State management

A special word should be mentioned about state management. Managing state is the first step in creating an undo system.

The state system consists of two parts:

  1. A basic observer (the @observed decorator)
  2. A reverser


The observer simply dispatches the function called (as <function ..>, not as <unbound method..>!) to each handler registered in an observers list.

>>> from gaphas import state
>>> state.observers.clear()
>>> state.subscribers.clear()

Let’s start with creating a Canvas instance and some items:

>>> from gaphas.canvas import Canvas
>>> from gaphas.item import Item
>>> canvas = Canvas()
>>> item1, item2 = Item(), Item()

For this demonstration let’s use the Canvas class (which contains an add/remove method pair).

It works (see how the add method automatically schedules the item for update):

>>> def handler(event):
...     print 'event handled', event
>>> state.observers.add(handler)
>>> canvas.add(item1)                              
event handled (<function add at ...>, (<gaphas.canvas.Canvas object at 0x...>, <gaphas.item.Item object at 0x...>, None, None), {})
>>> canvas.add(item2, parent=item1)                
event handled (<function add at ...>, (<gaphas.canvas.Canvas object at 0x...>, <gaphas.item.Item object at 0x...>, <gaphas.item.Item object at 0x...), {})
>>> canvas.get_all_items()                         
[<gaphas.item.Item object at 0x...>, <gaphas.item.Item object at 0x...>]

Note that the handler is invoked before the actual call is made. This is important if you want to store the (old) state for an undo mechanism.

Remember that this observer is just a simple method call notifier and knows nothing about the internals of the Canvas class (in this case the remove() method recursively calls remove() for each of it’s children). Therefore some careful crafting of methods may be necessary in order to get the right effect (items should be removed in the right order, child first):

>>> canvas.remove(item1)                           
event handled (<function _remove at ...>, (<gaphas.canvas.Canvas object at 0x...>, <gaphas.item.Item object at 0x...>), {})
event handled (<function _remove at ...>, (<gaphas.canvas.Canvas object at 0x...>, <gaphas.item.Item object at 0x...>), {})
>>> canvas.get_all_items()

The @observed decorator can also be applied to properties, as is done in gaphas/’s Handle class:

>>> from gaphas.solver import Variable
>>> var = Variable()
>>> var.value = 10                                  
event handled (<function set_value at 0x...>, (Variable(0, 20), 10), {})

(this is simply done by observing the setter method).

Off course handlers can be removed as well (only the default revert handler is present now):

>>> state.observers.remove(handler)
>>> state.observers                                 

What should you know:

  1. The observer always generates events based on ‘function’ calls. Even for class method invocations. This is because, when calling a method (say Tree.add) it’s the im_func field is executed, which is a function type object.
  2. It’s important to know if an event came from invoking a method or a simple function. With methods, the first argument always is an instance. This can be handy when writing an undo management systems in case multiple calls from the same instance do not have to be registered (e.g. if a method set_point() is called with exact coordinates (in stead of deltas), only the first call to set_point() needs to be remembered).


The reverser requires some registration.

  1. Property setters should be declared with reversible_property()
  2. Method (or function) pairs that implement each others reverse operation (e.g. add and remove) should be registered as reversible_pair()’s in the reverser engine. The reverser will construct a tuple (callable, arguments) which are send to every handler registered in the subscribers list. Arguments is a dict().

First thing to do is to actually enable the revert_handler:

>>> state.observers.add(state.revert_handler)

This handler is not enabled by default because:

  1. it generates quite a bit of overhead if it isn’t used anyway
  2. you might want to add some additional filtering.

Point 2 may require some explanation. First of all observers have been added to almost every method that involves a state change. As a result loads of events are generated. In most cases you’re only interested in the first event, since that one contains the state before it started changing.

Handlers for the reverse events should be registered on the subscribers list:

>>> events = []
>>> def handler(event):
...     events.append(event)
...     print 'event handler', event
>>> state.subscribers.add(handler)

After that, signals can be received of undoable (reverse-)events:

>>> canvas.add(Item())                              
event handler (<function _remove at ...>, {'item': <gaphas.item.Item object at 0x...>, 'self': <gaphas.canvas.Canvas object at 0x...>})
>>> canvas.get_all_items()                          
[<gaphas.item.Item object at 0x...>]

As you can see this event is constructed of only two parameters: the function that does the inverse operation of add() and the arguments that should be applied to that function.

The inverse operation is easiest performed by the function saveapply(). Of course an inverse operation is emitting a change event too:

>>> state.saveapply(*events.pop())                  
event handler (<function add at 0x...>, {'item': <gaphas.item.Item object at 0x...>, 'self': <gaphas.canvas.Canvas object at 0x...>, 'parent': None, 'index': 0})
>>> canvas.get_all_items()

Just handling method pairs is one thing. Handling properties (descriptors) in a simple fashion is another matter. First of all the original value should be retrieved before the new value is applied (this is different from applying the same arguments to another method in order to reverse an operation).

For this a reversible_property has been introduced. It works just like a property (in fact it creates a plain old property descriptor), but also registers the property as being reversible.

>>> var = Variable()
>>> var.value = 10                                  
event handler (<function set_value at 0x...>, {'self': Variable(0, 20), 'value': 0.0})

Handlers can be simply removed:

>>> state.subscribers.remove(handler)
>>> state.observers.remove(state.revert_handler)

What is Observed

As far as Gaphas is concerned, only properties and methods related to the model (e.g. Canvas, Item) emit state changes. Some extra effort has been taken to monitor the Matrix class (which is from Cairo).
Canvas: add() and remove()

Position: x and y properties

Handle: connectable, movable, visible, connected_to and disconnect properties

Item: matrix property

Element: min_height and min_width properties

Line: line_width, fuzziness, orthogonal and horizontal properties

Variable: strength and value properties

Solver: add_constraint() and remove_constraint()
Matrix: invert(), translate(), rotate() and scale()

Test cases are described in undo.txt.