.. _lang:

Kv language
===========

Concept behind the language
---------------------------

As your application grow more complex, it's common that the construction of
widget trees and explicit declaration of bindings, becomes verbose and hard to
maintain. The `KV` Language is a attempt to overcome these short-comings.

The `KV` language (sometimes called kvlang, or kivy language), allows you to
create your widget tree in a declarative way and to bind widget properties
to each other or to callbacks in a natural manner. It allows for very fast
prototyping and agile changes to your UI. It also facilitates a good
separation between the logic of your application and it's User Interface.

How to load KV
--------------

There are two ways to load Kv code into your application:

- By name convention:

  Kivy looks for a Kv file with the same name as your App class in
  lowercase,  minus "App" if it ends with 'App'. E.g::

    MyApp -> my.kv.

  If this file defines a `Root Widget` it will be attached to the App's `root`
  attribute and used as the base of the application widget tree.

- :obj:`~kivy.lang.Builder`:
  You can tell Kivy to directly load a string or a file. If this string or file
  defines a root widget, it will be returned by the method::

    Builder.load_file('path/to/file.kv')

  or::

    Builder.load_string(kv_string)

Rule context
------------

A Kv source constitutes of `rules`, which are used to describe the content
of a Widget, you can have one `root` rule, and any number of `class` or
`template` rules.

The `root` rule is declared by declaring the class of your root widget, without
any indentation, followed by `:` and will be set as the `root` attribute of the
App instance::

    Widget:

A `class` rule, declared by the name of a widget class between `< >` and
followed by `:`, defines how any instance of that class will be
graphically represented::

    <MyWidget>:

Rules use indentation for delimitation, as python, indentation should be of
four spaces per level, like the python good practice recommendations.

There are three keywords specific to Kv language:

- `app`: always refers to the instance of your application.
- `root`: refers to the base widget/template in the current rule
- `self`: always refer to the current widget

Special syntaxes
----------------

There are two special syntax to define values for the whole Kv context:

To import something from python::

    #:import name x.y.z

Is equivalent to::

    from x.y import z as name

in python.

To set a global value::

    #:set name value

Is equivalent to::

    name = value

in python.

Instantiate children
--------------------

To declare the widget has a child widget, instance of some class, just declare
this child inside the rule:

.. code-block:: kv

    MyRootWidget:
        BoxLayout:
            Button:
            Button:

The example above defines that our root widget, an instance of `MyRootWidget`,
which has a child that is an instance of the
:class:`~kivy.uix.boxlayout.BoxLayout`.  That BoxLayout further has two
children, instances of the :class:`~kivy.uix.button.Button` class.

A python equivalent of this code could be:

.. code-block:: python

    root = MyRootWidget()
    box = BoxLayout()
    box.add_widget(Button())
    box.add_widget(Button())
    root.add_widget(box)

Which you may find less nice, both to read and to write.

Of course, in python, you can pass keyword arguments to your widgets at
creation to specify their behaviour.  For example, to set the number of columns
of a :mod:`~kivy.uix.gridlayout`, we would do::

    grid = GridLayout(cols=3)

To do the same thing in kv, you can set properties of the child widget directly
in the rule:

.. code-block:: kv

    GridLayout:
        cols: 3

The value is evaluated as a python expression, and all the properties used in
the expression will be observed, that means that if you had something like this
in python (this assume `self` is a widget with a `data`
:class:`~kivy.property.ListProperty`)::

    grid = GridLayout(cols=len(self.data))
    self.bind(data=grid.setter('cols'))

To have your display updated when your data change, you can now have just:

.. code-block:: kv

    GridLayout:
        cols: len(root.data)

Event Bindings
--------------

You can bind to events in Kv using the ":" syntax, that is, associating a
callback to an event:

.. code-block:: kv

    Widget:
        on_size: my_callback()

You can pass the values dispatched by the signal using the `args` keyword:

.. code-block:: kv

    TextInput:
        on_text: app.search(args[1])

More complex expressions can be used, like:

.. code-block:: kv

    pos: self.center_x - self.texture_size[0] / 2., self.center_y - self.texture_size[1] / 2.

This expression listens for a change in ``center_x``, ``center_y``,
and ``texture_size``. If one of them changes, the expression will be
re-evaluated to update the ``pos`` field.

You can also handle ``on_`` events inside your kv language.
For example the TextInput class has a ``focus`` property whose auto-generated
``on_focus`` event can be accessed inside the kv language like so:

.. code-block:: kv

    TextInput:
        on_focus: print(args)


Extend canvas
-------------

Kv lang can be used to define the canvas instructions of your widget like this:

.. code-block:: kv

    MyWidget:
        canvas:
            Color:
                rgba: 1, .3, .8, .5
            Line:
                points: zip(self.data.x, self.data.y)

And they get updated when properties values change.

Of course you can use `canvas.before` and `canvas.after`.

Referencing Widgets
-------------------

In a widget tree there is often a need to access/reference other widgets.
Kv Language provides a way to do this using id's. Think of them as class
level variables that can only be used in the Kv language. Consider the
following:

.. code-block:: kv

    <MyFirstWidget>:
        Button:
            id: f_but
        TextInput:
            text: f_but.state

    <MySecondWidget>:
        Button:
            id: s_but
        TextInput:
            text: s_but.state

An ``id`` is limited in scope to the rule it is declared in, so in the
code above ``s_but`` can not be accessed outside the <MySecondWidget>
rule.

An ``id`` is a ``weakref`` to the widget and not the widget itself. As a
consequence, storing the ``id`` is not sufficient to keep the widget from being
garbage collected. To demonstrate:

.. code-block:: kv

    <MyWidget>:
        label_widget: label_widget
        Button:
            text: 'Add Button'
            on_press: root.add_widget(label_widget)
        Button:
            text: 'Remove Button'
            on_press: root.remove_widget(label_widget)
        Label:
            id: label_widget
            text: 'widget'

Although a reference to ``label_widget`` is stored in ``MyWidget``, it is not
sufficient to keep the object alive once other references have been removed
because it's only a weakref.
Therefore, after the remove button is clicked (which removes
any direct reference to the widget) and the window is resized (which calls the
garbage collector resulting in the deletion of ``label_widget``), when the add
button is clicked to add the widget back, a ``ReferenceError: weakly-referenced
object no longer exists`` will be thrown.

To keep the widget alive, a direct reference to the ``label_widget`` widget
must be kept. This is achieved using ``id.__self__`` or ``label_widget.__self__``
in this case. The correct way to do this would be:

.. code-block:: kv

    <MyWidget>:
        label_widget: label_widget.__self__

Accessing Widgets defined inside Kv lang in your python code
------------------------------------------------------------

Consider the code below in my.kv:

.. code-block:: kv

    <MyFirstWidget>:
        # both these variables can be the same name and this doesn't lead to
        # an issue with uniqueness as the id is only accessible in kv.
        txt_inpt: txt_inpt
        Button:
            id: f_but
        TextInput:
            id: txt_inpt
            text: f_but.state
            on_text: root.check_status(f_but)


In myapp.py:

.. code-block:: py

    ...
    class MyFirstWidget(BoxLayout):

        txt_inpt = ObjectProperty(None)

        def check_status(self, btn):
            print('button state is: {state}'.format(state=btn.state))
            print('text input text is: {txt}'.format(txt=self.txt_inpt))
    ...

`txt_inpt` is defined as a :class:`~kivy.properties.ObjectProperty` initialized
to `None` inside the Class.::

    txt_inpt = ObjectProperty(None)

At this point self.txt_inpt is `None`. In Kv lang this property is updated to
hold the instance of the :class:`~kivy.uix.TextInput` referenced by the id
`txt_inpt`.::

    txt_inpt: txt_inpt

From this point onwards, `self.txt_inpt` holds a reference to the widget
identified by the id `txt_input` and can be used anywhere in the class, as in
the function `check_status`. In contrast to this method you could also just pass
the `id` to the function that needs to use it, like in case of `f_but` in the
code above.

There is a simpler way to access the ids as defined in the kv language for example:

.. code-block:: kv

    <Marvel>
      Label:
        id: loki
        text: 'loki: I AM YOUR GOD!'
      Button:
        id: hulk
        text: "press to smash loki"
        on_release: root.hulk_smash()

In your python code:

.. code-block:: python

    class Marvel(BoxLayout):

        def hulk_smash(self):
            self.ids.hulk.text = "hulk: puny god!"
            self.ids.loki.text = "loki: >_<!!!"

When your kv file is parsed, kivy collects all the widgets tagged with id's
and places them in this `self.ids` dictionary type property. That means you
can also iterate over these widgets and access them dictionary style::

    for key, val in self.ids.items():
        print("key={0}, val={1}".format(key, val))

Dynamic Classes
---------------
Consider the code below:

.. code-block:: kv

    <MyWidget>:
        Button:
            text: "Hello world, watch this text wrap inside the button"
            text_size: self.size
            font_size: '25sp'
            markup: True
        Button:
            text: "Even absolute is relative to itself"
            text_size: self.size
            font_size: '25sp'
            markup: True
        Button:
            text: "Repeating the same thing over and over in a comp = fail"
            text_size: self.size
            font_size: '25sp'
            markup: True
        Button:

Instead of having to repeat the same values for every button, we can just use a
template instead, like so:

.. code-block:: kv

    <MyBigButt@Button>:
        text_size: self.size
        font_size: '25sp'
        markup: True

    <MyWidget>:
        MyBigButt:
            text: "Hello world, watch this text wrap inside the button"
        MyBigButt:
            text: "Even absolute is relative to itself"
        MyBigButt:
            text: "repeating the same thing over and over in a comp = fail"
        MyBigButt:

This class, created just by the declaration of this rule, inherits from the
Button class and allows us to change default values and create bindings for all
its instances without adding any new code on the Python side.

Re-using styles in multiple widgets
-----------------------------------

Consider the code below in my.kv:

.. code-block:: kv

    <MyFirstWidget>:
        Button:
            on_press: self.text(txt_inpt.text)
        TextInput:
            id: txt_inpt

    <MySecondWidget>:
        Button:
            on_press: self.text(txt_inpt.text)
        TextInput:
            id: txt_inpt

In myapp.py:

.. code-block:: py

    class MyFirstWidget(BoxLayout):

        def text(self, val):
            print('text input text is: {txt}'.format(txt=val))

    class MySecondWidget(BoxLayout):

        writing = StringProperty('')

        def text(self, val):
            self.writing = val

Because both classes share the same .kv style, this design can be simplified
if we reuse the style for both widgets. You can do this in .kv as follows.
In my.kv:

.. code-block:: kv

    <MyFirstWidget,MySecondWidget>:
        Button:
            on_press: self.text(txt_inpt.text)
        TextInput:
            id: txt_inpt

By separating the class names with a comma, all the classes listed in the
declaration will have the same kv properties.

Designing with the Kivy Language
--------------------------------

One of aims of the Kivy language is to
`Separate the concerns <https://en.wikipedia.org/wiki/Separation_of_concerns>`_
of presentation and logic. The presentation (layout) side is addressed by your
kv file and the logic by your py file.

The code goes in py files
~~~~~~~~~~~~~~~~~~~~~~~~~

Let's start with a little example. First, the Python file named `main.py`:

.. include:: ../../../examples/guide/designwithkv/main.py
   :literal:

In this example, we are creating a Controller class with 2 properties:

    * ``info`` for receving some text
    * ``label_wid`` for receving the label widget

In addition, we are creating a ``do_action()`` method that will use both of
these properties. It will change the ``info`` text and change text in the
``label_wid`` widget.

The layout goes in controller.kv
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Executing this application without a corresponding `.kv` file will work, but
nothing will be shown on the screen. This is expected, because the
``Controller`` class has no widgets in it, it's just a ``FloatLayout``. We can
create the UI around the ``Controller`` class in a file named `controller.kv`,
which will be loaded when we run the ``ControllerApp``. How this is done and
what files are loaded is described in the :meth:`kivy.app.App.load_kv` method.

.. literalinclude:: ../../../examples/guide/designwithkv/controller.kv
    :language: kv
    :linenos:

One label and one button in a vertical ``BoxLayout``. Seems very simple. There
are 3 things going on here:

    1. Using data from the ``Controller``. As soon as the ``info`` property is
       changed in the controller, the expression ``text: 'My controller info
       is: ' + root.info`` will automatically be re-evaluated, changing the text
       in the ``Button``.

    2. Giving data to the ``Controller``. The expression ``id: my_custom_label``
       is assigning the created ``Label`` the id of ``my_custom_label``. Then,
       using ``my_custom_label`` in the expression ``label_wid:
       my_custom_label`` gives the instance of that ``Label`` widget to your
       ``Controller``.

    3. Creating a custom callback in the ``Button`` using the ``Controller``'s
       ``on_press`` method.

        * ``root`` and ``self`` are reserved keywords, useable anywhere.
          ``root`` represents the top widget in the rule and ``self`` represents
          the current widget.

        * You can use any id declared in the rule the same as ``root`` and
          ``self``. For example, you could do this in the ``on_press()``:

        .. code-block:: kv

            Button:
                on_press: root.do_action(); my_custom_label.font_size = 18

And that's that. Now when we run `main.py`, `controller.kv` will be loaded so
that the ``Button`` and ``Label`` will show up and respond to our touch events.