| 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 ``<body>`` to load jQuery: |
| |
| .. sourcecode:: html |
| |
| <script src="{{ url_for('static', filename='jquery.js') }}"></script> |
| |
| Another method is using Google's `AJAX Libraries API |
| <https://developers.google.com/speed/libraries/>`_ to load jQuery: |
|
|
| .. sourcecode:: html |
|
|
| <script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script> |
| <script>window.jQuery || document.write('<script src="{{ |
| url_for('static', filename='jquery.js') }}">\x3C/script>')</script> |
|
|
| 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 |
| |
| <script> |
| $SCRIPT_ROOT = {{ request.script_root|tojson }}; |
| </script> |
| |
| |
| JSON View Functions |
| ------------------- |
| |
| Now let's create a server side function that accepts two URL arguments of |
| numbers which should be added together and then sent back to the |
| application in a JSON object. This is a really ridiculous example and is |
| something you usually would do on the client side alone, but a simple |
| example that shows how you would use jQuery and Flask nonetheless:: |
|
|
| from flask import Flask, jsonify, render_template, request |
| app = Flask(__name__) |
|
|
| @app.route('/_add_numbers') |
| def add_numbers(): |
| a = request.args.get('a', 0, type=int) |
| b = request.args.get('b', 0, type=int) |
| return jsonify(result=a + b) |
|
|
| @app.route('/') |
| def index(): |
| return render_template('index.html') |
|
|
| As you can see I also added an `index` method here that renders a |
| template. This template will load jQuery as above and have a little form where |
| we can add two numbers and a link to trigger the function on the server |
| side. |
|
|
| Note that we are using the :meth:`~werkzeug.datastructures.MultiDict.get` method here |
| which will never fail. If the key is missing a default value (here ``0``) |
| is returned. Furthermore it can convert values to a specific type (like |
| in our case `int`). This is especially handy for code that is |
| triggered by a script (APIs, JavaScript etc.) because you don't need |
| special error reporting in that case. |
| |
| The HTML |
| -------- |
| |
| Your index.html template either has to extend a :file:`layout.html` template with |
| jQuery loaded and the `$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 |
|
|
| <script> |
| $(function() { |
| $('a#calculate').bind('click', function() { |
| $.getJSON($SCRIPT_ROOT + '/_add_numbers', { |
| a: $('input[name="a"]').val(), |
| b: $('input[name="b"]').val() |
| }, function(data) { |
| $("#result").text(data.result); |
| }); |
| return false; |
| }); |
| }); |
| </script> |
| <h1>jQuery Example</h1> |
| <p><input type=text size=5 name=a> + |
| <input type=text size=5 name=b> = |
| <span id=result>?</span> |
| <p><a href= |
|
|
| I won't go into detail here about how jQuery works, just a very quick |
| explanation of the little bit of code above: |
| |
| 1. ``$(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 <examples/javascript>` for a full |
| application demonstrating the code on this page, as well as the same |
| thing using ``XMLHttpRequest`` and ``fetch``. |
| |
