State machine design

1. Code example of basic state machine

1.1. Define the state interface (State): Define all states that the object may have, and declare the operations that each state may perform.

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// State.h
#import <Foundation/Foundation.h>

@protocol State <NSObject>

- (void)handleStateWithContext:(id)context;

@end

1.2. Implement concrete state class (ConcreteState): implement specific behavioral logic for each state.

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// ConcreteStateA.h
#import "State.h"

@interface ConcreteStateA : NSObject <State>

@end

// ConcreteStateA.m
#import "ConcreteStateA.h"

@implementation ConcreteStateA

- (void)handleStateWithContext:(id)context {
    NSLog(@"Handling state A...");
    // Perform actions specific to state A
}

@end

// ConcreteStateB.h
#import "State.h"

@interface ConcreteStateB : NSObject <State>

@end

// ConcreteStateB.m
#import "ConcreteStateB.h"

@implementation ConcreteStateB

- (void)handleStateWithContext:(id)context {
    NSLog(@"Handling state B...");
    // Perform actions specific to state B
}

@end

1.3. Define the Context class : The Context class holds the current state and transfers between states.

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// Context.h
#import <Foundation/Foundation.h>
#import "State.h"

@interface Context : NSObject

@property (nonatomic, strong) id<State> currentState;

- (void)transitionToState:(id<State>)state;

@end

// Context.m
#import "Context.h"

@implementation Context

- (void)transitionToState:(id<State>)state {
    NSLog(@"Transitioning to new state...");
    self.currentState = state;
}

- (void)request {
    [self.currentState handleStateWithContext:self];
}

@end

1.4. Use the state mode : Create a Context object in the client code and set the initial state, and then call the object’s method to perform the operation.

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// main.m
#import <Foundation/Foundation.h>
#import "Context.h"
#import "ConcreteStateA.h"
#import "ConcreteStateB.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // 
        Context *context = [[Context alloc] init];
        context.currentState = [[ConcreteStateA alloc] init];
        
        // 
        [context request];
        
        // 
        [context transitionToState:[[ConcreteStateB alloc] init]];
        
        // 
        [context request];
    }
    return 0;
}

In this way, a simple state pattern is implemented. When the request method of the Context object is called, the corresponding specific state class will be called to handle the behavior based on the current state. When you need to switch states, you can call the transitionToState method of the Context object to transfer the state.

2. The use of state machine mode in the classic library TransitionKit

There are also some code libraries in Objective-C that use the state machine design pattern. One of the famous libraries is TransitionKitthat it is a lightweight state machine library specially used to manage the state transitions of objects. TransitionKitAllows developers to define states and transition rules between states, and provides a simple API to trigger state transitions. This makes implementing complex state management in Objective-C applications much simpler and more maintainable.

2.1. Analyze the specific scenarios of using state machines in TransitionKit:

TransitionKit can be used in a variety of practical applications, some of which include:

1. User workflow management : For example, manage the user’s login and logout status in the application, as well as the operations that can be performed in different states, such as jumping to the homepage after logging in, returning to the login interface after logging out, etc.
2. Order status management : In e-commerce applications, manage the different statuses of orders (for example, pending payment, paid, shipped, completed, etc.), and trigger corresponding business logic when the status changes.
3. Game development : Used to manage the status of game characters (e.g., standing, walking, jumping, attacking, etc.) and trigger state transitions based on game events (e.g., pressing different buttons or encountering different enemies).
4. Process control : Manage complex business processes in applications, such as approval processes, workflows, etc., to ensure correct state transitions and process execution.
5. Device status management : In embedded systems or IoT applications, manage the different statuses of devices (for example, online, offline, fault, maintenance, etc.) and perform corresponding operations based on status changes.

In general, TransitionKit can be used in any scenario that requires managing states and state transitions, helping developers manage complex state logic more clearly and maintainably.

2.2. TransitionKit core code logic

The core code logic of TransitionKit includes the following aspects:

1. State management : TransitionKit provides state management functions, allowing the definition of different states and the transition relationships between states. This is usually achieved by defining a state machine, which includes the definition of states and state transition rules.
2. State transition rules : TransitionKit defines rules for state transitions, including transitioning from one state to another under specific conditions. These rules can be triggered based on events (such as user operations, system events, etc.) or conditional judgments (such as time conditions, data conditions, etc.).
3. State transition execution : When a state transition condition is triggered, TransitionKit will execute the corresponding state transition logic. This may involve performing specific actions, updating an object’s state information, triggering event notifications, etc.
4. Event processing : TransitionKit usually defines event processing logic to handle events generated during state transition. This may include triggering callback functions, sending notifications, updating the interface, etc.
5. Error handling : TransitionKit usually includes an error handling mechanism to handle error conditions that may occur during state transition. This includes verifying the legality of state transitions, handling invalid operations, recording error logs, etc.
6. Scalability and flexibility : TransitionKit is designed to be flexible, allowing developers to extend and customize state management and transition logic according to specific needs. This includes defining custom state types, adding new state transition rules, custom event handling logic, etc.

In general, the core code logic of TransitionKit revolves around state management, state transition rule definition, state transition execution and event processing to achieve management and control of complex state logic.

2.3. Write the core code example of TransitionKit

TransitionKit is a state machine library that allows you to define states and transition rules between states, and provides functionality to perform state transitions. The following is a simple Objective-C example that demonstrates TransitionKit’s core code logic:

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#import <Foundation/Foundation.h>
#import "TransitionKit.h"

// 
typedef NS_ENUM(NSInteger, State) {
    StateIdle,
    StateActive,
    StatePaused,
    StateStopped
};

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // 
        TKStateMachine *stateMachine = [TKStateMachine new];
        
        // 
        TKState *idleState = [TKState stateWithName:@"Idle"];
        TKState *activeState = [TKState stateWithName:@"Active"];
        TKState *pausedState = [TKState stateWithName:@"Paused"];
        TKState *stoppedState = [TKState stateWithName:@"Stopped"];
        
        // 
        [stateMachine addStates:@[idleState, activeState, pausedState, stoppedState]];
        
        // 
        [stateMachine addTransition:[TKTransition transitionFromState:idleState
                                                             toState:activeState
                                                             onEvent:@"Start"]];
        [stateMachine addTransition:[TKTransition transitionFromStates:@[activeState, pausedState]
                                                              toState:stoppedState
                                                              onEvent:@"Stop"]];
        [stateMachine addTransition:[TKTransition transitionFromState:activeState
                                                             toState:pausedState
                                                             onEvent:@"Pause"]];
        [stateMachine addTransition:[TKTransition transitionFromState:pausedState
                                                             toState:activeState
                                                             onEvent:@"Resume"]];
        
        // 
        [stateMachine activate];
        
        // 
        [stateMachine fireEvent:@"Start"];
        [stateMachine fireEvent:@"Pause"];
        [stateMachine fireEvent:@"Resume"];
        [stateMachine fireEvent:@"Stop"];
        
        // 
        NSLog(@"Current state: %@", stateMachine.currentState.name);
    }
    return 0;
}

Pseudocode of fireEvent

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- (void)fireEvent:(NSString *)event {
    // 
    State *currentState = self.currentState;
    
    // 
    NSArray *transitions = [self.transitions objectForKey:currentState];
    
    // 
    for (Transition *transition in transitions) {
        if ([transition.event isEqualToString:event]) {
            // 
            [self transitionToState:transition.toState];
            return; // 
        }
    }
    
    // 
    NSLog(@"No transition defined for event %@", event);
}

- (void)transitionToState:(State *)state {
    // 
    NSLog(@"Transitioning to state %@", state);
    self.currentState = state;
}

3. Comparative analysis of state machine mode and strategy mode

In the state machine mode, the behavior of the object will change according to the change of the internal state, which is similar to switching between different states; while in the strategy mode, the behavior of the object will change according to the external policy object passed in, similar to the switching between different states. Switching between strategies.

Core focus: Whether the object’s behavior is imported from the outside or converted from the inside;

3.1. State machine mode pseudocode example:

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// 
interface State {
    handleInput(input) // 
}

// 
class StateA implements State {
    handleInput(input) {
        if (input == "EventX") {
            //
            transitionToState(StateB) // 
        }
    }
}

class StateB implements State {
    handleInput(input) {
        if (input == "EventY") {
            // 
            transitionToState(StateC) // 
        }
    }
}

class StateC implements State {
    handleInput(input) {
        if (input == "EventZ") {
            // 
            transitionToState(StateA) // 
        }
    }
}

// 
class StateMachine {
    currentState // 

    handleInput(input) {
        currentState.handleInput(input) // 
    }

    transitionToState(newState) {
        currentState = newState // 
    }
}

// 
stateMachine = new StateMachine(StateA) // 
stateMachine.handleInput("EventX") // 

3.2. Strategy pattern pseudocode example:

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// 
interface Strategy {
    execute() // 
}

// 
class ConcreteStrategyA implements Strategy {
    execute() {
        // 
    }
}

class ConcreteStrategyB implements Strategy {
    execute() {
        // 
    }
}

class ConcreteStrategyC implements Strategy {
    execute() {
        // 
    }
}

// 
class Context {
    strategy // 

    setStrategy(strategy) {
        this.strategy = strategy // 
    }

    executeStrategy() {
        strategy.execute() // 
    }
}

// 
context = new Context()
context.setStrategy(new ConcreteStrategyA()) // 
context.executeStrategy() //