从Google V8引擎剖析Promise实现
本文阅读的源码为Google V8 Engine v3.29.45,此版本的promise实现为js版本,在后续版本Google继续对其实现进行了处理。引入了es6语法等,在7.X版本迭代后,逐渐迭代成了C版本实现。贴上源码地址:https://chromium.googlesource... 大家自觉传送。
代码中所有类似%functionName的函数均是C语言实现的运行时函数。
Define variables
首先定义了将要在JS作用域使用了一些变量,提高了编译器的效率。
var IsPromise;var PromiseCreate;var PromiseResolve;var PromiseReject;var PromiseChain;var PromiseCatch;var PromiseThen;var PromiseHasRejectHandler;
随后定义了一些全局私有变量供给和C语音交互,用于维护Promise的状态和进行Debug。
var promiseStatus = GLOBAL_PRIVATE("Promise#status");var promiseValue = GLOBAL_PRIVATE("Promise#value");var promiseOnResolve = GLOBAL_PRIVATE("Promise#onResolve");var promiseOnReject = GLOBAL_PRIVATE("Promise#onReject");var promiseRaw = GLOBAL_PRIVATE("Promise#raw");var promiseDebug = GLOBAL_PRIVATE("Promise#debug");var lastMicrotaskId = 0;
其中GLOBAL_PRIVATE是python进行实现的,运用python的宏定义(macro)来定义调用了C语言的CreateGlobalPrivateOwnSymbol方法。
macro GLOBAL_PRIVATE(name) = (%CreateGlobalPrivateOwnSymbol(name));
随后运用了一个自执行的匿名函数进行闭包逻辑。
(function() { // 主逻辑})();
在闭包逻辑的最后,在promise原型上挂载了三个方法:chain,then,catch。在promise对象上挂载了all,race等六个方法。将Promise对象注册到了global。
%AddNamedProperty(global, 'Promise', $Promise, DONT_ENUM);InstallFunctions($Promise, DONT_ENUM, [ "defer", PromiseDeferred, "accept", PromiseResolved, "reject", PromiseRejected, "all", PromiseAll, "race", PromiseOne, "resolve", PromiseCast]);InstallFunctions($Promise.prototype, DONT_ENUM, [ "chain", PromiseChain, "then", PromiseThen, "catch", PromiseCatch]);
Start from constructor
var $Promise = function Promise(resolver) { // 如果传入参数为全局promiseRaw变量的时候return if (resolver === promiseRaw) return; // 如果当前函数不是构造函数的化,抛出错误这不是一个promise if (!%_IsConstructCall()) throw MakeTypeError('not_a_promise', [this]); // 如果传入参数不是一个函数的话,抛出错误,传入参数不是一个function if (!IS_SPEC_FUNCTION(resolver)) throw MakeTypeError('resolver_not_a_function', [resolver]); var promise = PromiseInit(this); try { // debug相关忽略 %DebugPushPromise(promise); resolver(function(x) { PromiseResolve(promise, x) }, function(r) { PromiseReject(promise, r) }); } catch (e) { // 报错之后走到错误处理函数 PromiseReject(promise, e); } finally { // debug相关忽略 %DebugPopPromise(); }}
构造函数在做完额外的异常和参数判断后,进入主逻辑调用PromiseInit方法初始化promise,随后调用了resolver方法,传入了两个默认的处理函数。在promise在内部被调用时(PromiseDeferred方法被调用时)会实例化$promise,将默认方法return回去,使得创建的promise示例具有resolve和reject方法。
function PromiseDeferred() { if (this === $Promise) { // Optimized case, avoid extra closure. var promise = PromiseInit(new $Promise(promiseRaw)); return { promise: promise, resolve: function(x) { PromiseResolve(promise, x) }, reject: function(r) { PromiseReject(promise, r) } }; } else { var result = {}; result.promise = new this(function(resolve, reject) { result.resolve = resolve; result.reject = reject; }) return result; }}
PromiseInit
function PromiseSet(promise, status, value, onResolve, onReject) { // macro SET_PRIVATE(obj, sym, val) = (obj[sym] = val); // 设置promise的状态,SET_PRIVATE只有在给已经存在的对象设置已有属性值的时候才会被调用 SET_PRIVATE(promise, promiseStatus, status); SET_PRIVATE(promise, promiseValue, value); SET_PRIVATE(promise, promiseOnResolve, onResolve); SET_PRIVATE(promise, promiseOnReject, onReject); // debug代码忽略 if (DEBUG_IS_ACTIVE) { %DebugPromiseEvent({ promise: promise, status: status, value: value }); } return promise;}function PromiseInit(promise) { return PromiseSet( promise, 0, UNDEFINED, new InternalArray, new InternalArray)}
实质上是调用了PromiseSet方法给promise设置了当前的状态。
PromiseResolve
promiseResolve方法的调用暴露给外部的promise.accept(高版本为resolve)方法,如果当前的this指向promise的构造函数,则设置当前的promise状态,否则调用resolve函数执行。
function PromiseResolved(x) { if (this === $Promise) { // Optimized case, avoid extra closure. return PromiseSet(new $Promise(promiseRaw), +1, x); } else { return new this(function(resolve, reject) { resolve(x) }); }}
promiseResolve处理逻辑同promiseReject,不再赘述。
promise.then
PromiseThen方法的调用暴露给实例化后的promise.then方法调用。
PromiseThen = function PromiseThen(onResolve, onReject) { onResolve = IS_SPEC_FUNCTION(onResolve) ? onResolve : PromiseIdResolveHandler; onReject = IS_SPEC_FUNCTION(onReject) ? onReject : PromiseIdRejectHandler; var that = this; var constructor = this.constructor; return %_CallFunction( this, function(x) { x = PromiseCoerce(constructor, x); return x === that ? onReject(MakeTypeError('promise_cyclic', [x])) : IsPromise(x) ? x.then(onResolve, onReject) : onResolve(x); }, onReject, PromiseChain );}
首先判断传入的两个参数是否是函数,不是的话添加默认的处理函数,做良好的容错处理。而后调用了 %_CallFunction方法(第一个参数是this,最后一个参数是要调用的方法,中间是传入参数),类似Function.prototype.call()方法,调用了PromiseChain方法,传入了两个参数resolve和reject。在resolve方法内部调用了PromiseCoerce方法,生成对象如果是个thenable对象调用对象的then方法否则直接onResolve方法。
PromiseCoerce
function PromiseCoerce(constructor, x) { if (!IsPromise(x) && IS_SPEC_OBJECT(x)) { var then; try { then = x.then; } catch(r) { return %_CallFunction(constructor, r, PromiseRejected); } // macro IS_SPEC_FUNCTION(arg) = (%_ClassOf(arg) === 'Function'); // 如果是一个function if (IS_SPEC_FUNCTION(then)) { var deferred = %_CallFunction(constructor, PromiseDeferred); try { %_CallFunction(x, deferred.resolve, deferred.reject, then); } catch(r) { deferred.reject(r); } return deferred.promise; } } return x;}
核心的逻辑是如果传入对象的then属性是一个function,则调用then方法。若有报错走到reject方法。
PromiseChain
PromiseChain = function PromiseChain(onResolve, onReject) { // 补默认的处理函数 onResolve = IS_UNDEFINED(onResolve) ? PromiseIdResolveHandler : onResolve; onReject = IS_UNDEFINED(onReject) ? PromiseIdRejectHandler : onReject; var deferred = %_CallFunction(this.constructor, PromiseDeferred); switch (GET_PRIVATE(this, promiseStatus)) { case UNDEFINED: throw MakeTypeError('not_a_promise', [this]); case 0: // Pending GET_PRIVATE(this, promiseOnResolve).push(onResolve, deferred); GET_PRIVATE(this, promiseOnReject).push(onReject, deferred); break; case +1: // Resolved PromiseEnqueue(GET_PRIVATE(this, promiseValue), [onResolve, deferred], +1); break; case -1: // Rejected PromiseEnqueue(GET_PRIVATE(this, promiseValue), [onReject, deferred], -1); break; } // debug代码忽略 if (DEBUG_IS_ACTIVE) { %DebugPromiseEvent({ promise: deferred.promise, parentPromise: this }); } return deferred.promise;}
PromiseChain方法是promise实现的核心,判断当前定义的promise状态,如果是pending状态在promiseOnResolve数组中push当前的onResolve方法。如果是Resolved状态或者Rejected状态,则调用PromiseEnqueue函数进行微任务的添加。
PromiseEnqueue
function PromiseEnqueue(value, tasks, status) { var id, name, instrumenting = DEBUG_IS_ACTIVE; %EnqueueMicrotask(function() { if (instrumenting) { %DebugAsyncTaskEvent({ type: "willHandle", id: id, name: name }); } for (var i = 0; i < tasks.length; i += 2) { PromiseHandle(value, tasks[i], tasks[i + 1]) } if (instrumenting) { %DebugAsyncTaskEvent({ type: "didHandle", id: id, name: name }); } }); if (instrumenting) { id = ++lastMicrotaskId; name = status > 0 ? "Promise.resolve" : "Promise.reject"; %DebugAsyncTaskEvent({ type: "enqueue", id: id, name: name }); }}
此步骤其实是将PromiseHandle函数加入JS运行时的微任务队列中。微任务的队列列表是C语言进行维护的,应用%EnqueueMicrotask方法进行添加。
PromiseHandle
function PromiseHandle(value, handler, deferred) { try { %DebugPushPromise(deferred.promise); var result = handler(value); if (result === deferred.promise) throw MakeTypeError('promise_cyclic', [result]); else if (IsPromise(result)) %_CallFunction(result, deferred.resolve, deferred.reject, PromiseChain); else deferred.resolve(result); } catch (exception) { try { deferred.reject(exception); } catch (e) { } } finally { %DebugPopPromise(); }}
此函数处理了传入的方法,是指是调用了resolve方法,如果返回的结果依旧是一个promise则继续调用PromiseChain方法,否则调用新生成的promise实例的resolve方法,进而实现循坏调用。
promise.all
promise的all方法实现了发送多个promise请求,返回一个新的promise,所有promise打到resolve状态时触发resolve状态,若有一个promise被reject,则返回此promise的reject原因。
function PromiseAll(values) { var deferred = %_CallFunction(this, PromiseDeferred); var resolutions = []; if (!%_IsArray(values)) { deferred.reject(MakeTypeError('invalid_argument')); return deferred.promise; } try { var count = values.length; if (count === 0) { deferred.resolve(resolutions); } else { for (var i = 0; i < values.length; ++i) { this.resolve(values[i]).then( (function() { // Nested scope to get closure over current i (and avoid .bind). // TODO(rossberg): Use for-let instead once available. var i_captured = i; return function(x) { resolutions[i_captured] = x; if (--count === 0) deferred.resolve(resolutions); }; })(), function(r) { deferred.reject(r) } ); } } } catch (e) { deferred.reject(e) } return deferred.promise;}
首先判断传参的合理性,生成一个新的promise对象,利用遍历的方式给每个传入的promise的resolve方法后都追加了then方法,使得每个传入的promise执行then方法后凑执行判断逻辑,当计数count的flag为0的时候,所有promise resolve完成,调用新promise对象的resolve方法,传入新promise的reject方法作为then方法reject参数。使得所有promise的reject函数被调用时都会走到新promise对象的reject,最后返回新生成的promise。
Summary
Promise的状态和核心变量均托管到公共的作用域去维护,通过数组的push方法去添加Promise自定的resolve和reject方法。并将resolve和reject方法的执行加入微服务队列中,等到resolve方法被调用时执行resolve(value)方法进行调用。为了实现promise的循环嵌套调用,在每次处理value之前将处理逻辑之上包裹了一层新的promise逻辑,类似(new promise()).then(resolve(value)),思路如下。
var ref = function (value) { if (value && value.then) return value; return { then: function (callback) { // 实例化一个新的promise var result = defer(); // 进入宏任务队列 enqueue(function () { result.resolve(callback(value)); }); return result.promise; } };};
Reference
promise设计思想:https://github.com/kriskowal/...
JavaScript执行机制:https://www.jianshu.com/p/17c...