Most applications are main-window-style applications, that is, they have a menu bar, toolbars, a status bar, a central area, and possibly dock windows, to provide the user with a rich yet navigable and comprehensible user interface. In this chapter, we will see how to create a main-window-style application which demonstrates how to create and use all of these features.
We will use the Image Changer application shown in Figure 6.1 to demonstrate how to create a main-window-style application. Like most such applications it has menus, toolbars, and a status bar; it also has a dock window. In addition to seeing how to create all these user interface elements, we will cover how to relate user interactions with them, to methods that perform the relevant actions.
This chapter also explains how to handle the creation of new files and the opening of existing files, including keeping the user interface synchronized with the application's state. Also covered is how to give the user the opportunity to save unsaved changes, and how to manage a recently used files list. We will also show how to save and restore user preferences, including the sizes and positions of the main window and of the toolbars and dock windows.
Most applications have a data structure for holding their data, and use one or more widgets through which users can view and edit the data. The Image Changer application holds its data in a single QImage object, and uses a QLabel widget as its data viewer. In Chapter 8, we will see a main-window-style application that is used to present and edit lots of data items, and in Chapter 9, we will see how to create main window applications that can handle multiple documents.
Before looking at how to create the application, we will discuss some of the state that a user interface must maintain. Quite often, some menu options and toolbar buttons are "checkable", that is, they can be in one of two states. For example, in a word processor, a toolbar button for toggling italic text could be "on" (pushed down) or "off". If there is also an italic menu option, we must make sure that the menu option and the toolbar button are kept in sync. Fortunately, PyQt makes it easy to automate such synchronization.
Some options may be interdependent. For example, we can have text left-aligned, centered, or right-aligned, but only one of these can be "on" at any one time. So if the user switched on centered alignment, the left and right alignment toolbar buttons and menu options must be switched off. Again, PyQt makes it straightforward to synchronize such interdependent options. In this chapter, we will cover options that are noncheckable, such as "file open", and both independent and interdependent checkable options.
Although some menu and toolbar options can have an immediate effect on the application's data, others are used to invoke dialogs through which users can specify precisely what they want done. Since we have given so much coverage to dialogs in the preceding two chapters, here we will concentrate on how they are used rather than on how they are created. In this chapter we will see how to invoke custom dialogs, and also how to use many of PyQt's built-in dialogs, including dialogs for choosing a filename, the print dialog, and dialogs for asking the user for an item of data, such as a string or a number.
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