Sauvegarde des attributs des objets scripts

From FreeCAD Documentation
Revision as of 10:00, 5 December 2021 by David69 (talk | contribs) (Created page with "Par défaut, les attributs enregistrés dans une classe d'objets sont ceux du dictionnaire {{incode|__dict__}} de la classe.")
Other languages:

Introduction

Les Objets créés par script sont reconstruits à chaque fois qu'un document au format FCStd est ouvert. Pour ce faire, le document conserve une référence au module et à la classe Python qui ont été utilisés pour créer l'objet, ainsi que ses propriétés.

Les attributs de la classe utilisée pour créer l'objet peuvent également être enregistrés, c'est-à-dire "sérialisés". Ceci peut être contrôlé par les méthodes __getstate__ et __setstate__ de la classe.

Sauvegarde de tous les attributs

Par défaut, les attributs enregistrés dans une classe d'objets sont ceux du dictionnaire __dict__ de la classe.

# various_states.py
class VariousStates:
    def __init__(self, obj):
        obj.addProperty("App::PropertyLength", "Length")
        obj.addProperty("App::PropertyArea", "Area")
        obj.Length = 15
        obj.Area = 300
        obj.Proxy = self

        Type = dict()
        Type["Version"] = "Custom"
        Type["Release"] = "production"
        self.Type = Type
        self.Type = "Custom"
        self.ver = "0.18"
        self.color = (0, 0, 1)
        self.width = 2.5

    def execute(self, obj):
        pass

An object can be created using this class, and it can be saved to my_document.FCstd. If no particular viewprovider is assigned to the new object, its proxy class is simply set to a value different from None, in this case, to 1.

import FreeCAD as App
import various_states

doc = App.newDocument()
doc.FileName = "my_document.FCStd"

obj = doc.addObject("Part::FeaturePython", "Custom")
various_states.VariousStates(obj)

if App.GuiUp:
    obj.ViewObject.Proxy = 1

doc.recompute()
doc.save()

When we re-open the file we can inspect the dictionary of the object's class.

>>> obj = App.ActiveDocument.Custom
>>> print(obj.Proxy)
<various_states.VariousStates object at 0x7f0a899bde10>
>>> print(obj.Proxy.__dict__)
{'Type': {'Version': 'Custom', 'Release': 'production'}, 'ver': '0.18', 'color': [0, 0, 1], 'width': 2.5}

We see that all attributes that start with self in the class were saved. These can be of different types, including string, list, float, and dictionary. The original tuple for self.color was converted to a list, but otherwise all attributes were loaded the same.

Saving specific attributes

We can define a class similar to the first one, that implements specific attributes to save.

# various_states.py
class CustomStates:
    def __init__(self, obj):
        obj.addProperty("App::PropertyLength", "Length")
        obj.addProperty("App::PropertyArea", "Area")
        obj.Length = 15
        obj.Area = 300
        obj.Proxy = self

        Type = dict()
        Type["Version"] = "Custom"
        Type["Release"] = "production"
        self.Type = Type
        self.ver = "0.18"
        self.color = (0, 0, 1)
        self.width = 2.5

    def execute(self, obj):
        pass

    def __getstate__(self):
        return self.color, self.width

    def __setstate__(self, state):
        self.color = state[0]
        self.width = state[1]

The return value of __getstate__ is the object that will be serialized. This can be a single value, or a tuple of values. When the object is restored, the class calls the __setstate__ method, passing the state variable with the serialized content. In this case state is a tuple that is unpacked into the respective variables to reconstruct the "state" that original existed.

state = (self.color, self.width)
state = ((0, 0, 1), 2.5)

We can create an object with this class, and save the document, just like in the previous example. When we re-open the file we can inspect the dictionary of the object's class.

>>> obj2 = App.ActiveDocument.Custom2
>>> print(obj2.Proxy)
<various_states.CustomStates object at 0x7fb399496630>
>>> print(obj2.Proxy.__dict__)
{'color': [0, 0, 1], 'width': 2.5}

The original tuple for self.color was converted to a list, but otherwise the information was recovered fine. Instead of restoring all attributes like in the previous case, only the attributes that we specified in __getstate__ and __setstate__ were restored.

Usage

Identifying the type

Normally, scripted objects should use properties to store information, as these are automatically restored when the document is opened.

However, sometimes the class restore internal information which is not intended to be modified, but which is helpful to inspect.

For example, most objects of the Draft Workbench set up a Type attribute that can be used to determine the type of object that is under use.

class DraftObject:
    def __init__(self, obj, _type):
        self.Type = _type

    def __getstate__(self):
        return self.Type

    def __setstate__(self, state):
        if state:
            self.Type = state

All objects have a TypeId property, but for scripted objects this property is not useful, as it always refers to the parent C++ class, for example, Part::Part2DObjectPython or Part::FeaturePython. Therefore, having this additional Proxy.Type attribute in the class is useful to treat each object in a particular way.

Migrating the object

Version information can be stored inside the class in order to verify the origin of an object.

class CustomObject:
    def __init__(self, obj, _type):
        self.Type = _type
        self.version = "0.18"

    def __getstate__(self):
        return self.Type, self.version

    def __setstate__(self, state):
        if state:
            self.Type = state[0]
            self.version = state[1]

If the structure of the class changes, that is, if its properties or methods change, are renamed, or are removed, we could test the version attribute in order to migrate the older object to a new set of properties or to a new class. This can be done by implementing the onDocumentRestored method, as explained in Scripted objects migration.

class CustomObject:
    def onDocumentRestored(self, obj):
        if hasattr(obj.Proxy, "version") and obj.Proxy.version:
            if obj.Proxy.version == "0.18":
                self.migrate_from_018(obj)

    def migrate_from_018(self, obj):
        ...

Links