Callbacks are used to monitor the existence of a specific event and if that specific event occurs, some other special jobs are invoked. Callbacks are intended to provide additional monitoring capability to any user-defined composite types. As such, Callbacks are generally fields of user defined composite types. When a Callback is called, if the Callback is enabled and its condition function returns true, then its action function is invoked.

A Simple Example

Let's define a test object first that has a field named x of type Int and named callback of type Callback.

julia> mutable struct TestObject

To construct an instance of TestObject, we need to construct a Callback. For that purpose, the condition and action function must be defined. For this example, condition checks whether the x field is positive, and action prints a simple message saying that the x field is positive.

julia> condition(testobject) = testobject.x > 0 
condition (generic function with 1 method)

julia> action(testobject) = println("testobject.x is greater than zero") 
action (generic function with 1 method)

Now a test object can be constructed

julia> testobject = TestObject(-1, Callback(condition, action))  
TestObject(-1, Callback{typeof(condition),typeof(action)}(condition, action, true, "dac6f9eb-6daa-4622-a8fa-623f0f88780c"))

If the callback is called, no action is performed since the condition function returns false. Note the argument sent to the callback. The instance of the TestObject to which the callback is bound.

julia> testobject.callback(testobject) 

Now mutate the test object so that condition returns true.

julia> testobject.x = 3   

Now, if the callback is called, since the condition returns true and the callback is enabled, the action is invoked.

julia> testobject.callback(testobject) 
testobject.x is greater than zero

Full API

mutable struct Callback{CN, AC}

Constructs a Callback from condition and action. The condition and action must be a single-argument function. The condition returns true if the condition it checks occurs, otherwise, it returns false. action performs the specific action for which the Callback is constructed. A Callback can be called by passing its single argument which is mostly bound to the Callback.


  • condition::Any

    Condition function of callback. Expected to be a single-argument function

  • action::Any

    Action of the callback. Expected to be a single-argument fucntion

  • enabled::Bool

    If true, callback is activated

  • id::Base.UUID

    Unique identifier


julia> struct Object  # Define a dummy type.

julia> cond(obj) = obj.x > 0;  # Define callback condition.

julia> action(obj) = println("obj.x = ", obj.x); # Define callback action.

julia> obj = Object(1, Callback(condition=cond, action=action))
Object(1, Callback(condition:cond, action:action))

julia> obj.clb(obj)  # Call the callback bound `obj`.
obj.x = 1

Calls the callbacks of obj if the callbacks are not nothing.


julia> mutable struct MyType{CB}

julia> obj = MyType(5, Callback(condition=obj -> obj.x > 0, action=obj -> println("x is positive")));

julia> applycallbacks(obj)
x is positive

julia> obj.x = -1

julia> applycallbacks(obj)

Returns true if clb is enabled. Otherwise, returns false.