Android：关于下载文件写的完全？

Question

我正在开发一个必须从互联网上下载一些文件的项目，所有文件下载都很好，但是如果我在下载完成后直接打开下载的文件，我会得到未完成的文件。例如I got only half of the image, but if i wait a little more time, it's open correctly。
这是我的功能：

private boolean downloadFile(long id, String file_id,
        String file_name, boolean full) {
    currentFile = id;
    PowerManager pm = (PowerManager) context
            .getSystemService(Context.POWER_SERVICE);
    WakeLock mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,
            getClass().getName());
    mWakeLock.acquire();

    InputStream input = null;
    OutputStream output = null;
    HttpURLConnection connection = null;
    try {
        String _para = "?id=" + file_id + "&f=" + (full ? "1" : "0");
        URL url = new URL(Server.DOWNLOAD + _para);
        if (full)
            if (cancel) {
                if (mWakeLock.isHeld())
                    mWakeLock.release();
                currentFile = 0;
                return false;
            }
        connection = (HttpURLConnection) url.openConnection();
        connection.connect();
        if (full)
            if (cancel) {
                if (mWakeLock.isHeld())
                    mWakeLock.release();
                currentFile = 0;
                return false;
            }
        if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {
            if (mWakeLock.isHeld())
                mWakeLock.release();
            currentFile = 0;
            return false;
        }
        // file size
        int file_size = connection.getContentLength();
        // download the file
        input = connection.getInputStream();
        String path = DIR.DIR_THUMB + file_name;
        if (full)
            if (file_name.startsWith(Media.SMEDIA_TYPE_IMAGE))
                path = DIR.DIR_IMAGES + file_name;
            else if (file_name.startsWith(Files.S_TYPE_FILE))
                path = DIR.DIR_FILES + file_name;
        output = new FileOutputStream(path);

        byte data[] = new byte[4096];
        int count;
        int downloaded = 0;
        while ((count = input.read(data)) != -1) {
            if (full)
                if (cancel)
                    break;
            output.write(data, 0, count);
            if (full)
                if (cancel)
                    break;
            downloaded += count;
            if (full)
                if (file_size == -1)
                    progress.onUpdate(id, 0);
                else {
                    progress.onUpdate(
                            id,
                            (int) (((float) downloaded / (float) file_size) * 100));
                }
        }
        //edit
        output.flush();
        if (full)
            if (cancel) {
                if (mWakeLock.isHeld())
                    mWakeLock.release();
                currentFile = 0;
                try {
                    if (output != null)
                        output.close();
                } catch (IOException ignored) {
                }
                try {
                    input.close();
                } catch (IOException ignored) {
                }
                if (connection != null)
                    connection.disconnect();
                return false;
            }
    } catch (Exception e) {
        if (mWakeLock.isHeld())
            mWakeLock.release();
        currentFile = 0;
        return false;
    } finally {
        try {
            if (output != null)
                output.close();
        } catch (IOException ignored) {
        }
        try {
            input.close();
        } catch (IOException ignored) {
        }
        if (connection != null)
            connection.disconnect();
        if (mWakeLock.isHeld())
            mWakeLock.release();
    }
    if (mWakeLock.isHeld())
        mWakeLock.release();
    currentFile = 0;
    return true;
}

我允许用户在true返回后打开文件 问题是
我怎么知道文件何时完全写入SD卡？ 修改
下载完成后我直接打开下载的文件，我得到了：
incompletely image
我等了一会儿，我得到了：
completed image