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Controlling Property Access

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Property Access Methods Property Set Methods Property Get Methods Set and Get Method Execution and Property Events Access Methods and Subscripted Reference and Assignment Performing Additional Steps with Property Access Methods

Property Access Methods

Property access methods enable you to execute code whenever properties values are referenced or assigned a new value. These methods enable you to:

• Execute code before making an assignment to property values to impose range restrictions, check for proper types and dimensions, provide error handling, and so on.

• Execute code before returning the current value of properties to:

  • Calculate the value of properties that do not store values

  • Change the value of other properties

  • Trigger events, and so on

Property access methods automatically execute whenever you query or set object properties. However, you can define property access only for concrete properties (that is, properties that are not abstract).

MATLAB has no default set or get property access methods. Therefore, if you do not define property access methods, MATLAB software does not invoke any methods before assigning or returning property values.

Only the set and get methods can set and get the actual property values. You cannot call another function from the set or get method and attempt to set or get the property value from that function.

Note   Property set and get access methods are not equivalent to user-callable set and get methods used to access property values from an instance of the class. See Implementing a Set/Get Interface for Properties for information on user-callable set and get methods.

Access Methods Cannot Call Other Functions to Access Values

You can set and get property values only from within your property set or get access method. You cannot call another function from the set or get method and attempt to access the property value from that function.

For example, an anonymous function that calls another function to do the actual work cannot access the property value. Similarly, an ordinary access function cannot call another function to access the property value.

Defining Access Methods

Access methods have special names that include the property's name. Therefore, get.PropertyName executes whenever PropertyName is referenced and set.PropertyName executes whenever PropertyName is assigned a new value.

Define property access methods in a methods block that specifies no attributes. You cannot call these methods, MATLAB calls them when any code accesses the properties. Therefore, property access methods do not appear in the list of class methods returned by the methods command and are not included in the meta.class object's Methods property. However, the meta.property object's SetMethod property contains a function handle to the property's set method and the GetMethod property contains a function handle to the property's get method.

For example, if the class myClass defines a set function for its Text property, you can obtain a function handle to this method from the meta.class object:

m = ?myClass;
m.Properties{1}.SetMethod % Assuming Text is the first property in the cell array
ans = 
   @\mydir\@myClass\myClass.m>myClass.set.Text % This is a function handle

The meta.class object (m) contains meta.property objects corresponding to each class property in its Properties property. This example assumes that the Text property corresponds to the first meta.property object in the cell array of meta.property objects. The order of the class properties in the meta.class Properties property is the same as the order in which the class definition defines the properties.

Working with Meta-Classes provides more information on using meta-classes.

Function Handles discusses the use of function handles.

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Property Set Methods

Property set methods have the following syntax, where PropertyName is the name of the property.

methods % No method attributes
   function obj = set.PropertyName(obj,value) % Value class
end

Here obj is the object whose property is being assigned a value and value is the new value that is assigned to the property.

Value class set functions must return the object with the new value for the property assigned. Value classes replace the object whose property is being assigned with the object returned by the set method. Handle classes do not need to return the modified object.

methods % No method attributes
   function set.PropertyName(obj,value) % Handle class
end

The property set method can perform actions like error checking on the input value before taking whatever action is necessary to store the new property value.

function obj = set.PropertyName(obj,value)
   if ~(value > 0)
      error('Property value must be positive')
   else
      obj.PropertyName = value;
   end
end

See Restricting Properties to Specific Values for an example of a property set method.

Set Method Behavior

MATLAB software calls a property set method whenever a property value is assigned, if a set method for that property exists. However, property set methods are NOT called in the following cases:

• Assigning a value to a property from within its own property set method, to prevent recursive calling of the set method

• Assigning a property to its default initial value

• Initial values specified in class definitions do not invoke the set method

• Copying a value object (that is, not a handle object). Neither the set or get method is called when copying property values from one object to another.

It is possible for a set method from one property to assign values to other properties of the object. However, assignments made from property set methods cause the execution of any set methods defined for those properties.

When assigning a property value, the calling function's copy of the object that has been passed to the set method reflects the changed value.

Therefore, an assignment to even a single property is able to affect the whole object. This behavior enables a set method to change other properties in the object as well as its designated property. For example, a graphics window object can have a Units property and a Size property. Changing the Units property can also require a change to the values of the Size property to reflect the new units.

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Property Get Methods

MATLAB calls a property's get method whenever the property value is queried. For example, passing a property value in the following statement causes the method get.XYData to execute, if it exists.

plot(obj.XYData)

Property get methods have the following syntax, where PropertyName is the name of the property. The function must return the property value.

methods % No method attributes
   function value = get.PropertyName(obj)
end

Dependent Properties — Values Not Stored

One application of a property get method is to determine the value of a property only when it you need it, and avoid storing the value. To use this approach, set the property's Dependent attribute to true:

properties (Dependent = true)
   PropertyName
end

Now the get method for the PropertyName property determines the value of that property and assigns it to the object from within the method:

function value = get.PropertyName(obj)
   value = calculateValue;
   ...
end

The get method calls a function or static method calculateValue to calculate the property value and returns value to the code accessing the property. The property get method can take whatever action is necessary within the method to produce the output value.

Dependent Properties provide an example of a property get method.

When to Use Set Methods with Dependent Properties

While a dependent property does not store its value, there are situations in which you might want to define a set method for a dependent property.

For example, suppose you have a class that changes the name of a property from OldPropName to NewPropName. You want to continue to allow the use of the old name without exposing it to new users. You can make OldPropName a dependent property with set and get methods as show in the following example:

properties
   NewPropName
end
properties (Dependent, Hidden)
   OldPropName
end
methods
   function obj = set.OldPropName(obj,val)
      obj.NewPropName = val;
   end
   function value = get.OldPropName(obj)
      value = obj.NewPropName;
   endend

There is no memory wasted by storing both old and new property values, and code that accesses OldPropName continues to work as expected.

When to Use Private Set Access with Dependent Properties

If you use a dependent property only to return a value, then do not define a set access method for the dependent property. Instead, set the SetAccess attribute of the dependent property to private. For example, consider the following get method for the MaxValue property:

methods
   function mval = get.MaxValue(obj)
      mval = max(obj.BigArray(:));
   end
end

This example uses the MaxValue property to return a value that it calculates only when queried. For this application, define the MaxValue property as dependent and private:

properties (Dependent, SetAccess = private)
   MaxValue
end

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Set and Get Method Execution and Property Events

MATLAB software generates events before and after set and get operations. You can use these events to inform listeners that property values have been referenced or assigned. The timing of event generation is as follows:

• PreGet — Triggered before calling the property get method

• PostGet — Triggered after the property get method has returned its value

If a class computes a property value (Dependent = true), then the behaviors of its set events are like the get events:

• PreSet — Triggered before calling the property set method

• PostSet — Triggered after calling the property set method

If a property is not computed (Dependent = false, the default), then the assignment statement with the set method generates the events:

• PreSet — Triggered before assigning the new property value within the set method

• PostSet — Triggered after assigning the new property value within the set method

Events and Listeners — Concepts provides general information about events and listeners.

Creating Property Listeners provides information about using property events.

Implementing the PostSet Property Event and Listener shows an example of a property listener.

Responding to a Push Button is another example that uses property events.

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Access Methods and Subscripted Reference and Assignment

You can use subscripting as a way to reference or assign property values (that is, a = obj.prop(6) or obj.prop(6) = a) without interfering with property set and get methods. When using subscripted reference, the get method returns the whole property value and MATLAB accesses the value referenced by subscripting that object.

For subscripted assignment, MATLAB:

• Invokes the get method to get the property value

• Performs the subscripted assignment into the returned property

• Passes the new property value to the set method

MATLAB always passes scalar objects to set and get methods. When reference or assignment occurs on an object array, the set and get methods are called in a loop.

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Performing Additional Steps with Property Access Methods

Property access methods are useful in cases where you want to perform some additional steps before assigning or returning a property value. For example, the Testpoint class uses a property set method to check the range of a value. It then applies scaling if it is within a particular range, and set it to NaN if it is not.

The property get methods applies a scale factor before returning its current value:

classdef Testpoint
   properties
      expectedResult = [];
   end
   properties(Constant,SetAccess = private,GetAccess = private)
      scalingFactor = 0.001;
   end
   methods
      function obj = set.expectedResult(obj,erIn)
         if erIn >= 0 && erIn <= 100
            erIn = erIn.*obj.scalingFactor
            obj.expectedResult = erIn;
         else
            obj.expectedResult = NaN;
         end
      end
      function er = get.expectedResult(obj)
         er = obj.expectedResult/scalingFactor;
      end
   end
end

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Property Attributes

Dynamic Properties — Adding Properties to an Instance

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