How to use Multi-touch in Android 2

How to use Multi-touch in Android 2

Summary: This is the first in a series of articles on developing multi-touch applications with Android 2.x. It is excerpted from Chapter 11 of the book "Hello, Android! (3rd edition)", available in beta now at The Pragmatic Programmers.

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TOPICS: Hardware
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This is the first in a series of articles on developing multi-touch applications with Android 2.x. It is excerpted from Chapter 11 of the book "Hello, Android! (3rd edition)", available in beta now at The Pragmatic Programmers.

Introducing multi-touch

Multi-touch is simply an extension of the regular touch-screen user interface, using two or more fingers instead of one. We’ve used single-finger gestures before, although we didn’t call it that. In Chapter 4 we let the user touch a tile in the Sudoku game in order to change it. That’s called a "tap" gesture. Another gesture is called "drag". That’s where you hold one finger on the screen and move it around, causing the content under your finger to scroll.

Tap, drag, and a few other single-fingered gestures have always been supported in Android. But due to the popularity of the Apple iPhone, early Android users suffered from a kind of gesture envy. The iPhone supported multi-touch, in particular the "pinch zoom" gesture.

Three common touch gestures: a) tap, b) drag, and c) pinch zoom. (Image courtesy of GestureWorks.com)

With pinch zoom, you place two fingers on the screen and squeeze them together to make the item you’re viewing smaller, or pull them apart to make it bigger. Before Android 2.0 you had to use a clunky zoom control with icons that you pressed to zoom in and out (for example the setBuiltInZoomControls() in the MyMap example). But thanks to its new multi-touch support, you can now pinch to zoom on Android too! As long as the application supports it, of course.

Note: If you try to run the example in this chapter on Android 1.5 or 1.6, it will crash because those versions do not support multi-touch. We’ll learn how to work around that in chapter 13, "Write Once, Test Everywhere".

Warning: Multi-bugs ahead

Multi-touch, as implemented on current Android phones is extremely buggy. In fact it’s so buggy that it borders on the unusable. The API routinely reports invalid or impossible data points, especially during the transition from one finger to two fingers on the screen and vice-versa.

On the developer forums you can find complaints of fingers getting swapped, x and y axes flipping, and multiple fingers sometimes being treated as one. With a lot of trial and error, I was able to get the example in this chapter working because the gesture it implements is so simple. Until Google acknowledges and fixes the problems, thatmay be about all you can do. Luckily, pinch zoom seems to be the only multi-touch gesture most people want.

The Touch example

To demonstrate multi-touch, we’re going to build a simple image viewer application that lets you zoom in and scroll around an image. Here's a screenshot of the finished product:

The Touch example implements a simple image viewer with drag and pinch zoom.

Continue reading: Part 2: Building the Touch Example >

Copyright notice: This is an excerpt from Hello, Android 3rd edition, published by the Pragmatic Bookshelf. For more information or to purchase a paperback or PDF copy, please visit http://www.pragprog.com/titles/eband3.

Copyright © 2010 The Pragmatic Programmers, LLC. All rights reserved.

No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form, or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior consent of the publisher.

Topic: Hardware

Ed Burnette

About Ed Burnette

Ed Burnette is a software industry veteran with more than 25 years of experience as a programmer, author, and speaker. He has written numerous technical articles and books, most recently "Hello, Android: Introducing Google's Mobile Development Platform" from the Pragmatic Programmers.

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7 comments
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  • Multitouch bugs

    The problems you mention are not software issues, they are hardware
    issues. We are aware of these issues but unfortunately most of the
    problems cannot be fixed (at least reliably) in software.
    thinthalion
  • RE: How to use Multi-touch in Android 2

    I believe a lot of them could be addressed in operating system software. For example on my Droid Sholes I see debugging messages from the system about it dropping coords it thinks are wrong, but it seems to actually be dropping the good coords and keeping the bad. I've seen it give me move events with 2 pointers at almost the same coordinates instead of far apart like they are in reality, because according to the messages in LogCat it threw away the real coordinates for the 2nd finger thinking they were bogus. You can see this happen in the Touch example from the book (full source code is available at http://pragprog.com/titles/eband3/source_code) if you play around for a few seconds with lifting and dragging 2 fingers in different orders.

    (Sorry, that was meant as a reply to thinthalion's comment)
    Ed Burnette
  • RE: How to use Multi-touch in Android 2

    Android 2.0? What's that? Oh that's right. It is that version that Google/TMO can seem to squeeze out of their tight you-know-whats!

    I love my MyTouch Android, but communication from Google and TMO about ANYTHING related to the OS is horrible. If something doesn't change, I might have to defect when the contract is up.
    djmik
    • MyTouch update

      From what I've heard, MyTouch 3g (Magic/Ion) phones are expected to get an upgrade to 2.1 eventually. You're not missing much, though. The more important upgrade is a hardware one to a higher resolution screen and faster processor like the Droid Sholes or Nexus One.
      Ed Burnette
  • RE: How to use Multi-touch in Android 2

    Where can I download the example
    mobinauten
    • Downloading the example

      I was going to save the download link for the next post but since you asked, here it is:

      http://pragprog.com/titles/eband3/source_code
      Ed Burnette
  • Image from SDCard - Not moving/zooming

    Hi there,

    I am running similar code in my app where the picture is from a file, rather than from drawable. It is falling into the correct case statements and the coords returned in dump seem to be ok but my picture doesn't change. Any ideas?
    spengilley