BlobBuilder破坏二进制数据

Question

我有BlobBuilder(Chrome11)的问题我尝试从服务器获取XHR请求的图像.然后我尝试使用BlobBuilder/FileWriter将其保存到本地FS.互联网上的每个例子都是关于使用text/plain mime类型,这些例子工作得很好.但是当我尝试编写使用XHR获得的二进制数据时,文件大小变为原始文件大小的1.5-2倍.它无法在Picasa/Eye of Gnome中查看.

var xhr = new XMLHttpRequest();
var photoOrigUrl = 'http://www.google.ru/images/nav_logo72.png';
xhr.open('GET', photoOrigUrl, true);
xhr.onreadystatechange = function() {
    if (xhr.readyState == 4 && xhr.status == 200) {
        var contentType = xhr.getResponseHeader('Content-type');

        fsLink.root.getFile('nav_logo72.png', {'create': true}, function(fileEntry) {
            fileEntry.createWriter(function(fileWriter) {
                var BlobBuilderObj = new (window.BlobBuilder || window.WebKitBlobBuilder)();
                BlobBuilderObj.append(xhr.responseText);

                fileWriter.write(BlobBuilderObj.getBlob(contentType));
            }, function(resultError) {
                console.log('writing file to file system failed (   code ' + resultError.code + ')');
            });
        });
    }
}

xhr.send();

fsLink存在,这是扩展.

Answer 1

问题是将BlobBuilder.append(xhr.responseText)其参数检测为UTF-8字符串,这是XHR返回的,而不是二进制数据,这就是它的真实含义.有一些技巧可以让BlobBuilder将其读取为二进制数据而不是字符串数据:

var xhr = new XMLHttpRequest();
var photoOrigUrl = 'http://www.google.ru/images/nav_logo72.png';
xhr.open('GET', photoOrigUrl, true);

// CHANGE 1: This stops the browser from parsing the data as UTF-8:
xhr.overrideMimeType('text/plain; charset=x-user-defined');

xhr.onreadystatechange = function() {
    if (xhr.readyState == 4 && xhr.status == 200) {
        var contentType = xhr.getResponseHeader('Content-type');

        fsLink.root.getFile('nav_logo72.png', {'create': true}, function(fileEntry) {
            fileEntry.createWriter(function(fileWriter) {

                // CHANGE 2: convert string object into a binary object
                var byteArray = new Uint8Array(xhr.response.length);
                for (var i = 0; i < xhr.response.length; i++) {
                    byteArray[i] = xhr.response.charCodeAt(i) & 0xff;
                }

                var BlobBuilderObj = new (window.BlobBuilder || window.WebKitBlobBuilder)();

                // CHANGE 3: Pass the BlobBuilder an ArrayBuffer instead of a string
                BlobBuilderObj.append(byteArray.buffer);

                // CHANGE 4: not sure if it's needed, but keep only the necessary
                // part of the Internet Media Type string
                fileWriter.write(BlobBuilderObj.getBlob(contentType.split(";")[0]));
            }, function(resultError) {
                console.log('writing file to file system failed (   code ' + resultError.code + ')');
            });
        });
    }
}

xhr.send();

这给了我一个文件,其长度与xhr.getResponseHeader('Content-Length')它应该有的相同.