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"Gridded Geometry Management"

The second style of geometry management is called gridded. This approach occurs when one of the widgets of an application supports a range of useful sizes. This occurs, for example, in a text editor where the scrollbars, menus, and other adornments are fixed in size but the edit widget can support any number of lines of text or characters per line. In this case, it is usually desirable to let the user specify the number of lines or characters-per-line, either with the wm :geometry command or by interactively resizing the window. In the case of text, and in other interesting cases also, only discrete sizes of the window make sense, such as integral numbers of lines and characters-per-line; arbitrary pixel sizes are not useful.

Gridded geometry management provides support for this kind of application. Tk (and the window manager) assume that there is a grid of some sort within the application and that the application should be resized in terms of grid units rather than pixels. Gridded geometry management is typically invoked by turning on the setGrid option for a widget; it can also be invoked with the wm :grid command or by calling Tk_SetGrid. In each of these approaches the particular widget (or sometimes code in the application as a whole) specifies the relationship between integral grid sizes for the window and pixel sizes. To return to non-gridded geometry management, invoke wm :grid with empty argument strings.

When gridded geometry management is enabled then all the dimensions specified in wm :minsize, wm :maxsize, and wm :geometry commands are treated as grid units rather than pixel units. Interactive resizing is automatically enabled, and it will be carried out in even numbers of grid units rather than pixels. By default there are no limits on the minimum or maximum dimensions of a gridded window. As with ungridded windows, interactive resizing has exactly the same effect as invoking the wm :geometry command. For gridded windows, internally- and externally-requested dimensions work together: the externally-specified width and height determine the size of the window in grid units, and the information from the last wm :grid command maps from grid units to pixel units.

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