java - 我如何通知我的应用程序一个文件已从 SDCard (Android) 中删除？

Question

我正在将几首歌曲保存在播放列表中(在我的应用程序数据库中)。当从播放列表中已存在的 SDCard 中删除特定歌曲时，如何在我的数据库中反射(reflect)更改？

Answer 1

考虑使用 FileObserver

您可以监控单个文件或目录。所以您需要做的是确定您在哪些目录中有歌曲并监控每个目录。否则你可以监控你的外部存储目录，然后每次有任何变化时，检查它是否是你数据库中的文件之一。

它的工作原理非常简单，像这样应该可以工作:

import android.os.FileObserver;
public class SongDeletedFileObserver extends FileObserver {
    public String absolutePath;
    public MyFileObserver(String path) {
        //not sure if you need ALL_EVENTS but it was the only one in the doc listed as a MASK
        super(path, FileObserver.ALL_EVENTS);
        absolutePath = path;
    }
    @Override
    public void onEvent(int event, String path) {
        if (path == null) {
            return;
        }
        //a new file or subdirectory was created under the monitored directory
        if ((FileObserver.DELETE & event)!=0) {
            //handle deleted file
        }

        //data was written to a file
        if ((FileObserver.MODIFY & event)!=0) {
            //handle modified file (maybe id3 info changed?)
        }

        //the monitored file or directory was deleted, monitoring effectively stops
        if ((FileObserver.DELETE_SELF & event)!=0) {
           //handle when the whole directory being monitored is deleted
        }

        //a file or directory was opened
        if ((FileObserver.MOVED_TO & event)!=0) {
           //handle moved file
        }

        //a file or subdirectory was moved from the monitored directory
        if ((FileObserver.MOVED_FROM & event)!=0) {
            //?
        }

        //the monitored file or directory was moved; monitoring continues
        if ((FileObserver.MOVE_SELF & event)!=0) {
            //?
        }

    }
}

那么当然你需要让这个 FileObserver 一直运行才能有效，所以你需要把它放在一个服务中。从你会做的服务

SongDeletedFileObserver fileOb = new SongDeletedFileObserver(Environment.getExternalStorageDirectory());

您必须记住一些棘手的事情:

1. 如果你让它一直运行，这会更耗电。
2. 您必须在安装 SD 卡时(以及重新启动时)进行同步。这可能会很慢