Add example of how to use session to flash messages between requests (#270)

Author: bmwant

demo/flash_messages_example.py

@@ -0,0 +1,54 @@
+import base64
+from cryptography import fernet
+from aiohttp import web
+from aiohttp_session import setup, get_session
+from aiohttp_session.cookie_storage import EncryptedCookieStorage
+
+
+def flash(request, message):
+    request.setdefault('flash_outgoing', []).append(message)
+
+
+def get_messages(request):
+    return request.pop('flash_incoming')
+
+
+async def flash_middleware(app, handler):
+    async def process(request):
+        session = await get_session(request)
+        request['flash_incoming'] = session.pop('flash', [])
+        response = await handler(request)
+        session['flash'] = (request.get('flash_incoming', []) +
+                            request.get('flash_outgoing', []))
+        return response
+    return process
+
+
+async def flash_handler(request):
+    flash(request, 'You have just visited flash page')
+    return web.HTTPFound('/')
+
+
+async def handler(request):
+    text = 'No flash messages yet'
+    messages = get_messages(request)
+    if messages:
+        text = 'Messages: {}'.format(','.join(messages))
+    return web.Response(text=text)
+
+
+def make_app():
+    app = web.Application()
+    # secret_key must be 32 url-safe base64-encoded bytes
+    fernet_key = fernet.Fernet.generate_key()
+    secret_key = base64.urlsafe_b64decode(fernet_key)
+    setup(app, EncryptedCookieStorage(secret_key))
+    app.router.add_get('/', handler)
+    app.router.add_get('/flash', flash_handler)
+    # Install flash middleware
+    app.middlewares.append(flash_middleware)
+    return app
+
+
+web.run_app(make_app())
+