1.111 class obtain

Class ? is (pronounced as obtainer) is a template for the virtual machine instruction get in the same fashion as a message object is a template to the send operation. Obtainers are the most commonly used function objects.

Obtainers add a very powerful mechanism to PCE. Understanding them is critical for writing readable and fast PCE programs. Please study the examples. In the example below, an obtainer is used to combine dialog_item selections to create a single call to the application:

?- new(D, dialog),
   send(D, append, new(label)),		% for feedback
   send(D, append, new(Name, text_item(name, ''))),
   send(D, append, new(Age,  int_item(age))),
   send(Age, hor_stretch, 100),		% stretch to right of D
   send(D, append,
        button(create, message(@prolog, create_person,
                               Name?selection,
                               Age?selection))),
   send(D, open).

Of course, the application must define a predicate create_person/2.

See also

- @arg1
- topic Checking
- class get_method
- class block
- topic Obtainers
- object<-_slot
- object<-_references
- object<-_inspect
- object<-_flags
- object<-_class_name
- object<-_class
- object<-_arity
- object<-_arg
- object->_inspect
- object->_free

1.111.1 Instance variables

obtain <->_context:

Contains the Prolog module if the obtainer refers to a Prolog predicate. Here is an example of an obtainer using Prolog:

?- send(@pce, writeln, ?(@prolog,current_prolog_flag,version)). 100109

1.111.2 Send methods

obtain ->initialise: receiver=object|function, selector=name|function, argument=any|function ...

Create a get-template from its receiver, selector and argument list. Note that all arguments may be function objects. When the obtainer is evaluated it first evaluates all functions. Example:

?- new(B, box(100,100)),
   send(@pce, writeln, B?center?x).
50

See also class message.

1.111.3 Get methods

obtain <-_execute: ->

Evaluate the obtainer. Function objects in the obtain<-receiver, obtain<-selector or obtain<-arguments are evaluated first, after which the XPCE virtual machine operation

get(<-receiver, <-selector, <-arguments ..., Value)

is performed and the resulting Value is the result of the obtainer.