diff --git a/docs/patterns/index.rst b/docs/patterns/index.rst index f765cd8a..d7338ac0 100644 --- a/docs/patterns/index.rst +++ b/docs/patterns/index.rst @@ -29,7 +29,7 @@ collected in the following pages. wtforms templateinheritance flashing - jquery + javascript lazyloading mongoengine favicon diff --git a/docs/patterns/javascript.rst b/docs/patterns/javascript.rst new file mode 100644 index 00000000..046029f4 --- /dev/null +++ b/docs/patterns/javascript.rst @@ -0,0 +1,259 @@ +JavaScript, ``fetch``, and JSON +=============================== + +You may want to make your HTML page dynamic, by changing data without +reloading the entire page. Instead of submitting an HTML ``
`` and +performing a redirect to re-render the template, you can add +`JavaScript`_ that calls |fetch|_ and replaces content on the page. + +|fetch|_ is the modern, built-in JavaScript solution to making +requests from a page. You may have heard of other "AJAX" methods and +libraries, such as |XHR|_ or `jQuery`_. These are no longer needed in +modern browsers, although you may choose to use them or another library +depending on your application's requirements. These docs will only focus +on built-in JavaScript features. + +.. _JavaScript: https://developer.mozilla.org/Web/JavaScript +.. |fetch| replace:: ``fetch()`` +.. _fetch: https://developer.mozilla.org/Web/API/Fetch_API +.. |XHR| replace:: ``XMLHttpRequest()`` +.. _XHR: https://developer.mozilla.org/Web/API/XMLHttpRequest +.. _jQuery: https://jquery.com/ + + +Rendering Templates +------------------- + +It is important to understand the difference between templates and +JavaScript. Templates are rendered on the server, before the response is +sent to the user's browser. JavaScript runs in the user's browser, after +the template is rendered and sent. Therefore, it is impossible to use +JavaScript to affect how the Jinja template is rendered, but is is +possible to render data into the JavaScript that will run. + +To provide data to JavaScript when rendering the template, use the +:func:`~jinja-filters.tojson` filter in a `` + +A less common pattern is to add the data to a ``data-`` attribute on an +HTML tag. In this case, you must use single quotes around the value, not +double quotes, otherwise you will produce invalid or unsafe HTML. + +.. code-block:: jinja + +
+ + +Generating URLs +--------------- + +The other way to get data from the server to JavaScript is to make a +request for it. First, you need to know the URL to request. + +The simplest way to generate URLs is to continue to use +:func:`~flask.url_for` when rendering the template. For example: + +.. code-block:: javascript + + const user_url = {{ url_for("user", id=current_user.id)|tojson }} + fetch(user_url).then(...) + +However, you might need to generate a URL based on information you only +know in JavaScript. As discussed above, JavaScript runs in the user's +browser, not as part of the template rendering, so you can't use +``url_for`` at that point. + +In this case, you need to know the "root URL" under which your +application is served. In simple setups, this is ``/``, but it might +also be something else, like ``https://example.com/myapp/``. + +A simple way to tell your JavaScript code about this root is to set it +as a global variable when rendering the template. Then you can use it +when generating URLs from JavaScript. + +.. code-block:: javascript + + const SCRIPT_ROOT = {{ request.script_root|tojson }} + let user_id = ... // do something to get a user id from the page + let user_url = `${SCRIPT_ROOT}/user/${user_id}` + fetch(user_url).then(...) + + +Making a Request with ``fetch`` +------------------------------- + +|fetch|_ takes two arguments, a URL and an object with other options, +and returns a |Promise|_. We won't cover all the available options, and +will only use ``then()`` on the promise, not other callbacks or +``await`` syntax. Read the linked MDN docs for more information about +those features. + +By default, the GET method is used. If the response contains JSON, it +can be used with a ``then()`` callback chain. + +.. code-block:: javascript + + const room_url = {{ url_for("room_detail", id=room.id)|tojson }} + fetch(room_url) + .then(response => response.json()) + .then(data => { + // data is a parsed JSON object + }) + +To send data, use a data method such as POST, and pass the ``body`` +option. The most common types for data are form data or JSON data. + +To send form data, pass a populated |FormData|_ object. This uses the +same format as an HTML form, and would be accessed with ``request.form`` +in a Flask view. + +.. code-block:: javascript + + let data = new FormData() + data.append("name": "Flask Room") + data.append("description": "Talk about Flask here.") + fetch(room_url, { + "method": "POST", + "body": data, + }).then(...) + +In general, prefer sending request data as form data, as would be used +when submitting an HTML form. JSON can represent more complex data, but +unless you need that it's better to stick with the simpler format. When +sending JSON data, the ``Content-Type: application/json`` header must be +sent as well, otherwise Flask will return a 400 error. + +.. code-block:: javascript + + let data = { + "name": "Flask Room", + "description": "Talk about Flask here.", + } + fetch(room_url, { + "method": "POST", + "headers": {"Content-Type": "application/json"}, + "body": JSON.stringify(data), + }).then(...) + +.. |Promise| replace:: ``Promise`` +.. _Promise: https://developer.mozilla.org/Web/JavaScript/Reference/Global_Objects/Promise +.. |FormData| replace:: ``FormData`` +.. _FormData: https://developer.mozilla.org/en-US/docs/Web/API/FormData + + +Following Redirects +------------------- + +A response might be a redirect, for example if you logged in with +JavaScript instead of a traditional HTML form, and your view returned +a redirect instead of JSON. JavaScript requests do follow redirects, but +they don't change the page. If you want to make the page change you can +inspect the response and apply the redirect manually. + +.. code-block:: javascript + + fetch("/login", {"body": ...}).then( + response => { + if (response.redirected) { + window.location = response.url + } else { + showLoginError() + } + } + ) + + +Replacing Content +----------------- + +A response might be new HTML, either a new section of the page to add or +replace, or an entirely new page. In general, if you're returning the +entire page, it would be better to handle that with a redirect as shown +in the previous section. The following example shows how to a ``
`` +with the HTML returned by a request. + +.. code-block:: html + +
+ {{ include "geology_fact.html" }} +
+ + + +Return JSON from Views +---------------------- + +To return a JSON object from your API view, you can directly return a +dict from the view. It will be serialized to JSON automatically. + +.. code-block:: python + + @app.route("/user/") + def user_detail(id): + user = User.query.get_or_404(id) + return { + "username": User.username, + "email": User.email, + "picture": url_for("static", filename=f"users/{id}/profile.png"), + } + +If you want to return another JSON type, use the +:func:`~flask.json.jsonify` function, which creates a response object +with the given data serialized to JSON. + +.. code-block:: python + + from flask import jsonify + + @app.route("/users") + def user_list(): + users = User.query.order_by(User.name).all() + return jsonify([u.to_json() for u in users]) + +It is usually not a good idea to return file data in a JSON response. +JSON cannot represent binary data directly, so it must be base64 +encoded, which can be slow, takes more bandwidth to send, and is not as +easy to cache. Instead, serve files using one view, and generate a URL +to the desired file to include in the JSON. Then the client can make a +separate request to get the linked resource after getting the JSON. + + +Receiving JSON in Views +----------------------- + +Use the :attr:`~flask.Request.json` property of the +:data:`~flask.request` object to decode the request's body as JSON. If +the body is not valid JSON, or the ``Content-Type`` header is not set to +``application/json``, a 400 Bad Request error will be raised. + +.. code-block:: python + + from flask import request + + @app.post("/user/") + def user_update(id): + user = User.query.get_or_404(id) + user.update_from_json(request.json) + db.session.commit() + return user.to_json() diff --git a/docs/patterns/jquery.rst b/docs/patterns/jquery.rst index 0a75bb71..7ac6856e 100644 --- a/docs/patterns/jquery.rst +++ b/docs/patterns/jquery.rst @@ -1,148 +1,6 @@ +:orphan: + AJAX with jQuery ================ -`jQuery`_ is a small JavaScript library commonly used to simplify working -with the DOM and JavaScript in general. It is the perfect tool to make -web applications more dynamic by exchanging JSON between server and -client. - -JSON itself is a very lightweight transport format, very similar to how -Python primitives (numbers, strings, dicts and lists) look like which is -widely supported and very easy to parse. It became popular a few years -ago and quickly replaced XML as transport format in web applications. - -.. _jQuery: https://jquery.com/ - -Loading jQuery --------------- - -In order to use jQuery, you have to download it first and place it in the -static folder of your application and then ensure it's loaded. Ideally -you have a layout template that is used for all pages where you just have -to add a script statement to the bottom of your ```` to load jQuery: - -.. sourcecode:: html - - - -Another method is using Google's `AJAX Libraries API -`_ to load jQuery: - -.. sourcecode:: html - - - - -In this case you have to put jQuery into your static folder as a fallback, but it will -first try to load it directly from Google. This has the advantage that your -website will probably load faster for users if they went to at least one -other website before using the same jQuery version from Google because it -will already be in the browser cache. - -Where is My Site? ------------------ - -Do you know where your application is? If you are developing the answer -is quite simple: it's on localhost port something and directly on the root -of that server. But what if you later decide to move your application to -a different location? For example to ``http://example.com/myapp``? On -the server side this never was a problem because we were using the handy -:func:`~flask.url_for` function that could answer that question for -us, but if we are using jQuery we should not hardcode the path to -the application but make that dynamic, so how can we do that? - -A simple method would be to add a script tag to our page that sets a -global variable to the prefix to the root of the application. Something -like this: - -.. sourcecode:: html+jinja - - - - -JSON View Functions -------------------- - -Now let's create a server side function that accepts two URL arguments of -numbers which should be added together and then sent back to the -application in a JSON object. This is a really ridiculous example and is -something you usually would do on the client side alone, but a simple -example that shows how you would use jQuery and Flask nonetheless:: - - from flask import Flask, jsonify, render_template, request - app = Flask(__name__) - - @app.route('/_add_numbers') - def add_numbers(): - a = request.args.get('a', 0, type=int) - b = request.args.get('b', 0, type=int) - return jsonify(result=a + b) - - @app.route('/') - def index(): - return render_template('index.html') - -As you can see I also added an `index` method here that renders a -template. This template will load jQuery as above and have a little form where -we can add two numbers and a link to trigger the function on the server -side. - -Note that we are using the :meth:`~werkzeug.datastructures.MultiDict.get` method here -which will never fail. If the key is missing a default value (here ``0``) -is returned. Furthermore it can convert values to a specific type (like -in our case `int`). This is especially handy for code that is -triggered by a script (APIs, JavaScript etc.) because you don't need -special error reporting in that case. - -The HTML --------- - -Your index.html template either has to extend a :file:`layout.html` template with -jQuery loaded and the `$SCRIPT_ROOT` variable set, or do that on the top. -Here's the HTML code needed for our little application (:file:`index.html`). -Notice that we also drop the script directly into the HTML here. It is -usually a better idea to have that in a separate script file: - -.. sourcecode:: html - - -

jQuery Example

-

+ - = - ? -

calculate server side - -I won't go into detail here about how jQuery works, just a very quick -explanation of the little bit of code above: - -1. ``$(function() { ... })`` specifies code that should run once the - browser is done loading the basic parts of the page. -2. ``$('selector')`` selects an element and lets you operate on it. -3. ``element.bind('event', func)`` specifies a function that should run - when the user clicked on the element. If that function returns - `false`, the default behavior will not kick in (in this case, navigate - to the `#` URL). -4. ``$.getJSON(url, data, func)`` sends a ``GET`` request to `url` and will - send the contents of the `data` object as query parameters. Once the - data arrived, it will call the given function with the return value as - argument. Note that we can use the `$SCRIPT_ROOT` variable here that - we set earlier. - -Check out the :gh:`example source ` for a full -application demonstrating the code on this page, as well as the same -thing using ``XMLHttpRequest`` and ``fetch``. +Obsolete, see :doc:`/patterns/javascript` instead.