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We finished the last video by seeing a variety of touch events, and wondering how we could tell what the

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user was doing at the time.

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So the gesture detector can tell what type of gesture is happening.

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That's actually its purpose.

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So what we can do, 

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and I'm going to close down the log cat here, see in the onInterceptTouchEvent function is passed the event

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on to a gesture detector, and let it help us to call the appropriate functions for the events that we

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want to handle. Everything else

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it'll will just let through. So I'm going to change the onInterceptTouchEvent method first.

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So we're going to, we've got the log in there, we're going to leave that log call there,

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and we're going to put, on the next line, val result equals gestureDetector

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dot onTouchEvent parentheses e

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Then we're going to Log dot e,

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sorry Log.d parentheses,

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TAG comma double quotes dot onInterceptTouchEvent returning dollar result. Then instead of returning true

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we're going to return result.

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Now the theory is that anything the gestureDetectors onTouchEventvent method deals with, should return

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true. Anything it doesn't handle, should return false

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so that something else can deal with it.

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Now I wouldn't normally write a function in such a verbose way, and we can change it later when you're

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happy with what's going on.

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But I wanted to capture the return value from the call to onTouchEvent, so that we could log it.

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So let's create the gestureDetector and override the functions we're interested in, and get rid of that error.

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So we're going to create a new gesture detector after the interface declaration, before the onIntercept

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TouchEvent. So add the gestureDetector. Now

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it's going to be a private 

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val gestureDetector equals, GestureDetectorCompat is the one we want, parentheses

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context comma object colon, then it's going to be GestureDetector dot SimpleOnGestureListener parentheses.

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Then we want a code block,

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and then we want a closing right brace,

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a right parentheses rather. Then we've got our override function on the next line.

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So what I've done there is very similar to adding an onClickListener to a button.

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We're creating an anonymous class that extends a simple on Gesture Listener,

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then overriding the methods we're interested in.

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Now we're going to use the generator to generate the functions we want to override using control o, making

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sure that I'm actually in that definition.

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So the two that we want to override is onSingleTapUp and onLongPress.

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Now one thing you may want to do is implement all the functions, and put logging in to see when they're

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called.

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That's a very useful thing to experiment to get an understanding of how things work.

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Now from reading the documentation, we may consider using onSingleTapConfirmed rather than on 

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SingleTapUp,

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and that's because a single tap may be followed by another one to make a double tap.

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Now if we'd already responded to the first tap we'd miss the second one.

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Now if we were going to allow some other action or a double tap, then onSingleTapConfirmed would definitely

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be the one to use for that reason, and just before I OK it I'm going to have a quick check of the

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documentation.

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And if we scroll down to the bottom two, it says that onSingleTapUp results in the listener being notified

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when a tap occurs,

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whereas for onSingleTapConfirmed, unlike onSingleTapUp,

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this will only be called after the detector is confident that the user's first tap is not followed by

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a second tap, leading to a double tap gesture.

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Now as we're not checking for double taps,

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we can make the app a bit more responsive by removing the need for Android to wait to be sure that

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there won't be a second tap.

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But you can see why there's two methods there.

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So I respond to one or the other, but not both. Alright so back to Android studio.

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So I've already selected onSingleTapUp and onLongPress. Now before I click OK, have a look at

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the function signatures for all these functions. They nearly all return a boolean value,

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True or false, to indicate whether they've handled the event or not,

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which is exactly what we relied on in our onInterceptTouchEvent function.

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Unfortunately though, the onLongPress and onShowpress functions don't return a value, but we'll come back

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to that. Because we're using onLongPress,

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it's important that we understand what's going on. Now I'm going to click OK here, but I'm not going to

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add any useful code just yet, because we should really try to understand what's happening with all this.

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So let's just add some logging initially.

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So firstly for the onSingleTapUp, what I'm going to do is change the return to be just return true, then

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the previous line we'll just add a logging statement.

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So Log.d parentheses TAG comma, double, double quotes dot onSingleTapUp colon starts, and lets take

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a copy of that line and I'm going to remove the super call for onLongPress, paste in the log and change

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that to the right name.

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So that's going to be onLongPress starts.

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Alright, so let's run this and see how we go, and whether it works OK.

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Alright, so let's try a single click, and you can see we've got onSingleTapUp starts, and single click's working fine,

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and the onInterceptTouchEvent, and I'm just going to scroll over

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so we can see that, is returning true, which would be correct there. We got that from the call to the onTouch

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Event method, because that's what our overridden onSingleTapUp function's returning.

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So nothing else will attempt to handle the single click.

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In this case that's fine.

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We're going to deal with that,

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so we don't want anyone else interfering. But note that false is returned first when the action down event

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happens. You can see there, returning false. Our gesture detector isn't handling that,

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so it returns false.

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Now a long press though is a different matter.

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That doesn't return a value,

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so if we actually run that and check it,

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we get false returned as you can see there. That means anything else that might be listening for a long press

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will also try to handle it. Now

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one reason I can think of for this behavior with long press, is if you hold your finger still for too

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long before scrolling. Now to make the log cat a little bit easier to read, click at the end,

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press enter and type in long pressing, and press enter again. Breaking up the log entries like this makes

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it a bit easier to work out what's happened.

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Now this might be hard to see in the video, but I'm going to click and hold the button down. I'm going to go back

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to my emulator.

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I'm holding the button down.

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After a while we see that the onLongPress function being called in the log cat,

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and you can see that it was there. Still holding the mouse button down,

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I can happily scroll, even though we've theoretically handled the event.

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Now if our functioned had returned true, we wouldn't actually be able to scroll as we saw earlier.

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So that's the rationale behind the onLongPress and onShowPress methods not returning a boolean. Alright.

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So now that we understand how it all works,

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and by the way if you're still not sure experiment, but don't worry because this took me quite a while to get

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my head around. There's a lot of redirection going on here, with the Android framework calling the onIntercept

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TouchEvent function, which in turn calls the onTouch function of the gesture detector.

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Now that then calls an appropriate function for the gesture in question, which may or may not have

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been overridden, by our anonymous gesture detector compat class.

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So you may well have to watch this bit of the video a few times, and work through the entries in the

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log cat for it all to make sense.

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But anyway let's start doing something a little bit more useful than just logging, when we respond to

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a touch of it.

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Now in this class, I'm just going to go back to our Android Studio code and close the log cat, what we're doing

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currently is pretty simple.

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So we're basically going to call back the functions that we created in the listener, which if you recall

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is main activity.

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So what I'm going to do is start by adding the code to those overridden functions of the gesture detector

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onSingleTapUp first.

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So I'm just going to make a bit of space here so

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we can see what we're doing. There's onSingleTapUp.

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Now we've got our log in there so we're going to leave that in there,

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and we're going to put the code in here. I'm going to type val childView equals recyclerView dot findChildViewUnder,

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and it's going to be parentheses e dot x comma

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e dot y. We have got some errors there that we'll talk about shortly.

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Next line we're going to add another log, a Log.d parentheses TAG comma double quotes dot onSingle

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TapUp

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calling listener dot onItemClick. Then the next line, it's going to be listener dot onItemClick parentheses

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childView Comma recyclerView dot getChildAdapterPosition,

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parentheses will be childView, and then we return true.

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Now we've got these two errors on line 30,

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when we try to access the properties of the motion event. Now that's passed in as a nullable type.

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Now once again that's down to the parameters in the Java code not being annotated.

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Now is it possible to get a single tap without an event?

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Well actually no. If this function's called, something must have been tapped.

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So this is another case where we can safely modify the parameter declaration to use a non-null type.

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Let's go ahead and do that,

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and that fixes the errors.

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Alright so let's now make the change for onLongPress.

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So we're going to do the same thing there, we're going to remove the question mark, because it's the same issue there. It's

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not possible to get a long press without an event,

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so we can safely remove that. Leaving the logging in there,

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we're going to do, basically, a very similar code,

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and what we can probably do there to save a bit of time is copy those three lines

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into our onLongPress, and just make a few changes. So we've got val childView equals recyclerView dot

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findChild

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ViewUnder, e dot x e dot y but that's exactly the same. But the function that's being called

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here is onLongPress calling listener, it's going to be onItemLongClick.

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Then the next line is going to be the onItemLongClick call,

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and with the same arguments, childView recyclerView dot getChildAdapterPosition and a passing childView.

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So here essentially, both functions do the same thing, except that onSingleTapUp also returns the value true.

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Now using the motion event that's passed as a parameter,

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they check to see which view is underneath it,

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and that's done by using the coordinates on the screen that were tapped.

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So this findChildViewUnder function call does pretty much what it says. It checks to see what

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was underneath the x y coordinates of the tap, and returns the view that it found.

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Now if the listener's been set with a valid object, we just log the fact and call the onItemClick or

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onItemLongClick functions that we created in main activity.

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So most of the work's going to be done in main activity, but let's run the app again just to see that

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those calls are being made successfully.

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So we're going to run it. So we'll break up the log cat with a comment before each operation.

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So firstly what we're going to do is go to the end, press enter, type in single tap,

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press enter again, then go back to our emulator.

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Then we're just going to tap the screen.

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So basically a click causes the onSingleTapUp method to be, function to be called, which then in turn calls

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the main activity's onItemClick.

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So I'm going to come down to the end of the log again and type in long tap,

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and let's go back and check if that works, do a long

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tap, and we can see there that the long press caused the onLongPress function to then call main activity's on

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ItemLongClick,

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but you can see that there. Now I haven't been showing you that because I've been going off screen,

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but we can also see if we do the long taps or short taps, we can see the Toast messages popping up,

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normal tap, long tap. And note that it's also showing the position of the photos that we clicked on long press -

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position 8 there, long press, position 10 and so on.

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So we're good to go this point.

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We've got a listener that's correctly responding to events and notifying main activity,

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when the ones we're interested in, occur. Phew!

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This is one of the most complicated things you'll come across in the Android framework, but it's incredibly

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flexible, and it lets you do all sorts of cool things with animations in a recycler view,

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but that power comes with a price.

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So it's certainly not the easiest thing to understand. Now if you are struggling to make sense of it,

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don't worry. You have some code that works, and you can just use that as a template for responding to

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touch events in your apps.

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Hopefully though the logging will help in working out what's happening, and what's being called when events

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happen.

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Now there's a few other ways to check for clicks on items in a recycler view, and a bit of Googling

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will throw up some alternatives.

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A popular approach is to implement the listener in either the adapter or the view holder, and that can

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be easier to implement, depending on what you want to do when you get an event.

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Now those approaches are well documented,

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but this approach doesn't seem to be, perhaps because many people feel it's just too hard.

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And so for that reason we use this approach because it's the one that Google suggest,

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and if you understand this, then frankly you'll have no trouble understanding the other functions because

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they're actually easier.

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Alright so let's stop the video here, and in the next video we're going to add the code in main activity to do something

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more useful than just displaying a Toast message.

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So I'll see you in the next video.