# The Interface Kit

Most applications have an interactive graphic user interface. When an app starts, it displays a set of windows in which the user can click and type. The application responds to the user's actions, and updates its window to show the user that it's listening.

To run this kind of user interface, an application has to:

- Manage a set of windows
- Draw within the windows
- Respond to the users's actions (reported as interface messages)

The Interface Kit defines a set of C++ classes that provide a structure for these operations. This chapter first introduces the conceptual framework for the user interface, then describes all the classes, functions, types, and constants the kit defines.

## Framework for the User Interface

A graphical user interface is organized around windows. In a multitasking environment, any number of applications might be running at the same time, each with its own set of windows on-screen. The windows of all running applications must cooperate in a common interface. The Application Server manages this mess. It's the conduit for an application's message input and drawing output:

- It monitors the keyboard and mouse and sends messages reporting each user keystroke and mouse action to the application.

- It receives drawing instructions from the application and interprets them to render images within windows.

### BWindow Objects

Every window in an application is represented by a separate BWindow object. Constructing the BWindow establishes a connection to the Application Server. When you call BWindow's window-manipulating functions (`Show()`, `MoveTo()`, `SetTitle()` and so on), the object sends a message to the server, which performs the actual manipulation.

The BWindow class inherits from BLooper. Every BWindow object spawns a thread (in the application's address space) where it receives and responds to interface messages from the server.

All other Interface Kit objects play roles that depend on a BWindow. They draw in a window, respond to interface messages received by a window, or act in support of other objects that draw and respond to messages.

### BView Objects

A window is divided into smaller rectangular areas called views. Each view corresponds to one part of what the window displays--a scroll bar, a document, a list, and so on.

An application sets up a view by constructing a BView object and associating it with a particular BWindow. The BView object is responsible for drawing within the view rectangle and for handling interface messages directed at that area.

#### Drawing in a View

A window can retain and display rendered images, but it can't draw them; for that it needs a set of BViews. A BView is an agent for drawing, but it can't render the images it creates; for that it needs a BWindow. The two objects work hand in hand.

Each BView object is an autonomous graphics environment for drawing: It has its own coordinate system, current colors, drawing mode, clipping region, font, pen position, and so on. The BView class also defines functions that represent elemental drawing operations such as line stroking, character drawing, and image blitting.

#### Handling Messages in a View

When the user acts, system messages that report the resulting events are sent to the BWindow object, which determines which BView elicited the user action and should respond to it. For example, a BView that draws typed text can expect to respond to messages reporting the user's keystrokes. A BView that draws a button gets to handle the messages that are generated when the button is clicked. The BView class derives from BHandler, so BView objects are eligible to handle messages dispatched by the BWindow.

### The View Hierarchy

A window typically contains a number of different views--all arranged in a hierarchy beneath the top view, a view that's exactly the same size as the content area of the window. The top view is a companion of the window; it's created by the BWindow object when the BWindow is constructed. When the window is resized, the top view is resized to match. Unlike other views, the top view doesn't draw or respond to messages; it serves merely to connect the window to the views that the application creates and places in the hierarchy.

As illustrated in the following diagram, the view hierarchy can be represented as a branching tree structure with the top view at its root. All views in the hierarchy (except the top view) have one, and only one, parent view. Each view (including the top view) can have any number of child views.

In this diagram, the top view has four children, the container view has three, and the border view one.

When a new BView object is created, it isn't attached to a window and it has no parent. It's added to a window by making it a child of a view already in the view hierarchy. This is done with the `AddChild()` function. A view can be made a child of the window's top view by calling BWindow's version of `AddChild()`.

Until it's assigned to a window, a BView can't draw and won't receive reports of events. BViews know how to produce images, but it takes a window to display and retain the images they create.

#### Drawing and Message Handling in the View Hierarchy

The view hierarchy determines what's displayed where on-screen, and also how user actions are associated with the responsible BView object:

- When the views in a window are called on to draw, parents draw before their children; children draw in front of their ancestors.

- Mouse events are associated with the view where the cursor is located.

#### Overlapping Siblings

Siblings don't draw in a predefined order. If they overlap, it's indeterminate which view will draw last--that is, which one will draw in front of the others. Similarly, it's indeterminate which view will receive the mouse events that occur in the area the siblings share.

Therefore, it's strongly recommended that you arrange your sibling views so they don't overlap.

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Last modified March 26, 1998.