利用監聽者模式實現後臺多文件下載的監聽

項目聊天功能中，需要監聽圖片的下載進度。類型於QQ的聊天圖片查看。 當退出查看界面時， 下載進程仍在後臺運行，再次進入查看界面，如何監聽到圖片的下載進度呢。 如圖

/**
 * 下載文件
 *
 * @param url
 * @param outputStream     輸出流
 * @param onUpdateListener 下載進度監聽
 * @return
 */
public boolean downLoadFile(String url, OutputStream outputStream, OnUpdateListener onUpdateListener) {

    if (StringUtils.isStrEmpty(url)) {
        return false;
    }
    if (onUpdateListener != null) {
        downloadObserver.put(url, onUpdateListener);
    }
    HttpGet get = new HttpGet(url);
    // Header headerRange = new BasicHeader("connection", "close");
    // get.addHeader(headerRange);
    get.addHeader("connection", "close");
    InputStream in = null;
    try {
        HttpResponse response = getDownloadClient(url).execute(get);
        if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
            HttpEntity he = response.getEntity();
            if (onUpdateListener != null) {
                onUpdateListener.onTotal(he.getContentLength());
            }
            in = he.getContent();
            byte[] buffer = new byte[1024];
            int len = 0;
            long total = 0;
            while ((len = in.read(buffer)) != -1) {
                outputStream.write(buffer, 0, len);
                if (onUpdateListener != null) {
                    onUpdateListener.onProcess(total);
                }
            }
            return true;
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        try {
            if (in != null)
                in.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    return false;
}

這是原來 的下載代碼， 這裏的監聽只能在第一次進放查看頁面時生效。那麼如何在重新進入時仍然能監聽到下載的進度呢。 這裏借鑑到監聽者模式的思想。使用一個集合來管理 監聽者對象

private Map<String, OnUpdateListener> downloadObserver = new HashMap<String, OnUpdateListener>();

提供添加監聽的方法， 一個url對應一public void addObserver(String url, OnUpdateListener onUpdateListener) {
if (onUpdateListener != null) {
downloadObserver.put(url, onUpdateListener);
onUpdateListener.onTotal(-1);
}
}的 Listener動態獲取

/**
 * 下載文件
 *
 * @param url
 * @param outputStream     輸出流
 * @param onUpdateListener 下載進度監聽
 * @return
 */
public boolean downLoadFile(String url, OutputStream outputStream, OnUpdateListener onUpdateListener) {

    if (StringUtils.isStrEmpty(url)) {
        return false;
    }
    if (onUpdateListener != null) {
        downloadObserver.put(url, onUpdateListener);
    }
    HttpGet get = new HttpGet(url);
    // Header headerRange = new BasicHeader("connection", "close");
    // get.addHeader(headerRange);
    get.addHeader("connection", "close");
    InputStream in = null;
    try {
        HttpResponse response = getDownloadClient(url).execute(get);
        if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
            HttpEntity he = response.getEntity();
            if (downloadObserver.get(url) != null) {
                downloadObserver.get(url).onTotal(he.getContentLength());
            }
            in = he.getContent();
            byte[] buffer = new byte[1024];
            int len = 0;
            long total = 0;
            while ((len = in.read(buffer)) != -1) {
                outputStream.write(buffer, 0, len);
                if (downloadObserver.get(url) != null) {
                    total += len;
                    downloadObserver.get(url).onProcess(total);
                }
            }
            return true;
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        try {
            if (in != null)
                in.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    return false;
}

這樣，在任意地方都可以監聽到子線程的下載進度。