Mastering Perl/Tk

Marks

There are several ways to refer to different positions throughout the Text widget. An index value refers to a character. A tag is a named reference to a specific character or group of characters. The term mark refers to the spaces between characters. Similar to tags, a mark has a name. For example, the "insert" mark refers to the position of the insert cursor. However, tags refer to the actual characters, and if those characters are deleted, the tag is no longer associated with those characters. The mark stays in place whether the characters surrounding it are deleted or other characters are added. Marks can refer only to one location within the Text widget at a time.

Once a mark is created, you can use it as an index. The gravity of the mark will determine if the mark moves or not when you insert text with it. Using a gravity of 'right' (the default) means that any text inserted using that mark will go to the left of the mark. A gravity of 'left' means that text inserted using that mark will go to the right of the mark. Another way to think about it is the gravity tells you which side the mark will stick to. For example, suppose we have the string "abcdef" and place the mark in between the c and the d. We’ll represent the mark as a ^ character. Using 'right' gravity, "abc^def" becomes "abc1232^def" because the mark sticks to the character to its right. Using 'left' gravity, "abc^def" becomes "abc^123def" because the mark sticks to the character to its left. ...