Quoted from http://android-developers.blogspot.de/2008/12/touch-mode.html
Imagine a simple application, ApiDemos for example, that shows a list of text items. The user can freely navigate through the list using the trackball but also, alternatively, scroll and fling the list using the touch screen. The issue in this scenario is how to handle the selection properly when the user manipulates the list through the touch screen.
In this case, if the user selects an item at the top of the list and then flings the list towards the bottom, what should happen to the selection? Should it remain on the item and scroll off the screen? What should happen if the user then decided to move the selection with the trackball? Or worse, what should happen if the user presses the trackball to act upon the currently selected item, which is not shown on screen anymore?
After careful consideration, we decided to remove the selection altogether, when the user manipulates the UI through the touch screen.
In touch mode, there is no focus and no selection. Any selected item in a list of in a grid becomes unselected as soon as the user enters touch mode. Similarly, any focused widgets become unfocused when the user enters touch mode. The image below illustrates what happens when the user touches a list after selecting an item with the trackball.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…