我有一个适用于iOS的phonegap应用程序，可以从服务器下载图像和pdf文件，因为它们必须在本地可用。
我使用jQuery解析我需要下载的资产列表，然后使用Phonegap API启动FileTransfer，如下所示：
// assets is an array that has all pdf and image urls to be downloaded.
assets.map(downloadFile);
function downloadFile(file_url){
console.log('Downloading file:'+file_url);
var fileTransfer = new FileTransfer();
fileTransfer.download(
file_url,
get_local_file_path(file_url),
download_success,
function(error) {
stopspin();
console.log('ERROR downloading: '+ error.source);
$('#notify-user').html('Downloading failed. <a href="#" onclick="checkLatestIssue()">Try again?</a>')
}
);
}
现在，每次资产最多可达50个。我可以看到所有的文件传输请求都是立即发送的，有时，一些文件传输请求会超时，从而导致下载不完整，我可以通过本地资产进行渲染。
有没有办法可以通过大约5次并行下载来连续下载所有这些问题？
答案 0 :(得分：2)
我在我的应用程序上做的是创建一个我需要下载的全局文件数组，然后启动代码，从代码前面抓取第一个元素并下载它。完成后，它会检查数组中是否还有另一个要处理的项目，如果是，则再次启动该过程。如果没有，它会完成下载并继续处理下一个应用程序需要做的事情。
为我们要下载的文件创建一个空白数组
data_to_process = [];
将图像和视频添加到从API
返回的数组“结果”中// add images to array
$.each( result.images_to_download, function( index, value ) {
index = index + 1;
data_to_process.push(
{
"type": "images",
"filename": value,
"msg": "Downloading image "+index+" of "+result.images_to_download.length+"..."
}
);
});
// add videos to array
$.each( result.videos_to_download, function( index, value ) {
index = index + 1;
data_to_process.push(
{
"type": "videos",
"filename": value,
"msg": "Downloading video "+index+" of "+result.videos_to_download.length+"..."
}
);
});
这是处理文件的代码 注意：这里有一些不需要的代码，用于根据文件将文件放在某些目录中，更新下载进度的百分比指示符等：
// values for percentages
start_data_length = data_to_process.length;
current_iteration = 0;
// set writing to false
writing = false;
// what we do after each iteration in the data array
var finish_processing = function() {
// increment iteration count
current_iteration++;
// set percent value
var percent = current_iteration / start_data_length * 100;
percent = Math.round( percent );
$('#update_percent').html( percent );
// remove from array after process
data_to_process.splice( 0, 1 );
// set writing back to false so we can start another iteration
writing = false;
}
// checks if we need to process the next part of the array or not
var check_to_process = function() {
// console.log('checking if we need to process');
// if we still have items in the array to process
if ( data_to_process.length > 0 ) {
// if we're currently not working on an item
if ( writing === false ) {
// process next item in array
process_download_data();
} else {
// not ready yet, wait .5 seconds and check again
// console.log('busy - waiting to process');
setTimeout( check_to_process, 500 );
}
} else {
// console.log('nothing left to process');
}
}
// processes the downloaded data
var process_download_data = function() {
// set writing to true
writing = true;
// current item to process is first item in array
var current_item = data_to_process[0];
if ( current_item.msg ) {
$('#update_item').html(current_item.msg);
} else {
$('#update_item').html('Processing update data...');
}
// console.log( 'processing this: '+ current_item.filename );
// if something didn't work right..
var broke = function(error) {
// console.log( 'error' );
// console.log( error );
// cycle to next item in array
finish_processing();
}
// get the directory of current item's "type"
var get_directory = function( fileSystem ) {
fileSystem.root.getDirectory( current_item.type, {create:true,exclusive:false}, get_file, broke );
}
// get the file
var get_file = function( dirEntry ){
// if texts
if ( current_item.type == 'texts' ) {
dirEntry.getFile( current_item.filename+".txt", {create: true, exclusive: false}, create_text_writer, broke);
// if images
} else if ( current_item.type == 'images' ) {
var localFileName = current_item.filename.substring(current_item.filename.lastIndexOf('/')+1);
dirEntry.getFile( localFileName, {create: true, exclusive: false}, create_file_writer, broke);
// if videos
} else if ( current_item.type == 'videos' ) {
var localFileName = current_item.filename.substring(current_item.filename.lastIndexOf('/')+1);
dirEntry.getFile( localFileName, {create: true, exclusive: false}, create_file_writer, broke);
// if audios
} else if ( current_item.type == 'resources' ) {
var localFileName = current_item.filename.substring(current_item.filename.lastIndexOf('/')+1);
dirEntry.getFile( localFileName, {create: true, exclusive: false}, create_file_writer, broke);
// if audios
} else if ( current_item.type == 'audios' ) {
var localFileName = current_item.filename.substring(current_item.filename.lastIndexOf('/')+1);
dirEntry.getFile( localFileName, {create: true, exclusive: false}, create_file_writer, broke);
}
}
// write to file using filetransfer
var create_file_writer = function( fileEntry ) {
if ( current_item.remove === true ) {
fileEntry.remove( function(entry) {
// what to do when image is deleted
finish_processing();
},broke);
} else {
var localPath = fileEntry.fullPath;
// console.log('Writing to: '+localPath+' from: '+current_item.filename);
var ft = new FileTransfer();
ft.download( current_item.filename, localPath, function(entry) {
// what to do when image is downloaded
finish_processing();
},broke);
}
}
// create writer for "texts" type
var create_text_writer = function( fileEntry ) {
fileEntry.createWriter( write_text_file, broke );
}
// write to file for "texts" type
var write_text_file = function( writer ) {
// write new contents
writer.write( current_item.data );
// add item where it needs to be in the document
if ( current_item.append === true ) {
$('#'+current_item.marker).nextAll().remove();
// appending
// console.log('Appending data to: #'+current_item.id);
$('#'+current_item.id).append( current_item.data );
} else {
// adding
// console.log('Adding data to: #'+current_item.id);
$('#'+current_item.id).html( current_item.data );
}
// console.log('finished writing: '+current_item.filename);
finish_processing();
}
// request persistent filesystem
window.requestFileSystem( LocalFileSystem.PERSISTENT, 0, get_directory, broke );
// check if we need to process the next item yet
check_to_process();
}
// process data from the update
process_download_data();