原帖地址；http://luoji10000.blog.163.com/blog/static/166439382201068104029647/
State(狀態)模式 - 2
行為型模式
2010-07-08 10:40:29
閱讀23
評論0
字型大小：大
中
小
訂閱
如果您不瞭解TCP的連線方式,在看Gof的書介紹
State 狀態模式
時,大概會看得一頭霧水吧! TCP的連線狀態圖,光是要瞭解就要花點精神了,它的連線狀態很多,用來說明狀態模式確實很適合,但不適合教導初學模式的人.
由
簡單的開始會比較好理解狀態模式的作用,先來看一個例子,如果您有一個只能順時針轉動的瓦斯開關,轉動一次的狀態為off, small
fire,medium fire與large
fire,您如何在程式中控制狀態的變化與行為呢?一個最簡單的方式就是用if...else...或是switch流程來控制,例如:
State.java
public class State {
private int state;
public State() {
state = 0;
}
public void switchFire() {
if (state == 0) {
state = 1; System.out.println( "small fire" );
} else if (state == 1) {
state = 2; System.out.println( "medium fire" );
} else if (state == 2) {
state = 3; System.out.println( "large fire" ); } else {
state = 0; System.out.println( "turning off" ); }
}
}
Main.java
public class Main {
public static void main(String[] args) {
State state = new State();
state.switchFire();
state.switchFire();
state.switchFire();
state.switchFire(); }
}
這
個方法很簡單,每個人都會,但如果您的狀態變化並不是流水式的變化,而是像TCP連線狀態一樣,會是一個網狀圖的時候,用if...else或
switch來寫的話,您的程式就會亂的不像話了;來考慮如何讓物件控制自己的狀態轉換與所應表現的行為,這個程式可以這樣改寫:
IState.java
public interface IState { public void switchFire(FireSwitch sw); }
OffState
public class OffState implements IState {
public void switchFire(FireSwitch sw) {
sw.setState(new SmallState());
System.out.println( "small fire" ); }
}
SmallState.java
public class SmallState implements IState {
public void switchFire(FireSwitch sw) {
sw.setState(new MediumState());
System.out.println( "medium fire" ); }
}
MediumState.java
public class MediumState implements IState {
public void switchFire(FireSwitch sw) {
sw.setState(new LargeState());
System.out.println( "large fire" ); }
}
LargeState.java
public class LargeState implements IState {
public void switchFire(FireSwitch sw) {
sw.setState(new OffState()); System.out.println( "off fire" ); }
}
FireSwitch.java
public class FireSwitch {
private State current;
public FireSwitch() { current = new OffState(); }
public void setState(State s) { current = s; }
public void switchFire() { current.switchFire(this); }
}
Main.java
public class Main {
public static void main(String[] args) {
FireSwitch fireSwitch = new FireSwitch();
fireSwitch.switchFire();
fireSwitch.switchFire();
fireSwitch.switchFire();
fireSwitch.switchFire(); }
}
程式執行結果與上一個例子是一樣的,但這次並沒有用流程式控制制來進行狀態轉換,而由物件自行控制自己的狀態,與必須表現的行為,這個方式就是State模式,將這個例子的UML類別結構畫出就如下所示:
再進一步考慮開關可以順時針與逆時針轉動,這時如果您仍以if...else或switch來寫,就會讓流程顯示複雜,來看看如何使用狀態模式來撰寫:
IState.java
public interface IState {
public void switchClockWise(FireSwitch sw);
public void switchCountClock(FireSwitch sw);
}
OffState.java
public
class OffState implements IState { public void
switchClockWise(FireSwitch sw) { sw.setState(new SmallState());
System.out.println("small fire"); } public void
switchCountClock(FireSwitch sw) { sw.setState(new LargeState());
System.out.println("large fire"); } }
SmallState.java
public class SmallState implements IState {
public void switchClockWise(FireSwitch sw) { sw.setState(new MediumState()); System.out.println("medium fire"); }
public void switchCountClock(FireSwitch sw) { sw.setState(new OffState()); System.out.println("off fire"); }
}
MediumState.java
public class MediumState implements IState {
public
void switchClockWise(FireSwitch sw) { sw.setState(new LargeState());
System.out.println("large fire"); } public void
switchCountClock(FireSwitch sw) { sw.setState(new SmallState());
System.out.println("small fire"); }
}
LargeState.java
public class LargeState implements State {
public void switchClockWise(FireSwitch sw) { sw.setState(new OffState()); System.out.println("off fire"); }
public void switchCountClock(FireSwitch sw) { sw.setState(new MediumState()); System.out.println("mediumfire"); }
}
FireSwitch.java
public class FireSwitch {
private State current;
public FireSwitch() { current = new OffState(); }
public void setState(State s) { current = s; }
public void switchClockWise() { current.switchClockWise(this); }
public void switchCountClock() { current.switchCountClock(this); }
}
Main.java
public class Main {
public static void main(String[] args) {
FireSwitch fireSwitch = new FireSwitch();
fireSwitch.switchClockWise();
fireSwitch.switchClockWise();
fireSwitch.switchClockWise();
fireSwitch.switchClockWise();
System.out .println();
fireSwitch.switchCountClock();
fireSwitch.switchCountClock();
fireSwitch.switchCountClock();
fireSwitch.switchCountClock(); }
}
接
下來您可以任意的轉動開關了,無論是順時針轉動或是逆時針轉動,狀態的轉換都由物件自己來表現,這是雙向狀態轉換下的例子,如果一個狀態可能轉換至三個以
上的狀態,使用State模式就更可以看出它的好處了,就像Gof的TCP連線例子一樣,如果您瞭解TCP連線,可以看看原書是如何?TCP連線之間的
狀態轉換的.
State模式的UML結構圖如下:
