這次我們來看一下angular的Sandboxing Angular Expressions。關于內置方法的，核心有兩塊：Lexer和Parser。其中大家對$parse可能更了解一點。好了不多廢話，先看Lexer的內部結構：
1.Lexer
//構造函數var Lexer = function(options) { this.options = options;};//原型 Lexer.prototype = { constructor: Lexer, lex: function(){}, is: function(){}, peek: function(){ /* 返回表達式的下一個位置的數據，如果沒有則返回false */ }, isNumber: function(){ /* 判斷當前表達式是否是一個數字 */ }, isWhitespace: function(){/* 判斷當前表達式是否是空格符 */}, isIdent: function(){/* 判斷當前表達式是否是英文字符(包含_和$) */}, isExpOperator: function(){/* 判斷當時表達式是否是-，+還是數字 */}, throwError: function(){ /* 拋出異常 */}, readNumber: function(){ /* 讀取數字 */}, readIdent: function(){ /* 讀取字符 */}, readString: function(){ /*讀取攜帶''或""的字符串*/ }};這里指出一點，因為是表達式。所以類似"123"這類的東西，在Lexer看來應該算是數字而非字符串。表達式中的字符串必須使用單引號或者雙引號來標識。Lexer的核心邏輯在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 === "'") { /* 嘗試判斷是否是字符串 */ this.readString(ch); } else if (this.isNumber(ch) || ch === '.' && this.isNumber(this.peek())) { /* 嘗試判斷是否是數字 */ this.readNumber(); } else if (this.isIdent(ch)) { /* 嘗試判斷是否是字母 */ this.readIdent(); } else if (this.is(ch, '(){}[].,;:?')) { /* 判斷是否是(){}[].,;:? */ this.tokens.push({index: this.index, text: ch}); this.index++; } else if (this.isWhitespace(ch)) { /* 判斷是否是空白符 */ this.index++; } else { /* 嘗試匹配操作運算 */ 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運算。這里源碼中會調用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實際上存儲的是操作符和操作符函數的鍵值對。根據操作符返回對應的操作符函數。我們看一下調用例子：
var _l = new Lexer({});var a = _l.lex("a = a + 1");console.log(a);結合之前的lex方法，我們來回顧下代碼執行過程：
1.index指向'a'是一個字母。匹配isIdent成功。將生成的token存入tokens中
2.index指向空格符，匹配isWhitespace成功，同上
3.index指向=，匹配操作運算符成功，同上
4.index指向空格符，匹配isWhitespace成功，同上
5.index指向'a'是一個字母。匹配isIdent成功。同上
7.index指向+，匹配操作運算符成功，同上
8.index指向空格符，匹配isWhitespace成功，同上
9.index指向1，匹配數字成功，同上
以上則是"a = a + 1"的代碼執行過程。9步執行結束之后，跳出while循環。剛才我們看到了，每次匹配成功，源碼會生成一個token。因為匹配類型的不同，生成出來的token的鍵值對略有不同：
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是表達式，而value才是實際的值number和string其實都有相對應的真實值，意味著如果我們表達式是2e2，那number生成的token的值value就應該是200。到此我們通過lexer類獲得了一個具有token值得數組。從外部看，實際上Lexer是將我們輸入的表達式解析成了token json?？梢岳斫鉃樯闪吮磉_式的語法樹（AST）。但是目前來看，我們依舊還沒有能獲得我們定義表達式的結果。那就需要用到parser了。
2.Parser
先看一下Parser的內部結構：
//構造函數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(){ /* 語法拋錯 */}, peekToken: function(){}, peek: function(){/*返回tokens中的第一個成員對象 */}, peekAhead: function(){ /* 返回tokens中指定成員對象，否則返回false */}, expect: function(){ /* 取出tokens中第一個對象，否則返回false */ }, consume: function(){ /* 取出第一個，底層調用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(){ /* 邏輯或 */}, logicalAND: function(){ /* 邏輯與 */ }, equality: function(){ /* 等于 */ }, relational: function(){ /* 比較關系 */ }, additive: function(){ /* 加法，減法 */ }, 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，內部執行了statements方法。來看下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 $parseStatements(self, locals) { var value; for (var i = 0, ii = statements.length; i < ii; i++) { value = statements[i](self, locals); } return value; }; } } }這里我們將tokens理解為表達式，實際上它就是經過表達式通過lexer轉換過來的。statements中。如果表達式不以},),;,]開頭，將會執行filterChain方法。當tokens檢索完成之后，最后返回了一個$parseStatements方法。其實Parser中很多方法都返回了類似的對象，意味著返回的內容將需要執行后才能得到結果。
看一下filterChain：
filterChain: function() { /* 針對angular語法的filter */ var left = this.expression(); var token; while ((token = this.expect('|'))) { left = this.filter(left); } return left; }其中filterChain是針對angular表達式獨有的"|"filter寫法設計的。我們先繞過這塊，進入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; }我們看到了ternary方法。這是一個解析三目操作的方法。與此同時，assignment將表達式以=劃分成left和right兩塊。并且兩塊都嘗試執行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; }在解析三目運算之前，又根據?將表達式劃分成left和right兩塊。左側再去嘗試執行logicalOR，實際上這是一個邏輯與的解析，按照這個執行流程，我們一下有了思路。這有點類似我們一般寫三目時。代碼的執行情況，比如： 2 > 2 ? 1 : 0。如果把這個當成表達式，那根據?劃分left和right，left就應該是2 > 2，right應該就是 1: 0。然后嘗試在left看是否有邏輯或的操作。也就是，Parser里面的方法調用的嵌套級數越深，其方法的優先級則越高。好，那我們一口氣看看這個最高的優先級在哪？
logicalOR -> logicalAND -> equality -> relational -> additive -> multiplicative -> unary
好吧，嵌套級數確實有點多。那么我們看下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(); } }這邊需要看兩個主要的方法，一個是binaryFn和primay。如果判斷是-，則必須通過binaryFn去添加函數。看下binaryFn
binaryFn: function(left, op, right, isBranching) { var fn = OPERATORS[op]; return extend(function $parseBinaryFn(self, locals) { return fn(self, locals, left, right); }, { constant: left.constant && right.constant, inputs: !isBranching && [left, right] }); }其中OPERATORS是之前聊Lexer也用到過，它根據操作符存儲相應的操作函數?？匆幌耭n(self, locals, left, right)。而我們隨便取OPERATORS中的一個例子：
'-':function(self, locals, a, b) { a=a(self, locals); b=b(self, locals); return (isDefined(a) ? a : 0) - (isDefined(b) ? b : 0); }其中a和b就是left和right，他們其實都是返回的跟之前類似的$parseStatements方法。默認存儲著token中的value。經過事先解析好的四則運算來生成最終答案。其實這就是Parser的基本功能。至于嵌套，我們可以把它理解為js的操作符的優先級。這樣就一目了然了。至于primay方法。塔刷選{ ( 對象做進一步的解析過程。
Parser的代碼并不復雜，只是函數方法間調用密切，讓我們再看一個例子：
var _l = new Lexer({});var _p = new Parser(_l);var a = _p.parse("1 + 1 + 2");console.log(a()); //4我們看下1+1+2生成的token是什么樣的：
[{"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根據lexer生成的tokens嘗試解析。tokens每一個成員都會生成一個函數，其先后執行邏輯按照用戶輸入的1+1+2的順序執行。注意像1和2這類constants為true的token，parser會通過constant生成需要的函數$parseConstant，也就是說1+1+2中的兩個1和一個2都是返回$parseConstant函數，通過$parseBinaryFn管理加法邏輯。
constant: function() { var value = this.consume().value; return extend(function $parseConstant() { return value; //這個函數執行之后，就是將value值返回。 }, { constant: true, literal: true }); },binaryFn: function(left, op, right, isBranching) { var fn = OPERATORS[op];//加法邏輯 return extend(function $parseBinaryFn(self, locals) { return fn(self, locals, left, right);//left和right分別表示生成的對應函數 }, { constant: left.constant && right.constant, inputs: !isBranching && [left, right] }); }那我們demo中的a應該返回什么函數呢？當然是$parseBinaryFn。其中的left和right分別是1+1的$parseBinaryFn，right就是2的$parseConstant。
再來一個例子：
var _l = new Lexer({});var _p = new Parser(_l);var a = _p.parse('{"name": "hello"}');console.log(a);這邊我們傳入一個json，理論上我們執行完a函數，應該返回一個{name: "hello"}的對象。它調用了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取出來 keys.push(token.value); } else if (token.identifier) { keys.push(token.text); } else { this.throwError("invalid key", token); } this.consume(':'); //冒號之后，則是值，將值存在valueFns中 valueFns.push(this.expression()); //根據逗號去迭代下一個 } while (this.expect(',')); } this.consume('}'); return extend(function $parseObjectLiteral(self, locals) { var object = {}; for (var i = 0, ii = valueFns.length; i < ii; i++) { object[keys[i]] = valueFns[i](self, locals); } return object; }, { literal: true, constant: valueFns.every(isConstant), inputs: valueFns }); }比方我們的例子{"name": "hello"}，object會將name存在keys中，hello則會生成$parseConstant函數存在valueFns中，最終返回$parseObjectLiternal函數。
下一個例子：
var a = _p.parse('{"name": "hello"}["name"]');這個跟上一個例子的差別在于后面嘗試去讀取name的值，這邊則調用parser中的objectIndex方法。
objectIndex: function(obj) { var expression = this.text; var indexFn = this.expression(); this.consume(']'); return extend(function $parseObjectIndex(self, locals) { var o = obj(self, locals), //parseObjectLiteral，實際就是obj i = indexFn(self, locals), //$parseConstant，這里就是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; } }); }很簡單吧，obj[xx]和obj.x類似。大家自行閱讀，我們再看一個函數調用的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));我們傳入一個test的調用。這邊調用了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);...這是通過字符串創建一個匿名函數的方法。我們看下demo的test生成了一個什么匿名函數：
function('s', 'l', 'eso', 'fe'){if(s == null) return undefined;s=((l&&l.hasOwnProperty("test"))?l:s).test;return s;}這個匿名函數的意思，需要傳入一個上下文，匿名函數通過查找上下文中是否有test屬性，如果沒有傳上下文則直接返回未定義。這也就是為什么我們在生成好的a函數在執行它時需要傳入demo對象的原因。最后補一個functionCall
functionCall: function(fnGetter, contextGetter) { var argsFn = []; if (this.peekToken().text !== ')') { /* 確認調用時有入參 */ do { //形參存入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 $parseFunctionCall(scope, locals) { var context = contextGetter ? contextGetter(scope, locals) : isDefined(contextGetter) ? undefined : scope; //或者之前創建生成的匿名函數 var fn = fnGetter(scope, locals, context) || noop; if (args) { var i = argsFn.length; while (i--) { args[i] = ensureSafeObject(argsFn[i](scope, locals), expressionText); } } ensureSafeObject(context, expressionText); ensureSafeFunction(fn, expressionText); // IE doesn't have apply for some native functions //執行匿名函數的時候需要傳入上下文 var v = fn.apply ? fn.apply(context, args) : fn(args[0], args[1], args[2], args[3], args[4]); if (args) { // Free-up the memory (arguments of the last function call). args.length = 0; } return ensureSafeObject(v, expressionText); }; }下面我們看一下$ParseProvider，這是一個基于Lex和Parser函數的angular內置provider。它對scope的api提供了基礎支持。
...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; //調用lexer和parser var lexer = new Lexer(parseOptions); var parser = new Parser(lexer, $filter, parseOptions); parsedExpression = parser.parse(exp); //添加$$watchDelegate,為scope部分提供支持 if (parsedExpression.constant) { parsedExpression.$$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; } //做相關緩存 cache[cacheKey] = parsedExpression; } return addInterceptor(parsedExpression, interceptorFn); case 'function': return addInterceptor(exp, interceptorFn); default: return addInterceptor(noop, interceptorFn); } };總結：Lexer和Parser的實現確實讓我大開眼界。通過這兩個函數，實現了angular自己的語法解析器。邏輯部分還是相對復雜
以上所述是小編給大家介紹的Angularjs 1.3 中的$parse實例代碼，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對武林網網站的支持！
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