這個或許叫源-***（source/listener）模式更加形象。
觀察者模式定義了一種一對多的依賴關係，讓多個觀察者物件同時監聽某乙個主題物件。這個主題物件在狀態上發生變化時，會通知所有觀察者物件，使它們能夠自動更新自己。
the classes and/or objects participating in this pattern are:
concretesubject(ibm)
observer(iinvestor)
concreteobserver(investor)
// observer pattern -- real world example
using system;
using system.collections.generic;
namespace dofactory.gangoffour.observer.realworld
///
//////
observer design pattern.
///class
//////
///static
void main()
// create ibm stock and attach investors
ibm ibm = new
ibm("ibm", 120.00);
ibm.attach(new
investor("sorros"));
ibm.attach(new
investor("berkshire"));
// fluctuating prices will notify investors
ibm.price = 120.10;
ibm.price = 121.00;
ibm.price = 120.50;
ibm.price = 120.75;
// wait for user
console.readkey();
///
///the 'subject' abstract class
///abstract
class
stock
private
string _symbol;
private
double _price;
private
list
_investors = new
list
();// constructor
public stock(string symbol, double price)
this._symbol = symbol;
this._price = price;
public
void attach(iinvestor investor)
_investors.add(investor);
public
void detach(iinvestor investor)
_investors.remove(investor);
public
void notify()
foreach (iinvestor investor in _investors)
investor.update(this);
console.writeline("");
// gets or sets the price
public
double price
get
set
if (_price != value)
_price = value;
notify();
// gets the symbol
public
string symbol
get
///
///the 'concretesubject' class
///class
ibm : stock
// constructor
public ibm(string symbol, double price)
: base(symbol, price)
///
///the 'observer' inte***ce
///inte***ce
iinvestor
void update(stock stock);
///
///the 'concreteobserver' class
///class
investor : iinvestor
private
string _name;
private
stock _stock;
// constructor
public investor(string name)
this._name = name;
public
void update(stock stock)
console.writeline("notified of 's " +
"change to ", _name, stock.symbol, stock.price);
// gets or sets the stock
public
stock stock
get
set
output
notified sorros of ibm's change to $120.10
notified berkshire of ibm's change to $120.10
notified sorros of ibm's change to $121.00
notified berkshire of ibm's change to $121.00
notified sorros of ibm's change to $120.50
notified berkshire of ibm's change to $120.50
notified sorros of ibm's change to $120.75
notified berkshire of ibm's change to $120.75
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