resolve() 和 then() 比异步的东西先来

Question

我读了一篇关于 promise 的指南，上面写着：

const promise = new Promise((resolve, reject) => {

// Do some async stuff

// If async opp successful
resolve();

// If async opp fails
reject();

});

所以，我正在尝试使用它，但没有成功：

let async = new Promise((resolve, reject)=>{
  this.getPerson(page).subscribe( data => {
    console.log("First")
  });
  resolve();
})
async.then((result) =>{
  console.log("Last")
});

在控制台，我先是 "Last"，有人给我提示吗？

Answer 1

您的 resolve(); 应该在 this.getPerson(page).subscribe( data => {

之内

赞：

let async = new Promise((resolve, reject)=>{
    this.getPerson(page).subscribe( data => {
      console.log("First") 
      resolve(); // called asap the result gets from api and above line executed
    });
    // resolve(); will be called before the above async function get's its response
});

async.then((result) =>{
    console.log("Last")
});

Note : Don't use variable name such async , as it already reserved keyword.

Answer 2

您正在查询完成之前解决承诺。检查下面的代码以获取正确的解析位置

let async = new Promise((resolve, reject)=>{
  this.getPerson(page).subscribe( data => {
    console.log("First");
    resolve(); // this is the expected place
  });
});
async.then((result) =>{
  console.log("Last")
});

Answer 3

而不是使用 reserve async 关键字，在整个

 let async = new Promise((resolve, reject)=>{
    this.getPerson(page).subscribe( data => {
      console.log("First") 
    });
    resolve();
        })
   async.then((result) =>{
     console.log("Last")
   });

您可以将代码重写为：

const promise = new Promise((resolve, reject) => {

// Do some async stuff

// If async opp successful
setTimeout(function(){
resolve(console.log("First") 
    )});
  });

promise.then((result) =>{
     console.log("Last")
   });

这应该可以解决您的问题。

resolve() 和 then() 比异步的东西先来

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