Unit 4- State Machine in mobile programming
- 1. UNIT 4: UNDERSTANDING COMMUNICATIONS VIA NETWORK AND WEB State Machine
- 2. A state machine • A state machine defines an application process flow at runtime. When you activate the application, the process flow is converted to a state machine. • A state machine main purpose is reading a series of inputs and switching to different states after reading a new input.
- 3. Example: State machine • Fetching data from an endpoint. • The program can have 3 states: loading, success, and error. • A state machine also has a starting state, which coincidentally here is named “not started” but you can name it anything you want. • Here, the starting state indicates that the API is not called yet. • When a user opens a screen, the screen begins fetching the data, so the state moves to “loading”. • If the API call returns success/error, the state moves to SUCCESS/ERROR respectively. • After that, if the user refreshes the screen, the state goes back to LOADING. • We could call the annotation “fetch”, “error”, “success”, “refresh” an event or input. • The transition function receives the current state and an input and determines the next state.
- 4. Example: State machine of a Washer Dryer • We want this machine to have 4 distinct states that we go from one to the next in this sequence: 1. Initial State – Where we are when we are waiting to start the wash 2. Wash – Where we wash with soap and water 3. Dry – Where we dry the clothes 4. Done
- 5. State Diagram for Washer-Dryer Machine
- 6. State Diagram for Washer-Dryer Machine… 1. We start at an “Initial” state whenever we start up the machine • Let’s also assume this stage is when you’d put in the soap and clothes • Once input “Coin is in” is 1, GTNS is now • This event triggers leaving the current state to go to the next state
- 7. State Diagram for Washer-Dryer Machine… 2. This is followed by the next state, “Wash” • “Coin inserted” is now 0 at this point (so GTNS goes back to 0) • While soap is still present, GTNS goes back to 0 – When the input “Soap is no longer present” goes to 1, GTNS goes to 1 • This event triggers leaving the current state to go to the next state
- 8. State Diagram for Washer-Dryer Machine… 3. This is followed by the next state, “Dry” • This new state sets an output that triggers a timer • The input “Soap is no longer present” goes to 0, so GTNS is 0 also • While the input “Clothes are now dry” is 0 , GTNS remains at 0 too • When the input “Clothes are now dry” is 1, GTNS changes to 1 • This event triggers leaving the current state to go to the next state
- 9. 4. This is followed by the next and last state, “Done” • When you’re here, you go back to the “initial” state • No inputs to consider: you do move this regardless
- 10. Components of a State Machine • A state machine consists of the following components: • State: basically a status of the machine. While being in a state, a machine waits for the event that will trigger a transition to a different state. • Event: the actual inputs that the machine reads. If we speak about a mobile application as an example, an event would be an action performed by the user. • Transition: the movement of the machine from one state to another after a certain event occurs.
- 11. States of a state machine • States are elements of a state-machine system. • An application usually has many states, and can include different types of states. • Each state has a previous state and a following state, unless it is the initial state or the final state. • There can be only one initial state, but, depending on user interaction, there can be many final states.
- 12. initial state • An initial state is the first state in an application, and only handles state transitions to follow-up states, based on transition rules. • The initial-state is Start Application, which is created automatically when you create an application, and cannot be deleted. • By default, the name of the initial state is the same as the name of the application.
- 13. Finite State Machine/Synchronous state machine • A finite state machine is an abstract machine that can be in exactly one of a finite number of states at any given time. • Finite State Machine is a state machine, with a finite number of states. • It can be in one state at a time, and can transition to a different state once something happens. • A finite state machine must have: • an initial state which provides a starting point, and • a current state which remembers the product of the last state transition.
- 14. Example: Finite State Machine Figure: state diagram
- 15. Class Activity • Write a state transition table for the state machine diagram below
- 16. Class Activity • Write a state transition table for the state machine diagram below
- 17. Two types/models of FSM • Moore machine: its output is a function of present state only • Mealy machine: its output is a function of present state and present input
- 18. its output is a function of present state only
- 19. its output is a function of present state and present input
- 20. Comparison of Mealy and Moore FSM
- 21. Building a state machine for Android app • Step 1: Define the states • Step 2: Define the actions • Step 3: Connect the events with actions • Step 4: Write a bit of code • Step 5: Implement the states and build the activity • Example: • EventList+ AddEventClicked = AddEvent • AddEvent + EventSelected = NameEvent • NameEvent+ SubmitEventClicked = EventList
- 22. The benefits of using state machine pattern in your mobile application • Allows developers to focus only on the configuration of states and transitions without digging into the “how” aspect of the process. • once a developer configures the needed states and the conditions for the transitions, the work is done and the machine will function in accordance with the set conditions. • Ensures a high level of predictability and eliminates the possibility of any malfunctions since every action is predefined. • It allows developers to visualize the work and makes the whole system more transparent. • Elimination of hard coding conditions in the code • Easy tracking of the processes due to the finite number of states and machine’s predictability • An option to isolate certain pieces of code due to the independency of every action • High stability of the system.
- 23. Disadvantages of using state machine pattern in your mobile application • Not very suitable for asynchronous execution • Possibility to have complex code if several transitions are executed from one state • Limited support from the community • Tricky data management and storage • A possibility to obtain your own persistence layer in order to load balance properly
- 24. You can look at feather explanation on state machine