state

State

State Pattern allows the object to alter its behavior when its internal state changes.

Code

Process has a current ProcessState and, when a state-changing action happens to the process, it forwards the call the current ProcessState. The ProcessState - implemented by RunningProcessState, StoppedProcessState, and StoppedProcessState - moves sets the current state of Process to the next expected one.

Diagram

Note

Extensibility

The example code contains 3 states and 3 actions. So, there are 9 combinations of <current state, action>. It's possible to write in a single file or class that makes all the decision, but with the design in this example, we exact the decision to each process state class, which is easy to maintain and scale.

Encapsulation

Process::SetState is a public method in the code so the implementations of ProcessState can use that to set the current state to the next; however, it exposes the availability for callers to mutates the state directly without gaurd. I think this part can be improved.

Reference

• https://en.wikipedia.org/wiki/State_pattern
• https://www.robertlarsononline.com/2017/05/11/state-pattern-using-cplusplus/
• https://stackoverflow.com/questions/32427684/what-are-the-possible-states-for-a-docker-container
• https://google.aip.dev/216