event.ClassInstanceEvent Class
Namespace: event
Superclasses: event.EventData
Event data for InstanceCreated
and InstanceDestroyed
events
Description
This class defines the event data that is passed to listeners of the
matlab.metadata.Class
InstanceCreated
and InstanceDestroyed
events.
The event.ClassInstanceEvent
class is a handle
class.
Class Attributes
Sealed | true |
ConstructOnLoad | true |
HandleCompatible | true |
RestrictsSubclassing | true |
For information on class attributes, see Class Attributes.
Creation
MATLAB® creates an event.ClassInstanceEvent
object when triggering a InstanceCreated
or InstanceDestroyed
event.
Properties
Instance
— Object being created or deleted
object
Object being created or deleted.
Attributes:
GetAccess | public |
SetAccess | private |
GetObservable | true |
SetObservable | true |
Data Types: object
Source
— matlab.metadata.Class
object that is the source of the event
matlab.metadata.Class
matlab.metadata.Class
object that is the source of the event,
returned as a matlab.metadata.Class
object.
Attributes:
GetAccess | public |
SetAccess | private |
GetObservable | true |
SetObservable | true |
EventName
— Name of the event
character vector
Name of the event, returned as a character vector that is either InstanceCreated
or InstanceDestroyed
.
Attributes:
GetAccess | public |
SetAccess | private |
GetObservable | true |
SetObservable | true |
Data Types: char
Examples
Count Number of Instances
Use matlab.metadata.Class
events to count the number
of instances as objects are created and destroyed.
Create a class with a callback function for the InstanceCreated
and
InstanceDestroyed
events. The eventCallback
static
method uses a persistent variable to store the number of instances of the class that
exist. The addEventListeners
method adds the listeners to the
matlab.metadata.Class
object for the CountInstances
class.
classdef CountInstances methods (Static) function eventCallback(~,eventData) % Callback for InstanceCreated and InstanceDestroyed persistent instanceCount if ~isempty(instanceCount) switch eventData.EventName case "InstanceCreated" instanceCount = instanceCount + 1; case "InstanceDestroyed" if ~instanceCount == 0 instanceCount = instanceCount - 1; end end else instanceCount = 1; end fprintf('%s %d \n',... 'Number of Instances: ',instanceCount) end function addEventListeners(mc) % Add listeners addlistener(mc,"InstanceCreated",... @(src,evnt)CountInstances.eventCallback(src,evnt)); addlistener(mc,"InstanceDestroyed",... @(src,evnt)CountInstances.eventCallback(src,evnt)); end end end
Create a matlab.metadata.Class
object for the
CountInstances
class and add listeners for the
InstanceCreated
and InstanceDestroyed
events. This
example uses the same callback for both events.
mc = ?CountInstances; CountInstances.addEventListeners(mc)
Whenever you create or destroy an object of the CountInstances
class,
the event causes an update to the instanceCount
persistent
variable.
obj1 = CountInstances;
Number of Instances: 1
obj2 = CountInstances;
Number of Instances: 2
clear obj1
Number of Instances: 1
Observe Creation and Destruction Events
Use matlab.metadata.Class
events to observe the
creation and destruction of objects
Create a class with a callback function for the InstanceCreated
and
InstanceDestroyed
events. The eventCallback
static
method serves as the callback function for both events. The
addEventListeners
method adds the listeners to the
matlab.metadata.Class
object for the ClassInstanceEvent
class.
classdef ClassInstanceListeners properties Prop end methods function obj = ClassInstanceListeners(p) obj.Prop = p; end end methods (Static) function eventCallback(~,eventData) % Callback for InstanceCreated and InstanceDestroyed I = eventData.Instance; S = eventData.Source; E = eventData.EventName; dashLine = sprintf('%s\n','--------------------'); fprintf('%s',dashLine) fprintf('%s \n',['Class: ',class(I)]) fprintf('%s %d \n',[S.PropertyList.Name ': '],I.Prop) fprintf('%s%s \n','Event: ', E) fprintf('%s',dashLine) end function addEventListeners(mc) % Add listeners addlistener(mc,"InstanceCreated",... @(src,evnt)ClassInstanceListeners.eventCallback(src,evnt)); addlistener(mc,"InstanceDestroyed",... @(src,evnt)ClassInstanceListeners.eventCallback(src,evnt)); end end end
Create a matlab.metadata.Class
object for the
CreateInstanceListeners
class and add listeners for the
InstanceCreated
and InstanceDestroyed
events. This
example uses the same callback for both events. Construct an instance of the
CreateInstanceListeners
class and assign an identifier to the property
Prop
.
mc = ?ClassInstanceListeners; ClassInstanceListeners.addEventListeners(mc) obj = ClassInstanceListeners(1334);
-------------------- Class: ClassInstanceListeners Prop: 1334 Event: InstanceCreated --------------------
Constructing another instance that is assigned to the same variable creates a new object and destroys the old object.
obj = ClassInstanceListeners(7335);
-------------------- Class: ClassInstanceListeners Prop: 7335 Event: InstanceCreated -------------------- -------------------- Class: ClassInstanceListeners Prop: 1334 Event: InstanceDestroyed --------------------
If you modify the class definition, MATLAB deletes the matlab.metadata.Class
object because it is no
longer a valid description of the class. After modifying the class, you must create a
new matlab.metadata.Class
object and add listeners to it.
Version History
Introduced in R2008a
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