Event dispatching of Android

  次阅读 来源:互联网(转载协议) 2015-10-02 16:08 我要评论(0)

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.

- Show quoted text -

--

Dianne Hackborn

Android framework engineer

hackbod@android.com

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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