Tag List

A Tag List is a way to pass a variable number of arguments to a C function. It's like using an associative array in a scripting language.

It's implemented as C varargs containing a list of TagItems and ending with the special TAG_DONE value:

TagItem1, TagItem2, ..., TAG_DONE

In turn, a TagItem is a struct containing 2 fields:

• a key, ti_Tag, an identifier (unsigned integer), commonly called the Tag;
• a value, ti_Data, able to contain either an integer or a pointer.

Finally, in a C source code, using a Tag List looks like:

function(some, args, Tag1, value1, Tag2, value2, Tag3, value3, TAG_DONE);

Tag Lists are used in many places in AROS, especially in functions related to the windowed environment. BOOPSI (and thus MUI/Zune) is built upon them:

• all the method calls are done with the same function: the method and its arguments are passed as a Tag List;
• class instantiations, attribute setters and getters all use Tag Lists.

Class

A class is defined by its name, its parent class and a dispatcher.

• Name: this is either a string for the public classes, so that they may be used by any program in the system, or none if its a private class used only by a single application.
• Parent class: all BOOPSI classes are part of a hierarchy rooted in the class aptly named rootclass. It allows each subclass to implement its own version of a specific parent operation, or to fall back on the one provided by its parent. A parent class is also called a base class or super class.
• Dispatcher: this gives access to all operations (called methods) provided by this class, ensuring that each operation is handled by the proper code or, if it is unknown to the current class, passed to its super class.

The BOOPSI type for a class is Class *, also known as IClass.

Object

An object is an instance of class: each object has its specific data, but all objects of the same class share the same behavior (through a pointer to their IClass). An object has several classes if we count the parents of its true class (the most derived one) up to the rootclass.

The BOOPSI type for an object is Object *. It has no fields tthat can be accessed directly.

Attribute

An attribute is related to the instance data of each object: you can't access these data directly, you can only set or get the attributes provided by an object to modify its internal state. An attribute is implemented as a Tag (ULONG value or'ed with TAG_USER).

GetAttr() and SetAttrs() are used to modify an object's attributes.

Attributes can be one or more of the following:

• Initialization-settable (I) : the attribute can be given as parameter at the object creation.
• Settable (S) : You can set this attribute at any time (or at least, not only creation).
• Gettable (G) : You can get the value of this attribute.
• Private (P) : Your application shouldn't use this attribute, because its behaviour is not guaranteed outside of a certain context, not guaranteed in future release, may break the framework or some other reason.

Method

A BOOPSI method is a function which receives as parameters an object, a class and a message:

• object: the object you act on
• class: the considered class for this object.
• message: contains a method ID which determines the function to call within a dispatcher, and is followed by its parameters.

To send a message to an object, use DoMethod(). It will use the true class first. If the class implements this method, it will handle it. Else it will try its parent class, until the message is handled or the rootclass is reached (in this case, the unknown message is silently discarded). If a method is documented as private, you shouldn't call it in your own application, unless you want to risk portability and/or reliablity.

Identifiers

In this object-oriented framework, both attributes and methods are identified by a Tag (see above). Attribute access and method dispatch are based on those identifiers (usually by comparison in a C switch statement).

As you won't use both attributes and methods in the same context, both have their own identifier space: identifier for a method can be the same as that for an attribute without fear of conflict.

Creating identifiers is only a concern when creating your own classes. If you intend to do a public class, you should allocate some space in the Zune identifier space. Else you have to make sure you use a range not reserved for Zune, MUI legacy or AROS classes.

Getting an attribute

Querying a MUI String object for its content:

void f(Object *string)
{
    IPTR result;

    GetAttr(string, MUIA_String_Contents, &result);
    printf("String content is: %s\n", (STRPTR)result);
}

• Object * is the type of BOOPSI objects.
• IPTR must be used for the type of the result, which can be an integer or a pointer. An IPTR is always written in memory, so using a smaller type could lead to memory corruption!
• This is to query a MUI String object for its content: MUIA_String_Contents, as any other attribute, is a ULONG (it's a Tag).

Most Zune applications use the get() and XGET() macros instead:

get(string, MUIA_String_Contents, &result);

result = XGET(string, MUIA_String_Contents);

Setting an attribute

Changing the content of the string:

SetAttrs(string, MUIA_String_Contents, (IPTR)"hello", TAG_DONE);

• Pointer parameters must be cast to IPTR to avoid warnings.
• After the object parameter, a taglist is passed to SetAttrs; the list ends with TAG_DONE.

Again, there's a macro - set():

set(string, MUIA_String_Contents, (IPTR)"hello");

But it's only with SetAttrs() that you can set several attributes at once:

SetAttrs(string,
         MUIA_Disabled, TRUE,
         MUIA_String_Contents, (IPTR)"hmmm...",
         TAG_DONE);

Calling a method

Here's the most called method in a Zune program, the event processing method called in your main loop:

result = DoMethod(obj, MUIM_Application_NewInput, (IPTR)&sigs);

• Parameters are not a taglist, and thus don't end with TAG_DONE.
• You have to cast pointers to IPTR to avoid warnings.

General

We don't manually open libraries; it's done automatically for us.

GUI creation

We use a macro-based language to easily build our GUI. A Zune application has always 1 and only 1 Application object:

:    app = ApplicationObject,

An application can have 0, 1 or more Window objects. Most often a single one:

:        SubWindow, wnd = WindowObject,

Be nice, give a title to the window:

:        MUIA_Window_Title, "Hello world!",

A window must have 1 and only 1 child, usually a group. This one is vertical, that means that its children will be arranged vertically:

:        WindowContents, VGroup,

A group must have at least 1 child, here it's just a text:

:            Child, TextObject,

Zune accepts various escape codes (here, to center the text) and newlines:

:            MUIA_Text_Contents, "\33cHello world!\nHow are you?",

An End macro must match every xxxObject macro (here, TextObject):

:            End,

As a second child of the group, a button! With a keyboard shortcut o indicated by an underscore:

:            Child, but = SimpleButton("_Ok"),

Finish the group:

:            End,

Finish the window:

:        End,

Finish the application:

:        End;

All done, without a GUI builder.

Error handling

If any of the object in the application tree can't be created, Zune destroys all the objects already created and application creation fails. If not, you have a fully working application:

:    if (app != NULL)
:    {
:        ...

When you're done, just call MUI_DisposeObject() on your application object to destroy all the objects currently in the application, and free all the resources:

:       ...
:        MUI_DisposeObject(app);
:    }

Notifications

Notifications are the simplest way to react on events. The principle? We want to be notified when a certain attribute of a certain object is set to a certain value:

:        DoMethod(wnd, MUIM_Notify, MUIA_Window_CloseRequest, TRUE,

This listens to the MUIA_Window_CloseRequest of the Window object and get notified whenever this attribute is set to TRUE. So what happens when a notification is triggered? A message is sent to an object; in this case the Application is told to return MUIV_Application_ReturnID_Quit on the next event loop iteration:

:                 (IPTR)app, 2,
:                 MUIM_Application_ReturnID, MUIV_Application_ReturnID_Quit);

As anything could be specified here, the number of extra parameters we are supplying to MUIM_Notify has to be specified first; in this case: 2 parameters.

For the button, the program listens to the MUIA_Pressed attribute: it's set to FALSE whenever the button is being released (reacting when it's pressed is bad practice, you may want to release the mouse outside of the button to cancel your action - plus the user would want to see how it looks when it's pressed). The action is the same as the previous, send a message to the application:

:        DoMethod(but, MUIM_Notify, MUIA_Pressed, FALSE,
:                 (IPTR)app, 2,
:                 MUIM_Application_ReturnID, MUIV_Application_ReturnID_Quit);

Opening the window

Windows aren't open until you ask them to:

:        set(wnd, MUIA_Window_Open, TRUE);

If all goes well, your window should be displayed at this point. But it can fail! So don't forget to check by querying the attribute, which should be TRUE:

:        if (XGET(wnd, MUIA_Window_Open))

Main loop

Introducing - the ideal Zune event loop:

:        ULONG sigs = 0;

Don't forget to initialize the signals to 0 ... The test of the loop is the MUIM_Application_NewInput method:

:        ...
:        while((LONG) DoMethod(app, MUIM_Application_NewInput, (IPTR)&sigs)
:              != MUIV_Application_ReturnID_Quit)

It takes as input the signals of the events it has to process (result from Wait(), or 0), will modify this value to place the signals Zune is waiting for (for the next Wait()) and will return a value. This return value mechanism was historically the only way to react on events, but it was ugly and has been deprecated in favor of custom classes and object-oriented design.

The body of the loop is quite empty, we only wait for signals and handle Ctrl-C to break out of the loop:

:        {
:            if (sigs)
:            {
:                sigs = Wait(sigs | SIGBREAKF_CTRL_C);
:                if (sigs & SIGBREAKF_CTRL_C)
:                    break;
:            }
:        }

Conclusion

This program gets you started with Zune, and allows you to toy with GUI design, but not more.

Compiling

To compile this simple program with i386-aros-gcc cross-compiler please use this command:

i386-aros-gcc -o hello -D__AROS__ hello.c -lmui