Javascript 通过回调更新全局变量
我有以下功能Javascript 通过回调更新全局变量,javascript,ajax,Javascript,Ajax,我有以下功能 var urls = ''; handleFiles(f,function(url){ urls = urls + url + ','; console.log("urls is " + urls); }); 我在上传和更新我的url后获得url。但是我的url永远不会更新,它显示最后上传的文件的url 更新1 这是我现在的完整代码 var urls = ''; document.getElementById('questio
var urls = '';
handleFiles(f,function(url){
urls = urls + url + ',';
console.log("urls is " + urls);
});
我在上传和更新我的url后获得url。但是我的url永远不会更新,它显示最后上传的文件的url
更新1
这是我现在的完整代码
var urls = '';
document.getElementById('question-file-selector').addEventListener('change',handleFileSelect,false);
function handleFileSelect(evt){
var files = evt.target.files; //File list object
// Loop through file list, render image files as thumbnails
for (var i = 0,f;f = files[i];i++){
handleFiles(f,function(url){
urls = urls + url + ',';
console.log("urls is " + urls);
});
// Only process image files
if (!f.type.match('image.*')){
$('#list').append('<img class="file-thumb" src="/static/download168.png"/>');
continue;
}
var reader = new FileReader();
//Closure to capture file information
reader.onload = (function(theFile){
return function(e){
//Render thumbnail
$('#list').append('<img class="thumb" src="'+e.target.result+'" title="'+escape(theFile.name)+'"/>');
};
})(f);
reader.readAsDataURL(f);
}
}
console.log("Url is",urls);`
协调多个异步操作是最好使用promises之类的工具来解决的工作。所以,从长远来看,我建议你仔细阅读承诺以及如何使用承诺 在没有承诺的情况下,这里有一种蛮力的方式,你可以告诉你所有的
handleFiles()
操作是通过使用计数器知道上次异步操作何时完成,然后使用上次回调中的URL
变量或从那里调用函数并将URL
变量传递给该函数来完成的:
document.getElementById('question-file-selector').addEventListener('change', handleFileSelect, false);
function handleFileSelect(evt) {
var files = evt.target.files; //File list object
var urls = "";
// Loop through file list, render image files as thumbnails
var doneCnt = 0;
for (var i = 0, f; f = files[i]; i++) {
handleFiles(f, function (url) {
urls = urls + url + ',';
console.log("urls is " + urls);
++doneCnt;
if (doneCnt === files.length) {
// The last handleFiles async operation is now done
// final value is in urls variable here
// you can use it here and ONLY here
// Note: this code here will get executed LONG after the
// handleFileSelect() function has already finished executing
}
});
// Only process image files
if (!f.type.match('image.*')) {
$('#list').append('<img class="file-thumb" src="/static/download168.png"/>');
continue;
}
var reader = new FileReader();
//Closure to capture file information
reader.onload = (function (theFile) {
return function (e) {
//Render thumbnail
$('#list').append('<img class="thumb" src="' + e.target.result + '" title="' + escape(theFile.name) + '"/>');
};
})(f);
reader.readAsDataURL(f);
}
}
// You can't use the urls variable here. It won't be set yet.
由于我无法运行此代码来测试它,这里可能有一两个错误,但是您应该能够调试这些错误,或者告诉我们错误发生的位置,我们可以帮助您调试它。这些是从概念上解决协调多个异步操作的问题的方法。use:var url='';为什么不做
var-url=[];推送(url)然后是url.join(',')代码>。关于您当前的方法,如果我们看到包装此代码的代码,我们可能会有一个想法。可能是某个变量覆盖问题。看起来handleFiles
是一种ajax方法,它使用url参数进行回调。您可能必须在外部作用域中声明var URL
。@SonuSindhu这是一个很好的建议,但没有解释为什么这不起作用@哦,我真的不知道handleFiles
应该做什么,但是有可能handleFiles
永远不会把传递给它的函数作为第二个参数调用。这可能是一个时间问题,存在一个异步操作,在更新之前,您正在查看URL
。每当在回调中更新全局文件时,这总是一个危险信号。如果回调是异步的,那么您就有麻烦了。如果console.log()
确实在回调中,如您在问题中所示,它应该显示所有URL。如果它在回调之外,您将遇到问题。请参阅删除了async:false
。使用了此代码。但它不起作用。我要看看承诺是什么。@learner-我使用jQuery中内置的承诺添加了一个版本。如果第一个不起作用,那么可能只是某个地方的打字错误。如果您调试它并告诉我们您发现了什么,我们可能会帮助您修复它。当然,您还必须向我们展示您在适当位置使用URL
变量执行操作时实际使用的代码。我是否需要带Promissions的回调函数?@learner-否。Promissions使用。然后()
处理程序进行自己的回调。这些是你在承诺中使用的回扣。我从promises实现中删除了回调
参数。它没有被使用,只是意外地从原始代码中遗留下来。我尝试了所有的组合。承诺只提供最后一个url。我做$.when.apply($,promises).then(函数(结果){console.log(results.toString());document.getElementById('id_file_URL')。value=results.toString();})代码>
document.getElementById('question-file-selector').addEventListener('change', handleFileSelect, false);
function handleFileSelect(evt) {
var files = evt.target.files; //File list object
var urls = "";
// Loop through file list, render image files as thumbnails
var doneCnt = 0;
for (var i = 0, f; f = files[i]; i++) {
handleFiles(f, function (url) {
urls = urls + url + ',';
console.log("urls is " + urls);
++doneCnt;
if (doneCnt === files.length) {
// The last handleFiles async operation is now done
// final value is in urls variable here
// you can use it here and ONLY here
// Note: this code here will get executed LONG after the
// handleFileSelect() function has already finished executing
}
});
// Only process image files
if (!f.type.match('image.*')) {
$('#list').append('<img class="file-thumb" src="/static/download168.png"/>');
continue;
}
var reader = new FileReader();
//Closure to capture file information
reader.onload = (function (theFile) {
return function (e) {
//Render thumbnail
$('#list').append('<img class="thumb" src="' + e.target.result + '" title="' + escape(theFile.name) + '"/>');
};
})(f);
reader.readAsDataURL(f);
}
}
// You can't use the urls variable here. It won't be set yet.
document.getElementById('question-file-selector').addEventListener('change', handleFileSelect, false);
function handleFileSelect(evt) {
var files = evt.target.files; //File list object
// Loop through file list, render image files as thumbnails
var promises = [];
for (var i = 0, f; f = files[i]; i++) {
promises.push(handleFiles(f));
// Only process image files
if (!f.type.match('image.*')) {
$('#list').append('<img class="file-thumb" src="/static/download168.png"/>');
continue;
}
var reader = new FileReader();
//Closure to capture file information
reader.onload = (function (theFile) {
return function (e) {
//Render thumbnail
$('#list').append('<img class="thumb" src="' + e.target.result + '" title="' + escape(theFile.name) + '"/>');
};
})(f);
reader.readAsDataURL(f);
}
$.when.apply($, promises).then(function() {
var results = Array.prototype.slice.call(arguments);
// all handleFiles() operations are complete here
// results array contains list of URLs (some could be empty if there were errors)
});
}
function handleFiles(file) {
var filename = file.name;
return $.ajax({
type: 'GET',
data: {
"filename": file.name,
"FileType": "question_file"
},
url: '/dashboard/generateuploadurl/',
contentType: "application/json",
dataType: "json"
}).then(function(data) {
if (data.UploadUrl) {
console.log("upload url successfully created for " + file.name + " file");
console.log(data.UploadUrl);
return handleUpload(data.UploadUrl, file, data.Filename);
}
}, function(err) {
console.log("error occured while creating upload url for " + file.name + ' file');
console.log(err);
// allow operation to continue upon error
});
}
function handleUpload(UploadUrl, file, Filename) {
return $.ajax({
xhr: xhr_with_progress,
url: UploadUrl,
type: 'PUT',
data: file,
cache: false,
contentType: false,
processData: false
}).then(function(data) {
console.log('https://7c.ssl.cf6.rackcdn.com/' + Filename);
return 'https://7c.ssl.cf6.rackcdn.com/' + Filename;
}, function(err) {
console.log("error occured while uploading " + file.name);
console.log(err);
// allow operation to continue upon error
});
}