I have a parent Viewgroup with a child View inside of it. I'm am noticing some behavior during touch events that differs from my mental model of how touch events are dispatched.
Viewgroup A takes up the full screen and View B is programmatically added to A (using A.addView(B)) and takes up only part of the screen. For both A and B I am overriding the onTouchEvent(MotionEvent event) method, but nothing else.
If I touch my finger to the screen and lift it within the bounds of View B, I notice that the touch event is directly routed to B's onTouchEvent() method without ever hitting A's onTouchEvent() method. B receives the DOWN, MOVE and UP actions and returns true in for each action.
If I then touch my finger to the screen and lift within the bounds of Viewgroup A but outside of the bounds of View B then I see behavior that violates my mental model of how touch events are dispatched. When I do this View B again receives the DOWN, MOVE and UP actions in its onTouchEvent() even though B was not clicked. I would have expected A to receive the actions. If I then do the exact same motion with my finger (touch and lift inside of A but outside of B) then A's onTouchEvent() receives the DOWN, MOVE and UP actions as expected.
It's almost as if after one of the Views receives a touch event, it is setup by the system to receive the next touch regardless of the location of the touch.
Is there something basic I'm not understanding here?
Also, one of my resources mentions that I need to manually manage hit testing within my Viewgroup / Views to determine if when a viewgroup receives a hit if it should dispatch the hit down to a child or not. That makes sense to me, however, it seems like the system already does that for me in the first case I mention as the hit is automatically dispatched to the child. Which is the right way to do it, let the system handle it or handle the hit logic myself?
I used these resources to get me this far but they don't seem to explicitly cover this:
http://codetheory.in/understanding-android-input-touch-events/
https://guides.codepath.com/android/Gestures-and-Touch-Events
Lets assume I've a ViewGroup that handles a vertical scroll event. I already implemented the logic within onInterceptTouchEvent which will return true when the desired gesture is identified.
Lets say this gesture is a "Scroll down". When onInterceptTouchEvent returns true (scroll down is started) all subsequent touch events are going to be sent to my ViewGroup (correct :))
The problem is that as soon as the user scrolls up again (within the same gesture) i want to cancel myself as touch handler target and pass the events to my child view instead.
What should I use? calling dispatchTouchEvent within myViewGroup.onTouch as soon as i find the user started to "move up" ?
One way to stop handling a gesture previously "intercepted" through onInterceptTouchEvent->true is to call dispatchTouchEvent within your onTouch method.
You'll need to forge your own MotionEvent (ACTION_CANCEL).
I want to fire a method after a touch has been initiated and left the screen. I can detect the beginning of a touch by setting a OnTouch event to the View but can't detect when the hand leave the screen. I want to detect when does the touch stops. How can i detect this?
What you're looking for is http://developer.android.com/reference/android/view/MotionEvent.html - Specifically the use of MotionEvent.ACTION_UP. There is a lot of information on the link I've provided :)
A gesture starts with a motion event with ACTION_DOWN that provides the location of the first pointer down. As each additional pointer that goes down or up, the framework will generate a motion event with ACTION_POINTER_DOWN or ACTION_POINTER_UP accordingly. Pointer movements are described by motion events with ACTION_MOVE. Finally, a gesture end either when the final pointer goes up as represented by a motion event with ACTION_UP or when gesture is canceled with ACTION_CANCEL.
you can use MotionEvent.ACTION_UP to detect touch up.
I would look at this answer. Basically it says that you need to return true to process anything other than ACTION_DOWN.
I am overriding both onTouch and onClick. Each is supposed to result in different actions (very important). I don't want a touch to be both a touch and a click: it must be either-or. I am not having much luck. How do I get my app to discriminate between onTouch and onClick? Right now, either I get onTouch by itself or I get both onTouch and onClick together (based on whether I change onTouch to return true or false).
An easy solution is to just use onTouch(). Look for ACTION_UP and ACTION_DOWN. Store the position of the touch in ACTION_DOWN, and in ACTION_UP if the distance between the touch and the stored touch are < X then fire a custom click event, otherwise fire the drag event.
I'm not asking how to handle touch events, but what is going on behind the scenes? If there are several nested widgets, what order do they see the events in? Does the developer have any control over it? Ideally I would like a document on the subject.
Let's take a look at a visual example.
When a touch event occurs, first everyone is notified of the event, starting at the Activity and going all the way to the view on top. Then everyone is given a chance to handle the event, starting with the view on top (view having highest Z order in the touch region) and going all the way back to the Activity. So the Activity is the first to hear of it and the last to be given a chance to handle it.
If some ViewGroup wants to handle the touch event right away (and not give anyone else down the line a chance at it) then it can just return true in its onInterceptTouchEvent(). An Activity doesn't have onInterceptTouchEvent() but you can override dispatchTouchEvent() to do the same thing.
If a View (or a ViewGroup) has an OnTouchListener, then the touch event is handled by OnTouchListener.onTouch(). Otherwise it is handled by onTouchEvent(). If onTouchEvent() returns true for any touch event, then the handling stops there. No one else down the line gets a chance at it.
More detailed explanation
The above diagram makes things a little more simple than they actually are. For example, between the Activity and ViewGroup A (the root layout) there is also the Window and the DecorView. I left them out above because we generally don't have to interact with them. However, I will include them below. The description below follows a touch event through the source code. You can click a link to see the actual source code.
(Update: the source code has been updated so the line numbers are off now, but clicking the links will still get you to the right file. Just do a search for the method name.)
The Activity's dispatchTouchEvent() is notified of a touch event. The touch event is passed in as a MotionEvent, which contains the x,y coordinates, time, type of event, and other information.
The touch event is sent to the Window's superDispatchTouchEvent(). Window is an abstract class. The actual implementation is PhoneWindow.
The next in line to get the notification is DecorView's superDispatchTouchEvent(). DecorView is what handles the status bar, navigation bar, content area, etc. It is actually just a FrameLayout subclass, which is itself a subclass of ViewGroup.
The next one to get the notification (correct me if I'm wrong) is the content view of your activity. That is what you set as the root layout of your activity in xml when you create the layout in the Android Studio's Layout Editor. So whether you choose a RelativeLayout, a LinearLayout, or a ConstraintLayout, they are all subclasses of ViewGroup. And ViewGroup gets notified of the touch event in dispatchTouchEvent(). This is the ViewGroup A in my diagrams above.
The ViewGroup will notify any children it has of the touch event, including any ViewGroup children. This is ViewGroup B in my diagrams above.
Anywhere along the way, a ViewGroup can short-circuit the notification process by returning true for onInterceptTouchEvent().
Assuming no ViewGroup cut the notifications short, the natural end of the line for the notifications is when the View's dispatchTouchEvent() get's called.
Now it is time, to start handling the events. If there is an OnTouchListener, then it gets the first chance at handling the touch event with onTouch(). Otherwise, the View's onTouchEvent() gets to handle it.
Now all the ViewGroups recursively up the line get a chance to handle the touch event in the same way that View did. Although, I didn't indicate this in the diagram above, a ViewGroup is a View subclass, so everything I described about OnTouchListener.onTouch() and onTouchEvent() also applies to ViewGroups.
Finally, if no one else wanted it, the Activity also gets the last chance to handle the event with onTouchEvent().
FAQ
When would I ever need to override dispatchTouchEvent()?
Override it in the Activity if you want to catch a touch event before any of the views get a chance at it. For a ViewGroup (including the root view), then just override onInterceptTouchEvent() and onTouchEvent().
When would I ever need to override onInterceptTouchEvent()?
If you just want to spy of the touch notifications that are coming in, you can do it here and return false.
However, the main purpose of overriding this method is to let the ViewGroup handle a certain type of touch event while letting the child handle another type. For example, a ScrollView does this to handle scrolling while letting its child handle something like a Button click. Conversely, if the child view doesn't want to let its parent steal its touch event, it can call requestDisallowTouchIntercept().
What are the touch event types?
The main ones are
ACTION_DOWN - This is the start of a touch event. You should always return true for the ACTION_DOWN event in onTouchEvent if you want to handle a touch event. Otherwise, you won't get any more events delivered to you.
ACTION_MOVE - This event is continuously fired as you move your finger across the screen.
ACTION_UP - This is the last event of a touch event.
A runner up is ACTION_CANCEL. This gets called if a ViewGroup up the tree decides to intercept the touch event.
You can view the other kinds of MotionEvents here. Since Android is multi-touch, events are also fired when other fingers ("pointers") touch the screen.
Further study
Android onTouchEvent Part 1, Part 2, and Part 3 (YouTube video - good summary of some of the links below)
Mastering the Android Touch System (thorough video by Google developer)
Android UI Internal : Pipeline of View's Touch Event Handling
Managing Touch Events in a ViewGroup (Android docs)
Input Events (Android docs)
Gestures and Touch Events
From Activity viewpoint:
Touch events are delivered first to Activity.dispatchTouchEvent. It's where you may catch them first.
Here they get dispatched to Window, where they traverse View hierarchy, in such order that Widgets that are drawn last (on top of other widgets) have chance to process touch in View.onTouchEvent first. If some View returns true in onTouchEvent, then traversal stops and other Views don't receive touch event.
Finally, if no View consumes touch, it's delivered to Activity.onTouchEvent.
That's all your control. And it's logical that what you see drawn on top of something else, has chance to process touch event before something drawn below it.
Android Touch event
I have prepared a high level diagram that should illustrate a simple flow.
dispatchTouchEvent() - Activity, ViewGroup, View
onInterceptTouchEvent() - ViewGroup
onTouch() - ViewGroup, View. Using setOnTouchListener()
onTouchEvent() - Activity, ViewGroup, View
[iOS onTouch]
following Suragch's answer,
pseudocode:
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean consume = false;
if (onInterceptTouchEvent(ev) {
consume = onTouchEvent(ev);
} else {
consume = child.dispatchTouchEvent(ev);
}
return consume;
}
ref:Android开发艺术探索