============== Defining Views ============== Views in BFG are typically simple Python functions that accept two parameters: :term:`context`, and :term:`request`. A view is assumed to return a :term:`response` object. .. note:: A :mod:`repoze.bfg` view can also be defined as callable which accepts *one* arguments: a :term:`request`. You'll see this two-argument pattern used in other :mod:`repoze.bfg` tutorials and applications. Either calling convention will work in any :mod:`repoze.bfg` application. In :term:`traversal` based applications, such as this tutorial, the context is used frequently within the body of a view method, so it makes sense to use the two-argument syntax in this application. However, in :term:`url dispatch` based applications, however, the context object is rarely used in the view body itself, so within code that uses URL-dispatch-only, it's common to define views as callables that accept only a request to avoid the visual "noise". Adding View Functions ===================== We're going to add four :term:`view` functions to our ``views.py`` module. One view (named ``view_wiki``) will display the wiki itself (it will answer on the root URL), another named ``view_page`` will display an individual page, another named ``add_page`` will allow a page to be added, and a final view named ``edit_page`` will allow a page to be edited. .. note:: There is nothing automagically special about the filename ``views.py``. A project may have many views throughout its codebase in arbitrarily-named files. Files implementing views often have ``view`` in their filenames (or may live in a Python subpackage of your application package named ``views``), but this is only by convention. The ``view_wiki`` view function ------------------------------- The ``view_wiki`` function will respond as the default view of a ``Wiki`` model object. It always redirects to the ``Page`` object named "FrontPage". It returns an instance of the ``webob.exc.HTTPFound`` class (instances of which implement the WebOb :term:`response` interface), and the ``repoze.bfg.model_url`` API. ``model_url`` constructs a URL to the ``FrontPage`` page (e.g. ``http://localhost:6543/FrontPage``), and uses it as the "location" of the HTTPFound response, forming an HTTP redirect. The ``view_page`` view function ------------------------------- The ``view_page`` function will respond as the default view of a ``Page`` object. The ``view_page`` function renders the :term:`ReStructuredText` body of a page (stored as the ``data`` attribute of the context, which will be a Page object) as HTML. Then it substitutes an HTML anchor for each *WikiWord* reference in the rendered HTML using a compiled regular expression. The curried function named ``check`` is used as the first argument to ``wikiwords.sub``, indicating that it should be called to provide a value for each WikiWord match found in the content. If the wiki (our page's ``__parent__``) already contains a page with the matched WikiWord name, the ``check`` function generates a view link to be used as the substitution value and returns it. If the wiki does not already contain a page with with the matched WikiWord name, the function generates an "add" link as the substitution value and returns it. As a result, the ``content`` variable is now a fully formed bit of HTML containing various view and add links for WikiWords based on the content of our current page object. We then generate an edit URL (because it's easier to do here than in the template), and we wrap up a number of arguments in a dictionary and return it. The arguments we wrap into a dictionary include ``page``, ``content``, and ``edit_url``. As a result, the *template* associated with this view will be able to use these names to perform various rendering tasks. The template associated with this view will be a template which lives in ``templates/view.pt``, which we'll associate with this view via the ``configure.zcml`` file. Note the contrast between this view and the ``view_wiki`` view. In the ``view_wiki`` view, we return a *response* object. In this view, we return a *dictionary*. It is *always* fine to return a response object from a :mod:`repoze.bfg` view. Returning a dictionary is allowed only when there is a ``renderer`` associated with the view in the view configuration. The ``add_page`` view function ------------------------------ The ``add_page`` function will be invoked when a user clicks on a WikiWord which isn't yet represented as a page in the system. The ``check`` function within the ``view_page`` view generates URLs to this view. It also acts as a handler for the form that is generated when we want to add a page object. The ``context`` of the ``add_page`` view is always a Wiki object (*not* a Page object). The request :term:`subpath` in BFG is the sequence of names that are found *after* the view name in the URL segments given to BFG as the result of a request. If our add view is invoked via, e.g. ``http://localhost:6543/add_page/SomeName``, the :term:`subpath` will be ``['SomeName']``. The add view takes the zeroth element of the subpath (the wiki page name), and aliases it to the name attribute in order to know the name of the page we're trying to add. If the view rendering is *not* a result of a form submission (if the expression ``'form.submitted' in request.params`` is False), the view renders a template. To do so, it generates a "save url" which the template use as the form post URL during rendering. We're lazy here, so we're trying to use the same template (``templates/edit.pt``) for the add view as well as the page edit view, so we create a dummy Page object in order to satisfy the edit form's desire to have *some* page object exposed as ``page``, and we'll render the template to a response. If the view rendering *is* a result of a form submission (if the expression ``'form.submitted' in request.params`` is True), we scrape the page body from the form data, create a Page object using the name in the subpath and the page body, and save it into "our context" (the wiki) using the ``__setitem__`` method of the context. We then redirect back to the ``view_page`` view (the default view for a page) for the newly created page. The ``edit_page`` view function ------------------------------- The ``edit_page`` function will be invoked when a user clicks the "Edit this Page" button on the view form. It renders an edit form but it also acts as the handler for the form it renders. The ``context`` of the ``edit_page`` view will *always* be a Page object (never a Wiki object). If the view execution is *not* a result of a form submission (if the expression ``'form.submitted' in request.params`` is False), the view simply renders the edit form, passing the request, the page object, and a save_url which will be used as the action of the generated form. If the view execution *is* a result of a form submission (if the expression ``'form.submitted' in request.params`` is True), the view grabs the ``body`` element of the request parameter and sets it as the ``data`` attribute of the page context. It then redirects to the default view of the context (the page), which will always be the ``view_page`` view. Viewing the Result of Our Edits to ``views.py`` =============================================== The result of all of our edits to ``views.py`` will leave it looking like this: .. literalinclude:: src/views/tutorial/views.py :linenos: :language: python Adding Templates ================ The views we've added all reference a :term:`template`. Each template is a :term:`Chameleon` template. The default templating system in :mod:`repoze.bfg` is a variant of :term:`ZPT` provided by Chameleon. These templates will live in the ``templates`` directory of our tutorial package. The ``view.pt`` Template ------------------------ The ``view.pt`` template is used for viewing a single wiki page. It is used by the ``view_page`` view function. It should have a div that is "structure replaced" with the ``content`` value provided by the view. It should also have a link on the rendered page that points at the "edit" URL (the URL which invokes the ``edit_page`` view for the page being viewed). Once we're done with the ``view.pt`` template, it will look a lot like the below: .. literalinclude:: src/views/tutorial/templates/view.pt :linenos: :language: xml .. note:: The names available for our use in a template are always those that are present in the dictionary returned by the view callable. But our templates make use of a ``request`` object that none of our tutorial views return in their dictionary. This value appears as if "by magic". However, ``request`` is one of several names that are available "by default" in a template when a template renderer is used. See :ref:`chameleon_template_renderers` for more information about other names that are available by default in a template when a Chameleon template is used as a renderer. The ``edit.pt`` Template ------------------------ The ``edit.pt`` template is used for adding and editing a wiki page. It is used by the ``add_page`` and ``edit_page`` view functions. It should display a page containing a form that POSTs back to the "save_url" argument supplied by the view. The form should have a "body" textarea field (the page data), and a submit button that has the name "form.submitted". The textarea in the form should be filled with any existing page data when it is rendered. Once we're done with the ``edit.pt`` template, it will look a lot like the below: .. literalinclude:: src/views/tutorial/templates/edit.pt :linenos: :language: xml Static Resources ---------------- Our templates name a single static resource named ``style.css``. We need to create this and place it in a file named ``style.css`` within our package's ``templates/static`` directory: .. literalinclude:: src/views/tutorial/templates/static/style.css :linenos: :language: css This CSS file will be accessed via e.g. ``http://localhost:6543/static/style.css`` by virtue of the ``static_view`` view we've defined in the ``views.py`` file. Any number and type of static resources can be placed in this directory (or subdirectories) and are just referred to by URL within templates. Testing the Views ================= We'll modify our ``tests.py`` file, adding tests for each view function we added above. As a result, we'll *delete* the ``ViewTests`` test in the file, and add four other test classes: ``ViewWikiTests``, ``ViewPageTests``, ``AddPageTests``, and ``EditPageTests``. These test the ``view_wiki``, ``view_page``, ``add_page``, and ``edit_page`` views respectively. Once we're done with the ``tests.py`` module, it will look a lot like the below: .. literalinclude:: src/views/tutorial/tests.py :linenos: :language: python Running the Tests ================= We can run these tests by using ``setup.py test`` in the same way we did in :ref:`running_tests`. Assuming our shell's current working directory is the "tutorial" distribution directory: On UNIX: .. code-block:: bash $ ../bin/python setup.py test -q On Windows: .. code-block:: bash c:\bigfntut\tutorial> ..\Scripts\python setup.py test -q The expected result looks something like: .. code-block:: bash ......... ---------------------------------------------------------------------- Ran 9 tests in 0.203s OK Mapping Views to URLs in ``configure.zcml`` =========================================== The ``configure.zcml`` file contains ``view`` declarations which serve to map URLs (via :term:`traversal`) to view functions. You'll need to add four ``view`` declarations to ``configure.zcml``. #. Add a declaration which maps the "Wiki" class in our ``models.py`` file to the view named ``view_wiki`` in our ``views.py`` file with no view name. This is the default view for a Wiki. It does not use a ``renderer`` because the ``view_wiki`` view callable always returns a *response* object rather than a dictionary. #. Add a declaration which maps the "Wiki" class in our ``models.py`` file to the view named ``add_page`` in our ``views.py`` file with the view name ``add_page``. Associate this view with the ``templates/edit.pt`` template file via the ``renderer`` attribute. This view will use the :term:`Chameleon` ZPT renderer configured with the ``templates/edit.pt`` template to render non-*response* return values from the ``add_page`` view. This is the add view for a new Page. #. Add a declaration which maps the "Page" class in our ``models.py`` file to the view named ``view_page`` in our ``views.py`` file with no view name. Associate this view with the ``templates/view.pt`` template file via the ``renderer`` attribute. This view will use the :term:`Chameleon` ZPT renderer configured with the ``templates/view.pt`` template to render non-*response* return values from the ``view_page`` view. This is the default view for a Page. #. Add a declaration which maps the "Page" class in our ``models.py`` file to the view named ``edit_page`` in our ``views.py`` file with the view name ``edit_page``. Associate this view with the ``templates/edit.pt`` template file via the ``renderer`` attribute. This view will use the :term:`Chameleon` ZPT renderer configured with the ``templates/edit.pt`` template to render non-*response* return values from the ``edit_page`` view. This is the edit view for a page. As a result of our edits, the ``configure.zcml`` file should look something like so: .. literalinclude:: src/views/tutorial/configure.zcml :linenos: :language: xml Examining ``tutorial.ini`` ========================== Let's take a look at our ``tutorial.ini`` file. The contents of the file are as follows: .. literalinclude:: src/models/tutorial.ini :linenos: :language: ini The WSGI Pipeline ----------------- Within ``tutorial.ini``, note the existence of a ``[pipeline:main]`` section which specifies our WSGI pipeline. This "pipeline" will be served up as our WSGI application. As far as the WSGI server is concerned the pipeline *is* our application. Simpler configurations don't use a pipeline: instead they expose a single WSGI application as "main". Our setup is more complicated, so we use a pipeline. "egg:repoze.zodbconn#closer" is at the "top" of the pipeline. This is a piece of middleware which closes the ZODB connection opened by the PersistentApplicationFinder at the end of the request. "egg:repoze.tm#tm" is the second piece of middleware in the pipeline. This commits a transaction near the end of the request unless there's an exception raised. Adding an Element to the Pipeline --------------------------------- Let's add a piece of middleware to the WSGI pipeline. "egg:Paste#evalerror" middleware which displays debuggable errors in the browser while you're developing (not recommended for deployment). Let's insert evalerror into the pipeline right below "egg:repoze.zodbconn#closer", making our resulting ``tutorial.ini`` file look like so: .. literalinclude:: src/views/tutorial.ini :linenos: :language: ini Viewing the Application in a Browser ==================================== Once we've set up the WSGI pipeline properly, we can finally examine our application in a browser. The views we'll try are as follows: - Visiting `http://localhost:6543/ `_ in a browser invokes the ``view_wiki`` view. This always redirects to the ``view_page`` view of the FrontPage page object. - Visiting `http://localhost:6543/FrontPage/ `_ in a browser invokes the ``view_page`` view of the front page page object. This is because it's the *default view* (a view without a ``name``) for Page objects. - Visiting `http://localhost:6543/FrontPage/edit_page `_ in a browser invokes the edit view for the front page object. - Visiting `http://localhost:6543/add_page/SomePageName `_ in a browser invokes the add view for a page. - To generate an error, visit `http://localhost:6543/add_page `_ which will generate an ``IndexError`` for the expression ``request.subpath[0]``. You'll see an interactive traceback facility provided by evalerror.