Flutter动态特性之analyzer分析AST

AST（抽象语法树）是Flutter对Dart文件分词（类似）的一种结果，它可以把程序员编写的Dart语句逐个分解为json格式，如下最基本的一个 Hello world 页面被转换后的结果：

通过调用analyzer parseString/parseFile方法可以对Dart代码字符串分析成Json类型的AST，官方并没有把AST中各个字段所表达的意义一一说明，这给做Fflutter动态能力的增加了不少障碍，这里将对照源码一一分析它的意义。

Dart源代码：

class DemoWidget extends StatefulWidget {
  @override
  _DemoWidgetState createState() => _DemoWidgetState();
}

class _DemoWidgetState extends State {
  @override
  Widget build(BuildContext context) {
    return Text('Hello world!');
  }
}

如上所示是一个简单的StatefulWidget Hello world!页面，转换成AST后如下所示：

[
  {
    "type": "ClassDeclaration",
    "id": {
      "type": "Identifier",
      "name": "DemoWidget"
    },
    "superClause": {
      "type": "TypeName",
      "name": "StatefulWidget"
    },
    "implementsClause": null,
    "mixinClause": null,
    "metadata": [],
    "body": [
      {
        "type": "MethodDeclaration",
        "id": {
          "type": "Identifier",
          "name": "createState"
        },
        "parameters": {
          "type": "FormalParameterList",
          "parameterList": []
        },
        "typeParameters": null,
        "body": null,
        "isAsync": false,
        "returnType": {
          "type": "TypeName",
          "name": "_DemoWidgetState"
        }
      }
    ]
  },
  {
    "type": "ClassDeclaration",
    "id": {
      "type": "Identifier",
      "name": "_DemoWidgetState"
    },
    "superClause": {
      "type": "TypeName",
      "name": "State"
    },
    "implementsClause": null,
    "mixinClause": null,
    "metadata": [],
    "body": [
      {
        "type": "MethodDeclaration",
        "id": {
          "type": "Identifier",
          "name": "build"
        },
        "parameters": {
          "type": "FormalParameterList",
          "parameterList": [
            {
              "type": "SimpleFormalParameter",
              "paramType": {
                "type": "TypeName",
                "name": "BuildContext"
              },
              "name": "context"
            }
          ]
        },
        "typeParameters": null,
        "body": {
          "type": "BlockStatement",
          "body": [
            {
              "type": "ReturnStatement",
              "argument": {
                "type": "MethodInvocation",
                "callee": {
                  "type": "Identifier",
                  "name": "Text"
                },
                "typeArguments": null,
                "argumentList": {
                  "type": "ArgumentList",
                  "argumentList": [
                    {
                      "type": "StringLiteral",
                      "value": "Hello world!"
                    }
                  ]
                }
              }
            }
          ]
        },
        "isAsync": false,
        "returnType": {
          "type": "TypeName",
          "name": "Widget"
        }
      }
    ]
  }
]

analyzer只支持顶级类转换，不支持顶级变量、方法或mixin等类型。

AST关键属性讲解：

类Class

  {
    "type": "ClassDeclaration",
    "id": {
      "type": "Identifier",
      "name": "DemoWidget"
    },
    "superClause": {
      "type": "TypeName",
      "name": "StatefulWidget"
    },
    "implementsClause": null,
    "mixinClause": null,
    "metadata": [],
    "body": [
    ]
  }