現在不會用Promise都不好意思說自己是前端,Promise爲什麼火起來,一句話解決了回調嵌套和執行順序問題最重要的我感覺是解決順序問題。
不過開始寫之前我們先看看,promise怎麼解決問題,怎麼用。
列舉一個順序加載圖片demo:
//需求 加載三張圖片 img1,img2,img3,加載順序是1,2,3
let url1 = "https://ss0.bdstatic.com/70cFvHSh_Q1YnxGkpoWK1HF6hhy/it/u=3293635140,3955114282&fm=26&gp=0.jpg"
let url2 = "https://ss0.bdstatic.com/70cFuHSh_Q1YnxGkpoWK1HF6hhy/it/u=1019333328,1858047370&fm=26&gp=0.jpg"
let url3 = "https://ss1.bdstatic.com/70cFvXSh_Q1YnxGkpoWK1HF6hhy/it/u=4226468334,723383513&fm=26&gp=0.jpg"
let oImg1 = new Image()
oImg1.onload = function() {
console.log('img1加載完畢')
let oImg2 = new Image()
oImg2.onload = function() {
console.log('img2加載完畢')
let oImg3 = new Image()
oImg3.onload = function() {
console.log('img3加載完畢')
console.log('全部加載完畢')
}
oImg3.src = url3
}
oImg2.src = url2
}
oImg1.src = url1
//結果沒毛病
接下來我們試試promise怎麼做,
function loadImg(url) {
let img = new Image()
img.src = url
return new Promise((resolve, reject) => {
img.onload = () => {
console.log(url)
resolve()
}
img.onerror = (e) => {
reject(e)
}
})
}
loadImg(url1).then(() => {
return loadImg(url2)
}).then(() => {
return loadImg(url3)
})
接下來再看看Promise.all,
//需求 加載三張圖片 img1,img2 全部加載完成做一些事情
let urls = ["https://ss0.bdstatic.com/70cFvHSh_Q1YnxGkpoWK1HF6hhy/it/u=3293635140,3955114282&fm=26&gp=0.jpg",
"https://ss0.bdstatic.com/70cFuHSh_Q1YnxGkpoWK1HF6hhy/it/u=1019333328,1858047370&fm=26&gp=0.jpg"
]
function loadImg(url) {
let img = new Image()
img.src = url
return new Promise((resolve, reject) => {
img.onload = () => {
resolve(img)
}
img.onerror = (e) => {
reject(e)
}
})
}
function loadAll(arr) {
let result = []
arr.forEach(item => {
let p = loadImg(item).then((img) => {
console.log(img)
})
//存儲當前Promise對象
console.log(p)
result.push(p)
});
Promise.all(result).then(() => {
//全部完成
console.log("done")
}).catch((err) => {
// 有失敗的情況
console.log(err)
});
}
loadAll(urls)
完成。。。