原文链接: Promise的奇怪用法和自己实现一个Promise

使用Promise实现一个页面所有图片加载完毕的回调

import React, { useEffect } from "react";
export default () => {
  useEffect(() => {
    const imageDomList = Array.from(document.getElementsByClassName("image-item"));

    const imagePromiseList = imageDomList.map(
      (i) => new Promise((r) => (i.onload = r))
    );
    const imageLoaded = Promise.all(imagePromiseList);
    imageLoaded.then(() => {
      console.log("所有图片加载完成");
    });
  }, []);

  const imageSrcList = [
    "https://oscimg.oschina.net/oscnet/up-3ebf22d4756e0a1935f2549eeb604f69d2f.gif",
    "https://oscimg.oschina.net/oscnet/up-afc4f6c5faf60f807bed43129a9baaa122a.gif",
  ];
  return (
    <div>
      c1
      {imageSrcList.map((i) => (
        <img class="image-item" key={i} src={i}></img>
      ))}
    </div>
  );
};

将resolve函数提出来

仅演示用, 还有的优化空间, 使用context将resolve函数传递到子组件中, 在子组件完成渲染后调用resolve 通知上层组件渲染完毕

import React, { useEffect, useContext, useState } from "react";

const resolveMapContext = React.createContext({});

const Card = ({ id, time }) => {
  const resolveMap = useContext(resolveMapContext);
  useEffect(() => {
    const r = resolveMap[id];
    console.log("card", id, r);
    setTimeout(() => {
      r && r(id);
    }, time);
  }, [resolveMap]);
  return <div>id:{id}</div>;
};

export default () => {
  const [defaultContext, setDefaultContext] = useState({});

  const cardList = [
    { id: "card1", time: 2000 },
    { id: "card2", time: 4000 },
  ];
  useEffect(() => {
    console.log("App");
    const context = {};
    const cardPromiseList = cardList.map(
      ({ id }) => new Promise((r) => (context[id] = r))
    );

    setDefaultContext(context);
    const cardRendered = Promise.all(cardPromiseList);
    cardRendered.then(() => {
      console.log("所有组件加载完毕");
    });
  }, []);
  return (
    <resolveMapContext.Provider value={defaultContext}>
      <div>
        c1
        {cardList.map(({ id, time }) => (
          <Card key={id} id={id} time={time} />
        ))}
      </div>
    </resolveMapContext.Provider>
  );
};

上述代码有个问题, 在组件切换的时候, 由于promise不能被取消, 所以我们不能在useEffect的return函数中终止promise的执行,当然可以引入变量实现, 不过有更加优雅的方式

自己实现一个Promise, 提供 cancel方法用于取消promise

https://zhuanlan.zhihu.com/p/58428287
https://github.com/YvetteLau/Blog/issues/2
https://juejin.im/post/6844903625769091079

const PENDING = "pending";
const FULFILLED = "fulfilled";
const REJECTED = "rejected";

function resolvePromise(promise2, x, resolve, reject) {
  //PromiseA+ 2.3.1
  if (promise2 === x) {
    reject(new TypeError("Chaining cycle"));
  }
  if ((x && typeof x === "object") || typeof x === "function") {
    let used; //PromiseA+2.3.3.3.3 只能调用一次
    try {
      let then = x.then;
      if (typeof then === "function") {
        //PromiseA+2.3.3
        then.call(
          x,
          (y) => {
            //PromiseA+2.3.3.1
            if (used) return;
            used = true;
            resolvePromise(promise2, y, resolve, reject);
          },
          (r) => {
            //PromiseA+2.3.3.2
            if (used) return;
            used = true;
            reject(r);
          }
        );
      } else {
        //PromiseA+2.3.3.4
        if (used) return;
        used = true;
        resolve(x);
      }
    } catch (e) {
      //PromiseA+ 2.3.3.2
      if (used) return;
      used = true;
      reject(e);
    }
  } else {
    //PromiseA+ 2.3.3.4
    resolve(x);
  }
}

class Promise {
  constructor(executor) {
    this.status = PENDING;
    this.onFulfilled = [];
    this.onRejected = [];

    try {
      executor(this.resolve, this.reject);
    } catch (e) {
      this.reject(e);
    }
  }
  resolve = (value) => {
    if (this.status === PENDING) {
      this.status = FULFILLED;
      this.value = value;
      this.onFulfilled.forEach((fn) => fn()); //PromiseA+ 2.2.6.1
    }
  };

  reject = (reason) => {
    if (this.status === PENDING) {
      this.status = REJECTED;
      this.reason = reason;
      this.onRejected.forEach((fn) => fn()); //PromiseA+ 2.2.6.2
    }
  };
  then = (onFulfilled, onRejected) => {
    //PromiseA+ 2.2.1 / PromiseA+ 2.2.5 / PromiseA+ 2.2.7.3 / PromiseA+ 2.2.7.4
    onFulfilled =
      typeof onFulfilled === "function" ? onFulfilled : (value) => value;
    onRejected =
      typeof onRejected === "function"
        ? onRejected
        : (reason) => {
            throw reason;
          };
    //PromiseA+ 2.2.7
    let promise2 = new Promise((resolve, reject) => {
      if (this.status === FULFILLED) {
        //PromiseA+ 2.2.2
        //PromiseA+ 2.2.4 --- setTimeout
        setTimeout(() => {
          try {
            //PromiseA+ 2.2.7.1
            let x = onFulfilled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            //PromiseA+ 2.2.7.2
            reject(e);
          }
        });
      } else if (this.status === REJECTED) {
        //PromiseA+ 2.2.3
        setTimeout(() => {
          try {
            let x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        });
      } else if (this.status === PENDING) {
        this.onFulfilled.push(() => {
          setTimeout(() => {
            try {
              let x = onFulfilled(this.value);
              resolvePromise(promise2, x, resolve, reject);
            } catch (e) {
              reject(e);
            }
          });
        });
        this.onRejected.push(() => {
          setTimeout(() => {
            try {
              let x = onRejected(this.reason);
              resolvePromise(promise2, x, resolve, reject);
            } catch (e) {
              reject(e);
            }
          });
        });
      }
    });
    return promise2;
  };
  cancel = () => {
    // console.log("cancel");
    this.status = FULFILLED;
  };
}

Promise.defer = Promise.deferred = function () {
  let dfd = {};
  dfd.promise = new Promise((resolve, reject) => {
    dfd.resolve = resolve;
    dfd.reject = reject;
  });
  return dfd;
};

Promise.resolve = function (param) {
  if (param instanceof Promise) {
    return param;
  }
  return new Promise((resolve, reject) => {
    if (param && param.then && typeof param.then === "function") {
      setTimeout(() => {
        param.then(resolve, reject);
      });
    } else {
      resolve(param);
    }
  });
};

Promise.reject = function (reason) {
  return new Promise((resolve, reject) => {
    reject(reason);
  });
};

Promise.prototype.catch = function (onRejected) {
  return this.then(null, onRejected);
};

Promise.prototype.finally = function (callback) {
  return this.then(
    (value) => {
      return Promise.resolve(callback()).then(() => {
        return value;
      });
    },
    (err) => {
      return Promise.resolve(callback()).then(() => {
        throw err;
      });
    }
  );
};

Promise.all = function (promises) {
  return new Promise((resolve, reject) => {
    let index = 0;
    let result = [];
    if (promises.length === 0) {
      resolve(result);
    } else {
      function processValue(i, data) {
        result[i] = data;
        if (++index === promises.length) {
          resolve(result);
        }
      }
      for (let i = 0; i < promises.length; i++) {
        //promises[i] 可能是普通值
        Promise.resolve(promises[i]).then(
          (data) => {
            processValue(i, data);
          },
          (err) => {
            reject(err);
            return;
          }
        );
      }
    }
  });
};

Promise.race = function (promises) {
  return new Promise((resolve, reject) => {
    if (promises.length === 0) {
      return;
    } else {
      for (let i = 0; i < promises.length; i++) {
        Promise.resolve(promises[i]).then(
          (data) => {
            resolve(data);
            return;
          },
          (err) => {
            reject(err);
            return;
          }
        );
      }
    }
  });
};
module.exports = Promise;
// 跑测试
// npm install -g promises-aplus-tests
// promises-aplus-tests promise.js

测试可以取消的方法以及和原生Promise的兼容性

const CPromise = require("./promise");

const p = new Promise((r) =>
  setTimeout(() => {
    r("p");
  }, 4000)
);

const cp = new CPromise((r) =>
  setTimeout(() => {
    r("cp");
  }, 8000)
);
p.then((v) => console.log("p then", v, new Date().toTimeString()));
cp.then((v) => console.log("cp then", v, new Date().toTimeString()));
Promise.all([p, cp]).then((data) =>
  console.log(data, new Date().toTimeString())
);

setTimeout(() => {
//   cp.cancel();
}, 2000);

/*
调用cancel
p then p 23:51:05 GMT+0800 (China Standard Time)


不调用cancel
p then p 23:51:49 GMT+0800 (China Standard Time)
cp then cp 23:51:53 GMT+0800 (China Standard Time)
[ 'p', 'cp' ] 23:51:53 GMT+0800 (China Standard Time)
*/