代码技巧1监听器从子线程切换主线程

举个例子：

在请求一个接口或者进行IO处理的时候，因为这都属于耗时行为，所以都要放到子线程中去，当处理完毕的时候，通常会回调一个接口，

因为接口是在子线程中回调的，所以在回调的地方如果直接操作主线程中的控件会造成BUG。这个BUG我就不贴了。今天看FilerDownloader源码的时候，发现了一个巧妙的写法，就是在接口回调用也定义一个静态内部类，类中的方法就是接口中的方法，然后通过Handler进行线程切换。

/**
 * OnDetectBigUrlFileListener
 * 

 * 探测网络文件监听器（可以支持大文件监听）
 */
public interface OnDetectBigUrlFileListener {

    /**
     * the url file need to create(no database record for this url file)
     *
     * @param url      file url
     * @param fileName file name
     * @param saveDir  saveDir
     * @param fileSize fileSize
     */
    public abstract void onDetectNewDownloadFile(String url, String fileName, String saveDir, long fileSize);

    /**
     * the url file exist(it is in database record)
     *
     * @param url file url
     */
    void onDetectUrlFileExist(String url);

    /**
     * DetectUrlFileFailed
     *
     * @param url        file url
     * @param failReason fail reason
     */
    void onDetectUrlFileFailed(String url, DetectBigUrlFileFailReason failReason);

    /**
     * Callback helper for main thread
     */
    public static class MainThreadHelper {

        /**
         * the url file need to create(no database record for this url file)
         *
         * @param url      file url
         * @param fileName file name
         * @param saveDir  saveDir
         * @param fileSize fileSize
         */
        public static void onDetectNewDownloadFile(final String url, final String fileName, final String saveDir,
                                                   final long fileSize, final OnDetectBigUrlFileListener
                                                           nnDetectBigUrlFileListener) {
            if (nnDetectBigUrlFileListener == null) {
                return;
            }
            Handler handler = new Handler(Looper.getMainLooper());
            handler.post(new Runnable() {
                @Override
                public void run() {
                    if (nnDetectBigUrlFileListener == null) {
                        return;
                    }
                    nnDetectBigUrlFileListener.onDetectNewDownloadFile(url, fileName, saveDir, fileSize);
                }
            });
        }

        /**
         * the url file exist(it is in database record)
         *
         * @param url file url
         */
        public static void onDetectUrlFileExist(final String url, final OnDetectBigUrlFileListener
                nnDetectBigUrlFileListener) {
            if (nnDetectBigUrlFileListener == null) {
                return;
            }
            Handler handler = new Handler(Looper.getMainLooper());
            handler.post(new Runnable() {
                @Override
                public void run() {
                    if (nnDetectBigUrlFileListener == null) {
                        return;
                    }
                    nnDetectBigUrlFileListener.onDetectUrlFileExist(url);
                }
            });
        }

        /**
         * DetectUrlFileFailed
         *
         * @param url        file url
         * @param failReason fail reason
         */
        public static void onDetectUrlFileFailed(final String url, final DetectBigUrlFileFailReason failReason, final
        OnDetectBigUrlFileListener nnDetectBigUrlFileListener) {
            if (nnDetectBigUrlFileListener == null) {
                return;
            }
            Handler handler = new Handler(Looper.getMainLooper());
            handler.post(new Runnable() {
                @Override
                public void run() {
                    if (nnDetectBigUrlFileListener == null) {
                        return;
                    }
                    nnDetectBigUrlFileListener.onDetectUrlFileFailed(url, failReason);
                }
            });
        }
    }

    /**
     * DetectUrlFileFailReason
     */
    public static class DetectBigUrlFileFailReason extends HttpFailReason {
        /**
         * URL illegal
         */
        public static final String TYPE_URL_ILLEGAL = DetectBigUrlFileFailReason.class.getName() + "_TYPE_URL_ILLEGAL";
        /**
         * url over redirect count
         */
        public static final String TYPE_URL_OVER_REDIRECT_COUNT = DetectBigUrlFileFailReason.class.getName() +
                "_TYPE_URL_OVER_REDIRECT_COUNT";
        /**
         * bad http response code, not 2XX
         */
        public static final String TYPE_BAD_HTTP_RESPONSE_CODE = DetectBigUrlFileFailReason.class.getName() +
                "_TYPE_BAD_HTTP_RESPONSE_CODE";
        /**
         * the file need to download does not exist
         */
        public static final String TYPE_HTTP_FILE_NOT_EXIST = DetectBigUrlFileFailReason.class.getName() +
                "_TYPE_HTTP_FILE_NOT_EXIST";

        public DetectBigUrlFileFailReason(String url, String detailMessage, String type) {
            super(url, detailMessage, type);
        }

        public DetectBigUrlFileFailReason(String url, Throwable throwable) {
            super(url, throwable);
        }
    }

}

调用方式

    /**
     * notifyDetectNewDownloadFile
     */
    private boolean notifyDetectNewDownloadFile(String fileName, String saveDir, long fileSize) {

        Log.d(TAG, "探测文件不存在，需要创建，url：" + mUrl);

        if (mOnDetectBigUrlFileListener != null) {
            // main thread notify caller
            OnDetectBigUrlFileListener.MainThreadHelper.onDetectNewDownloadFile(mUrl, fileName, saveDir, fileSize, 
                    mOnDetectBigUrlFileListener);
            // return true;
        }
        // force return true
        return true;
    }

是不是很方便，下次如果遇到这样的情况直接这样写就好了