ios - 在 iOS Phonegap 应用程序上序列化文件传输

Question

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 进行解析以查找我需要下载的 Assets 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， Assets 每次最多可达 50 个。我可以看到所有文件传输请求都立即发送，有时，一些文件传输请求超时，因此下载不完整，无法通过本地 Assets 呈现。

有没有一种方法可以使所有这些问题的下载连续进行大约 5 次并行下载？

Answer 1

我在我的应用程序上所做的是创建一个我需要下载的文件的全局数组，然后启动代码从数组的前面获取第一个元素并下载它。完成后，它会检查数组中是否还有要处理的项目，如果有，它会重新开始该过程。如果没有，它将完成下载并继续执行应用程序接下来需要执行的任何操作。

为我们的文件创建一个空白数组来下载data_to_process = [];

将图像和视频添加到从 API 返回的数组形式的“result”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，我用来根据文件的内容将文件放在某些目录中，更新下载进度的百分比指示器等:

// 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();