co源碼

co模塊整體包括三部分

1. 對於幾種參數類型的判斷,主要判斷是否object,array,promise,generator,generatorFunction這幾種;
2. 將幾種不同的參數類型轉換爲promise

3. 遞歸執行promise

    module.exports = co['default'] = co.co = co;
    let slice = [].slice; // 多次用到,所以定義成變量; 原文是Array.prototype.slice
   
    // 1. 對於幾種參數類型的判斷,主要判斷是否object,array,promise,generator,generatorFunction這幾種;
   
    /**
    * 判斷是否爲對象
    * 任何數據類型的constructor屬性返回創建該對象的函數的引用,實例的constructor屬性指向其構造函數,
    */
    function isObject(obj) {
        return obj.constructor === Object;
    }
   
    /**
    * 是否promise
    * promise具有.then方法,.catch方法
    */
    function isPromise(obj) {
        return 'function' === typeof obj.then;   //也可以用 obj.constructor === Promise
    }
   
    /**
    * 是否generator對象,
    * generator具有next,throw,return方法
    */
    function isGenerator(obj) {
        return 'function' === typeof obj.next() && 'function' === typeof obj.throw() && 'function' === typeof obj.return();
    }
   
    /**
    * generator的constructor的name爲GeneratorFunction,displayName(非標準)如果設置也爲* GeneratorFunction,
    * display相關參考https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Function/displayName
    *【注意】generator的constructor !== GeneratorFunction , 參考自這裏https://blog.csdn.net/sinat_30552885/article/details/85247120
    */
    function isGeneratorFunction(obj) {
        let constructor = obj.constructor;
        if ( !constructor ) return false;
        if ( constructor.name === 'GeneratorFunction' || constructor.displayName === 'GeneratorFunction') return true;
        return isGenerator(obj);
    }
   
    // 2. 將幾種不同的參數類型轉換爲promise
   
   
    // 總括
    function toPromise(obj) {
        if (!obj) return obj;
        if ( isPromise(obj) ) return obj;
        if ( isGenerator(obj) || isGeneratorFunction(obj) ) return co.call(this, obj);  // co的入參接收promise或者generator，因此如果是generator,則直接調用一遍co;
        if ( 'function' === typeof obj) return thunkToPromise.call(this, obj);
        if ( Array.isArray(obj) ) return arrayToPromise.call(this, obj);
        if ( isObject(obj) )  return objToPromise.call(this, obj);
        return obj;
    }
   
    /**
    * thunk => promise
    */
    function thunkToPromise(fn) {
        let ctx = this;
        return new Promise(function(resolve, reject) {
            fn.call(ctx, function(err, res) {
                if ( err ) reject(err);
                if ( arguments.length > 2) res = slice.call(arguments,1);
                resolve(res);
            })
        })
    }
   
    /**
    *  arr => promise
    *  將數組的每一元素變成promise,然後放入到promise.all裏統一存儲
    */
    function arrayToPromise(arr) {
        return Promise.all(arr.map(toPromise, this));
    }
   
    /**
    *  obj => promise;
    *  obj.constructor 返回創建該對象的函數的引用
    */
    function objToPromise(obj) {
        let results = new obj.constructor();
        let promises = [];
        let keys = Object.keys(obj);
        for ( let i = 0; i < keys.length; i++ ) {
            let key = keys[i];
            let promise = toPromise(this, obj[key]);
            if ( promise && isPromise(promise) ) {
                defer(promise, key);
            } else {
                results[key] = obj[key];
            }
        }
        return Promise.all(promises).then(function() {
            return results;
        })
        function defer(promise, key) {
            promises.push(promise.then(function(res) {
                results[key] = res;
            }))
        }
    }
   
    // 3. 遞歸執行promise
   
    /**
    * 
    * @param {*} fn 入參是一個promise或者generator
    * @return Promise;
    */
    function co(gen) {
        let ctx = this;
        let args = slice.call(arguments, 1);
        return new Promise(function(resolve, reject) {
            if ( typeof gen === 'function') gen = gen.apply(ctx, args);
            if ( !gen || typeof gen !== 'function' ) resolve(gen);
            onFulfilled();
            function onFulfilled() {
                let ret;
                try {
                    ret = gen.next();
                } catch (e) {
                    return reject(e);
                }
                next(ret);
            }
            function onRejected() {
                let ret;
                try {
                    ret = gen.throw();
                } catch (e) {
                    return reject(e)
                }
                next(ret);
            }
            function next(ret) {
                if ( ret.done ) return resolve(ret.value);
                let value = toPromise.call(this, ret.value);
                if ( value && isPromise(value)) return value.then(onFulfilled, onRejected);
            }
   
            return onRejected(new TypeError('You may only yield a function, promise, generator, array, or object, but the following object was passed: "' + String(ret.value) + '"'));
        })
    }
   
    co.wrapper = function(fn) {
        createPromise.__generatorFunction__ = fn;
        return createPromise;
        function createPromise() {
            return co.call(this, fn.apply(this,arguments))
        }
    }