Promise初探

遭遇“恶魔金字塔”

项目需要，封装了一个省市区的地址选择器组件。

可以根据省份id、城市id和区id对组件设置默认值。逻辑是这样的：

1. 获取省份列表，选中默认省份；
2. 第一步成功后，根据默认省份id获取城市列表，选中默认城市；
3. 第二部成功后，根据默认城市id获取区列表，选中默认区。

 getDefaultOptions = () = >{
    let {
        province: provinceId,
        city: cityId
    } = this.props.defaultValue;
        
    //获取省份
    this.props.dispatch({
        type: 'basic/getProvinceList',
        params: {},
        successCallBack: (rs) = >{
            let provinceList = rs.provinces;

            //获取城市
            let params = {
                province_id: +provinceId
            };
            this.props.dispatch({
                type: 'storage/getCityList',
                params,
                successCallBack: (rs2) = >{
                    let cityList = rs2.cities;
                    if (cityList == null) {
                        cityList = [];
                    }

                    if ( + cityId == 0) {
                        this._getOptions(provinceList, [], []);
                        return;
                    }
                    //获取区
                    let params = {
                        city_id: +cityId,
                    };
                    this.props.dispatch({
                        type: 'storage/getDistrictList',
                        params,
                        successCallBack:  (rs3) = >{
                            let districtList = rs3.districts;
                            if (districtList == null) {
                                districtList = [];
                            }
                            this._getOptions(provinceList, cityList, districtList);
                    };
                    });

                }
            });

        }
    });
 };

出现3层嵌套的回调，这就是传说中的“恶魔金字塔”。确实是恶魔呀，可读性巨差，自己看着都有点晕，更别说其他人了。

都说ES6的Promise对象让“恶魔金字塔”闻风丧胆，忍不住来体验一下。

Promise登场

MDN上这样定义Promise:
The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.

废话不多说，来看看使用了Promise后的代码是怎样的：

sendRequest = (type, params) = >{
    return new Promise((resolve, reject) = >{
        this.props.dispatch({
            type,
            params,
            successCallBack: (rs) = >{
                resolve(rs);
            }
        });
    });
};

getDefaultOptions = () = >{
    let {
        province: provinceId,
        city: cityId
    } = this.props.defaultValue;
    let provinceList,
    cityList,
    districtList;
    let _promise = this.sendRequest('basic/getProvinceList', {});
    _promise.then(rs = >{
        provinceList = rs.provinces ? rs.provinces: [];
        return this.sendRequest('storage/getCityList', {
            province_id: +provinceId
        })
    }).then(rs = >{
        cityList = rs.cities ? rs.cities: [];
        //只有省份的情况
        if ( + cityId == 0) {
            this._getOptions(provinceList, cityList, []);
            return Promise.reject({
                notRealPromiseException: true,
            });
        }
        return this.sendRequest('storage/getDistrictList', {
            city_id: +cityId
        });
    }).then(rs = >{
        districtList = rs.districts ? rs.districts: [];
        return this._getOptions(provinceList, cityList, districtList);
    }).
    catch(ex = >{
        if (ex.notRealPromiseException) {
            return true;
        }
        return false;
    });
};

需要有序地执行异步操作的场景，Promise再适合不过了。相比回调嵌套，层次更分明，可读性强。

Promise基本原理学习

无论是在异步操作的执行之前或执行之后，用Promise对象的then方法注册回调，回调都能一致执行。

很好奇它是怎么做到的，于是自己尝试写了个简易的Promise, 模拟Promise对异步操作的值的代理：
https://gist.github.com/anonymous/402271e9e9c59958279d0fe096e0a277