狀態模式 State Pattern - LinZiyou Dev Blog

目的#

讓物件的外顯行為隨內部狀態的改變而改變，彷彿連類別也變了似的。

適用#

當物件的行為取決於它的狀態，連執行期行為也得隨狀態而改變時。如果某些操作裡面，依據物件狀態而做的條件判斷式太過龐大時。狀態通常會以enum表示，許多操作裡面也常會有相同的條件判斷結構。State Pattern將每一個條件分支抽取成獨立的類別，以便將物件狀態也視為另一個獨立的物件，可獨立改變。

結構及成員#

Collaborations: Context將與狀態有關的訊息委託給目前持有的ConcreteState去處理。 Context可能把自己當作傳參傳給State。 Context是主要的對外窗口。Client可用State物件來設定Context的組態，設好之後便不再直接插手State物件。可令Context或ConcreteState的子類別決定在什麼情況下該切換成什麼狀態。

Context: like the CruiseControl, TCPConnection制定外界感興趣的介面。持有一個ConcreteState。State:此介面負責封裝當Context處於特定狀態時所該展現的行為。ConcreteState: like the CruisingCanceled, TCPEstablished, TCPClosed, TCPListen, etc.每一個子類別都針對某一種Context狀態實作出該展現的行為。

影響結果#

好處#

集中處理與狀態相依的行為並予以切割。只要定義新的State子類別，即可輕易增加新的狀態及狀態轉移邏輯。替每一種狀態設一個State物件，能突顯狀態轉移邏輯，也能避免Context不一致的內部狀態。（以Context來看，狀態轉移是個不可再細分的atomic動作）狀態物件可共用。若State物件不含實例變數，則Context可以共用State物件，此時State物件相當於沒有內在狀態、只有行為的Flyweight物件。

壞處#

需要額外新增類別。

實作#

誰來定義狀態轉移邏輯？#

若條件是固定的，可以全寫在Context裡；而彈性更好的做法，是讓State的子類別決定自己的下一個State是誰、何時才做轉移。Context需增加一個讓State物件可以設定Context的State的方法。只要定義新的State子類別，就能修改或擴充狀態轉移邏輯。但缺點是State子類別需要知道至少一個其他的State子類別，增加子類別之間的實作相依性。

State物件的生與死？#

取捨：有需要時才產生State物件，用完即砍。事先件立好，事後絕不殺掉。若在Run-time時不知道處於哪些狀態，且Context不常改變狀態時，第一種做法較好。若State物件含有大量資訊，且狀態常常改變，就會想避免一再刪除State物件，就該用第二種做法。

使用動態繼承#

若能在Run-time動態改變物件所隸屬的類別，就能針對特定訊息改變自身行為。

Example: Cruise Control System#

1
public abstract class State {
2
  State[] states;
3

4
  abstract void leverDown(CruiseControl c);
5
  abstract void leverUpAndHold(CruiseControl c);
6
  abstract void leverDownAndHold(CruiseControl c);
7
  abstract void leverReleased(CruiseControl c);
8
  abstract void leverPulled(CruiseControl c);
9
  abstract void brakeApplied(CruiseControl c);
10
  abstract void leverUp(CruiseControl c);
11
  abstract void onOffButtonPressed(CruiseControl c);
12

13
  void setDesiredSpeed(CruiseControl c) {
14
    System.out.println("setDesiredSpeed");
15
  }
16
}

1
public class CruiseDeactivated extends State {
2
  @Override
3
  void onOffButtonPressed(CruiseControl c) {
4
    // TODO Auto-generated method stub
5
    System.out.println("CruiseDeactivated change to CruiseActivated");
6
    System.out.println("CruiseDeactivated change to CruisingCancelled");
7
    c.setState(new CruisingCancelled());
8
  }
9
  @Override
10
  void leverDown(CruiseControl c) {}
11
  @Override
12
  void leverUp(CruiseControl c) {}
13
  @Override
14
  void brakeApplied(CruiseControl c) {}
15
  @Override
16
  void leverDownAndHold(CruiseControl c) {}
17
  @Override
18
  void leverUpAndHold(CruiseControl c) {}
19
  @Override
20
  void leverPulled(CruiseControl c) {}
21
  @Override
22
  void leverReleased(CruiseControl c) {}
23
  @Override
24
  void setDesiredSpeed(CruiseControl c) {}
25
}

1
public class CruiseActivated extends State {
2

3
  @Override
4
  void onOffButtonPressed(CruiseControl c) {
5
    // TODO Auto-generated method stub
6
    System.out.println("CruiseActivated change to CruiseDeactivated");
7
    c.setState(new CruiseDeactivated());
8
  }
9
  @Override
10
  void leverDown(CruiseControl c) {}
11
  @Override
12
  void leverUp(CruiseControl c) {}
13
  @Override
14
  void brakeApplied(CruiseControl c) {}
15
  @Override
16
  void leverDownAndHold(CruiseControl c) {}
17
  @Override
18
  void leverUpAndHold(CruiseControl c) {}
19
  @Override
20
  void leverPulled(CruiseControl c) {}
21
  @Override
22
  void leverReleased(CruiseControl c) {}
23
}

1
public class Cruising extends CruiseActivated {
2
  @Override
3
  void leverDown(CruiseControl c) {
4
    System.out.println("Cruising change to Cruising");
5
    setDesiredSpeed(c);
6
    c.setState(new Cruising());
7
  }
8
  @Override
9
  void brakeApplied(CruiseControl c) {
10
    System.out.println("Cruising change to CruisingCancelled");
11
    c.setState(new CruisingCancelled());
12
  }
13
  @Override
14
  void leverDownAndHold(CruiseControl c) {
15
    System.out.println("Cruising change to DecreasingSpeed");
16
    c.setState(new DecreasingSpeed());
17
  }
18

19
  @Override
20
  void leverUpAndHold(CruiseControl c) {
21
    System.out.println("Cruising change to IncreasingSpeed");
22
    c.setState(new IncreasingSpeed());
23
  }
24

25
  @Override
26
  void leverPulled(CruiseControl c) {
27
    System.out.println("Cruising change to CruisingCancelled");
28
    c.setState(new CruisingCancelled());
29
  }
30
}

1
public class CruisingCancelled extends CruiseActivated {
2
  @Override
3
  void leverDown(CruiseControl c) {
4
    System.out.println("CruisingCancelled change to Cruising");
5
    setDesiredSpeed(c);
6
    c.setState(new Cruising());
7
  }
8
  @Override
9
  void leverUp(CruiseControl c) {
10
    System.out.println("CruisingCancelled change to Cruising");
11
    c.setState(new Cruising());
12
  }
13
  @Override
14
  void leverDownAndHold(CruiseControl c) {
15
  System.out.println("CruisingCancelled change to DecreasingSpeed");
16
    c.setState(new DecreasingSpeed());
17
  }
18
  @Override
19
  void leverUpAndHold(CruiseControl c) {
20
  System.out.println("CruisingCancelled change to IncreasingSpeed");
21
    c.setState(new IncreasingSpeed());
22
  }
23
}

1
public class DecreasingSpeed extends CruiseActivated{
2
    @Override
3
  void leverReleased(CruiseControl c){
4
    System.out.println("DecreasingSpeed change to Cruising");
5
    setDesiredSpeed(c);
6
    c.setState(new Cruising());
7
  }
8
}
9

10
public class IncreasingSpeed extends CruiseActivated{
11
  void leverReleased(CruiseControl c){
12
    System.out.println("IncreasingSpeed change to Cruising");
13
    c.setState(new Cruising());
14
  }
15
}

1
public class CruiseControl {
2
  State state;
3
  public CruiseControl() {
4
    // TODO Auto-generated constructor stub
5
    this.state = new CruiseDeactivated();
6
  }
7
  public State getState() {
8
    return state;
9
  }
10
  public void setState(State state) {
11
    this.state = state;
12
  }
13
  void onOffButtonPressed() {
14
    state.onOffButtonPressed(this);
15
  }
16
  void leverDown() {
17
    state.leverDown(this);
18
  }
19
  void leverUp() {
20
    state.leverUp(this);
21
  }
22
  void brakeApplied() {
23
    state.brakeApplied(this);
24
  }
25
  void leverDownAndHold() {
26
    state.leverDownAndHold(this);
27
  }
28
  void leverUpAndHold() {
29
    state.leverUpAndHold(this);
30
  }
31
  void leverPulled() {
32
    state.leverPulled(this);
33
  }
34
  void leverReleased() {
35
    state.leverReleased(this);
36
  }
37
}

1
public class Main {
2

3
  public static void main(String[] args) {
4
    // TODO Auto-generated method stub
5
    CruiseControl cruiseControl = new CruiseControl();
6
    cruiseControl.onOffButtonPressed();
7
    cruiseControl.leverDown();
8
    cruiseControl.onOffButtonPressed();
9
  }
10

11
}

1
Result:
2

3
CruiseDeactivated change to CruiseActivated
4
CruiseDeactivated change to CruisingCancelled
5
CruisingCancelled change to Cruising
6
setDesiredSpeed
7
CruiseActivated change to CruiseDeactivated

目的#

適用#