android touch system (input events )
original source : https://stackoverflow.com/questions/9586032/android-difference-between-onintercepttouchevent-and-dispatchtouchevent
#1 answer
dispatchTouchEvent is actually defined on Activity, View and ViewGroup. Think of it as a controller which decides how to route the touch events.
For example, the simplest case is that of View.dispatchTouchEvent which will route the touch event to either OnTouchListener.onTouch if it's defined or to the extension method onTouchEvent.
View, ViewGroup, Activity에 연결된 listener(위의 예시에서 OnTouchListener.onTouch)나 그 element내부에 가지고 있는 handler(위의 예시에서onTouchEvent)를 이용해 처리할수있는데 dispatchTouchEvent를 통해 어느 element가 처리할지가 결정되지 전까지 계속 이동하게(route) 된다.
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참조사항) https://stackoverflow.com/a/12646163
The basic difference is that event handlers let the originating object itself do something in response to the event, whereas event listeners let other objects do something in response to events originating in the object.
For example: your activity has a button. If you want your activity to handle when someone touches the button, you use an event listener (by doing btn.setOnTouchListener(...)). BUT, if you want to create a specialized button (e.g. one that looks like a dog and barks when touched), you can create a subclass of Button and implement its event handler, onTouchEvent(...). In this latter case, the button itself will handle its touch event.
setOnTouchListener 를 통해 이벤트를 처리할 listener를 연결할수 있는데 이벤트를 처리할 listener가 view 그자체에 있지 않고 다른 element에 있을수 있다. 즉 element내에서 직접처리하지 않고 외부에 있는 경우 listener라고 한다. element내부에서 처리할수 있는 경우는이를 handler라고 하고 onTouchEvent()가 그 예시가 될수 있다.
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For ViewGroup.dispatchTouchEvent things are way more complicated. It needs to figure out which one of its child views should get the event (by calling child.dispatchTouchEvent). This is basically a hit testing algorithm where you figure out which child view's bounding rectangle contains the touch point coordinates.
But before it can dispatch the event to the appropriate child view, the parent can spy and/or intercept the event all together. This is what onInterceptTouchEvent is there for. So it calls this method first before doing the hit testing and if the event was hijacked (by returning true from onInterceptTouchEvent) it sends a ACTION_CANCEL to the child views so they can abandon their touch event processing (from previous touch events) and from then onwards all touch events at the parent level are dispatched to onTouchListener.onTouch (if defined) or onTouchEvent(). Also in that case, onInterceptTouchEvent is never called again.
Would you even want to override [Activity|ViewGroup|View].dispatchTouchEvent? Unless you are doing some custom routing you probably should not.
The main extension methods are ViewGroup.onInterceptTouchEvent if you want to spy and/or intercept touch event at the parent level and View.onTouchListener/View.onTouchEvent for main event handling.
All in all its overly complicated design imo but android apis lean more towards flexibility than simplicity.
#2 answer
Because this is the first result on Google. I want to share with you a great Talk by Dave Smith on Youtube: Mastering the Android Touch System (이 동영상은 1시간분량)and the slides are available here. It gave me a good deep understanding about the Android Touch System:
How the Activity handles touch:
Activity.dispatchTouchEvent()
Always first to be called
Sends event to root view attached to Window
onTouchEvent()
Called if no views consume the event
Always last to be called
How the View handles touch:
View.dispatchTouchEvent()
Sends event to listener first, if exists
If not consumed, processes the touch itself
View.OnTouchListener.onTouch()
View.onTouchEvent()
How a ViewGroup handles touch:
ViewGroup.dispatchTouchEvent()
Intercepted events jump over the child step
onInterceptTouchEvent()
For each child view (in reverse order they were added)
If no children handles the event, the listener gets a chance
If there is no listener, or its not handled
Check if it should supersede children
Passes ACTION_CANCEL to active child
If it returns true once, the ViewGroup consumes all subsequent events
If touch is relevant (inside view), child.dispatchTouchEvent()
If it is not handled by a previous, dispatch to next view
OnTouchListener.onTouch()
onTouchEvent()
He also provides example code of custom touch on github.com/devunwired/.
Answer: Basically the dispatchTouchEvent() is called on every View layer to determine if a View is interested in an ongoing gesture. In a ViewGroup the ViewGroup has the ability to steal the touch events in his dispatchTouchEvent()-method, before it would call dispatchTouchEvent() on the children. The ViewGroup would only stop the dispatching if the ViewGroup onInterceptTouchEvent()-method returns true. The difference is that dispatchTouchEvent()is dispatching MotionEvents and onInterceptTouchEvent tells if it should intercept (not dispatching the MotionEvent to children) or not (dispatching to children).
You could imagine the code of a ViewGroup doing more-or-less this (very simplified):
public boolean dispatchTouchEvent(MotionEvent ev) { if(!onInterceptTouchEvent()){ for(View child : children){ if(child.dispatchTouchEvent(ev)) return true; } } return super.dispatchTouchEvent(ev); }
#3 answer