Obtaining the current Android view and forcing it to be redrawn

问题

How can I get the current Android view when it displays data that has been updated, and force it to be redrawn? I worked through Android's Notepad tutorial and completed lesson three without any problems — the solution is provided, after all — but I'm stuck on my first non-trivial modification.

I added a new button to the menu, next to the Add note button. When pressed, that button adds a letter to the title of each note in the system. However, the new titles don't show up in the list of notes no matter how long I wait. I know the updater works because the changes do appear if I dismiss the app and bring it back up.

So far, I've discovered that I have to use some kind of invalidation method to make the program redraw itself with the new values. I know that invalidate() is used from the UI thread and postInvalidate() is used from non-UI threads ^{1, 2}, but I don't even know which thread I'm in. Also, both of those methods have to be called from the View object that needs drawing, and I don't know how to obtain that object. Everything I try returns null.

My main class:

public boolean onMenuItemSelected(int featureId, MenuItem item) {
    switch(item.getItemId()) {
        case INSERT_ID:
            createNote();
            return true;
        case NEW_BUTTON:
            expandTitles();
            return true;
        default:
            // Intentionally empty
    }
    return super.onMenuItemSelected(featureId, item);
}

private void expandTitles() {
    View noteListView = null;

    // noteListView = findViewById(R.layout.notes_list); // null

    // noteListView =
    //   getWindow().getDecorView().findViewById(android.R.id.content);
    // From SO question 4486034

    noteListView = findViewById(R.id.body); // Fails

    mDbHelper.expandNoteTitles(noteListView);
}

My DAO class:

public void expandNoteTitles(View noteListView) {
    Cursor notes = fetchAllNotes();
    for(int i = 1; i <= notes.getCount(); i++) {
        expandNoteTitle(i);
    }

    // NPE here when attempt to redraw is not commented out
    noteListView.invalidate(); // Analogous to AWT's repaint(). Not working.
    // noteListView.postInvalidate(); // Like repaint(). Not working.
}

public void expandNoteTitle(int i) {
    Cursor note = fetchNote(i);
    long rowId =
      note.getLong(note.getColumnIndexOrThrow(NotesDbAdapter.KEY_ROWID));
    String title =
      note.getString(note.getColumnIndexOrThrow(NotesDbAdapter.KEY_TITLE)) + "W";
    String body =
      note.getString(note.getColumnIndexOrThrow(NotesDbAdapter.KEY_BODY));
    updateNote(rowId, title, body);
}

What do I have to do to get the updated note titles to show up as soon as I press the button?

Obviously, I'm a complete newbie to Android. I point this out to encourage you to use small words and explain even obvious things. I know this is the millionth "Android not redrawing" question, but I've read dozens of existing posts and they either don't apply or don't make sense to me.

^{1: What does postInvalidate() do?}
^{2: What is the difference between Android's invalidate() and postInvalidate() methods?}

回答1:

According to the tutorial, the list of existing notes are presented in a ListView. That is an adapter based View, so the items it shows are sourced from an adapter extending theBaseAdapter class. In these cases, you should notify the adapter that the contents have changed by calling its notifyDatasetChanged method. This'll signal the ListView to update and redraw its rows.

Edit:

Sorry, I now realize that this example uses CursorAdapters. These source the items to show from a Cursor object that was obtained from a database query. Now, what the notifyDatasetChanged() tells the adapter is, that the data that backs the adapter has changed, so Views that show stuff based on this adapter need to redraw their contents. In the case of a CursorAdapter, this data is coming from a cursor. So you also need to requery that cursor, refreshing it from the DB, like this:

private void expandTitles() {
        mDbHelper.expandNoteTitles();

        CursorAdapter adapter = (CursorAdapter)getListAdapter();
        adapter.getCursor().requery();
    }

The requery() method automatically calls the notifyDatasetChanged() in this case, so you don't need to worry about that, the list will update itself. See this thread also: https://groups.google.com/forum/?fromgroups#!topic/android-developers/_FrDcy0KC-w%5B1-25%5D.

来源：https://stackoverflow.com/questions/11912390/obtaining-the-current-android-view-and-forcing-it-to-be-redrawn

标签

android

android-layout

invalidation