Text Selection

The two actions Extend Selection and Shrink Selection in IntelliJ Platform IDEs let you adjust selected text based on the structure of the source code. This makes it easy to select not only expressions, blocks, and function definitions, but also code like whole lines or tags in Javadoc comments.

When implementing a custom language, the IntelliJ Platform provides basic implementations of this EP, allowing you to select code based on your PSI structure and to select whole lines. In many cases this is sufficient to provide a good user experience. However, sometimes it's advantageous to provide additional regions that the user may wish to be able to select when extending or shrinking a selection.

This EP has two methods that need to be implemented:

1. canSelect(PsiElement) is called on each PSI element, starting from the element at the cursor and walking up each of its parents. Return true for a particular element to indicate that further text-ranges should be included for the PSI element.

2. select(PsiElement, CharSequence, int, Editor) returns the text-ranges within the PSI element of interest are calculated and returned.

A possible use-case for custom language developers is a function call f(a, b) where the function call node has its two arguments as children. If the cursor is located at argument a, extending the selection would first select argument a itself and in the next step grow to cover the whole function call. However, you might want to select the list of all arguments as an intermediate step. This can be achieved by implementing this EP in the following way:

1. Create a class that implements the ExtendWordSelectionHandler interface and register it in a com.intellij.extendWordSelectionHandler EP in your plugin.xml.

2. The canSelect(PsiElement) method should return true for the function call node. That indicates that select(PsiElement, CharSequence, int, Editor) will be called for the function-call node.

3. When the select() method is called, you can use the function call PSI element or the editor text to extract the text range that spans the arguments a and b and return it as List<TextRange> with one element.

Looking at other implementations can be an effective way to get a better understanding of how this EP works. To get further insight into this EP, you may want to take a look at DocTagSelectioner. It provides the ability to select tag names like @param in Javadoc comments. Additionally, the IntelliJ Platform Explorer provides a list of open-source plugins with implementations of the com.intellij.extendWordSelectionHandler EP.

There are also some important places in the IntelliJ Platform to add breakpoints during debugging. When Extend Selection is called by the user, it is handled by SelectWordHandler. The majority of the work, however, is then done inside SelectWordUtil, where processElement() checks which implementations of this EP apply for the current PSI element. If one of them returns true from its canSelect() method, the additional text ranges are extracted in the askSelectioner() function. These places are good candidates to set breakpoints and investigate during debugging.