Implementing Plugin Systems with UnknownAction

Explain how to implement a plugin system in Redux where plugins can declare how to handle UnknownAction.

const unknownActionHandler = (state = {}, action) => {
    // Default case for unhandled actions
    return state;
};

This is a reducer function that acts as a default case for handling unknown actions. It should be used as a fallback in the Redux store. It takes the current state and the action as arguments and simply returns the state unchanged if the action is not recognized.

const applyMiddlewareWithPlugins = (store) => {
    const originalDispatch = store.dispatch;
    store.dispatch = (action) => {
        // Check if the action type is not known
        if (action.type === 'UnknownAction') {
            // Allow plugins to handle the action
            pluginSystem.handle(action);
        } else {
            // If the action is known, proceed as normal
            originalDispatch(action);
        }
    };
    return store;
};

This function enhances the store's `dispatch` method to allow plugins to handle unknown actions. It wraps the original dispatch method so that if an 'UnknownAction' is encountered, the plugin system gets a chance to handle the action before the default dispatch behavior.

const pluginSystem = {
    plugins: [],

    register: function(plugin) {
        this.plugins.push(plugin);
    },

    handle: function(action) {
        this.plugins.forEach(plugin => {
            if (plugin.canHandle(action.type)) {
                plugin.handle(action);
            }
        });
    }
};

This object represents a simple plugin system. Plugins can register themselves with the system. The handle function is called when the store dispatches an unknown action. Each plugin gets a chance to check if it can handle the action and act accordingly.

const examplePlugin = {
    canHandle: function(actionType) {
        // Determine if this plugin can handle the action
        // based on the action type
        return actionType === 'SomeSpecificUnknownAction';
    },
    handle: function(action) {
        // Handle the action here
        console.log('Action handled by examplePlugin:', action);
    }
};

// Register the plugin with the plugin system
pluginSystem.register(examplePlugin);

This is a sample plugin that can handle a specific action type. The `canHandle` function checks if the action type matches 'SomeSpecificUnknownAction'. If it does, the plugin's `handle` function will log the action to the console. The plugin is registered with the plugin system using the `register` function.

<html>
<head>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/redux/4.0.5/redux.min.js"></script>
    <script>
        // Include all the above JavaScript code here

        // Create Redux store
        const store = Redux.createStore(unknownActionHandler);
        
        // Apply the middleware to enable the plugin system
        applyMiddlewareWithPlugins(store);
    </script>
</head>
<body>
    <!-- HTML content here -->
</body>
</html>

This HTML snippet includes the Redux library and sets up the Redux store with the `unknownActionHandler` as the root reducer. The `applyMiddlewareWithPlugins` function is then used to enhance the store with the plugin system. HTML content can be added to the body as needed for the application.