Android—在WebView中下载Blob协议文件

之前有个需求是要下载Blob协议的gif,让我苦恼了好久。平时下载http协议的文件时直接获取输入流即可,但是Java无法获得Blob协议的文件流,无法直接处理。不过JavaScript处理Blob协议非常方便,可以考虑通过前端将该文件转化为Base64的字符串。

那么下载Blob协议文件时,可以先将链接传给前端,前端通过JS处理后得到Base64的文件流,再将文件流通过Android的JavaScript方法传给客户端。

按这个逻辑,JS处理文件流这块需要前端开发,不过有意思的是,这部分功能我们可以直接在客户端写。如下方的getBase64StringFromBlobUrl()方法,将JS代码拼成String,调用WebView.loadUrl(...)加载这段JS代码即可,那么所有的流程都能在客户端实现。

public class DownloadBlobFileJSInterface {

    private Context mContext;
    private DownloadGifSuccessListener mDownloadGifSuccessListener;

    public DownloadBlobFileJSInterface(Context context) {
        this.mContext = context;
    }

    public void setDownloadGifSuccessListener(DownloadGifSuccessListener listener) {
        mDownloadGifSuccessListener = listener;
    }

    @JavascriptInterface
    public void getBase64FromBlobData(String base64Data)  {
        convertToGifAndProcess(base64Data);
    }

    public static String getBase64StringFromBlobUrl(String blobUrl) {
        if (blobUrl.startsWith("blob")) {
            return "javascript: var xhr = new XMLHttpRequest();" +
                    "xhr.open('GET', '" + blobUrl + "', true);" +
                    "xhr.setRequestHeader('Content-type','image/gif');" +
                    "xhr.responseType = 'blob';" +
                    "xhr.onload = function(e) {" +
                    "    if (this.status == 200) {" +
                    "        var blobFile = this.response;" +
                    "        var reader = new FileReader();" +
                    "        reader.readAsDataURL(blobFile);" +
                    "        reader.onloadend = function() {" +
                    "            base64data = reader.result;" +
                    "            Android.getBase64FromBlobData(base64data);" +
                    "        }" +
                    "    }" +
                    "};" +
                    "xhr.send();";
        }
        return "javascript: console.log('It is not a Blob URL');";
    }

    private void convertToGifAndProcess(String base64) {
        String currentDateTime = DateFormat.getDateTimeInstance().format(new Date());
        File gifFile = new File(Environment.getExternalStoragePublicDirectory(
                Environment.DIRECTORY_DOWNLOADS) + "/Test_" + currentDateTime + "_.gif");
        saveGifToPath(base64, gifFile);
        Toast.makeText(mContext, R.string.save_image_success, Toast.LENGTH_SHORT).show();
        if (mDownloadGifSuccessListener != null) {
            mDownloadGifSuccessListener.downloadGifSuccess(gifFile.getAbsolutePath());
        }
    }

    private void saveGifToPath(String base64, File gifFilePath) {
        try {
            byte[] fileBytes = Base64.decode(base64.replaceFirst(
                    "data:image/gif;base64,", ""), 0);
            FileOutputStream os = new FileOutputStream(gifFilePath, false);
            os.write(fileBytes);
            os.flush();
            os.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public interface DownloadGifSuccessListener {
        void downloadGifSuccess(String absolutePath);
    }
}

随后新建WebView,并为WebView添加JavascriptInterface,最后通过loadUrl()方法执行JS代码即可。

// 1. 新建WebView
mWebView = new WebView(this);
WebSettings webSettings = mWebView.getSettings();
webSettings.setJavaScriptEnabled(true);
webSettings.setAppCachePath(getApplicationContext().getCacheDir().getAbsolutePath());
webSettings.setCacheMode(WebSettings.LOAD_DEFAULT);
webSettings.setDatabaseEnabled(true);
webSettings.setDomStorageEnabled(true);
webSettings.setUseWideViewPort(true);
webSettings.setLoadWithOverviewMode(true);
webSettings.setPluginState(WebSettings.PluginState.ON);

// 2. 添加JavascriptInterface
mDownloadBlobFileJSInterface = new DownloadBlobFileJSInterface(this);
mWebView.addJavascriptInterface(mDownloadBlobFileJSInterface, "Android");

// 3. 执行JS代码
mWebView.loadUrl(DownloadBlobFileJSInterface.getBase64StringFromBlobUrl(url));

参考

  1. Download Blob file from Website inside Android WebViewClient