使用 XHR2 请求而不是cordova-file-transfer 将二进制数据下载到应用程序沙箱中

Question

Cordova is "sunsetting" (going to deprecate) cordovan-plugin-file, see their blogpost .

No more work will be done on the file-transfer plugin by the Cordova development community. You can continue to use the file-transfer plugin if you wish - it should work fine as-is for the foreseeable future. We highly suggest Cordova users transition to using the standards-compliant way of sending and receiving binary data.

They are encouraging a transition to use XHR2 requests (XHR Requests where the responseType is set to Blob or ArrayBuffer .

The blog post wants to provide an example how binary data can be fetched using XHR2:

window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, function (fs) {
console.log('file system open: ' + fs.name);
fs.root.getFile('bot.png', { create: true, exclusive: false }, function (fileEntry) {
    console.log('fileEntry is file? ' + fileEntry.isFile.toString());
    var oReq = new XMLHttpRequest();
    // Make sure you add the domain name to the Content-Security-Policy <meta> element.
    oReq.open("GET", "http://cordova.apache.org/static/img/cordova_bot.png", true);
    // Define how you want the XHR data to come back
    oReq.responseType = "blob";
    oReq.onload = function (oEvent) {
        var blob = oReq.response; // Note: not oReq.responseText
        if (blob) {
            // Create a URL based on the blob, and set an <img> tag's src to it.
            var url = window.URL.createObjectURL(blob);
            document.getElementById('bot-img').src = url;
            // Or read the data with a FileReader
            var reader = new FileReader();
            reader.addEventListener("loadend", function() {
               // reader.result contains the contents of blob as text
            });
            reader.readAsText(blob);
        } else console.error('we didnt get an XHR response!');
    };
    oReq.send(null);
}, function (err) { console.error('error getting file! ' + err); });}, function (err) { console.error('error getting persistent fs! ' + err); });

I have some issues understanding the code above and the intention of cordova to drop the file-tranfer plugin in favour of directly fetching the Blobs via Ajax.

Am I seeing this right: fs.root.getFile creates a file. The download success handler ( oReq.onload ) does not attempt to write the fetched blob to the created file. There is no clear reason why the fileEntry is created. If I would want to save the fetched blob to the created FileEntry, within oReq.onload I could go on using a FileWriter, but only for small (I read up to 5 MB) files (since the Blob is handled in-memory). The blog post is more about how a blob can be fetched in general and not about it can be downloaded into the filesystem. If I would want to download bigger files (like a couple of 100 MB), moving away from cordova-plugin-filetransfer is not an option at the moment.

Answer 1

With this code you can download big images as they are written by blocks of 1MB instead of doing the whole write at once. Without the 1MB writting I wasn't able to write files bigger than 4MB, but with this I've tested with files up to 40MB without problems

 window.resolveLocalFileSystemURL(cordova.file.externalDataDirectory, 
    function (dirEntry) {
        console.log('file system open: ' + dirEntry.name);
        createFile(dirEntry, "downloadedImage.jpg");
    }, onFSError);
    function onFSError(error) {
        alert(JSON.stringify(error));
    }
    function createFile(dirEntry, fileName) {
        // Creates a new file or returns the file if it already exists.
        dirEntry.getFile(fileName, {create: true, exclusive: false}, function(fileEntry) {

            var xhr = new XMLHttpRequest();
            xhr.open('GET', 'https://static.vix.com/es/sites/default/files/styles/large/public/imj/3/30-cosas-de-los-gatos-que-no-sabias-3.jpg', true);
            xhr.responseType = 'blob';

            xhr.onload = function() {
                if (this.status == 200) {
                    var blob = new Blob([this.response], { type: 'image/jpeg' });
                    writeFile(fileEntry, blob);
                }
            };
            xhr.send();
        }, onFSError);
    }

    function writeFile(fileEntry, data) {
       // Create a FileWriter object for our FileEntry (log.txt).
       fileEntry.createWriter(function (fileWriter) {

            fileWriter.onerror = function(e) {
                console.log("Failed file write: " + e.toString());
            };

            function writeFinish() {
                function success(file) {
                    alert("Wrote file with size: " + file.size);
                }
                function fail(error) {
                    alert("Unable to retrieve file properties: " + error.code);
                }
                fileEntry.file(success, fail);
            }
            var written = 0;
            var BLOCK_SIZE = 1*1024*1024; // write 1M every time of write
            function writeNext(cbFinish) {
                fileWriter.onwrite = function(evt) {
                    if (written < data.size)
                        writeNext(cbFinish);
                    else
                        cbFinish();
                };
                if (written) fileWriter.seek(fileWriter.length);
                fileWriter.write(data.slice(written, written + Math.min(BLOCK_SIZE, data.size - written)));
                written += Math.min(BLOCK_SIZE, data.size - written);
            }
            writeNext(writeFinish);
        });
    }