官术网_书友最值得收藏!

Declaring properties within a class

Here we want to highlight an important difference between traditional Python coding and Kivy, and the usefulness of this change.

Getting ready

We need to remember the traditional form to declare properties in Python. Usually, if we want to declare a property in Python, we do something such as:

class MyClass(object):
    def __init__(self):
        super(MyClass,self).__init__()
        self._numeric_var = 1
@property
    def numeric_var(self): 
        return self._numeric_var

We are declaring a numeric one, whereas if we use MyClass().numeric_var in the Python shell, we get 1 in return.

How to do it…

Now, to declare this property in Kivy we follow these steps:

  1. Import Kivy and its properties
  2. Define the class
  3. Reference the Kivy property, in this case the numeric one:
    import kivy
    
    from kivy.event import EventDispatcher
    from kivy.properties import *
    
    class MyClass(EventDispatcher):
        numeric_var = NumericProperty(1.0)

How it works…

The idea behind this is that you inherit the declaration from Kivy's properties, which reduces the number of code lines.

To use them, you have to declare them at a class level. That is, directly in the class, not in any method for the class. A property is a class attribute that will automatically create instance attributes. Each property, by default, provides an on_<propertyname> event that is called whenever the property's state/value changes.

Something additional to point out is that NumericProperty accepts all the Python numeric values: ints, floats, and longs.

In general, Kivy properties can be overridden easily when creating the instance of the class, using keyword arguments such as ClassName(property=newvalue).

There's more…

They help you to:

  • Easily manipulate widgets defined in the Kv language
  • Automatically observe any changes
  • Check and validate values
  • Optimize memory management

Kivy provides more properties as follows:

  • NumericProperty
  • StringProperty
  • ListProperty
  • ObjectProperty
  • BooleanProperty
  • BoundedNumericProperty
  • OptionProperty
  • ReferenceListProperty
  • AliasProperty
  • DictProperty

See also

These properties actually implement the Observer pattern; if you want to learn more about patterns, you can find information online at http://www.oodesign.com/observer-pattern.html.

主站蜘蛛池模板: 从化市| 博乐市| 金川县| 民乐县| 郯城县| 贵州省| 荣成市| 勐海县| 津市市| 苍梧县| 绥化市| 吴桥县| 晋宁县| 岚皋县| 托克托县| 科技| 陆川县| 宁安市| 乳山市| 佛学| 张家界市| 双流县| 雷波县| 新平| 依安县| 新疆| 南澳县| 天台县| 丰县| 江西省| 神木县| 谷城县| 三亚市| 运城市| 河西区| 奉化市| 光泽县| 民和| 重庆市| 新乡县| 阳泉市|