2.5 Parse-Tree Listeners and Visitors

ANTLR 运行时支持 2 种 parse tree 遍历方式：

• 默认 built-in tree walker
  • 自动生成 parse-tree listener 接口，响应 walker 触发的事件
• 使用 visitor 设计模式的 tree walker

Parse-Tree Listeners

• ParseTreeWalker
• ParseTreeListener

ANTLR 为 each grammar 生成 ParseTreeListener 子类，对 grammar 中的每条规则，生成对应的 enter 和 exit 方法：

• ANTLR 为 assign 规则生成 enterAssign() 和 exitAssign() 方法
• AssignContext 对象作为参数传递给 enterAssign() 和 exitAssign() 方法

在上面的深度优先遍历中，ParseTreeWalker 对监听器方法的完整调用如下：

• 根节点：enterXXX 和 exitXXX
• 叶子结点：visitTerminal

listener 方式优点：

• 无需手动编写 遍历 代码；
• 全自动

Parse-Tree Visitors

有时需要控制 遍历本身，通过 函数调用 来访问子节点（listener 是通过 回调函数），此时可以通过 -visitor 生成 visitor 接口：

• 每个 grammar 对应一个 visitor interface
• 一条规则 对应 visitor interface 中的一个 visit 方法

下面是 assign 语法生成的 visitor 访问模式：

• 深度优先
• 当遇到根节点时，ANTLR 会调用 visitStat 函数

visitor 接口使用如下：

ParseTree tree = ...;  // parse tree is the result of parsing
MyVisitor v = new MyVisitor();
v.visit(tree);

• 必须手动调用 visit 函数

术语

• Language
  • A language is a set of valid sentences
  • Sentences are composed of phrases, which are composed of subphrases, and so on
• Grammar
  • A grammar formally defines the syntax rules of a language
  • Each rule in a grammar expresses the structure of a subphrase
• Syntax tree/parse tree
  • This represents the structure of the sentence where each subtree root gives an abstract name to the elements beneath it.
  • The subtree roots correspond to grammar rule names.
  • The leaves of the tree are symbols or tokenss of the sentence.
• Token
  • A token is a vocabulary symbol in a language;
  • these can represent a category of symbols such as "identifier" or can represent s single operator or keyword.
• Lexer or tokenizer
  • This breaks up an input character stream into tokens;
  • A lexer performs lexical analysis.
• Parser
  • A parser checks sentences for membership in a specific language by checking the sentence's structure against the rules of a grammar.
  • ANTLR generates top-down parsers called *ALL() that can use all remaining input symbols to make decisions**.
  • Top-down parser are goal-oriented and start matching at the rule associated with the coarsest, such as program or inputFile.
• Recursive-descent parser
  • This is a specific kind of top-down parser implemented with a function for each rule in the grammar.
• Lookahead
  • Parsers use lookahead to make decisions by comparing the symbols that begin each alternatives.