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So in the last lesson, we learned about planes and anchors and implemented this delegate method to be
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able to detect horizontal planes in the real world.
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Now, in this lesson, I want to look at how we can start detecting touches on the screen and interpret it
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as locations in the real world.
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So to do that, I'm going to use a delegate method called touchesBegan and this gets called when a touch
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is detected in the view or in the window.
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So in here is where we're going to be receiving those touches that are coming from the user and use
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ARKit to convert it into a real world location.
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So the first thing we need to do is to check if there indeed were touches detected and this method wasn't
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just called in error.
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So to do that, we're going to use a "if let."
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And we're going to call our object touch and it's going to be set to equal touches.first.
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Now, if you option-click on touchesBegan, you can see that touches is actually a set of UI touch instances,
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but they only have more than one object when you're using multi-touch. And to do that, you need to enable
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multi-touch. But in our case, because that's not enabled and we're not interested in detecting multiple
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touches,
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we only want the first object in that set which is the location that the user touched on the screen.
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So if this touches.first does indeed contain an object, then we're going to sign it to touch and
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we're going to be using it in order to detect its location.
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So let's create another constant code touchLocation and set it to the touch that we--the UITouch that
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we got from earlier on, .location, and the "in" is where that location was detected. And you can see that
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the location is expecting a node that is a descendant of a scene present in the window that receive
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the touch event.
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So this is going to be our sceneView because that is where our touch event was initiated.
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All right.
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So once we've got the touch location remember that this is a 2D coordinate because your iPhone or your
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iPod screen is 2D.
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You can't actually push through the iPad and touch a place in 3D.
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So in order to convert that touch location into a 3D location inside our scene, we're going to use a
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method that is from ARKit called hitTest.
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And I'm going to show you how it works.
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So we're going to call the results, results, and it's going to be created by calling one of the 
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sceneView methods called hitTest. And hitTest searches for real world objects or ARAnchors which is true
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in our case because we're looking for planeAnchors that is in the captured camera image corresponding
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to a point in the SceneKit view.
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So, basically, we're converting a point that we're touching in 2D space on the screen into a 3D coordinate
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that corresponds to one of our ARPlaneAnchors. And the hitTest method that we want
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is this one where it expects a point that's going to be our touchLocation and it's also expecting a
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type which is the type of result we're looking for.
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So the point that we're going to run out hitTest on is going to be touchLocation. And the type is going
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to be .existingPlanes using extent, and this is a planeAnchor that's already in the scene because
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we put it on in this delegate method and it's respecting the plane's limited size.
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And now, I just want to briefly explain how this hitTest works.
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So let's say that you've got your app running and you tap a point on your screen. Now, that tap is interpreted
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as a touchLocation and that will trigger the touchesBegan method,
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and we're going to look for the location of that touch. Now that touchLocation is a point in 2D space.
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It has a x and a y component on the iPhone or on the iPad screen.
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Now, what we want, though, is we want to convert that 2D location and we want to let it traverse through
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the camera image and hit a point in 3D space.
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So a place on our plane that we've already added to the scene.
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So, now that point that the hitTest has detected within the scene has a third positional component,
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the z component. So it is now a position in 3D space.
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And this is the point that we're going to be placing our dice.
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So depending on the z component, your dice is going to be rendered larger or smaller.
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So if the z was closer to you, then your dice would get rendered larger.
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And if the z was further away, then it would be scaled down smaller.
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So it's being placed in our 3D plane based on our real horizontal plane that was detected.
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Now, if you're interested in finding out more about how ARHitTest works, then have a look in the
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class documentation on Apple.com and you can read more about what are the different types of hitTests
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and what are the parameters for this particular method that we're using, and how it works behind
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the scenes.
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Now, once we've gotten our result back, I want to be able to print to the console and, say, if the result
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is empty, i.e., we didn't get one, then that means that our hit or our touch probably didn't hit one of our
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existing planes.
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So I'm going to say you touched somewhere else.
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But if it was not empty, i.e., we did get a result back, then I'm going to print that you touched a point
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on the plane.
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So let's say if results.isEmpty, and we're using "isEmpty" because results is an array type.
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So I'm going to put the exclamation mark in front of 
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!results.isEmpty to reverse it,
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so we're now checking to see if results is not empty.
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And in that case, that means we did get a result back from our hitTest and it did find a position on
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an existing plane.
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So we're going to print "touched the plane," but otherwise, our touchesBegan was triggered.
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So the user did touch somewhere on the screen, just not on an existing plane,
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then we're going to print "touched somewhere else."
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All right, let's try that.
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So, as you can see, when I touched the plane that is level with the desk, it says, "touch the plane."
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And if I touch somewhere else, like, say, this slightly more elevated box that's wrapped in a pink shirt,
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"touched somewhere else," and it works even if I move the camera somewhere else, and the original plane
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is out of the view.