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Classic Tips: Class Up Those Widgets With Form Layouts


By Dwight Deugo
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July 30, 2008 —  (Page 1 of 3)
With a nice layout, applications can be not only useful, but also attractive, fun and easy to use. Fortunately, Eclipse has many layout classes to help you position widgets in a view.

The major ones include FillLayout, RowLayout, GridLayout and FormLayout. FillLayout is the simplest layout class and lays out widgets in a single row or column, forcing them to be the same size. Instances of RowLayout determine the size and position of the widgets by placing them in horizontal rows or vertical columns. RowLayout has a number of fields, including height and width, that can be specified by adding a RowData object to the widget using this layout.

Instances of GridLayout arrange widgets in a grid. Like RowLayout, GridLayout has a number of configuration fields, and the widgets it lays out can have an associated layout data object called a GridData.

The layout class I like to use the most is FormLayout. If I am not arranging widgets in a grid, that’s the layout I always choose. A FormLayout controls the position of widgets using a combination of FormData and FormAttachment objects assigned to each widget, which enables me to precisely configure the left, top, right and bottom edges of each widget that is using the layout.

Let’s start with an example. Listing 1 (less the getters and setters for the instance variables) produces the window shown in Figure 1. Notice in the createLayout(..) method that each Button is assigned an instance of FormData, e.g., getFirstButton().setLayoutData(data).

The FormData instance has fields top, bottom, left and right assigned to instances of FormAttachment. The FormAttachment constructor lets me specify a percentage and pixel offset placing the widget’s edge relative to the composite object containing the widget, which in this case is the Shell.

Based on the code below, my FirstButton has its left and top edges connect to the left and top edges of the shell (with a margin offset) and the right edge is 75 percent of the way across. You can also set the height and width of the widget providing you don’t attach a FormAttachment to the FormData bottom field. In the code below, I made my FirstButton 30 pixels in height. The SecondButton was placed using different attachments.



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