angular源码剖析之Provider系列--QProvider

QProvider 简介

源码里是这么描述的：

A service that helps you run functions asynchronously, and use their return values (or exceptions) when they are done processing.

大概意思是帮助你异步执行方法，且当他们执行完后可以使用他们的返回值。

This is an implementation of promises/deferred objects inspired by Kris Kowal's Q.

这是一个 promises/deferred 对象的实现,灵感来自于 Kris Kowal's Q

QProvider 用法

下面的例子假设$q和asyncGreet在当前作用域内是有效的.

function asyncGreet(name) {
// perform some asynchronous operation, resolve or reject the promise when appropriate.
return $q(function(resolve, reject) {
setTimeout(function() {
if (okToGreet(name)) {
resolve('Hello, ' + name + '!');
} else {
reject('Greeting ' + name + ' is not allowed.');
}
}, 1000);
});
}

var promise = asyncGreet('Robin Hood');
//then函数放入pending数组
promise.then(function(greeting) {
alert('Success: ' + greeting);
}, function(reason) {
alert('Failed: ' + reason);
});

下面我们深入源码内部去一探究竟：

$QProvider 定义

function $QProvider() {

this.$get = ['$rootScope', '$exceptionHandler', function($rootScope, $exceptionHandler) {
return qFactory(function(callback) {
$rootScope.$evalAsync(callback);
}, $exceptionHandler);
}];
}

$evalAsync: function(expr, locals) {
// if we are outside of an $digest loop and this is the first time we are scheduling async
// task also schedule async auto-flush
//如果当前不处于$digest或者$apply的过程中(只有在$apply和$digest方法中才会设置$$phase这个字段)，并且asyncQueue数组中还不存在任务时，
//就会异步调度一轮digest循环来确保asyncQueue数组中的表达式会被执行
if (!$rootScope.$$phase && !asyncQueue.length) {
$browser.defer(function() {
//最终调用的是setTimeout
if (asyncQueue.length) {
$rootScope.$digest();//执行消化功能
}
});
}
asyncQueue.push({scope: this, expression: $parse(expr), locals: locals});
}

由之前 cacheFactory的分析
，再结合上面源码我们就知道 注入$q时调用了qFactory工厂方法：

qFactory

function qFactory(nextTick, exceptionHandler) {

function Promise() {
//初始化Promise的状态对象
this.$$state = { status: 0 };
}
//扩展Promise类原型
extend(Promise.prototype, {
//then主要是把一个defer对象和fullfiled reject 函数 放入pending数组
then: function(onFulfilled, onRejected, progressBack) {
if (isUndefined(onFulfilled) && isUndefined(onRejected) && isUndefined(progressBack)) {
return this;
}
var result = new Deferred();

this.$$state.pending = this.$$state.pending || [];
//把一个新的 Defer 对象push进pending数组
this.$$state.pending.push([result, onFulfilled, onRejected, progressBack]);

if (this.$$state.status > 0) scheduleProcessQueue(this.$$state);

//返回这个新建Defer对象的promise
//可以形成promise chain
return result.promise;
}
//代码省略
}
//通过$q注入时返回Q函数
//随后Q函数 传入resolver参数调用
//调用resolver函数时传入包装deferred对象的resolve和 reject函数
//随后返回promise对象
//promise对象调用then函数时放入pending队列
var $Q = function Q(resolver) {
if (!isFunction(resolver)) {
throw $qMinErr('norslvr', "Expected resolverFn, got '{0}'", resolver);
}
//构造一个Deferred对象
var deferred = new Deferred();
function resolveFn(value) {
deferred.resolve(value);
}
function rejectFn(reason) {
deferred.reject(reason);
}
//调用resolver参数函数
//resolveFn供外部调用
resolver(resolveFn, rejectFn);
//返回一个promise对象
//供调用then函数
return deferred.promise;
};
// Let's make the instanceof operator work for promises, so that
// `new $q(fn) instanceof $q` would evaluate to true.
//使得new $q(fn) 也可以调用promise的方法
$Q.prototype = Promise.prototype;

//暴露内部方法
$Q.defer = defer;
$Q.reject = reject;
$Q.when = when;
$Q.resolve = resolve;
//$q.all是用于执行多个异步任务进行回调，它可以接受一个promise的数组，
//或是promise的hash(object)。任何一个promise失败，都会导致整个任务的失败。
//https://blog.csdn.net/shidaping/article/details/52398925
$Q.all = all;
//$q.race() 是 Angular 里面的一个新方法，和 $q.all() 类似，但是它只会返回第一个处理完成的 Promise 给你///。假定 API 调用 1 和 API 调用 2 同时执行，而 API 调用 2 在 API 调用 1 之前处理完成，那么你就只会得到 //API 调用 2 的返回对象。换句话说，最快（处理完成）的 Promise 会赢得返回对象的机会：
$Q.race = race;

return $Q;
}

调用then方法时实际上是新建一个defer对象放入pending数组，在调用defer.resolve的时候
去调度这个数组中的元素，也就是任务.

resolve 方法

extend(Deferred.prototype, {
resolve: function(val) {
//第一次resolve的时候为0 所以会往下走
if (this.promise.$$state.status) return;
if (val === this.promise) {
this.$$reject($qMinErr(
'qcycle',
"Expected promise to be resolved with value other than itself '{0}'",
val));
} else {
//调度pending数组里的任务
this.$$resolve(val);
}
},

$$resolve: function(val) {
var then;
var that = this;
var done = false;
try {
if ((isObject(val) || isFunction(val))) then = val && val.then;
if (isFunction(then)) {
//val.then方法 val是promise的时候
//resolvePromise函数里放入了当前defer对象
this.promise.$$state.status = -1;
then.call(val, resolvePromise, rejectPromise, simpleBind(this, this.notify));
} else {

//更新promise的状态对象
this.promise.$$state.value = val;
this.promise.$$state.status = 1;
//调度的时候pending为空就返回了
scheduleProcessQueue(this.promise.$$state);
}
} catch (e) {
rejectPromise(e);
exceptionHandler(e);
}

function resolvePromise(val) {
if (done) return;
done = true;
that.$$resolve(val);
}
function rejectPromise(val) {
if (done) return;
done = true;
that.$$reject(val);
}
}
}

scheduleProcessQueue 方法

//传入promise的state对象
function scheduleProcessQueue(state) {
if (state.processScheduled || !state.pending) return;
state.processScheduled = true;
//nextTick里调用$browser.defer函数

nextTick(function() { processQueue(state); });
}

processQueue 方法

实际最终处理的还是processQueue函数，里面循环调用pending数组

function processQueue(state) {
var fn, deferred, pending;

pending = state.pending;
state.processScheduled = false;
state.pending = undefined;
for (var i = 0, ii = pending.length; i < ii; ++i) {

//获取pending数组的元素 元素本身也是数组
deferred = pending[i][0];
fn = pending[i][state.status];
try {
if (isFunction(fn)) {
deferred.resolve(fn(state.value));
} else if (state.status === 1) {
deferred.resolve(state.value);
} else {
deferred.reject(state.value);
}
} catch (e) {
deferred.reject(e);
exceptionHandler(e);
}
}
}

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参考资料：

1. a tiny implementation of Promises/A+.
2. Angular中的$q的形象解释及深入用法
3. 关于 Angular 里的 $q 和 Promise
4. 理解状态机