FormLayout, FormData and controls

Question

can someone please explain to me how FormData on FormLayout works? I'm trying to understand the FormAttachment constructors and its parameters (numerator, offset, denominator). When do you need only one of them, two of them or all of the parameters... Also, if i have a group (or a composite) which will add widgets like buttons and labels which will need to be resized as per resolution using FormData, does the Group need to have a FormLayout also? I tried specifying a width of a group with form layout but no change

Answer 1

FormAttachment has several different constructors, I generally use two typically:

new FormAttachment (int numerator, int offset) - If there's only two integer parameters then the first parameter represents the percentage from the edge of the parent. Which edge is determined by which slot in the FormData object you insert the FormAttachment into: FormData.top, data.bottom, data.left, and FormData.right.

new FormAttachment (Control control, int offset) - Instead of positioning the object against the parent, this positions the object next to another child object. This works in conjunction with the previous constructor so that you effectively "anchor" one element against the parent, then build your layout by positioning other elements relative to the anchor. This also allows you to easily move and insert elements into the overall layout without having to rebuild the entire layout.

The others are variations on the themes of these two. FormAttachment (int numerator, int denominator, int offset) is the same as the FormAttachment with two integer parameters, but with two the denominator just becomes "100", i.e. it turns the numerator into a percentage. But you could say something like (1, 2, 0) for 1/2 of the way across or (1, 3, 0) for 1/3 of the space.

I'm not sure what you mean about whether the Group will need to have a layout defined but in general every parent Composite must have a layout defined on it to display children elements. If you want elements to resize as the parent resizes, one option is to attach the child to both the left and right sides of the element:

FormData formData = new FormData();
formData.left = new FormAttachment (0, 0); // Attach at the 0% left with 0 offset
formData.right = new FormAttachment (100, 0); // Attach at the 100% right with 0 offset

The Eclipse site has a lot of good snippets showing FormLayout, check out:

http://www.eclipse.org/swt/snippets/

Scroll down to the FormLayout section.