Android Difference between onInterceptTouchEvent and dispatchTouchEvent

Question

What is the difference between onInterceptTouchEvent and dispatchTouchEvent in Android   According to the android developer guide  both methods can be used to intercept a touch event  MotionEvent   but what is the difference   How do onInterceptTouchEvent  dispatchTouchEvent and onTouchEvent interact together within a hierarchy of Views  ViewGroup

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I came accross very intuitive explanation at this webpage http   doandroids com blogs tag codeexample   Taken from there         boolean onTouchEvent MotionEvent ev  - called whenever a touch event with this View as target is detected   boolean onInterceptTouchEvent MotionEvent ev  - called whenever a touch event is detected with this ViewGroup or a child of it as target  If this function returns true  the MotionEvent will be intercepted  meaning it will be not be passed on to the child  but rather to the onTouchEvent of this View

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Small answer   onInterceptTouchEvent comes before setOnTouchListener

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The following code within a ViewGroup subclass would prevent it s parent containers from receiving touch events      Override   public boolean dispatchTouchEvent MotionEvent ev           Normal event dispatch to this container s children  ignore the return value     super dispatchTouchEvent ev           Always consume the event so it is not dispatched further up the chain     return true        I used this with a custom overlay to prevent background views from responding to touch events

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You can find the answer in this video https   www youtube com watch v SYoN-OvdZ3M amp list PLonJJ3BVjZW6CtAMbJz1XD8ELUs1KXaTD amp index 19 and the next 3 videos  All the touch events are explained very well  it s very clear and full of examples

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The best place to demystify this is the source code  The docs are woefully inadequate about explaining this   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   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

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Both Activity and View have method dispatchTouchEvent   and onTouchEvent The ViewGroup have this methods too  but have another method called onInterceptTouchEvent  The return type of those methods are boolean  you can control the dispatch route through the return value   The event dispatch in Android starts from Activity- ViewGroup- View

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ViewGroup s onInterceptTouchEvent   is always the entry point for ACTION DOWN event which is first event to occur    If you want ViewGroup to process this gesture  return true from onInterceptTouchEvent    On returning true  ViewGroup s onTouchEvent   will receive all subsequent events till next ACTION UP or ACTION CANCEL  and in most cases  the touch events between ACTION DOWN and ACTION UP or ACTION CANCEL are ACTION MOVE  which will normally be recognized as scrolling fling gestures   If you return false from onInterceptTouchEvent    the target view s onTouchEvent   will be called  It will be repeated for subsequent messages till you return true from onInterceptTouchEvent     Source  http   neevek net posts 2013 10 13 implementing-onInterceptTouchEvent-and-onTouchEvent-for-ViewGroup html

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public boolean dispatchTouchEvent MotionEvent ev       boolean consume  false      if onInterceptTouchEvent ev           consume   onTouchEvent ev        else          consume   child dispatchTouchEvent ev

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Short answer  dispatchTouchEvent   will be called first of all   Short advice  should not override dispatchTouchEvent   since it s hard to control  sometimes it can slow down your performance  IMHO  I suggest overriding onInterceptTouchEvent       Because most answers mention pretty clearly about the flow touch event on activity  view group  view  I only add more detail about code on these methods in ViewGroup  ignoring dispatchTouchEvent      onInterceptTouchEvent   will be called first  ACTION event will be called respectively down -  move -  up  There are 2 cases    If you return false in 3 cases  ACTION DOWN  ACTION MOVE  ACTION UP   it will consider as the parent won t need this touch event  so onTouch   of the parents never calls  but onTouch   of the children will call instead  however please notice     The onInterceptTouchEvent   still continue to receive touch event  as long as its children don t call requestDisallowInterceptTouchEvent true    If there are no children receiving that event  it can happen in 2 cases  no children at the position the users touch  or there are children but it returns false at ACTION DOWN   the parents will send that event back to onTouch   of the parents   Vice versa  if you return true  the parent will steal this touch event immediately  and onInterceptTouchEvent   will stop immediately  instead onTouch   of the parents will be called as well as all onTouch   of children will receive the last action event - ACTION CANCEL  thus  it means the parents stole touch event  and children cannot handle it from then on   The flow of onInterceptTouchEvent   return false is normal  but there is a little confusion with return true case  so I list it here    Return true at ACTION DOWN  onTouch   of the parents will receive ACTION DOWN again and following actions  ACTION MOVE  ACTION UP   Return true at ACTION MOVE  onTouch   of the parents will receive next ACTION MOVE  not the same ACTION MOVE in onInterceptTouchEvent    and following actions  ACTION MOVE  ACTION UP   Return true at ACTION UP  onTouch   of the parents will NOT called at all since it s too late for the parents to steal touch event     One more thing important is ACTION DOWN of the event in onTouch   will determine if the view would like to receive more action from that event or not  If the view returns true at ACTION DOWN in onTouch    it means the view is willing to receive more action from that event  Otherwise  return false at ACTION DOWN in onTouch   will imply that the view won t receive any more action from that event

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The main difference           Activity dispatchTouchEvent MotionEvent  - This allows your Activity   to intercept all touch events before they are dispatched to the   window        ViewGroup onInterceptTouchEvent MotionEvent  - This allows a   ViewGroup to watch events as they are dispatched to child Views

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dispatchTouchEvent handles before onInterceptTouchEvent   Using this simple example      main   new LinearLayout this            Override         public boolean onInterceptTouchEvent MotionEvent ev                System out println  Event - onInterceptTouchEvent                return super onInterceptTouchEvent ev                 return false    event get propagated                    Override         public boolean dispatchTouchEvent MotionEvent ev                System out println  Event - dispatchTouchEvent                return super dispatchTouchEvent ev                 return false    event DONT get propagated                       main setBackgroundColor Color GRAY       main setLayoutParams new LinearLayout LayoutParams 320 480              viewA   new EditText this       viewA setBackgroundColor Color YELLOW       viewA setTextColor Color BLACK       viewA setTextSize 16       viewA setLayoutParams new LinearLayout LayoutParams 320 80        main addView viewA        setContentView main     You can see that the log willl be like   I System out 25900   Event - dispatchTouchEvent I System out 25900   Event - onInterceptTouchEvent   So in case you are working with these 2 handlers use dispatchTouchEvent to handle on first instance the event  which will go to onInterceptTouchEvent   Another difference is that if dispatchTouchEvent return  false  the event dont get propagated to the child  in this case the EditText  whereas if you return false in onInterceptTouchEvent the event still get dispatch to the EditText

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There is a lot of confusion about these methods  but it is actually not that complicated  Most of the confusion is because    If your View ViewGroup or any of its children do not return true in onTouchEvent  dispatchTouchEvent and onInterceptTouchEvent will ONLY be called for MotionEvent ACTION DOWN  Without a true from onTouchEvent  the parent view will assume your view does not need the MotionEvents  When none of the children of a ViewGroup return true in onTouchEvent  onInterceptTouchEvent will ONLY be called for MotionEvent ACTION DOWN  even if your ViewGroup returns true in onTouchEvent    Processing order is like this    dispatchTouchEvent is called  onInterceptTouchEvent is called for MotionEvent ACTION DOWN or when any of the children of the ViewGroup returned true in onTouchEvent  onTouchEvent is first called on the children of the ViewGroup and when none of the children returns true it is called on the View ViewGroup    If you want to preview TouchEvents MotionEvents without disabling the events on your children  you must do two things    Override dispatchTouchEvent to preview the event and return super dispatchTouchEvent ev   Override onTouchEvent and return true  otherwise you won   t get any MotionEvent  except MotionEvent ACTION DOWN    If you want to detect some gesture like a swipe event  without disabling other events on your children as long as you did not detect the gesture  you can do it like this    Preview the MotionEvents as described above and set a flag when you detected your gesture  Return true in onInterceptTouchEvent when your flag is set to cancel MotionEvent  processing by your children  This is also a convenient place to reset your flag  because onInterceptTouchEvent won   t be called again until the next MotionEvent ACTION DOWN    Example of overrides in a FrameLayout  my example in is C  as I   m programming with Xamarin Android  but the logic is the same in Java    public override bool DispatchTouchEvent MotionEvent e           Preview the touch event to detect a swipe      switch  e ActionMasked                case MotionEventActions Down               processingSwipe   false               touchStartPosition   e RawX              break          case MotionEventActions Move              if    processingSwipe                                float move   e RawX -  touchStartPosition                  if  move  gt    swipeSize                                         processingSwipe   true                       cancelChildren   true                      ProcessSwipe                                                break            return base DispatchTouchEvent e      public override bool OnTouchEvent MotionEvent e           To make sure to receive touch events  tell parent we are handling them      return true     public override bool OnInterceptTouchEvent MotionEvent e           Cancel all children when processing a swipe      if   cancelChildren                   Reset cancel flag here  as OnInterceptTouchEvent won t be called until the next MotionEventActions Down           cancelChildren   false          return true            return false

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Supplemental Answer Here are some visual supplements to the other answers  My full answer is here    The dispatchTouchEvent   method of a ViewGroup uses onInterceptTouchEvent   to choose whether it should immediately handle the touch event  with onTouchEvent    or continue notifying the dispatchTouchEvent   methods of its children

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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 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         View OnTouchListener onTouch        If  not  consumed   processes  the  touch  itself         View onTouchEvent              How a ViewGroup handles touch         ViewGroup dispatchTouchEvent           onInterceptTouchEvent           Check  if  it  should  supersede  children   Passes  ACTION CANCEL  to  active  child   If it returns  true  once   the ViewGroup consumes  all  subsequent  events      For  each  child  view   in  reverse  order  they  were  added          If  touch  is  relevant   inside  view    child dispatchTouchEvent     If  it is not  handled  by  a previous   dispatch  to  next  view      If  no  children  handles the  event  the listener  gets  a  chance         OnTouchListener onTouch        If there is no listener  or its not handled         onTouchEvent           Intercepted  events  jump  over the child step      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

[java] Android: Difference between onInterceptTouchEvent and dispatchTouchEvent?

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