概念-提炼承诺是如何运作的?

从根本上说,promise只是一个对象,它有一个标志,说明它是否已被解决,以及它维护的一系列函数,以在解决时通知它。代码有时可以说的不仅仅是单词,所以这里有一个 非常基本,不是真实的世界 示例纯粹是为了帮助传达概念:

// See notes following the code for why this isn't real-world code
function Promise() {
    this.settled = false;
    this.settledValue = null;
    this.callbacks = [];
}
Promise.prototype.then = function(f) {
    if (this.settled) {
        f(this.settledValue);                // See notes 1 and 2
    } else {
        this.callbacks.push(f);
    }
                                             // See note 3 about `then`
                                             // needing a return value
};
Promise.prototype.settle = function(value) { // See notes 4 and 5
    var callback;

    if (!this.settled) {
        this.settled = true;
        this.settledValue = value;
        while (this.callbacks.length) {
            callback = this.callbacks.pop();
            callback(this.settledValue);      // See notes 1 and 2
        }
    }
};

所以 Promise 保存状态,以及在约定完成时要调用的函数。的行为 沉淀 承诺通常是外部的 许诺 对象本身(当然,这取决于实际的使用,您可以将它们组合起来——例如,与jQuery的 ajax [ jqXHR ]对象)。

同样,以上内容纯粹是概念性的,缺少了在任何现实世界的承诺实现中都必须存在的几个重要内容,这些内容才会有用:

1. then 和 settle 应该 总是 异步调用回调,即使promise已经解决。 然后 应该,因为否则调用者不知道回调是否是异步的。 解决 应该,因为回调应该在之后才运行 解决 已返回。(ES2015的承诺做到了这两件事。jQuery的 Deferred 没有。)

2. 然后 和 解决 应确保回调中的失败(例如,异常)不会直接传播到代码调用 然后 或 解决 这与上面的#1部分相关,并且与下面的#3更相关。

3. 然后 应返回 新 promise基于调用回调的结果(当时或以后)。这对于构成承诺化操作是相当基本的,但会使上述操作明显复杂化。任何合理的承诺实施都是如此。

4. 我们需要不同类型的“结算”操作:“解析”(基本操作成功)和“拒绝”(失败)。有些用例可能有更多的状态,但已解决和已拒绝是基本的两个状态。(ES2015的承诺有决心也有拒绝。)

5. 我们可能会 解决 (或单独的 resolve 和 reject )在某种程度上是私有的,这样只有承诺的创建者才能解决它。(ES2015承诺和其他几个承诺通过 许诺 构造函数接受一个回调,该回调接收 决定 和 拒绝 作为参数值,因此只有回调中的代码才能解析或拒绝[除非回调中的编码以某种方式公开它们]。)

等等。

有人能用几句话实现最基本的承诺吗?

它在这里:

function Promise(exec) {
    // takes a function as an argument that gets the fullfiller
    var callbacks = [], result;
    exec(function fulfill() {
        if (result) return;
        result = arguments;
        for (let c;c=callbacks.shift();)
            c.apply(null, arguments);
    });
    this.addCallback = function(c) {
        if (result)
            c.apply(null, result)
        else
            callbacks.push(c);
    }
}

附加的 then 带有链接(您将需要 the answer ):

Promise.prototype.then = function(fn) {
    return new Promise(fulfill => {
        this.addCallback((...args) => {
            const result = fn(...args);
            if (result instanceof Promise)
                result.addCallback(fulfill);
            else
                fulfill(result);
        });
    });
};

这两个片段是如何关联的?

ajax 从调用 getPromiseForAjaxResult 功能:

function getPromiseForAjaxResult(ressource) {
    return new Promise(function(callback) {
        ajax({url:ressource}, callback);
    });
}

我已经在ES7中实现了一个。有了链接,就有70行了,如果算少的话。我认为状态机是实现承诺的正确范例。生成的代码比许多代码更容易理解 if s国际海事组织。在中详细描述 this article .

这是代码:

const states = {
    pending: 'Pending',
    resolved: 'Resolved',
    rejected: 'Rejected'
};

class Nancy {
    constructor(executor) {
        const tryCall = callback => Nancy.try(() => callback(this.value));
        const laterCalls = [];
        const callLater = getMember => callback => new Nancy(resolve => laterCalls.push(() => resolve(getMember()(callback))));
        const members = {
            [states.resolved]: {
                state: states.resolved,
                then: tryCall,
                catch: _ => this
            },
            [states.rejected]: {
                state: states.rejected,
                then: _ => this,
                catch: tryCall
            },
            [states.pending]: {
                state: states.pending,
                then: callLater(() => this.then),
                catch: callLater(() => this.catch)
            }
        };
        const changeState = state => Object.assign(this, members[state]);
        const apply = (value, state) => {
            if (this.state === states.pending) {
                this.value = value;
                changeState(state);
                for (const laterCall of laterCalls) {
                    laterCall();
                }
            }
        };

        const getCallback = state => value => {
            if (value instanceof Nancy && state === states.resolved) {
                value.then(value => apply(value, states.resolved));
                value.catch(value => apply(value, states.rejected));
            } else {
                apply(value, state);
            }
        };

        const resolve = getCallback(states.resolved);
        const reject = getCallback(states.rejected);
        changeState(states.pending);
        try {
            executor(resolve, reject);
        } catch (error) {
            reject(error);
        }
    }

    static resolve(value) {
        return new Nancy(resolve => resolve(value));
    }

    static reject(value) {
        return new Nancy((_, reject) => reject(value));
    }

    static try(callback) {
        return new Nancy(resolve => resolve(callback()));
    }
}

这里有一个轻量级的promise实现,称为“sequence”,我在日常工作中使用它:

(function() {
    sequence = (function() {
        var chained = [];
        var value;
        var error;

        var chain = function(func) {
            chained.push(func);
            return this;
        };

        var execute = function(index) {
            var callback;
            index = typeof index === "number" ? index : 0;

            if ( index >= chained.length ) {
                chained = [];
                return true;
            }

            callback = chained[index];

            callback({
                resolve: function(_value) {
                    value = _value;
                    execute(++index);
                },
                reject: function(_error) {
                    error = _error;
                    execute(++index);
                },
                response: {
                    value: value,
                    error: error
                }
            });
        };

        return {
            chain: chain,
            execute: execute
        };
    })();
})();

初始化后,您可以按以下方式使用序列:

sequence()
    .chain(function(seq) {
        setTimeout(function() {
            console.log("func A");
            seq.resolve();
        }, 2000);
    })
    .chain(function(seq) {
        setTimeout(function() {
            console.log("func B");
        }, 1000)
    })
    .execute()

要启用实际的链接,您需要调用seq对象的resolve()函数,回调必须将其用作参数。

序列公开两个公共方法:

• chain-这个方法只需将回调推送到一个私有数组
• execute-此方法使用递归来实现回调的正确顺序执行。它基本上是按照链接它们的顺序执行回调,方法是将seq对象传递给每个回调。一旦当前回调被解析/拒绝,就执行下一次回调。

“execute”方法是魔法发生的地方。它将“seq”对象传递给您的所有回调。因此,当您调用seq.resolve()或seq.rejectt()时,您实际上会调用下一个链式回调。

请注意,此实现仅存储来自先前执行的回调的响应。

有关更多示例和文档,请参阅: https://github.com/nevendyulgerov/sequence

这里有一个简单的Promise实现,对我来说很有效。

    function Promise(callback) {
        this._pending = [];
        this.PENDING = "pending";
        this.RESOLVED = "resolved";
        this.REJECTED = "rejected";
        this.PromiseState = this.PENDING;
        this._catch = function (error) {
            console.error(error);
        };
        setTimeout(function () {
            try {
                callback.call(this, this.resolve.bind(this), this.reject.bind(this));
            } catch (error) {
                this.reject(error);
            }
        }.bind(this), 0)
    };
    Promise.prototype.resolve = function (object) {
        if (this.PromiseState !== this.PENDING) return;
        while (this._pending.length > 0) {
            var callbacks = this._pending.shift();
            try {
                var resolve = callbacks.resolve;
                if (resolve instanceof Promise) {
                    resolve._pending = resolve._pending.concat(this._pending);
                    resolve._catch = this._catch;
                    resolve.resolve(object);
                    return resolve;
                }
                object = resolve.call(this, object);
                if (object instanceof Promise) {
                    object._pending = object._pending.concat(this._pending);
                    object._catch = this._catch;
                    return object;
                }
            } catch (error) {
                (callbacks.reject || this._catch).call(this, error);
                return;
            }
        }
        this.PromiseState = this.RESOLVED;
        return object;
    };
    Promise.prototype.reject = function (error) {
        if (this.PromiseState !== this.PENDING) return;
        this.PromiseState = this.REJECTED;
        try {
            this._catch(error);
        } catch (e) {
            console.error(error, e);
        }
    };
    Promise.prototype.then = function (onFulfilled, onRejected) {
        onFulfilled = onFulfilled || function (result) {
            return result;
        };
        this._catch = onRejected || this._catch;
        this._pending.push({resolve: onFulfilled, reject: onRejected});
        return this;
    };
    Promise.prototype.catch = function (onRejected) {
        // var onFulfilled = function (result) {
        //     return result;
        // };
        this._catch = onRejected || this._catch;
        // this._pending.push({resolve: onFulfilled, reject: onRejected});
        return this;
    };
    Promise.all = function (array) {
        return new Promise(function () {
            var self = this;
            var counter = 0;
            var finishResult = [];

            function success(item, index) {
                counter++;
                finishResult[index] = item;
                if (counter >= array.length) {
                    self.resolve(finishResult);
                }
            }
            for(var i in array) {
                var item = array[i];
                if (item instanceof Promise) {
                    item.then(function (result) {
                        success(result,this);
                    }.bind(i), function (error) {
                        array.map(function (item) {
                            item.PromiseState = Promise.REJECTED
                        });
                        self._catch(error);
                    })
                } else {
                    success(item, i);
                }
            }
        });
    };
    Promise.race = function (array) {
        return new Promise(function () {
            var self = this;
            var counter = 0;
            var finishResult = [];
            array.map(function (item) {
                if (item instanceof Promise) {
                    item.then(function (result) {
                        array.map(function (item) {
                            item.PromiseState = Promise.REJECTED
                        });
                        self.resolve(result);
                    }, function (error) {
                        array.map(function (item) {
                            item.PromiseState = Promise.REJECTED
                        });
                        self._catch(error);
                    })
                } else {
                    array.map(function (item) {
                        item.PromiseState = Promise.REJECTED
                    });
                    self.resolve(item);
                }
            })
        });
    };
    Promise.resolve = function (value) {
        return new Promise(function (resolve, reject) {
            try {
                resolve(value);
            } catch (error) {
                reject(error);
            }
        });
    };
    Promise.reject = function (error) {
        return new Promise(function (resolve, reject) {
            reject(error);
        });
    }

讨论 here . 小提琴: here .

这是promise体系结构的绝对最小值

function Promise(F) {
  var gotoNext = false;
  var stack = [];
  var args = [];

  var isFunction = function(f) {
    return f && {}.toString.call(f) === '[object Function]';
  };

  var getArguments = function(self, _args) {
    var SLICE = Array.prototype.slice;

    _args = SLICE.call(_args);
    _args.push(self);

    return _args;
  };

  var callNext = function() {
    var method = stack.shift();

    gotoNext = false;
    if (isFunction(method)) method.apply(null, args);
  };

  var resolve = [(function loop() {
    if (stack.length) setTimeout(loop, 0);
    if (gotoNext) callNext();
  })];

  this.return = function() {
    gotoNext = true;
    args = getArguments(this, arguments);
    if(resolve.length) resolve.shift()();

    return this;
  };

  this.then = function(fn) {
    if (isFunction(fn)) stack.push(fn);

    return this;
  };

  return this.then(F).return();
}


// --- below is a working implementation --- //

var bar = function(p) {
  setTimeout(function() {
    console.log("1");
    p.return(2);
  }, 1000);
};

var foo = function(num, p) {
  setTimeout(function() {
    console.log(num);
    p.return(++num);
  }, 1000);
};

new Promise(bar)
  .then(foo)
  .then(foo)
  .then(foo);