我一直在尝试一些我认为在Javascript中与回调和/或承诺相关的相对简单的东西(超过12个小时)。
注意:我正在对一个.js.erb
文件使用Rails,但这与我的问题无关,因为我更关注的是Javascript;只是为了说明更大的问题。
我正在使用jsmediatags从远程主机上的mp3s读取元数据(主要集中在收集专辑艺术作品上)。我站点上的每个“文章”都与多个音频文件(@related_titles
)相关,所以我循环遍历每个文件(本地变量url
),收集图片,然后在coverflow插件中显示它们--这意味着对每个处理的mp3执行一次jsmediatags.read
。问题是。在coverflow插件初始化之前,jsmediatags不会读完所有的图片。尽管我对js比较陌生,但它似乎是回调函数的完美用法。
现在,在执行makeitrain
函数(在coverflow中)之前,我不得不引入一个8500ms的setTimeout
函数。它可以工作,但并不是在所有设备上都可靠(当它们处于负载状态时),而且它不是一个优雅的解决方案。
让我们假设@related_titles
中有3个mp3s。为了显示代码没有按正确的顺序执行(不是在等待回调?),我添加了控制台日志。
接收到的输出:
Gathered all art! Now I will callback and render the Coverflow
Initing Coverflow since gatherArtwork completed/we have art!
Gathering album art for title...
Gathering album art for title...
Gathering album art for title...
预期输出:
Gathering album art for title...
Gathering album art for title...
Gathering album art for title...
Gathered all art! Now I will callback and render the Coverflow
Initing Coverflow since gatherArtwork completed/we have art!
这是我尝试过的:
function gatherArtwork(_callback) {
// Read each of the mp3s to gather artwork ('url' == local var for mp3)
<% @related_titles.each do |url| %>
jsmediatags.read("<%= url['mp3'] %>", {
onSuccess: function(tag) {
console.log('Gathering album art for title...');
// Convert the image contents to a Base64 encoded str
var tags = tag.tags;
albumartwork = _arrayBufferToBase64(tags.picture["data"]);
// Append to the coverflow list a `<ul><li> </li></ul>` for each mp3
// The 'albumartwork' Base64 image str is also appended here
$("#coverflow-item-list").append('<ul><li>...</li></li>');
}
});
<% end %>
console.log('Gathered all art! Now I will callback and render the Coverflow');
_callback();
}
// Gather artwork MUST complete fully before the `makeitrain` function runs (callback)
function renderCoverflow() {
gatherArtwork(function() {
console.log('Initing Coverflow since gatherArtwork completed/we have art!');
makeitrain();
});
}
// Execute 'renderCoverflow' which will gather art and then "makeitrain"!
renderCoverflow();
发布于 2018-06-04 05:10:48
最好的办法就是对此做出承诺。我用来自https://github.com/aadsm/jsmediatags的promise包装器复制了基本的new jsmediatags
。可能需要一些微调,但目的是为您提供基本的概念轮廓
我不是一个rails dev,所以我将把你的related_titles
数组输出到一个javascript变量,方法是打印JSON,然后由javascript自动读取为数组。
<script>
var urls = // echo json array
<script>
完成后,脚本的其余部分可以放在单独的文件中或直接放在页面中,无论您喜欢哪一个
// helper function to get tag info and return promise that resolves with the base64 result
function getTags(url) {
return new Promise((resolve, reject) => {
new jsmediatags.Reader(url)
.read({
onSuccess: (tag) => {
resolve( _arrayBufferToBase64(tag.tags.picture["data"]));
},
onError: (error) => {
reject(error);
}
});
});
}
// create array of getTags() promises
let promises = urls.map(url => getTags(url))
Promise.all(promises).then(results=>{
// `results` is array of all the base64 values same order as the urls array
// loop over the results and add what is needed to DOM
// then call cover flow
}).catch(err=> console.log('One of the requests failed'))
发布于 2018-06-04 05:16:42
首先,你不应该(永远)像你一样在服务器上复制你的js代码,结果是:
function gatherArtwork() {
// Read each of the mp3s to gather artwork ('url' == local var for mp3)
<% @related_titles.each do |url| %>
console.log("<%= url['mp3'] %>");
<% end %>
}
是:
function gatherArtwork() {
// Read each of the mp3s to gather artwork ('url' == local var for mp3)
console.log("url1");
console.log("url2");
....
console.log("urln");
}
因此,这意味着您正在向用户发送大量不必要的代码(您的JS文件现在更大了)。相反,您可以执行以下操作:
/**
* Load all arts from urls
*/
function gatherArtwork(urls) {
// Read each of the mp3s to gather artwork ('url' == local var for mp3)
for(url in urls) {
console.log(url);
}
}
使用这种方法的好处还在于,您现在可以将gatherArtwork
函数声明为async
并执行以下操作:
async function gatherArtwork(urls) {
for(url in urls) {
await new Promise(function(resolve, reject) {
jsmediatags.read("<%= url['mp3'] %>", {
onSuccess: function(tag) {
console.log('Gathering album art for title...');
// Convert the image contents to a Base64 encoded str
var tags = tag.tags;
albumartwork = _arrayBufferToBase64(tags.picture["data"]);
// Append to the coverflow list a `<ul><li> </li></ul>` for each mp3
// The 'albumartwork' Base64 image str is also appended here
$("#coverflow-item-list").append('<ul><li>...</li></li>');
resolve(tag);
},
onError: function(error) {
reject(error);
}
});
});
}
}
然后这样叫它:
await gatherArtwork(urls)
console.log('Gathered all art! Now I will callback and render the Coverflow');
makeitrain();
小心!触发上述代码的函数也应该是async
。如果您想要避免这种情况,您应该使用Promise.all
https://stackoverflow.com/questions/50670852
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