Types of tokens for lexers used in IDEA are defined by instances of
IElementType. A number of token types common for all languages are defined in the
TokenType interface; custom language plugins should reuse these token types wherever applicable. For all other token types, the plugin needs to create new
IElementType instances and associate with the language in which the token type is used. The same
IElementType instance should be returned every time a particular token type is encountered by the lexer.
An important feature which can be implemented at lexer level is mixing languages within a file (for example, embedding fragments of Java code in some template language). If a language supports embedding its fragments in another language, it needs to define the chameleon token types for different types of fragments which can be embedded, and these token types need to implement the
ILazyParseableElementType interface. The lexer of the enclosing language needs to return the entire fragment of the embedded language as a single chameleon token, of the type defined by the embedded language. To parse the contents of the chameleon token, IDEA will call the parser of the embedded language through a call to
Second, a PSI (Program Structure Interface) tree is built on top of the AST, adding semantics and methods for manipulating specific language constructs. Nodes of the PSI tree are represented by classes implementing the
PsiElement interface and are created by the language plugin in the
ParserDefinition.createElement() method. The top-level node of the PSI tree for a file needs to implement the
PsiFile interface, and is created in the
The lifecycle of the PSI is described in more detail in IntelliJ IDEA Architectural Overview.
The language plugin provides the parser implementation as an implementation of the
PsiParser interface, returned from
ParserDefinition.createParser(). The parser receives an instance of the
PsiBuilder class, which is used to get the stream of tokens from the lexer and to hold the intermediate state of the AST being built. The parser must process all tokens returned by the lexer up to the end of stream (until
null), even if the tokens are not valid according to the language syntax.
The parser works by setting pairs of markers (
PsiBuilder.Marker instances) within the stream of tokens received from the lexer. Each pair of markers defines the range of lexer tokens for a single node in the AST tree. If a pair of markers is nested in another pair (starts after its start and ends before its end), it becomes the child node of the outer pair.
The syntax and error highlighting is performed on multiple levels. The first level of syntax highlighting is based on the lexer output, and is provided through the
SyntaxHighlighter interface. The syntax highlighter returns the
TextAttributeKey instances for each token type which needs special highlighting. For highlighting lexer errors, the standard
TextAttributeKey for bad characters (
HighligherColors.BAD_CHARACTER) can be used.
The second level of error highlighting happens during parsing. If a particular sequence of tokens is invalid according to the grammar of the language, the
PsiBuilder.error() method can be used to highlight the invalid tokens and display an error message showing why they are not valid.
When the rename refactoring is performed, the method
PsiNamedElement.setName() is called for the renamed element, and
PsiReference.handleElementRename() is called for all references to the renamed element. Both of these methods perform basically the same action: replace the underlying AST node of the PSI element with the node containing the new text entered by the user. Creating a fully correct AST node from scratch is quite difficult. Thus, surprisingly, the easiest way to get the replacement node is to create a dummy file in the custom language so that it would contain the necessary node in its parse tree, build the parse tree and extract the necessary node from it.
Another interface related to the Rename refactoring is
NamesValidator. This interface allows a plugin to check if the name entered by the user in the Rename dialog is a valid identifier (and not a keyword) according to the custom language rules. If an implementation of this interface is not provided by the plugin, Java rules for validating identifiers are used. Implementations of
NamesValidator are registered in the
com.intellij.lang.namesValidator extension point.
PsiElement.delete()method for the PsiElement subclasses for which Safe Delete is available. Deleting PSI elements is implemented by deleting the underlying AST nodes from the AST tree (which, in turn, causes the text ranges corresponding to the AST nodes to be deleted from the document).
If needed, it's possible to further customize how Safe Delete is performed for a particular type of element (how references are searched, etc). This is done by implementing the