View source for Locating Objects in the Object Tree

''Grzegorz Kraszewski''
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<small>This article in other languages: [[Wyszukiwanie obiektów w drzewie|Polish]]</small>


After the complete object tree is created, there is no direct access to any object except the main ''Application'' object. A way to access other objects is needed. There are a few ways to do this:
* Storing pointers to objects in global variables. This is the simplest way and may work well in simple projects. The disadvantage is it breaks object oriented design principles (like data encapsulation) and creates a mess when the number of global variables reaches 50, 100 or more. 
* Store pointers in fields of some subclass instance data (for example the ''Application'' instance). A good idea, but a bit tedious to implement. An object's instance data area does not exist until the object is created (to be precise &ndash; until ''rootclass'' constructor is executed) and the ''Application'' object is created as the last one. Then pointers to subobjects have to be stored in some temporary variables. This technique also requires that a parent object of the cached one is an instance of a custom (subclassed) class and the parent creates its subobjects inside the constructor, which is not always true.
* Use the ''MUIA_UserData'' attribute and the ''MUIM_FindUData()'' method to find objects dynamically. This is the best solution when objects are accessed rarely (for example once, just to set [[Event Driven Programming, Notifications#Notifications in MUI|notifications]]). For frequently accessed objects (let's say several times a second) it may be combined with caching objects' pointers in an instance data of some subclassed object.
The last approach works as follows: every object to be searched has the ''MUIA_UserData'' attribute set to some predefined unique value. Then at any time the object may be found by this value using the ''MUIM_FindUData()'' method on a direct or indirect parent object, for example on the master ''Application'' object.

 #define OBJ_SOME_BUTTON 36
 
 /* Somewhere in the initial tags for the button */
 MUIA_UserData, OBJ_SOME_BUTTON,
 /* ... */
 
 /* Let's get the pointer to the button now */
 
 Object *some_button;
 
 some_button = (Object*)DoMethod(App, MUIM_FindUData, OBJ_SOME_BUTTON);

This operation is so common that it is usually encapsulated in a macro:

 #define findobj(id, parent) (Object*)DoMethod(parent, MUIM_FindUData, id)
 
 some_button = findobj(OBJ_SOME_BUTTON, App);

The macro may of course be used directly in other functions, like in this example changing the button label:

 set(findobj(OBJ_SOME_BUTTON, App), MUIA_Text_Contents, "Press Me");

Note that the ''findobj()'' macro is not defined in the system MUI headers, so it should be defined in the application code.