Ch.27 Touch Screens

Understanding MotionEvents

The MotionEvent Object

Receiving MotionEvent Objects

• When we want touch events to do something new with a particular View object, we can extend the class, override the onTouchEvent() method, and put our logic there. We can also implement the View.OnTouchListener interface and set up a callback handler on the View object. By setting up a callback handler with onTouch(), MotionEvents will be delivered there first before they go to the View’s onTouchEvent() method. Only if the onTouch() method returned false would our View’s onTouchEvent() method get called.

Understanding MotionEvent Contents

Recycling MotionEvents

• MotionEvent.obtaion,MotionEvent.recyle

Using VelocityTracker

• To use VelocityTracker, you first get an instance of a VelocityTracker by calling the static method VelocityTracker.obtain(). You can then add MotionEvent objects to it with the addMovement(MotionEvent ev) method. You would call this method in your handler that receives MotionEvent objects, from a handler method such as onTouch(), or from a view’s onTouchEvent(). The VelocityTracker uses the MotionEvent objects to figure out what is going on with the user’s touch sequence. Once VelocityTracker has at least two MotionEvent objects in it, we can use the other methods to find out what’s happening.
• When you are finished with the VelocityTracker object you got with the obtain() method, call the VelocityTracker object’s recycle() method.

Multitouch

The Basics of Multitouch

• At a basic level, the methods of MotionEvent are the same; that is, we call getAction(), getDownTime(), getX(), and so on. However, when more than one finger is touching the screen, the MotionEvent object must include information from all fingers, with some caveats. The action value from getAction() is for one finger, not all. The down time value is for the very first finger down and measures the time as long as at least one finger is down. The location values getX() and getY(), as well as getPressure() and getSize(), can take an argument for the finger; therefore, you need to use a pointer index value to request the information for the finger you’re interested in. There are method calls that we used previously that did not take any argument to specify a finger (for example, getX(), getY()), so which finger would the values be for if we used those methods? You can figure it out, but it takes some work. Therefore, if you don’t take into account multiple fingers all of the time, you might end up with some strange results.

Understanding Multitouch Contents

Touches with Maps