ASSUME_NONNULL_END

MyView.mm

#import "MyView.h"
#include "platform.h"@implementation MyView- (void)drawRect:(NSRect)dirtyRect {[super drawRect:dirtyRect];// Drawing code here.
}-(void)changeBackgroundColor:(NSView*)view withColor:(NSColor*)color1
{[view setWantsLayer:YES];CGColorSpaceRef space = [[color1 colorSpace] CGColorSpace];CGFloat components[] ={color1.redComponent,color1.greenComponent,color1.blueComponent,color1.alphaComponent};CGColorRef color_ref = CGColorCreate(space,components);[[view layer] setBackgroundColor:color_ref];CGColorRelease(color_ref);
}-(void)recvNotificationMessage:(NSNotification*) notification
{if([notification.name compare:@(MyNotificationChangeBackgroundColor)] == NSOrderedSame){NSDictionary* dict = notification.userInfo;NSColor* color = [dict objectForKey:@"color"];[self changeBackgroundColor:self withColor:color];}
}-(void)registerObserver:(id)subject
{[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(recvNotificationMessage:) name:@(MyNotificationChangeBackgroundColor) object:subject];
}-(void)removeObserver:(id)subject
{[[NSNotificationCenter defaultCenter] removeObserver:self name:@(MyNotificationChangeBackgroundColor) object:subject];
}-(void)awakeFromNib
{}@end

platform.h

#ifndef platform_h
#define platform_h#define MyNotificationChangeBackgroundColor "MyNotificationChangeBackgroundColor"#endif /* platform_h */

main.m

#import <Cocoa/Cocoa.h>int main(int argc, const char * argv[]) {return NSApplicationMain(argc, argv);
}

MainMenu.xib

<?xml version="1.0" encoding="UTF-8"?>
<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="14460.31" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES" customObjectInstantitationMethod="direct"><dependencies><plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="14460.31"/><capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/></dependencies><objects><customObject id="-2" userLabel="File's Owner" customClass="NSApplication"><connections><outlet property="delegate" destination="Voe-Tx-rLC" id="GzC-gU-4Uq"/></connections></customObject><customObject id="-1" userLabel="First Responder" customClass="FirstResponder"/><customObject id="-3" userLabel="Application"/><customObject id="Voe-Tx-rLC" customClass="AppDelegate"><connections><outlet property="view1_" destination="Gxg-KB-hXo" id="0fI-03-3v8"/><outlet property="view2_" destination="4YQ-ZK-Q0W" id="4bZ-O9-7Dg"/><outlet property="window" destination="QvC-M9-y7g" id="gIp-Ho-8D9"/></connections></customObject><customObject id="YLy-65-1bz" customClass="NSFontManager"/><menu title="Main Menu" systemMenu="main" id="AYu-sK-qS6"><items><menuItem title="NotificationTest" id="1Xt-HY-uBw"><modifierMask key="keyEquivalentModifierMask"/><menu key="submenu" title="NotificationTest" systemMenu="apple" id="uQy-DD-JDr"><items><menuItem title="About NotificationTest" id="5kV-Vb-QxS"><modifierMask key="keyEquivalentModifierMask"/><connections><action selector="orderFrontStandardAboutPanel:" target="-1" id="Exp-CZ-Vem"/></connections></menuItem><menuItem isSeparatorItem="YES" id="VOq-y0-SEH"/><menuItem title="Preferences…" keyEquivalent="," id="BOF-NM-1cW"/><menuItem isSeparatorItem="YES" id="wFC-TO-SCJ"/><menuItem title="Services" id="NMo-om-nkz"><modifierMask key="keyEquivalentModifierMask"/><menu key="submenu" title="Services" systemMenu="services" id="hz9-B4-Xy5"/></menuItem><menuItem isSeparatorItem="YES" id="4je-JR-u6R"/><menuItem title="Hide 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</document>

例子

图1

参考

1.观察者模式在项目中实际使用例子

2.观察者模式在项目中实际使用例子2

3.NSNotificationCenter通知中心发布接收消息注意事项