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angularjs-1.3代码学习-$parse

这次我们来看一下angular的Sandboxing Angular Expressions。关于内置方法的,核心有两块:Lexer和Parser。其中大家对$parse可能更了解一点。好了不多废话,先看Lexer的内部结构:

1.Lexer

//构造函数
var Lexer = function(options) {
  this.options = options;
};
//原型 
Lexer.prototype = {
    constructor: Lexer,
    lex: function(){},
    is: function(){},
    peek: function(){ /* 返回表达式的下一个位置的数据,如果没有则返回false */ },
    isNumber: function(){ /* 判断当前表达式是否是一个数字 */ },
    isWhitespace: function(){/* 判断当前表达式是否是空格符 */},
    isIdent: function(){/* 判断当前表达式是否是英文字符(包含_和$) */},
    isExpOperator: function(){/* 判断当时表达式是否是-,+还是数字 */},
    throwError: function(){ /* 抛出异常 */},
    readNumber: function(){ /* 读取数字 */},
    readIdent: function(){ /* 读取字符 */},
    readString: function(){ /*读取携带''或""的字符串*/ }
};

 这里指出一点,因为是表达式。所以类似"123"这类的东西,在Lexer看来应该算是数字而非字符串。表达式中的字符串必须使用单引号或者双引号来标识。Lexer的核心逻辑在lex方法中:

lex: function(text) {
    this.text = text;
    this.index = 0;
    this.tokens = [];

    while (this.index < this.text.length) {
      var ch = this.text.charAt(this.index);
      if (ch === '"' || ch === "'") {
        /* 尝试判断是否是字符串 */
        this.readString(ch);
      } else if (this.isNumber(ch) || ch === '.' && this.isNumber(this.peek())) {
        /* 尝试判断是否是数字 */
        this.readNumber();
      } else if (this.isIdent(ch)) {
        /* 尝试判断是否是字母 */
        this.readIdent();
      } else if (this.is(ch, '(){}[].,;:?')) {
        /* 判断是否是(){}[].,;:? */
        this.tokens.push({index: this.index, text: ch});
        this.index++;
      } else if (this.isWhitespace(ch)) {
        /* 判断是否是空白符 */
        this.index++;
      } else {
        /* 尝试匹配操作运算 */
        var ch2 = ch + this.peek();
        var ch3 = ch2 + this.peek(2);
        var op1 = OPERATORS[ch];
        var op2 = OPERATORS[ch2];
        var op3 = OPERATORS[ch3];
        if (op1 || op2 || op3) {
          var token = op3 ? ch3 : (op2 ? ch2 : ch);
          this.tokens.push({index: this.index, text: token, operator: true});
          this.index += token.length;
        } else {
          this.throwError('Unexpected next character ', this.index, this.index + 1);
        }
      }
    }
    return this.tokens;
  }

主要看一下匹配操作运算。这里源码中会调用OPERATORS。看一下OPERATORS:

var OPERATORS = extend(createMap(), {
    '+':function(self, locals, a, b) {
      a=a(self, locals); b=b(self, locals);
      if (isDefined(a)) {
        if (isDefined(b)) {
          return a + b;
        }
        return a;
      }
      return isDefined(b) ? b : undefined;},
    '-':function(self, locals, a, b) {
          a=a(self, locals); b=b(self, locals);
          return (isDefined(a) ? a : 0) - (isDefined(b) ? b : 0);
        },
    '*':function(self, locals, a, b) {return a(self, locals) * b(self, locals);},
    '/':function(self, locals, a, b) {return a(self, locals) / b(self, locals);},
    '%':function(self, locals, a, b) {return a(self, locals) % b(self, locals);},
    '===':function(self, locals, a, b) {return a(self, locals) === b(self, locals);},
    '!==':function(self, locals, a, b) {return a(self, locals) !== b(self, locals);},
    '==':function(self, locals, a, b) {return a(self, locals) == b(self, locals);},
    '!=':function(self, locals, a, b) {return a(self, locals) != b(self, locals);},
    '<':function(self, locals, a, b) {return a(self, locals) < b(self, locals);},
    '>':function(self, locals, a, b) {return a(self, locals) > b(self, locals);},
    '<=':function(self, locals, a, b) {return a(self, locals) <= b(self, locals);},
    '>=':function(self, locals, a, b) {return a(self, locals) >= b(self, locals);},
    '&&':function(self, locals, a, b) {return a(self, locals) && b(self, locals);},
    '||':function(self, locals, a, b) {return a(self, locals) || b(self, locals);},
    '!':function(self, locals, a) {return !a(self, locals);},

    //Tokenized as operators but parsed as assignment/filters
    '=':true,
    '|':true
});

可以看到OPERATORS实际上存储的是操作符和操作符函数的键值对。根据操作符返回对应的操作符函数。我们看一下调用例子:

var _l = new Lexer({});
var a = _l.lex("a = a + 1");
console.log(a);

 结合之前的lex方法,我们来回顾下代码执行过程:

1.index指向'a'是一个字母。匹配isIdent成功。将生成的token存入tokens中

2.index指向空格符,匹配isWhitespace成功,同上

3.index指向=,匹配操作运算符成功,同上

4.index指向空格符,匹配isWhitespace成功,同上

5.index指向'a'是一个字母。匹配isIdent成功。同上

7.index指向+,匹配操作运算符成功,同上

8.index指向空格符,匹配isWhitespace成功,同上

9.index指向1,匹配数字成功,同上

以上则是"a = a + 1"的代码执行过程。9步执行结束之后,跳出while循环。刚才我们看到了,每次匹配成功,源码会生成一个token。因为匹配类型的不同,生成出来的token的键值对略有不同:

number:{
      index: start,
      text: number,
      constant: true,
      value: Number(number)
    },
string: {
          index: start,
          text: rawString,
          constant: true,
          value: string
        },
ident: {
      index: start,
      text: this.text.slice(start, this.index),
      identifier: true /* 字符表示 */ 
    },
'(){}[].,;:?': {
    index: this.index,
    text: ch
},
"操作符": {
     index: this.index, 
     text: token, 
     operator: true
}
//text是表达式,而value才是实际的值

number和string其实都有相对应的真实值,意味着如果我们表达式是2e2,那number生成的token的值value就应该是200。到此我们通过lexer类获得了一个具有token值得数组。从外部看,实际上Lexer是将我们输入的表达式解析成了token json。可以理解为生成了表达式的语法树(AST)。但是目前来看,我们依旧还没有能获得我们定义表达式的结果。那就需要用到parser了。

2.Parser

先看一下Parser的内部结构:

//构造函数
var Parser = function(lexer, $filter, options) {
  this.lexer = lexer;
  this.$filter = $filter;
  this.options = options;
};

//原型
Parser.prototype = {
  constructor: Parser,
  parse: function(){},
  primary: function(){},
  throwError: function(){ /* 语法抛错 */},
  peekToken: function(){},
  peek: function(){/*返回tokens中的第一个成员对象 */},
  peekAhead: function(){ /* 返回tokens中指定成员对象,否则返回false */},
  expect: function(){ /* 取出tokens中第一个对象,否则返回false */ },
  consume: function(){ /* 取出第一个,底层调用expect */ },
  unaryFn: function(){ /* 一元操作 */},
  binaryFn: function(){ /* 二元操作 */},
  identifier: function(){},
  constant: function(){},
  statements: function(){},
  filterChain: function(){},
  filter: function(){},
  expression: function(){},
  assignment: function(){},
  ternary: function(){},
  logicalOR: function(){ /* 逻辑或 */},
  logicalAND: function(){ /* 逻辑与 */ },
  equality: function(){ /* 等于 */ },
  relational: function(){ /* 比较关系 */ },
  additive: function(){ /* 加法,减法 */ },
  multiplicative: function(){ /* 乘法,除法,求余 */ },
  unary: function(){ /* 一元 */ },
  fieldAccess: function(){},
  objectIndex: function(){},
  functionCall: function(){},
  arrayDeclaration: function(){},
  object: function(){}
}

Parser的入口方法是parse,内部执行了statements方法。来看下statements:

statements: function() {
    var statements = [];
    while (true) {
      if (this.tokens.length > 0 && !this.peek('}', ')', ';', ']'))
        statements.push(this.filterChain());
      if (!this.expect(';')) {
        // optimize for the common case where there is only one statement.
        // TODO(size): maybe we should not support multiple statements?
        return (statements.length === 1)
            ? statements[0]
            : function $parseStatements(self, locals) {
                var value;
                for (var i = 0, ii = statements.length; i < ii; i++) {
                  value = statements[i](self, locals);
                }
                return value;
              };
      }
    }
  }

这里我们将tokens理解为表达式,实际上它就是经过表达式通过lexer转换过来的。statements中。如果表达式不以},),;,]开头,将会执行filterChain方法。当tokens检索完成之后,最后返回了一个$parseStatements方法。其实Parser中很多方法都返回了类似的对象,意味着返回的内容将需要执行后才能得到结果。

看一下filterChain:

filterChain: function() {
    /* 针对angular语法的filter */
    var left = this.expression();
    var token;
    while ((token = this.expect('|'))) {
      left = this.filter(left);
    }
    return left;
  }

其中filterChain是针对angular表达式独有的"|"filter写法设计的。我们先绕过这块,进入expression

expression: function() {
    return this.assignment();
  }

再看assignment:

assignment: function() {
    var left = this.ternary();
    var right;
    var token;
    if ((token = this.expect('='))) {
      if (!left.assign) {
        this.throwError('implies assignment but [' +
            this.text.substring(0, token.index) + '] can not be assigned to', token);
      }
      right = this.ternary();
      return extend(function $parseAssignment(scope, locals) {
        return left.assign(scope, right(scope, locals), locals);
      }, {
        inputs: [left, right]
      });
    }
    return left;
  }

我们看到了ternary方法。这是一个解析三目操作的方法。与此同时,assignment将表达式以=划分成left和right两块。并且两块都尝试执行ternary。

ternary: function() {
    var left = this.logicalOR();
    var middle;
    var token;
    if ((token = this.expect('?'))) {
      middle = this.assignment();
      if (this.consume(':')) {
        var right = this.assignment();

        return extend(function $parseTernary(self, locals) {
          return left(self, locals) ? middle(self, locals) : right(self, locals);
        }, {
          constant: left.constant && middle.constant && right.constant
        });
      }
    }

    return left;
  }

在解析三目运算之前,又根据?将表达式划分成left和right两块。左侧再去尝试执行logicalOR,实际上这是一个逻辑与的解析,按照这个执行流程,我们一下有了思路。这有点类似我们一般写三目时。代码的执行情况,比如: 2 > 2 ? 1 : 0。如果把这个当成表达式,那根据?划分left和right,left就应该是2 > 2,right应该就是 1: 0。然后尝试在left看是否有逻辑或的操作。也就是,Parser里面的方法调用的嵌套级数越深,其方法的优先级则越高。好,那我们一口气看看这个最高的优先级在哪?

logicalOR -> logicalAND -> equality -> relational -> additive -> multiplicative ->  unary

好吧,嵌套级数确实有点多。那么我们看下unary。

unary: function() {
    var token;
    if (this.expect('+')) {
      return this.primary();
    } else if ((token = this.expect('-'))) {
      return this.binaryFn(Parser.ZERO, token.text, this.unary());
    } else if ((token = this.expect('!'))) {
      return this.unaryFn(token.text, this.unary());
    } else {
      return this.primary();
    }
  }

这边需要看两个主要的方法,一个是binaryFn和primay。如果判断是-,则必须通过binaryFn去添加函数。看下binaryFn

binaryFn: function(left, op, right, isBranching) {
    var fn = OPERATORS[op];
    return extend(function $parseBinaryFn(self, locals) {
      return fn(self, locals, left, right);
    }, {
      constant: left.constant && right.constant,
      inputs: !isBranching && [left, right]
    });
  }

其中OPERATORS是之前聊Lexer也用到过,它根据操作符存储相应的操作函数。看一下fn(self, locals, left, right)。而我们随便取OPERATORS中的一个例子:

'-':function(self, locals, a, b) {
          a=a(self, locals); b=b(self, locals);
          return (isDefined(a) ? a : 0) - (isDefined(b) ? b : 0);
        }

其中a和b就是left和right,他们其实都是返回的跟之前类似的$parseStatements方法。默认存储着token中的value。经过事先解析好的四则运算来生成最终答案。其实这就是Parser的基本功能。至于嵌套,我们可以把它理解为js的操作符的优先级。这样就一目了然了。至于primay方法。塔刷选{ ( 对象做进一步的解析过程。

Parser的代码并不复杂,只是函数方法间调用密切,让我们再看一个例子:

var _l = new Lexer({});
var _p = new Parser(_l);
var a = _p.parse("1 + 1 + 2");
console.log(a()); //4

我们看下1+1+2生成的token是什么样的:

[
{"index":0,"text":"1","constant":true,"value":1},{"index":2,"text":"+","operator":true},{"index":4,"text":"1","constant":true,"value":1},{"index":6,"text":"+","operator":true},{"index":8,"text":"2","constant":true,"value":2}
]

Parser根据lexer生成的tokens尝试解析。tokens每一个成员都会生成一个函数,其先后执行逻辑按照用户输入的1+1+2的顺序执行。注意像1和2这类constants为true的token,parser会通过constant生成需要的函数$parseConstant,也就是说1+1+2中的两个1和一个2都是返回$parseConstant函数,通过$parseBinaryFn管理加法逻辑。

constant: function() {
    var value = this.consume().value;

    return extend(function $parseConstant() {
      return value; //这个函数执行之后,就是将value值返回。
    }, {
      constant: true,
      literal: true
    });
  },
binaryFn: function(left, op, right, isBranching) {
    var fn = OPERATORS[op];//加法逻辑
    return extend(function $parseBinaryFn(self, locals) {
      return fn(self, locals, left, right);//left和right分别表示生成的对应函数
    }, {
      constant: left.constant && right.constant,
      inputs: !isBranching && [left, right]
    });
  }

那我们demo中的a应该返回什么函数呢?当然是$parseBinaryFn。其中的left和right分别是1+1的$parseBinaryFn,right就是2的$parseConstant。

再来一个例子:

var _l = new Lexer({});
var _p = new Parser(_l);
var a = _p.parse('{"name": "hello"}');
console.log(a);

这边我们传入一个json,理论上我们执行完a函数,应该返回一个{name: "hello"}的对象。它调用了Parser中的object

object: function() {
    var keys = [], valueFns = [];
    if (this.peekToken().text !== '}') {
      do {
        if (this.peek('}')) {
          // Support trailing commas per ES5.1.
          break;
        }
        var token = this.consume();
        if (token.constant) {
          //把key取出来
          keys.push(token.value);
        } else if (token.identifier) {
          keys.push(token.text);
        } else {
          this.throwError("invalid key", token);
        }
        this.consume(':');
        //冒号之后,则是值,将值存在valueFns中
        valueFns.push(this.expression());
        //根据逗号去迭代下一个
      } while (this.expect(','));
    }
    this.consume('}');

    return extend(function $parseObjectLiteral(self, locals) {
      var object = {};
      for (var i = 0, ii = valueFns.length; i < ii; i++) {
        object[keys[i]] = valueFns[i](self, locals);
      }
      return object;
    }, {
      literal: true,
      constant: valueFns.every(isConstant),
      inputs: valueFns
    });
  }

比方我们的例子{"name": "hello"},object会将name存在keys中,hello则会生成$parseConstant函数存在valueFns中,最终返回$parseObjectLiternal函数。

下一个例子:

var a = _p.parse('{"name": "hello"}["name"]');

这个跟上一个例子的差别在于后面尝试去读取name的值,这边则调用parser中的objectIndex方法。

objectIndex: function(obj) {
    var expression = this.text;

    var indexFn = this.expression();
    this.consume(']');

    return extend(function $parseObjectIndex(self, locals) {
      var o = obj(self, locals), //parseObjectLiteral,实际就是obj
          i = indexFn(self, locals), //$parseConstant,这里就是name
          v;

      ensureSafeMemberName(i, expression);
      if (!o) return undefined;
      v = ensureSafeObject(o[i], expression);
      return v;
    }, {
      assign: function(self, value, locals) {
        var key = ensureSafeMemberName(indexFn(self, locals), expression);
        // prevent overwriting of Function.constructor which would break ensureSafeObject check
        var o = ensureSafeObject(obj(self, locals), expression);
        if (!o) obj.assign(self, o = {}, locals);
        return o[key] = value;
      }
    });
  }

很简单吧,obj[xx]和obj.x类似。大家自行阅读,我们再看一个函数调用的demo

var _l = new Lexer({});
var _p = new Parser(_l, '', {});
var demo = {
  "test": function(){
    alert("welcome");
  }
};
var a = _p.parse('test()');
console.log(a(demo));

我们传入一个test的调用。这边调用了parser中的functionCall方法和identifier方法

identifier: function() {
    var id = this.consume().text;

    //Continue reading each `.identifier` unless it is a method invocation
    while (this.peek('.') && this.peekAhead(1).identifier && !this.peekAhead(2, '(')) {
      id += this.consume().text + this.consume().text;
    }

    return getterFn(id, this.options, this.text);
  }

看一下getterFn方法

...
forEach(pathKeys, function(key, index) {
      ensureSafeMemberName(key, fullExp);
      var lookupJs = (index
                      // we simply dereference 's' on any .dot notation
                      ? 's'
                      // but if we are first then we check locals first, and if so read it first
                      : '((l&&l.hasOwnProperty("' + key + '"))?l:s)') + '.' + key;
      if (expensiveChecks || isPossiblyDangerousMemberName(key)) {
        lookupJs = 'eso(' + lookupJs + ', fe)';
        needsEnsureSafeObject = true;
      }
      code += 'if(s == null) return undefined;\n' +
              's=' + lookupJs + ';\n';
    });
    code += 'return s;';

    /* jshint -W054 */
    var evaledFnGetter = new Function('s', 'l', 'eso', 'fe', code); // s=scope, l=locals, eso=ensureSafeObject
    /* jshint +W054 */
    evaledFnGetter.toString = valueFn(code);
...

这是通过字符串创建一个匿名函数的方法。我们看下demo的test生成了一个什么匿名函数:

function('s', 'l', 'eso', 'fe'){
if(s == null) return undefined;
s=((l&&l.hasOwnProperty("test"))?l:s).test;
return s;
}

这个匿名函数的意思,需要传入一个上下文,匿名函数通过查找上下文中是否有test属性,如果没有传上下文则直接返回未定义。这也就是为什么我们在生成好的a函数在执行它时需要传入demo对象的原因。最后补一个functionCall

functionCall: function(fnGetter, contextGetter) {
    var argsFn = [];
    if (this.peekToken().text !== ')') {
      /* 确认调用时有入参 */
      do {
        //形参存入argsFn
        argsFn.push(this.expression());
      } while (this.expect(','));
    }
    this.consume(')');

    var expressionText = this.text;
    // we can safely reuse the array across invocations
    var args = argsFn.length ? [] : null;

    return function $parseFunctionCall(scope, locals) {
      var context = contextGetter ? contextGetter(scope, locals) : isDefined(contextGetter) ? undefined : scope;
      //或者之前创建生成的匿名函数
      var fn = fnGetter(scope, locals, context) || noop;

      if (args) {
        var i = argsFn.length;
        while (i--) {
          args[i] = ensureSafeObject(argsFn[i](scope, locals), expressionText);
        }
      }

      ensureSafeObject(context, expressionText);
      ensureSafeFunction(fn, expressionText);

      // IE doesn't have apply for some native functions
      //执行匿名函数的时候需要传入上下文
      var v = fn.apply
            ? fn.apply(context, args)
            : fn(args[0], args[1], args[2], args[3], args[4]);

      if (args) {
        // Free-up the memory (arguments of the last function call).
        args.length = 0;
      }

      return ensureSafeObject(v, expressionText);
      };
  }

下面我们看一下$ParseProvider,这是一个基于Lex和Parser函数的angular内置provider。它对scope的api提供了基础支持。

...
return function $parse(exp, interceptorFn, expensiveChecks) {
      var parsedExpression, oneTime, cacheKey;

      switch (typeof exp) {
        case 'string':
          cacheKey = exp = exp.trim();

          var cache = (expensiveChecks ? cacheExpensive : cacheDefault);
          parsedExpression = cache[cacheKey];

          if (!parsedExpression) {
            if (exp.charAt(0) === ':' && exp.charAt(1) === ':') {
              oneTime = true;
              exp = exp.substring(2);
            }

            var parseOptions = expensiveChecks ? $parseOptionsExpensive : $parseOptions;
            //调用lexer和parser
            var lexer = new Lexer(parseOptions);
            var parser = new Parser(lexer, $filter, parseOptions);
            parsedExpression = parser.parse(exp);
            //添加$$watchDelegate,为scope部分提供支持
            if (parsedExpression.constant) {
              parsedExpression.$$watchDelegate = constantWatchDelegate;
            } else if (oneTime) {
              //oneTime is not part of the exp passed to the Parser so we may have to
              //wrap the parsedExpression before adding a $$watchDelegate
              parsedExpression = wrapSharedExpression(parsedExpression);
              parsedExpression.$$watchDelegate = parsedExpression.literal ?
                oneTimeLiteralWatchDelegate : oneTimeWatchDelegate;
            } else if (parsedExpression.inputs) {
              parsedExpression.$$watchDelegate = inputsWatchDelegate;
            }
            //做相关缓存
            cache[cacheKey] = parsedExpression;
          }
          return addInterceptor(parsedExpression, interceptorFn);

        case 'function':
          return addInterceptor(exp, interceptorFn);

        default:
          return addInterceptor(noop, interceptorFn);
      }
    };

总结:Lexer和Parser的实现确实让我大开眼界。通过这两个函数,实现了angular自己的语法解析器。逻辑部分还是相对复杂

时间不多,内容刚好,以上是个人阅读源码的一些理解,有不对或者偏差的地方,还希望园友们斧正。共同进步。

posted on 2017-09-13 20:52 无码帝 阅读(...) 评论(...) 编辑 收藏