Observe Model Signal Example

An example which demonstrates how to observe a model signal.

This examples uses a model with a signal to notify listeners about in place changes to a list. This pattern is interesting for the times when a ContainerList would emit too many synchronous notifications. A common example is reordering the elements in a list.

Tip

To see this example in action, download it from observe_model_signal and run:

$ enaml-run observe_model_signal.enaml

Screenshot

Example Enaml Code

#------------------------------------------------------------------------------
# Copyright (c) 2015, Nucleic Development Team.
#
# Distributed under the terms of the Modified BSD License.
#
# The full license is in the file LICENSE, distributed with this software.
#------------------------------------------------------------------------------
""" An example which demonstrates how to observe a model signal.

This examples uses a model with a signal to notify listeners about in
place changes to a list. This pattern is interesting for the times when
a ContainerList would emit too many synchronous notifications. A common
example is reordering the elements in a list.

<< autodoc-me >>
"""
from atom.api import Atom, List, Signal, Str
from enaml.core.api import Looper
from enaml.widgets.api import Window, Form, PushButton, Field, Menu, Action


class Item(Atom):
    """Object to store in a list and display.

    """
    text = Str()


class Model(Atom):
    """ A model which manages a collection of values.

    This model expose a list which should be considered read-only and
    methods to manipulate it. The `values_changed` signal is emitted
    when an in-place change occurs in the list.

    """
    values = List(default=[Item(text='val')])

    values_changed = Signal()

    def add_value(self, index, val):
        """ Add a value at a specified index.

        """
        self.values.insert(index, val)
        self.values_changed('add')

    def move_value(self, old, new):
        """ Move a value from one index to another.

        """
        val = self.values.pop(old)
        self.values.insert(new, val)
        self.values_changed('moved')

    def delete_value(self, index):
        """ Delete a value.

        """
        del self.values[index]
        self.values_changed('deleted')


enamldef EditMenu(Menu):
    """ A menu used to edit the content of the list.

    The visible menu items will vary to show appropriate actions based
    on the current model state.

    """
    attr model: Model
    attr index: int
    Action:
        text = 'Add before'
        triggered :: model.add_value(index,
                                     Item(text='item %d' % len(model.values)))
    Action:
        text = 'Add after'
        triggered :: model.add_value(index + 1,
                                     Item(text='item %d' % len(model.values)))
    Action:
        separator = True
    Action:
        visible = index != 0
        text = 'Move up'
        triggered :: model.move_value(index, index - 1)
    Action:
        visible = index != len(model.values) - 1
        text = 'Move down'
        triggered :: model.move_value(index, index + 1)
    Action:
        separator = True
    Action:
        visible = len(model.values) > 1
        text = 'Delete'
        triggered :: model.delete_value(index)


enamldef Main(Window):
    """ The main window which displays the list contents as a form.

    """
    attr model = Model()
    attr _values = model.values[:]

    # Subscribe to the model when the window initializes.
    initialized :: model.observe('values_changed', on_changed)

    func on_changed(kind):
        self._values = model.values[:]

    func open_menu(item):
        EditMenu(model=model, index=_values.index(item)).popup()

    Form:
        # Note that a Looper expects to iterate over unique values. Passing
        # duplicate values can lead to crashes.
        Looper:
            iterable << _values
            PushButton:
                text = '>'
                constraints = [width == 15, height == 20]
                font = 'bold 12pt Consolas'
                clicked :: open_menu(loop.item)
            Field:
                read_only = True
                text = loop.item.text