| This chapter introduces the basic concepts used in the Tk graphical user interface toolkit. Tk adds about 35 Tcl commands that let you create and manipulate widgets in a graphical user interface. Tk works with the X window system, Windows, and Macintosh. The same script can run unchanged on all of these major platforms. Tk is a toolkit for programming graphical user interfaces. It was designed for the X window system used on UNIX systems, and it was ported later to the Macintosh and Windows environments. Tk shares many concepts with other windowing toolkits, but you do not need to know much about graphical user interfaces to get started with Tk. Tk provides a set of Tcl commands that create and manipulate widgets. A widget is a window in a graphical user interface that has a particular appearance and behavior. The terms widget and window are often used interchangeably. Widget types include buttons, scrollbars, menus, and text windows. Tk also has a general-purpose drawing widget called a canvas that lets you create lighter-weight items such as lines, boxes, and bitmaps. The Tcl commands added by Tk are summarized at the end of this chapter. Tk widgets are organized in a hierarchy. To an application, the window hierarchy means that there is a primary window, and inside that window there can be a number of children windows. The children windows can contain more windows, and so on. Just as a hierarchical file system has directories (i.e., folders) that are containers for files and directories, a hierarchical window system uses windows as containers for other windows. The hierarchy affects the naming scheme used for Tk widgets as described later, and it is used to help arrange widgets on the screen. Widgets are under the control of a geometry manager that controls their size and location on the screen. Until a geometry manager learns about a widget, it will not be mapped onto the screen and you will not see it. Tk has powerful geometry managers that make it very easy to create nice screen layouts. The main trick with any geometry manager is that you use frame widgets as containers for other widgets. One or more widgets are created and then arranged in a frame by a geometry manager. By putting frames within frames you can create complex layouts. There are three different geometry managers you can use in Tk: grid, pack, and place. The Tk geometry managers are discussed in detail in Chapters 23, 24, and 25. A Tk-based application has an event-driven control flow, like most window system toolkits. The Tk widgets handle most events automatically, so programming your application remains simple. For specialized behaviors, you use the bind command to register a Tcl command that runs when an event occurs. There are lots of events, including mouse motion, keystrokes, window resize, and window destruction. You can also define virtual events, like Cut and Paste, that are caused by different events on different platforms. Bindings are discussed in detail in Chapter 26. Chapter 16 describes I/O events and the Tcl event loop, while Chapter 47 describes C programming and the event loop. Event bindings are grouped into classes, which are called bindtags. The bindtags command associates a widget with an ordered set of bindtags. The level of indirection between the event bindings and the widgets creates a flexible and powerful system for managing events. You can create your own bindtags and dynamically change the bindtags for a widget to support mode changes in your application. A concept related to binding is focus. At any given time, one of the widgets has the input focus, and keyboard events are directed to it. There are two general approaches to focusing: give focus to the widget under the mouse, or explicitly set the focus to a particular widget. Tk provides commands to change focus so you can implement either style of focus management. To support modal dialog boxes, you can forcibly grab the focus away from other widgets. Chapter 36 describes focus, grabs, and dialogs. The basic structure of a Tk script begins by creating widgets and arranging them with a geometry manager, and then binding actions to the widgets. After the interpreter processes the commands that initialize the user interface, the event loop is entered and your application begins running. If you use wish interactively, it creates and displays an empty main window and gives you a command-line prompt. With this interface, your keyboard commands are handled by the event loop, so you can build your Tk interface gradually. As we will see, you will be able to change virtually all aspects of your application interactively. | 
