Service服务实现下载功能_service downlown

Service生命周期最全面解
http://www.jianshu.com/p/8d0cde35eb10

Service服务史上最全面解析
http://www.jianshu.com/p/d963c55c3ab9

实例

DwonloadListener.class
下载过程中的各状态进行监听和回调

public interface DownloadListener {

    //下载进度
    void onProgress(int progress);

    //下载成功
    void onSuccess();

    //下载失败
    void onFailed();

    //下载暂停
    void onPaused();

    //下载取消
    void onCanceled();
}
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DownloadTask.class
下载类

public class DownloadTask extends AsyncTask <String,Integer,Integer>{

    public static final int TYPE_SUCCES = 0;

    public static final int TYPE_FAILED = 1;

    public static final int TYPE_PAUSED = 2;

    public static final int TYPE_CANCELED = 3;

    private DownloadListener listener;

    private boolean isCanceled = false;

    private boolean isPaused = false;

    private int lastProgress;

    public DownloadTask(DownloadListener listener){
        //接收一个接口实现类
        this.listener = listener;
    }

    //下载逻辑
    @Override
    protected Integer doInBackground(String... params) {
        InputStream is = null;
        //RandomAccessFile 是随机访问文件(包括读/写)的类。
        //它支持对文件随机访问的读取和写入，即我们可以从指定的位置读取/写入文件数据。
        //如果要想对文件内容进行操作，则可以使用RandomAccessFile类
        RandomAccessFile savedFile = null;
        File file = null;
        try{
            long downloadedLength = 0;
            String downloadUrl = params[0];
            String fileName = downloadUrl.substring(downloadUrl.lastIndexOf("/"));
            String directory = Environment.getExternalStoragePublicDirectory
                    (Environment.DIRECTORY_DOWNLOADS).getPath();
            file = new File(directory + fileName);
            if(file.exists()){
                //已下载的文件字节长度
                downloadedLength = file.length();
            }

            //服务器文件字节长度
            long contentLength = getContentLength(downloadUrl);
            if(contentLength == 0){
                //如果服务器文件字节长度等于0，说明下载文件有问题，下载失败
                return TYPE_FAILED;
            }else if(contentLength == downloadedLength){
                //如果服务器文件字节长度等于已下载文件字节长度，说明已经下载完成
                return TYPE_SUCCES;
            }
            OkHttpClient client = 
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