State pattern

The state pattern, which closely resembles Strategy Pattern, is a behavioral software design pattern, also known as the objects for states pattern. This pattern is used in computer programming to represent the state of an object. This is a clean way for an object to partially change its type at runtime.

Structure

State in UML

State in LePUS3

Example

Pseudocode

Take, for example, a drawing program. The program has a mouse cursor, which at any point in time can act as one of several tools. Instead of switching between multiple cursor objects, the cursor maintains an internal state representing the tool currently in use. When a tool-dependent method is called (say, as a result of a mouse click), the method call is passed on to the cursor's state.

Each tool corresponds to a state. The shared abstract state class is AbstractTool:

 class AbstractTool is
     function moveTo(point) is
         input:  the location point the mouse moved to
         (this function must be implemented by subclasses)
 
     function mouseDown(point) is
         input:  the location point the mouse is at
         (this function must be implemented by subclasses)
 
     function mouseUp(point) is
         input:  the location point the mouse is at
         (this function must be implemented by subclasses)

According to this definition, each tool must handle movement of the mouse cursor and also the start and end of any click or drag.

Using that base class, simple pen and selection tools could look like this:

 subclass PenTool of AbstractTool is
     last_mouse_position := invalid
     mouse_button := up
 
     function moveTo(point) is
         input:  the location point the mouse moved to
         if mouse_button = down
             (draw a line from the last_mouse_position to point)
             last_mouse_position := point
 
     function mouseDown(point) is
         input:  the location point the mouse is at
         mouse_button := down
         last_mouse_position := point
 
     function mouseUp(point) is
         input:  the location point the mouse is at
         mouse_button := up
 
 
 subclass SelectionTool of AbstractTool is
     selection_start := invalid
     mouse_button := up
 
     function moveTo(point) is
         input:  the location point the mouse moved to
         if mouse_button = down
             (select the rectangle between selection_start and point)
 
     function mouseDown(point) is
         input:  the location point the mouse is at
         mouse_button := down
         selection_start := point
 
     function mouseUp(point) is
         input:  the location point the mouse is at
         mouse_button := up

For this example, the class for the context is called Cursor. The methods named in the abstract state class (AbstractTool in this case) are also implemented in the context. In the context class, these methods invoke the corresponding method of the current state, represented by current_tool.

 class Cursor is
     current_tool := new PenTool
 
     function moveTo(point) is
         input:  the location point the mouse moved to
         current_tool.moveTo(point)
 
     function mouseDown(point) is
         input:  the location point the mouse is at
         current_tool.mouseDown(point)
 
     function mouseUp(point) is
         input:  the location point the mouse is at
         current_tool.mouseUp(point)
 
     function usePenTool() is
         current_tool := new PenTool
 
     function useSelectionTool() is
         current_tool := new SelectionTool

Notice how one Cursor object can act both as a PenTool and a SelectionTool at different points, by passing the appropriate method calls on to whichever tool is active. That is the essence of the state pattern. In this case, we could have combined state and object by creating PenCursor and SelectCursor classes, thus reducing the solution to simple inheritance, but in practice, Cursor may carry data which is expensive or inelegant to copy to a new object whenever a new tool is selected.

Java

The state interface and two implementations. The state's method has a reference to the context object and is able to change its state.

interface State { 
        void writeName(StateContext stateContext, String name);
} 
 
class StateA implements State { 
        public void writeName(StateContext stateContext, String name) { 
                System.out.println(name.toLowerCase()); 
                stateContext.setState(new StateB()); 
        } 
} 
 
class StateB implements State { 
        private int count=0; 
        public void writeName(StateContext stateContext, String name){ 
                System.out.println(name.toUpperCase()); 
                // change state after StateB's writeName() gets invoked twice
                if(++count>1) { 
                        stateContext.setState(new StateA()); 
                }
        }
}

The context class has a state variable which it instantiates in an initial state, in this case StateA. In its method, it uses the corresponding methods of the state object.

public class StateContext {
        private State myState; 
        public StateContext() { 
                setState(new StateA()); 
        } 
 
        // normally only called by classes implementing the State interface
        public void setState(State newState) { 
                this.myState = newState; 
        }
 
        public void writeName(String name) { 
                this.myState.writeName(this, name); 
        } 
}

And the usage:

public class TestClientState { 
        public static void main(String[] args) { 
                StateContext sc = new StateContext(); 
                sc.writeName("Monday"); 
                sc.writeName("Tuesday"); 
                sc.writeName("Wednesday"); 
                sc.writeName("Thursday"); 
                sc.writeName("Saturday"); 
                sc.writeName("Sunday"); 
        }
}

According to the above code, the output of main() from TestClientState should be:

monday
TUESDAY
WEDNESDAY
thursday
SATURDAY
SUNDAY

See also

• Finite State Machine
• Strategy pattern

External links

• State Design Pattern