七、promise

一

<script>
        var baseUrl = "https://music.aityp.com/";
        $.ajax({
            url:baseUrl+"top/playlist",
            type:"get",
            data:{
                cat:"华语"
            },
            dataType:"json",
            success:res=>{
                var {playlists}= res;
                var item = playlists[0];
                console.log(item)
                $.ajax({
                    url:`${baseUrl}playlist/detail?id=${item.id}`,
                    type:"get",
                    success:res=>{
                        console.log(res)
                        let id = res.playlist.trackIds[0].id;
                        $.ajax({
                            url:`${baseUrl}song/url?id=${id}`,
                            success:res=>{
                                console.log(res)
                            }
                        })
                    }
                })
            }
        })
    </script>

二、promise

• new Function

   new Array   new 实例化的对象<br />    **我们使用promise的时候会将函数的状态暂停/凝固**<br />       new promise<br />       1.使用New 关键字实现promise<br />       2.promise 有两种状态resolve,reject<br />       3.promise 函数不会马上触发，要通过一个then函数去触发<br />       4.then触发的是resolve的状态，catch触发失败的状态<br />       resolve  -->成功<br />       reject  -->失败

<script>
        function go(){
            return new Promise((resolve,reject)=>{
                resolve(1);
                reject(2)
            })
        }
        go().then(res=>{
            console.log(1)
        }).catch(err=>{
            console.log(err)
        })
    </script>

三、封装promise

<script>
        function http(){
            return new Promise((resolve,reject)=>{
                $.ajax({
                    url:"https://music.aityp.com/top/playlist?cat=华语",
                    type:"get",
                    success:res=>{
                        resolve(res)
                    },
                    error:err=>{
                        reject(err)
                    }
                })
            })
        }
        http().then(res=>{
            console.log(res.playists)
        })
    </script>

四、p

<script>
        var p = new Promise((resolve,reject)=>{
            resolve("success");
            reject("失败")
        })
        p.then(res=>{
            console.log(res)
        })
        console.log("4")
    </script>

五、-promise

使用promise之后将http请求由纵向变为横向的了

<script>
        http("top/playlist?cat=华语").then(res=>{
            let id = res.playlists[0].id;
            return id
        }).then(res=>{
            http(`playlist/detail?id=${res}`).then(res=>{
                let id = res.playlist.trackIds[0].id;
                return id
            }).then(res=>{
                http(`song/url?id=${res}`).then(res=>{
                    console.log(res)
                })
            })
        })
    </script>

六、Ajax-p

 <script>
        var url = "https://music.aityp.com/top/playlist?tag=华语"  
        /* ajax就是一个promise */
        var ajax= $.ajax({url,type:"get"})  
        console.log(ajax)
        ajax.then(res=>{
            console.log(res)
        })
    </script>