HorizontalScrollView: auto-scroll to end when new Views are added?

Question

I have a HorizontalScrollView containing a LinearLayout. On screen I have a Button that will add new Views to the LinearLayout at runtime, and I'd like the scroll view to scroll to the end of the list when a new View is added.

I almost have it working - except that it always scrolls one view short of the last view. It seems like it's scrolling without first calculating the inclusion of the new view.

In my app I am using a custom View object, but I made a small test application that uses ImageView and has the same symptom. I tried various things like requestLayout() on both the Layout and ScrollView, I tried scrollTo(Integer.MAX_VALUE) and it scrolled into the netherverse :) Am I violating a UI thread issue or something?

• Rick

======

    public class Main extends Activity {
        /** Called when the activity is first created. */
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.main);

             Button b = (Button) findViewById(R.id.addButton);
             b.setOnClickListener(new AddListener());

             add();
        }

        private void add() {
             LinearLayout l = (LinearLayout) findViewById(R.id.list);
             HorizontalScrollView s = 
                 (HorizontalScrollView) findViewById(R.id.scroller);

             ImageView i = new ImageView(getApplicationContext());
             i.setImageResource(android.R.drawable.btn_star_big_on);
             l.addView(i);

             s.fullScroll(HorizontalScrollView.FOCUS_RIGHT);
        }

        private class AddListener implements View.OnClickListener {
             @Override
             public void onClick(View v) {
                 add();
             }
        }
    }

Layout XML:

    <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent">

        <HorizontalScrollView
            android:id="@+id/scroller"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:scrollbarSize="50px">
            <LinearLayout
                android:id="@+id/list"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:padding="4px"/>
        </HorizontalScrollView>

        <LinearLayout
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"            
            android:layout_gravity="center">
            <Button
                android:id="@+id/addButton"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:paddingLeft="80px"
                android:paddingRight="80px"
                android:paddingTop="40px"
                android:paddingBottom="40px"
                android:text="Add"/>
        </LinearLayout>

    </LinearLayout>

Answer 1

I think there's a timing issue. Layout isn't done when a view is added. It is requested and done a short time later. When you call fullScroll immediately after adding the view, the width of the linearlayout hasn't had a chance to expand.

Try replacing:

s.fullScroll(HorizontalScrollView.FOCUS_RIGHT);

with:

s.postDelayed(new Runnable() {
    public void run() {
        s.fullScroll(HorizontalScrollView.FOCUS_RIGHT);
    }
}, 100L);

The short delay should give the system enough time to settle.

P.S. It might be sufficient to simply delay the scrolling until after the current iteration of the UI loop. I have not tested this theory, but if it's right, it would be sufficient to do the following:

s.post(new Runnable() {
    public void run() {
        s.fullScroll(HorizontalScrollView.FOCUS_RIGHT);
    }
});

I like this better because introducing an arbitrary delay seems hacky to me (even though it was my suggestion).