需求背景

当某一个目录的文件发生变化（创建、修改、删除、移动）时，需要给一个回调事件给其他端调用。
其他场景：阅后即焚等等。

比如在 Android 的 VR 设备中，有一个用于部署的文件，在Android 系统中发生变化时，需要给 Unity 端的一个回调，Unity 端基于该回调做相应的操作。

涉及到的技术点：

Unity 和 Android 端的数据交互，Android系统中 接口的设计、以及 AIDL 跨进程的通信等等，此处不在展开，后期再更新。本文只介绍一下，文件监听的使用及注意事项。

android.os下的FileObserver类是一个用于监听文件访问、创建、修改、删除、移动等操作的监听器，基于linux的inotify。

FileObserver 是个抽象类，必须继承它才能使用。每个FileObserver对象监听一个单独的文件或者文件夹，如果监视的是一个文件夹，那么文件夹下所有的文件和级联子目录的改变都会触发监听的事件。

所能监听的事件类型如下：

• ACCESS，即文件被访问
• MODIFY，文件被 修改
• ATTRIB，文件属性被修改，如 chmod、chown、touch 等
• CLOSE_WRITE，可写文件被 close
• CLOSE_NOWRITE，不可写文件被 close
• OPEN，文件被 open
• MOVED_FROM，文件被移走,如 mv
• MOVED_TO，文件被移来，如 mv、cp
• CREATE，创建新文件
• DELETE，文件被删除，如 rm
• DELETE_SELF，自删除，即一个可执行文件在执行时删除自己
• MOVE_SELF，自移动，即一个可执行文件在执行时移动自己
• CLOSE，文件被关闭，等同于(IN_CLOSE_WRITE | IN_CLOSE_NOWRITE)
• ALL_EVENTS，包括上面的所有事件

走进源码

/**
	FileObserver 类是一个用于监听文件访问、创建、修改、删除、移动等操作的监听器，基于linux的inotify。
    FileObserver 是个抽象类，必须继承它才能使用。
	每个FileObserver对象监听一个单独的文件或者文件夹，如果监视的是一个文件夹，那么文件夹下所有的文件和级联子目录的改变都会触发监听的事件。
**/
public abstract class FileObserver {
    /** @hide */
    @IntDef(flag = true, value = {
            ACCESS,
            MODIFY,
            ATTRIB,
            CLOSE_WRITE,
            CLOSE_NOWRITE,
            OPEN,
            MOVED_FROM,
            MOVED_TO,
            CREATE,
            DELETE,
            DELETE_SELF,
            MOVE_SELF
    })
    @Retention(RetentionPolicy.SOURCE)
    public @interface NotifyEventType {}

    /** Event type: Data was read from a file */
    public static final int ACCESS = 0x00000001;
    /** Event type: Data was written to a file */
    public static final int MODIFY = 0x00000002;
    /** Event type: Metadata (permissions, owner, timestamp) was changed explicitly */
    public static final int ATTRIB = 0x00000004;
    /** Event type: Someone had a file or directory open for writing, and closed it */
    public static final int CLOSE_WRITE = 0x00000008;
    /** Event type: Someone had a file or directory open read-only, and closed it */
    public static final int CLOSE_NOWRITE = 0x00000010;
    /** Event type: A file or directory was opened */
    public static final int OPEN = 0x00000020;
    /** Event type: A file or subdirectory was moved from the monitored directory */
    public static final int MOVED_FROM = 0x00000040;
    /** Event type: A file or subdirectory was moved to the monitored directory */
    public static final int MOVED_TO = 0x00000080;
    /** Event type: A new file or subdirectory was created under the monitored directory */
    public static final int CREATE = 0x00000100;
    /** Event type: A file was deleted from the monitored directory */
    public static final int DELETE = 0x00000200;
    /** Event type: The monitored file or directory was deleted; monitoring effectively stops */
    public static final int DELETE_SELF = 0x00000400;
    /** Event type: The monitored file or directory was moved; monitoring continues */
    public static final int MOVE_SELF = 0x00000800;

    /** Event mask: All valid event types, combined */
    @NotifyEventType
    public static final int ALL_EVENTS = ACCESS | MODIFY | ATTRIB | CLOSE_WRITE
            | CLOSE_NOWRITE | OPEN | MOVED_FROM | MOVED_TO | DELETE | CREATE
            | DELETE_SELF | MOVE_SELF;

    private static final String LOG_TAG = "FileObserver";

    private static class ObserverThread extends Thread {
        private HashMap<Integer, WeakReference> m_observers = new HashMap<Integer, WeakReference>();
        private int m_fd;

        public ObserverThread() {
            super("FileObserver");
            m_fd = init();
        }

        public void run() {
            observe(m_fd);
        }

        public int[] startWatching(List<File> files,
                @NotifyEventType int mask, FileObserver observer) {
            final int count = files.size();
            final String[] paths = new String[count];
            for (int i = 0; i < count; ++i) {
                paths[i] = files.get(i).getAbsolutePath();
            }
            final int[] wfds = new int[count];
            Arrays.fill(wfds, -1);

            startWatching(m_fd, paths, mask, wfds);

            final WeakReference<FileObserver> fileObserverWeakReference =
                    new WeakReference<>(observer);
            synchronized (m_observers) {
                for (int wfd : wfds) {
                    if (wfd >= 0) {
                        m_observers.put(wfd, fileObserverWeakReference);
                    }
                }
            }

            return wfds;
        }

        public void stopWatching(int[] descriptors) {
            stopWatching(m_fd, descriptors);
        }

        @UnsupportedAppUsage
        public void onEvent(int wfd, @NotifyEventType int mask, String path) {
            // look up our observer, fixing up the map if necessary...
            FileObserver observer = null;

            synchronized (m_observers) {
                WeakReference weak = m_observers.get(wfd);
                if (weak != null) {  // can happen with lots of events from a dead wfd
                    observer = (FileObserver) weak.get();
                    if (observer == null) {
                        m_observers.remove(wfd);
                    }
                }
            }

            // ...then call out to the observer without the sync lock held
            if (observer != null) {
                try {
                    observer.onEvent(mask, path);
                } catch (Throwable throwable) {
                    Log.wtf(LOG_TAG, "Unhandled exception in FileObserver " + observer, throwable);
                }
            }
        }

        private native int init();
        private native void observe(int fd);
        private native void startWatching(int fd, String[] paths,
                @NotifyEventType int mask, int[] wfds);
        private native void stopWatching(int fd, int[] wfds);
    }

    @UnsupportedAppUsage
    private static ObserverThread s_observerThread;

    static {
        s_observerThread = new ObserverThread();
        s_observerThread.start();
    }

    // instance
    private final List<File> mFiles;
    private int[] mDescriptors;
    private final int mMask;

    /**
     * Equivalent to FileObserver(path, FileObserver.ALL_EVENTS).
     *
     * @deprecated use {@link #FileObserver(File)} instead.
     */
    @Deprecated
    public FileObserver(String path) {
        this(new File(path));
    }

    /**
     * Equivalent to FileObserver(file, FileObserver.ALL_EVENTS).
     */
    public FileObserver(@NonNull File file) {
        this(Arrays.asList(file));
    }

    /**
     * Equivalent to FileObserver(paths, FileObserver.ALL_EVENTS).
     *
     * @param files The files or directories to monitor
     */
    public FileObserver(@NonNull List<File> files) {
        this(files, ALL_EVENTS);
    }

    /**
     * Create a new file observer for a certain file or directory.
     * Monitoring does not start on creation!  You must call
     * {@link #startWatching()} before you will receive events.
     *
     * @param path The file or directory to monitor
     * @param mask The event or events (added together) to watch for
     *
     * @deprecated use {@link #FileObserver(File, int)} instead.
     */
    @Deprecated
    public FileObserver(String path, @NotifyEventType int mask) {
        this(new File(path), mask);
    }

    /**
     * Create a new file observer for a certain file or directory.
     * Monitoring does not start on creation!  You must call
     * {@link #startWatching()} before you will receive events.
     *
     * @param file The file or directory to monitor
     * @param mask The event or events (added together) to watch for
     */
    public FileObserver(@NonNull File file, @NotifyEventType int mask) {
        this(Arrays.asList(file), mask);
    }

    /**
     * Version of {@link #FileObserver(File, int)} that allows callers to monitor
     * multiple files or directories.
     *
     * @param files The files or directories to monitor
     * @param mask The event or events (added together) to watch for
     */
    public FileObserver(@NonNull List<File> files, @NotifyEventType int mask) {
        mFiles = files;
        mMask = mask;
    }

    protected void finalize() {
        stopWatching();
    }

    /**
     * Start watching for events.  The monitored file or directory must exist at
     * this time, or else no events will be reported (even if it appears later).
     * If monitoring is already started, this call has no effect.
     */
    public void startWatching() {
        if (mDescriptors == null) {
            mDescriptors = s_observerThread.startWatching(mFiles, mMask, this);
        }
    }

    /**
     * Stop watching for events.  Some events may be in process, so events
     * may continue to be reported even after this method completes.  If
     * monitoring is already stopped, this call has no effect.
     */
    public void stopWatching() {
        if (mDescriptors != null) {
            s_observerThread.stopWatching(mDescriptors);
            mDescriptors = null;
        }
    }

    /**
     * The event handler, which must be implemented by subclasses.
     *
     * <p class="note">This method is invoked on a special FileObserver thread.
     * It runs independently of any threads, so take care to use appropriate
     * synchronization!  Consider using {@link Handler#post(Runnable)} to shift
     * event handling work to the main thread to avoid concurrency problems.</p>
     *
     * <p>Event handlers must not throw exceptions.</p>
     *
     * @param event The type of event which happened
     * @param path The path, relative to the main monitored file or directory,
     *     of the file or directory which triggered the event.  This value can
     *     be {@code null} for certain events, such as {@link #MOVE_SELF}.
     */
    public abstract void onEvent(int event, @Nullable String path);
}

源码解读及注意事项：

相关实现类并不复杂，代码也不多，这里可以完整看一下，学习一下实现原理。

1. ALL_EVENTS 这个事件由 “|”位运算实现，位运算相关知识回顾。这里用或运算，后面在监听时的回调 onEvent会用到。

   符号	描述	运算规则
   &	与	两个位都为1时，结果才为1
   |	或	两个位都为0时，结果才为0
   ^	异或	两个位相同为0，相异为1
   ~	取反	0变1，1变0
   <<	左移	各二进位全部左移若干位，高位丢弃，低位补0
   >>	右移	各二进位全部右移若干位，对无符号数，高位补0，有符号数，各编译器处理方法不一样，有的补符号位（算术右移），有的补0（逻辑右移）

2. 在onEvent的回调事件处理中，我们得注意 用 "&"来监听，否则会出现返回未确定定义的 event type.这里其实不是bug.是我们用错的方式。

   在

   @Override
   public void onEvent(int event, String path) {
       Log.d(TAG, "event: " + event);
       /* event的值是与 0x40000000 进行或运算后的值，所以在 case 之前需要先和 FileObserver.ALL_EVENTS进行与运算*/
       int e = event & FileObserver.ALL_EVENTS;
       switch (e) {
           case FileObserver.CREATE:
               break;
            case FileObserver.DELETE:
               break;
       }
   }
   
   

   如果不做 与&运算,你会得到以下的测试数字，以为是 bug. 其实不是。我们了解一下位运算就知道了。

类型值	含义
1073742080	“文件夹”的创建(Create)操作
1073742336	“文件夹”的删除(Delete)操作
1073741888	“文件夹”的移出（MOVE_FROM） 操作
1073741952	“文件夹”的移入（MOVE_TO） 操作
32768	“文件夹” 的打开操作 （OPEN） 操作

实现示例

FileObserver是一个抽象类，使用的时候我们需要自己实现一个类来继承FileObserver。

/**
 * <pre>
 *     @author : JuneYang
 *     time   : 2023/01/20
 *     desc   :
 *     version: 1.0
 * </pre>
 */
public class SDCardFileObServer extends FileObserver {
    public static final String TAG = SDCardFileObServer.class.getSimpleName();

    public SDCardFileObServer(String path) {
        /*
         * 这种构造方法是默认监听所有事件的,如果使用 super(String,int)这种构造方法，
         * 则int参数是要监听的事件类型.
         */
        super(path);
    }

    @RequiresApi(api = Build.VERSION_CODES.Q)
    public SDCardFileObServer(@NonNull File file, int mask) {
        super(file, mask);
    }

    @Override public void onEvent(int event, @Nullable String path) {
        //注意点
        int e = event & FileObserver.ALL_EVENTS;
        switch (e) {
            case FileObserver.CREATE:
                break;
            case FileObserver.DELETE:
                break;
           case FileObserver.MODIFY:
                break;
            default:
                break;
        }
    }

    // 调用
    public static void main(String[] args) {
        String path = "xx/xx/xx";
        // 初始化操作
        SDCardFileObServer sdCardFileObServer = new SDCardFileObServer(path);
        sdCardFileObServer.startWatching();

        // 服务结束后关闭监听
        sdCardFileObServer.stopWatching();
    }

测试用例：
以监听某个目录为例,当目录下发生文件的状态变化时，测试情况如下：

1. 拷贝文件时，如果文件过大，modify 方法会每 50ms 左右回调一次接口，因为文件在一直变化，直到不再变化为止。
2. 替换文件时,会回调 delete 和 create 和 modify 方法。
3. 该路径下的两个文件如果执行拷贝、删除、替换，有几个文件就会执行几个文件的几种状态的回调。
4. 文件夹删除时也会执行删除 delete回调，文件夹新建时会有 create 回调.
5. 文件夹合并时不会有回调

Tips: 在项目中，由于 FileObserver对象必须保持一个引用，确保不被垃圾收集器回收掉，否则就不会触发事件。我们可以考虑使用 Service 服务。

也就是说在 Service 中的 Oncreate中初始化（startWatching），在OnDestory中(stopWatching)。

参考

位运算在Java编程中的应用

Android中巧妙的位运算_钟秀的博客-CSDN博客_android 视图标志位 或运算

Android系统中Flag的位操作设计