FileObserver CREATE or DELETE received only for files

问题

I've registered a FileObserver for a directory.

this.observer = new DirectoryObserver(requested.getAbsolutePath(),
        FileObserver.CREATE | FileObserver.DELETE | FileObserver.DELETE_SELF);
this.observer.startWatching();

Tested on KitKat emulator. adb shell:

root@generic:/sdcard # echo "test" >> test.txt //notified CREATE
root@generic:/sdcard # rm test.txt //notified DELETE
root@generic:/sdcard # mkdir test //no events received
root@generic:/sdcard # rmdir test //no events received

The DirectoryObserver for reference

private final class DirectoryObserver extends FileObserver {

    private DirectoryObserver(String path, int mask) {
        super(path, mask);
    }

    @Override
    public void onEvent(int event, String pathString) {
        switch (event) {
            case FileObserver.DELETE_SELF:
                //do stuff
                break;

            case FileObserver.CREATE:
            case FileObserver.DELETE:
                //do stuff
                break;
        }
    }
}

From docs

CREATE
Event type: A new file or subdirectory was created under the monitored directory 

DELETE
Event type: A file was deleted from the monitored directory

So for CREATE I should receive for files and directories and on DELETE only for files? Well, I still don't receive CREATE for a subdirectory.

回答1:

The reason of this is that android does not abstract over underlying file system well enough and returns underlying event code with some of the flags raised (some of the higher bits of the event). This is why comparing the event value with the event type directly does not work.

To solve this you can drop extra flags by applying FileObserver.ALL_EVENTS event mask (using bitwise and) to actual event value stripping it down to event type.

Using the code you've provided in your question this will look something like this:

private final class DirectoryObserver extends FileObserver {

    private DirectoryObserver(String path, int mask) {
        super(path, mask);
    }

    @Override
    public void onEvent(int event, String pathString) {
        event &= FileObserver.ALL_EVENTS;
        switch (event) {
            case FileObserver.DELETE_SELF:
                //do stuff
                break;

            case FileObserver.CREATE:
            case FileObserver.DELETE:
                //do stuff
                break;
        }
    }
}

回答2:

I've tested on two devices, one with Ice Cream Sandwich and one with Lollipop. They always come out with the same int, so I just defined two new constants:

/**
 * Event type: A new subdirectory was created under the monitored directory<br>
 * For some reason this constant is undocumented in {@link FileObserver}.
 */
public static final int CREATE_DIR = 0x40000100;
/**
 * Event type: A subdirectory was deleted from the monitored directory<br>
 * For some reason this constant is undocumented in {@link FileObserver}.
 */
public static final int DELETE_DIR = 0x40000200;

Both of these are successfully received when filtering for CREATE and DELETE.

回答3:

Ther's an issue on that. Seems the doc is wrong https://code.google.com/p/android/issues/detail?id=33659

on this answer Android: FileObserver monitors only top directory someone published a recursive file observer that should work for you

来源：https://stackoverflow.com/questions/20455423/fileobserver-create-or-delete-received-only-for-files

标签

android

fileobserver