Avoid Property Initialization Order Dependency
Control Property Loading
Problems can occur if property values depend on the order in which
load sets the property values.
Suppose that your class design is such that both of the following are true:
A property set method changes another property value.
A property value is computed from other property values.
Then the final state of an object after changing a series of property values can depend on the order in which you set the properties. This order dependency can affect the result of loading an object.
load function sets property values in a particular order. This order can be different from the order in which you set the properties in the saved object. As a result, the loaded object can have different property values than the object had when it was saved.
Restore Nondependent Properties
If a property set function changes the values of other properties, then define the
Dependent attribute of that property as
true. MATLAB® does not save or restore dependent property values.
Use nondependent properties for storing the values set by the dependent property. Then the
load function restores the nondependent properties with the same values that were saved. The
load function does not call the dependent property set method because there is no value in the saved file for that property.
Dependent Property with Private Storage
Odometer class avoids order dependences when loading objects by controlling which properties are restored when loading:
Unitsproperty is dependent. Its property set method sets the
loaddoes not call the
Unitsproperty set method.
The load function restores
TotalDistanceto whatever value it had when you saved the object.
classdef Odometer properties(Constant) ConversionFactor = 1.6 end properties TotalDistance = 0 end properties(Dependent) Units end properties(Access=private) PrivateUnits = 'mi' end methods function unit = get.Units(obj) unit = obj.PrivateUnits; end function obj = set.Units(obj,newUnits) % validate newUnits to be a char vector switch(newUnits) case 'mi' if strcmp(obj.PrivateUnits,'km') obj.TotalDistance = obj.TotalDistance / ... obj.ConversionFactor; obj.PrivateUnits = newUnits; end case 'km' if strcmp(obj.PrivateUnits,'mi') obj.TotalDistance = obj.TotalDistance * ... obj.ConversionFactor; obj.PrivateUnits = newUnits; end otherwise error('Odometer:InvalidUnits', ... 'Units ''%s'' is not supported.', newUnits); end end end end
Suppose that you create an instance of
Odometer and set the following property values:
odObj = Odometer; odObj.Units = 'km'; odObj.TotalDistance = 16;
When you save the object:
ConversionFactoris not saved because it is a
Unitsis not saved because it is a
PrivateUnitsis saved and provides the storage for the current value of
When you load the object:
ConversionFactoris obtained from the class definition.
Unitsis not loaded, so its set method is not called.
PrivateUnitsis loaded from the saved object.
Units property was not
Dependent, loading it calls its set method and causes the
TotalDistance property to be set again.
Property Value Computed from Other Properties
TripDistance property depends only on the values of two other properties,
The class avoids order dependence when initializing property values during the load process by making the
TripDistance property dependent. MATLAB does not save or load a value for the
TripDistance property, but does save and load values for the two properties used to calculate
TripDistance in its property get method.
classdef Odometer2 properties TotalDistance = 0 TripMarker = 0 end properties(Dependent) TripDistance end methods function distance = get.TripDistance(obj) distance = obj.TotalDistance - obj.TripMarker; end end end