Event dispatching of Android

2012/02/26 15:12
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Event dispatching is relatively simple.  There are two major models: 

- Focus-based dispatching is used for key events and trackball events. 
- Position-based dispatching is used for touch events. 

In all cases, the dispatching you probably care about starts with an IPC 
coming from the window manager into your process, telling a specific window 
about an event.  This is received in the internal VIewRoot class.  This in 
turn simply calls the appropriate dispatchXxx() function on the top-level 
view of the hierarchy, which dispatches it down to child views based on 
either the current focus target or the x/y coordinate of the event as 
appropriate.  Most of the code for dispatching down the hierarchy is in 
ViewGroup.  Ultimately View will receive the dispatchXxx() call and turn it 
into an onKeyDown() etc.  The View class and its subclasses will also in 
variation situations look at the raw events they are receiving to perform 
higher-level callbacks like onClick(). 

Another important common pattern we have for low-level events is returning a 
boolean indicating whether the receiver handled it.  If you return false, 
the framework will try to perform a callback on the next target.  For both 
focus and position dispatching, this means doing a callback on the parents 
up the view hierarchy until someone handles it or we reach the top.  In 
addition, returning true on a down of a touch event "locks in" that view as 
the target, allowing to receive the following moves and final up event 
regardless of where they occur on the screen; if you return false you will 
not receive any more events until the next down. 

We don't use any messaging for dispatching an event and the corresponding 
actions that result -- this is all done through callbacks -- so in general 
if you break into the debugger from a callback you can see the primary chain 
from receiving the original raw event in ViewRoot through to the call you 
are getting. 

And of course you can look at the source code to see the actual 
implementation.  There isn't a -lot- to deal with -- ViewRoot for the 
initial receipt of the event, the dispatch functions on ViewRoot for 
deciding where the event goes, View for the low-level callbacks, and the 
appropriate class for higher-level callbacks. 

Dianne Hackborn 
Android framework engineer 

Note: please don't send private questions to me, as I don't have time to 
provide private support.  All such questions should be posted on public 
forums, where I and others can see and answer them. 



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