Developing offline applications using HTML5

Author：Chuan Chen 阅读数：26473人阅读分类： HTML

Basic Concepts of HTML5 Offline Applications

HTML5 provides a complete set of offline application solutions, enabling web applications to continue running without an internet connection. This solution primarily includes the following core technologies:

Application Cache
LocalStorage and SessionStorage
IndexedDB
Service Worker (a more modern alternative)

Offline applications are particularly suitable for the following scenarios:

Mobile web applications
Pages that require fast loading
Environments with unstable network connections
Applications that need to reduce server requests

Using Application Cache

Application Cache is the earliest offline solution provided in HTML5, which uses a manifest file to specify resources to be stored offline.

Example manifest file (cache.manifest):

CACHE MANIFEST
# v1.0.0

CACHE:
/css/style.css
/js/app.js
/images/logo.png
/index.html

NETWORK:
/api/

FALLBACK:
/ /offline.html

Referencing the manifest file in HTML:

<!DOCTYPE html>
<html manifest="cache.manifest">
<head>
    <title>Offline Application Example</title>
    <link rel="stylesheet" href="/css/style.css">
</head>
<body>
    <!-- Page content -->
    <script src="/js/app.js"></script>
</body>
</html>

JavaScript listening for cache events:

window.applicationCache.addEventListener('updateready', function(e) {
    if (window.applicationCache.status == window.applicationCache.UPDATEREADY) {
        // Cache update complete, prompt user to refresh
        if (confirm('A new version has been downloaded. Refresh now?')) {
            window.location.reload();
        }
    }
}, false);

LocalStorage and SessionStorage

Web Storage provides simple key-value pair storage, suitable for storing small amounts of data.

LocalStorage example:

// Store data
localStorage.setItem('username', 'John');
localStorage.setItem('userSettings', JSON.stringify({
    theme: 'dark',
    fontSize: 14
}));

// Retrieve data
const username = localStorage.getItem('username');
const settings = JSON.parse(localStorage.getItem('userSettings'));

// Delete data
localStorage.removeItem('username');

// Clear all data
localStorage.clear();

Difference between SessionStorage and LocalStorage:

// SessionStorage is only valid for the current session
sessionStorage.setItem('tempData', 'Cleared when session ends');

// Still exists after page refresh
console.log(sessionStorage.getItem('tempData'));

// Data disappears after closing the tab

Advanced Storage with IndexedDB

For applications that need to store large amounts of structured data, IndexedDB provides a more powerful solution.

IndexedDB basic operations example:

// Open or create a database
const request = indexedDB.open('myDatabase', 1);

request.onupgradeneeded = function(event) {
    const db = event.target.result;
    
    // Create an object store (similar to a table)
    const store = db.createObjectStore('customers', {
        keyPath: 'id',
        autoIncrement: true
    });
    
    // Create indexes
    store.createIndex('name', 'name', { unique: false });
    store.createIndex('email', 'email', { unique: true });
};

request.onsuccess = function(event) {
    const db = event.target.result;
    
    // Add data
    const transaction = db.transaction(['customers'], 'readwrite');
    const store = transaction.objectStore('customers');
    
    const customer = {
        name: 'Jane Doe',
        email: 'jane@example.com',
        phone: '13800138000'
    };
    
    const addRequest = store.add(customer);
    
    addRequest.onsuccess = function() {
        console.log('Data added successfully');
    };
    
    // Query data
    const getRequest = store.get(1);
    
    getRequest.onsuccess = function() {
        console.log('Query result:', getRequest.result);
    };
};

request.onerror = function(event) {
    console.error('Database error:', event.target.error);
};

Modern Offline Solution with Service Worker

Service Worker is a more modern offline solution that can intercept network requests and provide finer-grained cache control.

Service Worker registration:

// Register Service Worker in the main thread
if ('serviceWorker' in navigator) {
    navigator.serviceWorker.register('/sw.js')
        .then(registration => {
            console.log('ServiceWorker registered successfully:', registration.scope);
        })
        .catch(error => {
            console.log('ServiceWorker registration failed:', error);
        });
}

Service Worker script example (sw.js):

const CACHE_NAME = 'my-app-cache-v1';
const urlsToCache = [
    '/',
    '/index.html',
    '/css/style.css',
    '/js/app.js',
    '/images/logo.png'
];

// Installation phase
self.addEventListener('install', event => {
    event.waitUntil(
        caches.open(CACHE_NAME)
            .then(cache => {
                console.log('Cache opened');
                return cache.addAll(urlsToCache);
            })
    );
});

// Activation phase
self.addEventListener('activate', event => {
    const cacheWhitelist = [CACHE_NAME];
    event.waitUntil(
        caches.keys().then(cacheNames => {
            return Promise.all(
                cacheNames.map(cacheName => {
                    if (!cacheWhitelist.includes(cacheName)) {
                        return caches.delete(cacheName);
                    }
                })
            );
        })
    );
});

// Intercept requests
self.addEventListener('fetch', event => {
    event.respondWith(
        caches.match(event.request)
            .then(response => {
                // Return cached response if available, otherwise fetch from network
                return response || fetch(event.request);
            })
    );
});

Data Synchronization Strategy for Offline Applications

When the application regains online status, it needs to handle data changes made during the offline period.

Data synchronization example:

// Check network status
function checkOnlineStatus() {
    return navigator.onLine;
}

// Store data pending synchronization
function queueForSync(data) {
    const pendingSyncs = JSON.parse(localStorage.getItem('pendingSyncs') || '[]');
    pendingSyncs.push(data);
    localStorage.setItem('pendingSyncs', JSON.stringify(pendingSyncs));
}

// Attempt to synchronize data
function trySync() {
    if (!checkOnlineStatus()) return;
    
    const pendingSyncs = JSON.parse(localStorage.getItem('pendingSyncs') || '[]');
    if (pendingSyncs.length === 0) return;
    
    // Simulate API call
    pendingSyncs.forEach(data => {
        fetch('/api/sync', {
            method: 'POST',
            headers: {
                'Content-Type': 'application/json'
            },
            body: JSON.stringify(data)
        })
        .then(response => {
            if (response.ok) {
                // Synchronization successful, remove from queue
                const index = pendingSyncs.indexOf(data);
                if (index > -1) {
                    pendingSyncs.splice(index, 1);
                    localStorage.setItem('pendingSyncs', JSON.stringify(pendingSyncs));
                }
            }
        });
    });
}

// Listen for network status changes
window.addEventListener('online', trySync);

User Experience Optimization for Offline Applications

A good user experience is crucial for offline applications.

Offline status detection and notification:

// Detect network status and display notification
function updateOnlineStatus() {
    const statusElement = document.getElementById('network-status');
    
    if (navigator.onLine) {
        statusElement.textContent = 'Online';
        statusElement.className = 'online';
    } else {
        statusElement.textContent = 'Offline - Using local cache';
        statusElement.className = 'offline';
    }
}

// Initial detection
updateOnlineStatus();

// Listen for network status changes
window.addEventListener('online', updateOnlineStatus);
window.addEventListener('offline', updateOnlineStatus);

CSS styling example:

#network-status {
    position: fixed;
    bottom: 10px;
    right: 10px;
    padding: 5px 10px;
    border-radius: 3px;
    font-size: 12px;
}

.online {
    background-color: #4CAF50;
    color: white;
}

.offline {
    background-color: #f44336;
    color: white;
}

Performance Optimization and Caching Strategies

Reasonable caching strategies can significantly improve the performance of offline applications.

Caching strategy example:

// Advanced caching strategy in Service Worker
self.addEventListener('fetch', event => {
    event.respondWith(
        caches.match(event.request)
            .then(response => {
                // Cache-first, network fallback
                return response || fetch(event.request)
                    .then(response => {
                        // Cache responses for GET requests
                        if (event.request.method === 'GET') {
                            const responseToCache = response.clone();
                            caches.open(CACHE_NAME)
                                .then(cache => {
                                    cache.put(event.request, responseToCache);
                                });
                        }
                        return response;
                    });
            })
            .catch(() => {
                // For HTML documents, return offline page
                if (event.request.headers.get('accept').includes('text/html')) {
                    return caches.match('/offline.html');
                }
            })
    );
});

Cache version control:

// Using version control in Service Worker
const CACHE_VERSION = 'v2';
const CACHE_NAME = `my-app-cache-${CACHE_VERSION}`;

self.addEventListener('activate', event => {
    event.waitUntil(
        caches.keys().then(cacheNames => {
            return Promise.all(
                cacheNames.map(cacheName => {
                    if (cacheName.startsWith('my-app-cache-') && 
                        cacheName !== CACHE_NAME) {
                        return caches.delete(cacheName);
                    }
                })
            );
        })
    );
});

Debugging and Testing Offline Applications

Debugging and testing are essential when developing offline applications.

Chrome Developer Tools tips:

Use the Application panel to view cached content
Use the Network panel to simulate offline status
Use Lighthouse for PWA audits

Automated testing example:

// Testing Service Worker with Jest
describe('Service Worker', () => {
    beforeAll(() => {
        // Mock Service Worker environment
        global.self = {
            addEventListener: jest.fn(),
            caches: {
                open: jest.fn(() => Promise.resolve({
                    addAll: jest.fn(),
                    put: jest.fn(),
                    match: jest.fn()
                })),
                keys: jest.fn(),
                delete: jest.fn(),
                match: jest.fn()
            },
            registration: {
                waitUntil: jest.fn()
            }
        };
        
        require('../sw.js');
    });
    
    it('should listen for install event', () => {
        expect(self.addEventListener).toHaveBeenCalledWith(
            'install', expect.any(Function)
        );
    });
    
    it('should listen for activate event', () => {
        expect(self.addEventListener).toHaveBeenCalledWith(
            'activate', expect.any(Function)
        );
    });
    
    it('should listen for fetch event', () => {
        expect(self.addEventListener).toHaveBeenCalledWith(
            'fetch', expect.any(Function)
        );
    });
});

Case Study of a Practical Application

Taking a simple offline notes application as an example to demonstrate a complete implementation.

HTML structure:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Offline Notes App</title>
    <style>
        body { font-family: Arial, sans-serif; max-width: 800px; margin: 0 auto; padding: 20px; }
        #notes-list { margin: 20px 0; border: 1px solid #ddd; min-height: 200px; }
        .note { padding: 10px; border-bottom: 1px solid #eee; }
        #status { position: fixed; bottom: 10px; right: 10px; padding: 5px 10px; }
        .online { background: #cfc; }
        .offline { background: #fcc; }
    </style>
</head>
<body>
    <h1>Offline Notes</h1>
    
    <div>
        <input type="text" id="note-title" placeholder="Title">
        <textarea id="note-content" placeholder="Content"></textarea>
        <button id="save-note">Save</button>
    </div>
    
    <div id="notes-list"></div>
    
    <div id="status">Checking status...</div>
    
    <script src="app.js"></script>
</body>
</html>

JavaScript implementation (app.js):

// Initialize database
let db;

function initDB() {
    return new Promise((resolve, reject) => {
        const request = indexedDB.open('NotesDB', 1);
        
        request.onupgradeneeded = function(event) {
            db = event.target.result;
            if (!db.objectStoreNames.contains('notes')) {
                const store = db.createObjectStore('notes', {
                    keyPath: 'id',
                    autoIncrement: true
                });
                store.createIndex('title', 'title', { unique: false });
            }
        };
        
        request.onsuccess = function(event) {
            db = event.target.result;
            resolve(db);
        };
        
        request.onerror = function(event) {
            reject('Failed to open database: ' + event.target.error);
        };
    });
}

// Save note
function saveNote(note) {
    return new Promise((resolve, reject) => {
        const transaction = db.transaction(['notes'], 'readwrite');
        const store = transaction.objectStore('notes');
        
        const request = store.add(note);
        
        request.onsuccess = function() {
            resolve(request.result);
        };
        
        request.onerror = function(event) {
            reject('Failed to save: ' + event.target.error);
        };
    });
}

// Get all notes
function getAllNotes() {
    return new Promise((resolve, reject) => {
        const transaction = db.transaction(['notes'], 'readonly');
        const store = transaction.objectStore('notes');
        const request = store.getAll();
        
        request.onsuccess = function() {
            resolve(request.result);
        };
        
        request.onerror = function(event) {
            reject('Failed to retrieve notes: ' + event.target.error);
        };
    });
}

// Render notes list
async function renderNotes() {
    try {
        const notes = await getAllNotes();
        const notesList = document.getElementById('notes-list');
        
        notesList.innerHTML = notes.map(note => `
            <div class="note">
                <h3>${note.title}</h3>
                <p>${note.content}</p>
                <small>${new Date(note.timestamp).toLocaleString()}</small>
            </div>
        `).join('');
    } catch (error) {
        console.error(error);
    }
}

// Initialize application
async function initApp() {
    try {
        await initDB();
        await renderNotes();
        
        // Save button event
        document.getElementById('save-note').addEventListener('click', async () => {
            const title = document.getElementById('note-title').value;
            const content = document.getElementById('note-content').value;
            
            if (title && content) {
                const note = {
                    title,
                    content,
                    timestamp: Date.now()
                };
                
                await saveNote(note);
                await renderNotes();
                
                // Clear inputs
                document.getElementById('note-title').value = '';
                document.getElementById('note-content').value = '';
            }
        });
        
        // Network status detection
        function updateStatus() {
            const status = document.getElementById('status');
            if (navigator.onLine) {
                status.textContent = 'Online';
                status.className = 'online';
            } else {
                status.textContent = 'Offline - Data will sync when network is restored';
                status.className = 'offline';
            }
        }
        
        window.addEventListener('online', updateStatus);
        window.addEventListener('offline', updateStatus);
        updateStatus();
    } catch (error) {
        console.error('Application initialization failed:', error);
    }
}

// Launch application
initApp();

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