private int getSectionByPoint(float y) {
    if (mSections == null || mSections.length == 0) {
        return 0;
    }
    if (y <= mIndexbarRect.top + mIndexbarMargin) {
        return 0;
    }
    if (y >= mIndexbarRect.top + mIndexbarRect.height() - mIndexbarMargin) {
        return mSections.length - 1;
    }
    return (int) ((y - mIndexbarRect.top - mIndexbarMargin) / ((mIndexbarRect.height() - 2 * mIndexbarMargin) / mSections.length));
}

http://blog.csdn.net/wwj_748/article/details/17305195

/*

 * Copyright 2011 woozzu

 *

 * Licensed under the Apache License, Version 2.0 (the "License");

 * you may not use this file except in compliance with the License.

 * You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

package com.woozzu.android.widget;

import android.content.Context;

import android.graphics.Canvas;

import android.graphics.Color;

import android.graphics.Paint;

import android.graphics.RectF;

import android.os.Handler;

import android.os.Message;

import android.os.SystemClock;

import android.view.MotionEvent;

import android.widget.Adapter;

import android.widget.ListView;

import android.widget.SectionIndexer;

public class IndexScroller {

private float mIndexbarWidth;

private float mIndexbarMargin;

private float mPreviewPadding;

private float mDensity;

private float mScaledDensity;

private float mAlphaRate;

private int mState = STATE_HIDDEN;

private int mListViewWidth;

private int mListViewHeight;

private int mCurrentSection = -1;

private boolean mIsIndexing = false;

private ListView mListView = null;

private SectionIndexer mIndexer = null;

private String[] mSections = null;

private RectF mIndexbarRect;

private static final int STATE_HIDDEN = 0;

private static final int STATE_SHOWING = 1;

private static final int STATE_SHOWN = 2;

private static final int STATE_HIDING = 3;

public IndexScroller(Context context, ListView lv) {

mDensity = context.getResources().getDisplayMetrics().density;

mScaledDensity = context.getResources().getDisplayMetrics().scaledDensity;

mListView = lv;

setAdapter(mListView.getAdapter());

mIndexbarWidth = 20 * mDensity;

mIndexbarMargin = 10 * mDensity;

mPreviewPadding = 5 * mDensity;

}

public void draw(Canvas canvas) {

if (mState == STATE_HIDDEN)

return;

// mAlphaRate determines the rate of opacity

Paint indexbarPaint = new Paint();

indexbarPaint.setColor(Color.BLACK);

indexbarPaint.setAlpha((int) (64 * mAlphaRate));

indexbarPaint.setAntiAlias(true);

canvas.drawRoundRect(mIndexbarRect, 5 * mDensity, 5 * mDensity, indexbarPaint);

if (mSections != null && mSections.length > 0) {

// Preview is shown when mCurrentSection is set

if (mCurrentSection >= 0) {

Paint previewPaint = new Paint();

previewPaint.setColor(Color.BLACK);

previewPaint.setAlpha(96);

previewPaint.setAntiAlias(true);

previewPaint.setShadowLayer(3, 0, 0, Color.argb(64, 0, 0, 0));

Paint previewTextPaint = new Paint();

previewTextPaint.setColor(Color.WHITE);

previewTextPaint.setAntiAlias(true);

previewTextPaint.setTextSize(50 * mScaledDensity);

float previewTextWidth = previewTextPaint.measureText(mSections[mCurrentSection]);

float previewSize = 2 * mPreviewPadding + previewTextPaint.descent() - previewTextPaint.ascent();

RectF previewRect = new RectF((mListViewWidth - previewSize) / 2

, (mListViewHeight - previewSize) / 2

, (mListViewWidth - previewSize) / 2 + previewSize

, (mListViewHeight - previewSize) / 2 + previewSize);

canvas.drawRoundRect(previewRect, 5 * mDensity, 5 * mDensity, previewPaint);

canvas.drawText(mSections[mCurrentSection], previewRect.left + (previewSize - previewTextWidth) / 2 - 1

, previewRect.top + mPreviewPadding - previewTextPaint.ascent() + 1, previewTextPaint);

}

Paint indexPaint = new Paint();

indexPaint.setColor(Color.WHITE);

indexPaint.setAlpha((int) (255 * mAlphaRate));

indexPaint.setAntiAlias(true);

indexPaint.setTextSize(12 * mScaledDensity);

float sectionHeight = (mIndexbarRect.height() - 2 * mIndexbarMargin) / mSections.length;

float paddingTop = (sectionHeight - (indexPaint.descent() - indexPaint.ascent())) / 2;

for (int i = 0; i < mSections.length; i++) {

float paddingLeft = (mIndexbarWidth - indexPaint.measureText(mSections[i])) / 2;

canvas.drawText(mSections[i], mIndexbarRect.left + paddingLeft

, mIndexbarRect.top + mIndexbarMargin + sectionHeight * i + paddingTop - indexPaint.ascent(), indexPaint);

}

}

}

public boolean onTouchEvent(MotionEvent ev) {

switch (ev.getAction()) {

case MotionEvent.ACTION_DOWN:

// If down event occurs inside index bar region, start indexing

if (mState != STATE_HIDDEN && contains(ev.getX(), ev.getY())) {

setState(STATE_SHOWN);

// It demonstrates that the motion event started from index bar

mIsIndexing = true;

// Determine which section the point is in, and move the list to that section

mCurrentSection = getSectionByPoint(ev.getY());

mListView.setSelection(mIndexer.getPositionForSection(mCurrentSection));

return true;

}

break;

case MotionEvent.ACTION_MOVE:

if (mIsIndexing) {

// If this event moves inside index bar

if (contains(ev.getX(), ev.getY())) {

// Determine which section the point is in, and move the list to that section

mCurrentSection = getSectionByPoint(ev.getY());

mListView.setSelection(mIndexer.getPositionForSection(mCurrentSection));

}

return true;

}

break;

case MotionEvent.ACTION_UP:

if (mIsIndexing) {

mIsIndexing = false;

mCurrentSection = -1;

}

if (mState == STATE_SHOWN)

setState(STATE_HIDING);

break;

}

return false;

}

public void onSizeChanged(int w, int h, int oldw, int oldh) {

mListViewWidth = w;

mListViewHeight = h;

mIndexbarRect = new RectF(w - mIndexbarMargin - mIndexbarWidth

, mIndexbarMargin

, w - mIndexbarMargin

, h - mIndexbarMargin);

}

public void show() {

if (mState == STATE_HIDDEN)

setState(STATE_SHOWING);

else if (mState == STATE_HIDING)

setState(STATE_HIDING);

}

public void hide() {

if (mState == STATE_SHOWN)

setState(STATE_HIDING);

}

public void setAdapter(Adapter adapter) {

if (adapter instanceof SectionIndexer) {

mIndexer = (SectionIndexer) adapter;

mSections = (String[]) mIndexer.getSections();

}

}

private void setState(int state) {

if (state < STATE_HIDDEN || state > STATE_HIDING)

return;

mState = state;

switch (mState) {

case STATE_HIDDEN:

// Cancel any fade effect

mHandler.removeMessages(0);

break;

case STATE_SHOWING:

// Start to fade in

mAlphaRate = 0;

fade(0);

break;

case STATE_SHOWN:

// Cancel any fade effect

mHandler.removeMessages(0);

break;

case STATE_HIDING:

// Start to fade out after three seconds

mAlphaRate = 1;

fade(3000);

break;

}

}

public boolean contains(float x, float y) {

// Determine if the point is in index bar region, which includes the right margin of the bar

return (x >= mIndexbarRect.left && y >= mIndexbarRect.top && y <= mIndexbarRect.top + mIndexbarRect.height());

}

private int getSectionByPoint(float y) {

if (mSections == null || mSections.length == 0)

return 0;

if (y < mIndexbarRect.top + mIndexbarMargin)

return 0;

if (y >= mIndexbarRect.top + mIndexbarRect.height() - mIndexbarMargin)

return mSections.length - 1;

return (int) ((y - mIndexbarRect.top - mIndexbarMargin) / ((mIndexbarRect.height() - 2 * mIndexbarMargin) / mSections.length));

}

private void fade(long delay) {

mHandler.removeMessages(0);

mHandler.sendEmptyMessageAtTime(0, SystemClock.uptimeMillis() + delay);

}

private Handler mHandler = new Handler() {

@Override

public void handleMessage(Message msg) {

super.handleMessage(msg);

switch (mState) {

case STATE_SHOWING:

// Fade in effect

mAlphaRate += (1 - mAlphaRate) * 0.2;

if (mAlphaRate > 0.9) {

mAlphaRate = 1;

setState(STATE_SHOWN);

}

mListView.invalidate();

fade(10);

break;

case STATE_SHOWN:

// If no action, hide automatically

setState(STATE_HIDING);

break;

case STATE_HIDING:

// Fade out effect

mAlphaRate -= mAlphaRate * 0.2;

if (mAlphaRate < 0.1) {

mAlphaRate = 0;

setState(STATE_HIDDEN);

}

mListView.invalidate();

fade(10);

break;

}

}

};

}