布衣重九
布衣重九
5月前 · 13 人阅读

1.Promise解决什么问题?

1.1回到地狱

比如实现一个动画,页面上有多个球,每个球要等到上一个球的运动停止之后开始动画。

  function move(ball,target,cb){
      let timer = setInterval(function(){
        //先取到当前的left值
       let index=0;
      let timer = setInterval(function () {
        if(index++<target){
          ball.style.transform = `translateX(${index}px)`;
        }else{
          clearInterval(timer);
          resolve();
          cb();
        }
      }, 13)
        }
      },4)
    }
    move(ball1,100,function(){
      move(ball2,100,function(){
        move(ball3,100,function(){
          alert('over');
        });
      });
    });

问题:代码丑陋并且多层嵌套,Promise横空出世可以解决这个问题。下面用Promise进行实现这个动画

function move(ball, target) {
    return new Promise(function (resolve, reject) {
      let index=0;
      let timer = setInterval(function () {
        if(index++<target){
          ball.style.transform = `translateX(${index}px)`;
        }else{
          clearInterval(timer);
          resolve();
        }
      }, 13)
    });
  }
  move(ball1, 100)
    .then(function () {
      return move(ball2, 100);
    })
    .then(function () {
      return move(ball3, 100);
    })
    .then(function () {
      alert('over');
    })

2.实现Promise

实现一个符合 Promise/A+ 规范的 MyPromise,并实现 resolve、reject、catch、all、race、deferred等静态方法。

const PENDING =  'pending';//初始态
const FULFILLED =  'fulfilled';//初始态
const REJECTED =  'rejected';//初始态

2.1实现链式写法

function Promise(executor){
  let self = this;//先缓存当前promise实例
  self.status = PENDING;//设置状态
  //定义存放成功的回调的数组
  self.onResolvedCallbacks = [];
  //定义存放失败回调的数组
  self.onRejectedCallbacks = [];
  //当调用此方法的时候,如果promise状态为pending,的话可以转成成功态,如果已经是成功态或者失败态了,则什么都不做
  //2.1
  function resolve(value){ //2.1.1
    if(value!=null &&value.then&&typeof value.then == 'function'){
      return value.then(resolve,reject);
    }
    //如果是初始态,则转成成功态
    setTimeout(function(){
      if(self.status == PENDING){
        self.status = FULFILLED;
        self.value = value;//成功后会得到一个值,这个值不能改
        //调用所有成功的回调
        self.onResolvedCallbacks.forEach(cb=>cb(self.value));
      }
    })

  }
  function reject(reason){ //2.1.2
    setTimeout(function(){
      //如果是初始态,则转成失败态
      if(self.status == PENDING){
        self.status = REJECTED;
        self.value = reason;//失败的原因给了value
        self.onRejectedCallbacks.forEach(cb=>cb(self.value));
      }
    });

  }
  try{
    //因为此函数执行可能会异常,所以需要捕获,如果出错了,需要用错误 对象reject
    executor(resolve,reject);
  }catch(e){
    //如果这函数执行失败了,则用失败的原因reject这个promise
    reject(e);
  };
}
function resolvePromise(promise2,x,resolve,reject){
  if(promise2 === x){
    return reject(new TypeError('循环引用'));
  }
  let called = false;//promise2是否已经resolve 或reject了
  if(x instanceof Promise){
    if(x.status == PENDING){
      x.then(function(y){
        resolvePromise(promise2,y,resolve,reject);
      },reject);
    }else{
      x.then(resolve,reject);
    }
  //x是一个thenable对象或函数,只要有then方法的对象,
  }else if(x!= null &&((typeof x=='object')||(typeof x == 'function'))){
    //当我们的promise和别的promise进行交互,编写这段代码的时候尽量的考虑兼容性,允许别人瞎写
   try{
     let then = x.then;
     if(typeof then == 'function'){
       //有些promise会同时执行成功和失败的回调
       then.call(x,function(y){
         //如果promise2已经成功或失败了,则不会再处理了
          if(called)return;
          called = true;
          resolvePromise(promise2,y,resolve,reject)
       },function(err){
         if(called)return;
         called = true;
         reject(err);
       });
     }else{
       //到此的话x不是一个thenable对象,那直接把它当成值resolve promise2就可以了
       resolve(x);
     }
   }catch(e){
     if(called)return;
     called = true;
     reject(e);
   }

  }else{
    //如果X是一个普通 的值,则用x的值去resolve promise2
    resolve(x);
  }
}
//onFulfilled 是用来接收promise成功的值或者失败的原因
Promise.prototype.then = function(onFulfildeled,onRejected){
  //如果成功和失败的回调没有传,则表示这个then没有任何逻辑,只会把值往后抛
  //2.2.1
  onFulfilled = typeof onFulfilled == 'function'?onFulfilled:function(value){return  value};
  onRejected = typeof onRejected == 'function'?onRejected:reason=>{throw reason};
  //如果当前promise状态已经是成功态了,onFulfilled直接取值
  let self = this;
  let promise2;
  if(self.status == FULFILLED){
    return promise2 = new Promise(function(resolve,reject){
      setTimeout(function(){
        try{
          let x =onFulfilled(self.value);
          //如果获取到了返回值x,会走解析promise的过程
          resolvePromise(promise2,x,resolve,reject);
        }catch(e){
          //如果执行成功的回调过程中出错了,用错误原因把promise2 reject
          reject(e);
        }
      })

    });
  }
  if(self.status == REJECTED){
    return promise2 = new Promise(function(resolve,reject){
      setTimeout(function(){
        try{
          let x =onRejected(self.value);
          resolvePromise(promise2,x,resolve,reject);
        }catch(e){
          reject(e);
        }
      })
    });
  }
  if(self.status == PENDING){
   return promise2 = new Promise(function(resolve,reject){
     self.onResolvedCallbacks.push(function(){
         try{
           let x =onFulfilled(self.value);
           //如果获取到了返回值x,会走解析promise的过程
           resolvePromise(promise2,x,resolve,reject);
         }catch(e){
           reject(e);
         }

     });
     self.onRejectedCallbacks.push(function(){
         try{
           let x =onRejected(self.value);
           resolvePromise(promise2,x,resolve,reject);
         }catch(e){
           reject(e);
         }
     });
   });
  }

}

2.2 Promise.prototype.catch

//catch原理就是只传失败的回调
Promise.prototype.catch = function(onRejected){
  this.then(null,onRejected);
}

2.3 Promise.deferred

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

2.4 Promise.all

Promise.all = function(promises){
 return new Promise(function(resolve,reject){
   let done = gen(promises.length,resolve);
   for(let i=0;i<promises.length;i++){
     promises[i].then(function(data){
       done(i,data);
     },reject);
   }
 });
}

2.5 Promise.race

Promise.race = function(promises){
  return new Promise(function(resolve,reject){
    for(let i=0;i<promises.length;i++){
      promises[i].then(resolve,reject);
    }
  });
}

2.6 Promise.resolve

Promise.resolve = function(value){
  return new Promise(function(resolve){
    resolve(value);
  });
}

2.7 Promise.reject

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

参考资料

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