Pluggable Views
===============

.. versionadded:: 0.7

Flask 0.7 introduces pluggable views inspired by the generic views from
Django which are based on classes instead of functions.  The main
intention is that you can replace parts of the implementations and this
way have customizable pluggable views.

Basic Principle
---------------

Consider you have a function that loads a list of objects from the
database and renders into a template::

    @app.route('/users/')
    def show_users(page):
        users = User.query.all()
        return render_template('users.html', users=users)

This is simple and flexible, but if you want to provide this view in a
generic fashion that can be adapted to other models and templates as well
you might want more flexibility.  This is where pluggable class-based
views come into place.  As the first step to convert this into a class
based view you would do this::


    from flask.views import View

    class ShowUsers(View):

        def dispatch_request(self):
            users = User.query.all()
            return render_template('users.html', objects=users)

    app.add_url_rule('/users/', view_func=ShowUsers.as_view('show_users'))

As you can see what you have to do is to create a subclass of
:class:`flask.views.View` and implement
:meth:`~flask.views.View.dispatch_request`.  Then we have to convert that
class into an actual view function by using the
:meth:`~flask.views.View.as_view` class method.  The string you pass to
that function is the name of the endpoint that view will then have.  But
this by itself is not helpful, so let's refactor the code a bit::


    from flask.views import View

    class ListView(View):

        def get_template_name(self):
            raise NotImplementedError()

        def render_template(self, context):
            return render_template(self.get_template_name(), **context)

        def dispatch_request(self):
            context = {'objects': self.get_objects()}
            return self.render_template(context)

    class UserView(ListView):

        def get_template_name(self):
            return 'users.html'

        def get_objects(self):
            return User.query.all()

This of course is not that helpful for such a small example, but it's good
enough to explain the basic principle.  When you have a class-based view
the question comes up what ``self`` points to.  The way this works is that
whenever the request is dispatched a new instance of the class is created
and the :meth:`~flask.views.View.dispatch_request` method is called with
the parameters from the URL rule.  The class itself is instantiated with
the parameters passed to the :meth:`~flask.views.View.as_view` function.
For instance you can write a class like this::

    class RenderTemplateView(View):
        def __init__(self, template_name):
            self.template_name = template_name
        def dispatch_request(self):
            return render_template(self.template_name)

And then you can register it like this::

    app.add_url_rule('/about', view_func=RenderTemplateView.as_view(
        'about_page', template_name='about.html'))

Method Hints
------------

Pluggable views are attached to the application like a regular function by
either using :func:`~flask.Flask.route` or better
:meth:`~flask.Flask.add_url_rule`.  That however also means that you would
have to provide the names of the HTTP methods the view supports when you
attach this.  In order to move that information to the class you can
provide a :attr:`~flask.views.View.methods` attribute that has this
information::

    class MyView(View):
        methods = ['GET', 'POST']

        def dispatch_request(self):
            if request.method == 'POST':
                ...
            ...

    app.add_url_rule('/myview', view_func=MyView.as_view('myview'))

Method Based Dispatching
------------------------

For RESTful APIs it's especially helpful to execute a different function
for each HTTP method.  With the :class:`flask.views.MethodView` you can
easily do that.  Each HTTP method maps to a function with the same name
(just in lowercase)::

    from flask.views import MethodView

    class UserAPI(MethodView):

        def get(self):
            users = User.query.all()
            ...

        def post(self):
            user = User.from_form_data(request.form)
            ...

    app.add_url_rule('/users/', view_func=UserAPI.as_view('users'))

That way you also don't have to provide the
:attr:`~flask.views.View.methods` attribute.  It's automatically set based
on the methods defined in the class.

Decorating Views
----------------

Since the view class itself is not the view function that is added to the
routing system it does not make much sense to decorate the class itself.
Instead you either have to decorate the return value of
:meth:`~flask.views.View.as_view` by hand::

    def user_required(f):
        """Checks whether user is logged in or raises error 401."""
        def decorator(*args, **kwargs):
            if not g.user:
                abort(401)
            return f(*args, **kwargs)
        return decorator

    view = user_required(UserAPI.as_view('users'))
    app.add_url_rule('/users/', view_func=view)

Starting with Flask 0.8 there is also an alternative way where you can
specify a list of decorators to apply in the class declaration::

    class UserAPI(MethodView):
        decorators = [user_required]

Due to the implicit self from the caller's perspective you cannot use
regular view decorators on the individual methods of the view however,
keep this in mind.

Method Views for APIs
---------------------

Web APIs are often working very closely with HTTP verbs so it makes a lot
of sense to implement such an API based on the
:class:`~flask.views.MethodView`.  That said, you will notice that the API
will require different URL rules that go to the same method view most of
the time.  For instance consider that you are exposing a user object on
the web:

=============== =============== ======================================
URL             Method          Description
--------------- --------------- --------------------------------------
``/users/``     ``GET``         Gives a list of all users
``/users/``     ``POST``        Creates a new user
``/users/<id>`` ``GET``         Shows a single user
``/users/<id>`` ``PUT``         Updates a single user
``/users/<id>`` ``DELETE``      Deletes a single user
=============== =============== ======================================

So how would you go about doing that with the
:class:`~flask.views.MethodView`?  The trick is to take advantage of the
fact that you can provide multiple rules to the same view.

Let's assume for the moment the view would look like this::

    class UserAPI(MethodView):

        def get(self, user_id):
            if user_id is None:
                # return a list of users
                pass
            else:
                # expose a single user
                pass

        def post(self):
            # create a new user
            pass

        def delete(self, user_id):
            # delete a single user
            pass

        def put(self, user_id):
            # update a single user
            pass

So how do we hook this up with the routing system?  By adding two rules
and explicitly mentioning the methods for each::

    user_view = UserAPI.as_view('user_api')
    app.add_url_rule('/users/', defaults={'user_id': None},
                     view_func=user_view, methods=['GET',])
    app.add_url_rule('/users/', view_func=user_view, methods=['POST',])
    app.add_url_rule('/users/<int:user_id>', view_func=user_view,
                     methods=['GET', 'PUT', 'DELETE'])

If you have a lot of APIs that look similar you can refactor that
registration code::

    def register_api(view, endpoint, url, pk='id', pk_type='int'):
        view_func = view.as_view(endpoint)
        app.add_url_rule(url, defaults={pk: None},
                         view_func=view_func, methods=['GET',])
        app.add_url_rule(url, view_func=view_func, methods=['POST',])
        app.add_url_rule(f'{url}<{pk_type}:{pk}>', view_func=view_func,
                         methods=['GET', 'PUT', 'DELETE'])

    register_api(UserAPI, 'user_api', '/users/', pk='user_id')
