1 00:00:04,390 --> 00:00:05,440 ‫Hello and welcome. 2 00:00:05,440 --> 00:00:10,000 ‫In this lecture, we're going to be adding a new component to our BP player, the physics handle. 3 00:00:10,000 --> 00:00:16,060 ‫And we're going to be learning how we can get a pointer to it in our grabber component. 4 00:00:16,060 --> 00:00:20,800 ‫We're also going to look at how we can protect ourselves from accidentally crashing the engine. 5 00:00:20,800 --> 00:00:23,380 ‫Let's dive in and see how. 6 00:00:24,280 --> 00:00:29,380 ‫Now, in order to grab hold of an object, we need to interact with the physics system. 7 00:00:29,380 --> 00:00:37,090 ‫And there is actually a component that we can use called the physics handle, which allows us to grab 8 00:00:37,090 --> 00:00:43,090 ‫physical objects and tell them where we want to be in the world, but do it in such a way that it acknowledges 9 00:00:43,090 --> 00:00:45,760 ‫physical constraints so it doesn't pass through walls. 10 00:00:46,180 --> 00:00:53,200 ‫And it kind of looks like the object is on a spring rather than rigidly following a specific point that 11 00:00:53,200 --> 00:00:54,880 ‫we tell it to go to. 12 00:00:55,030 --> 00:00:57,910 ‫So we're going to use this physics handle components. 13 00:00:57,910 --> 00:01:02,230 ‫We're going to go and add it to our player, BP, along with the grabber. 14 00:01:02,230 --> 00:01:09,790 ‫So let's go to the add button under the components pane and look for physics handle and add it in. 15 00:01:09,790 --> 00:01:14,290 ‫And you can see that it's an actor component, not a scene component, because it goes down in the lower 16 00:01:14,290 --> 00:01:18,460 ‫section so it doesn't actually have a physical transform of its own. 17 00:01:18,460 --> 00:01:23,830 ‫What we do with the physics handle is we tell it the object to grab and then we tell it where we want 18 00:01:23,830 --> 00:01:26,170 ‫it to put the object that we've grabbed. 19 00:01:26,290 --> 00:01:32,050 ‫If you go over to the documentation, you can see this in the functions that are available. 20 00:01:32,060 --> 00:01:39,430 ‫If you scroll down to the function section, you can see that there is a release component and a grab 21 00:01:39,430 --> 00:01:41,320 ‫component at location function. 22 00:01:41,320 --> 00:01:45,940 ‫There's also a set target location and rotation. 23 00:01:45,940 --> 00:01:51,160 ‫This is where it wants to try and put the object, but obviously it will respect any physical constraints 24 00:01:51,160 --> 00:01:56,440 ‫in doing so and it will do it in a smooth way as if it were on a spring. 25 00:01:56,440 --> 00:02:02,440 ‫Now, we're not going to use these functions just yet, but we do need to find a way of getting hold 26 00:02:02,440 --> 00:02:05,470 ‫of this component in our grabber component. 27 00:02:05,590 --> 00:02:07,780 ‫So how do we do that? 28 00:02:07,780 --> 00:02:15,310 ‫Well, let's go over to our grab a CP and have a look in begin play and try and get hold of it. 29 00:02:15,310 --> 00:02:23,980 ‫Now there's a function on any actor which allows us to find a component by its type. 30 00:02:23,980 --> 00:02:27,190 ‫But what is the type of the physical handle? 31 00:02:27,190 --> 00:02:31,180 ‫Well, it is a you physics handle component. 32 00:02:31,180 --> 00:02:36,670 ‫Now, to get hold of this type, we need to use an include and you can see the include in the documentation 33 00:02:36,670 --> 00:02:40,180 ‫is physics engine forward slash physics handle components. 34 00:02:40,180 --> 00:02:46,420 ‫Let's copy that go over to our grab a CP and add it in under add draw debug helpers. 35 00:02:46,870 --> 00:02:53,470 ‫Then what we're going to do is we're going to do a get ONA to get hold of our owning act a pointer, 36 00:02:53,590 --> 00:03:00,370 ‫then do an arrow and cool find component by class. 37 00:03:00,370 --> 00:03:07,540 ‫Now this function is a special kind of function called a template function in C++, and this means that 38 00:03:07,540 --> 00:03:14,410 ‫you can pass it some parameters that actually get evaluated when you compile. 39 00:03:14,410 --> 00:03:19,090 ‫And the way you do this is using angle brackets like this. 40 00:03:19,090 --> 00:03:25,330 ‫So you use an angle bracket after the function and you give it the type that you want it to find, which 41 00:03:25,330 --> 00:03:30,910 ‫in our case is the you physics handle component like so. 42 00:03:30,970 --> 00:03:33,250 ‫And then you close the angle brackets. 43 00:03:33,250 --> 00:03:37,660 ‫But because it's a function, you do need to call it using regular parentheses as well. 44 00:03:37,660 --> 00:03:41,680 ‫So then you put the second set of parentheses after this. 45 00:03:41,680 --> 00:03:48,640 ‫So you can see that these compile time arguments are passed in using the angle brackets and then the 46 00:03:48,640 --> 00:03:52,330 ‫runtime ones are passed in using the parentheses. 47 00:03:52,630 --> 00:03:58,900 ‫Now, like a regular function, this will have a return value and in this case the return value is going 48 00:03:58,900 --> 00:04:01,390 ‫to be whatever type we've passed in here. 49 00:04:01,390 --> 00:04:08,680 ‫But as a pointer, so we can take a you physics handle component pointer and create a variable for that 50 00:04:08,680 --> 00:04:16,390 ‫which we're going to call the physics handle and set it equal to this get owner find component by class. 51 00:04:16,390 --> 00:04:21,910 ‫Now let's remember a semicolon at the end of this line and that is everything required in order for 52 00:04:21,910 --> 00:04:26,110 ‫us to get hold of another component on our same actor. 53 00:04:26,110 --> 00:04:32,740 ‫Now remember that I told you that pointers can be dangerous because sometimes they can be a null and 54 00:04:32,740 --> 00:04:34,720 ‫that can lead to a crash. 55 00:04:34,720 --> 00:04:42,100 ‫So imagine we'd forgotten to put a physics handle component on our actor, or we've put a grabber on 56 00:04:42,100 --> 00:04:46,180 ‫a new actor and we haven't hooked it up with a physics handle component yet. 57 00:04:46,180 --> 00:04:52,510 ‫If this is the case, then the physics handle pointer is going to be a null pointer. 58 00:04:52,510 --> 00:04:59,020 ‫And if we try to use the arrow operator or de referencing operator on that pointer, it will crash our 59 00:04:59,020 --> 00:05:03,340 ‫engine and it can be sometimes really hard to see what that crash looks like. 60 00:05:03,340 --> 00:05:10,000 ‫So I'm just going to go ahead and actually cause this crash just to show you and you might be able to 61 00:05:10,000 --> 00:05:15,700 ‫recognize it if it happens to you in the future, I'm going to reassign our physics handle pointer equal 62 00:05:15,700 --> 00:05:18,010 ‫to the null pointer. 63 00:05:19,060 --> 00:05:20,320 ‫Special keyword. 64 00:05:20,320 --> 00:05:21,630 ‫It's all null. 65 00:05:21,640 --> 00:05:24,550 ‫Peter That's how you spell null pointer in C++. 66 00:05:24,550 --> 00:05:28,030 ‫Then you put a semicolon at the end, and then what we're going to do is we're going to do physics, 67 00:05:28,030 --> 00:05:32,410 ‫handle arrow, get name like so. 68 00:05:32,620 --> 00:05:38,200 ‫And if we go ahead and compile using live coding and when that's finished compiling, we're going to 69 00:05:38,200 --> 00:05:43,790 ‫go ahead and make sure, first of all, that we save anything that we've got here going on in replay 70 00:05:43,840 --> 00:05:49,330 ‫because I'm about to crash the engine and if I go ahead and hit play, this is the result that we get 71 00:05:49,330 --> 00:05:50,140 ‫a crash. 72 00:05:50,140 --> 00:05:55,390 ‫Reporter And you may be tempted to think when you see this that something's gone wrong with unreal, 73 00:05:55,390 --> 00:05:58,440 ‫but it's actually your code that's at fault here. 74 00:05:58,450 --> 00:06:06,220 ‫You can see that in this crash report, the first line says that the issue is coming from you grab a 75 00:06:06,250 --> 00:06:08,230 ‫colon, colon, begin play. 76 00:06:08,230 --> 00:06:11,760 ‫And it's actually giving us the line number as well, which is line 28. 77 00:06:11,770 --> 00:06:18,490 ‫So you can see that at this point on line 28, when we referenced that pointer, in order to call a 78 00:06:18,490 --> 00:06:26,080 ‫function on it, we actually committed a grave error in not checking first whether that pointer was 79 00:06:26,080 --> 00:06:26,800 ‫null. 80 00:06:26,860 --> 00:06:28,330 ‫So that's what we need to do. 81 00:06:28,330 --> 00:06:29,710 ‫And we do that with an if statement. 82 00:06:29,710 --> 00:06:41,440 ‫We basically say if physics handle is not equal to null pointer, then we can go ahead and call stuff 83 00:06:41,440 --> 00:06:46,900 ‫on that physics handle so we can put the physical handle, get name inside of this if statement and 84 00:06:46,900 --> 00:06:52,120 ‫that protects us because that code, that D referencing code won't be executed in the case where we 85 00:06:52,120 --> 00:06:53,380 ‫have a null pointer. 86 00:06:53,380 --> 00:06:58,330 ‫So I'm going to go ahead and remove line 27 now where we are setting it to a null pointer. 87 00:06:58,330 --> 00:07:02,710 ‫And I'm going to challenge you to print the component name for yourself. 88 00:07:02,710 --> 00:07:10,930 ‫So only print if we have the actual component, basically we don't have a null pointer and use an else 89 00:07:10,930 --> 00:07:17,230 ‫to warn if we didn't manage to get hold of that component and try removing the component to check, 90 00:07:17,230 --> 00:07:19,180 ‫pause the video and have a go. 91 00:07:22,290 --> 00:07:22,770 ‫Okay. 92 00:07:22,770 --> 00:07:23,690 ‫Welcome back. 93 00:07:23,700 --> 00:07:33,600 ‫So we're going to create a new you log message here and we're going to say got physics handle colon 94 00:07:33,600 --> 00:07:40,200 ‫percent sx and we've got this physics handle name which I'm just going to cut and paste as the second 95 00:07:40,200 --> 00:07:43,260 ‫arguments or the fourth arguments into the U log. 96 00:07:43,320 --> 00:07:46,950 ‫And I'm going to put a star because it is a string. 97 00:07:46,950 --> 00:07:50,250 ‫Now let's add an else statement under our if statement. 98 00:07:50,250 --> 00:07:55,500 ‫And in here we're going to put another U log, which is going to be this time instead of a display, 99 00:07:55,500 --> 00:08:02,340 ‫we're going to put a warning type message that's going to come up in yellow and we're going to say No 100 00:08:02,640 --> 00:08:07,350 ‫physics handle found, exclamation point. 101 00:08:07,350 --> 00:08:12,960 ‫Now I haven't got the editor open, so I'm going to do a control shift B to build within Visual Studio 102 00:08:12,960 --> 00:08:17,820 ‫code, close down the terminal when it's successful and relaunch from the new project. 103 00:08:17,820 --> 00:08:24,990 ‫Once that's launched, let's go ahead and hit play, open up the output log and we should see the log 104 00:08:24,990 --> 00:08:25,440 ‫message. 105 00:08:25,440 --> 00:08:28,440 ‫Got physics handle which name the name is. 106 00:08:28,440 --> 00:08:29,790 ‫Funnily enough, physics handle. 107 00:08:29,970 --> 00:08:39,510 ‫Now what if we were to go over to our beep player and actually remove delete that physics handle and 108 00:08:39,510 --> 00:08:41,160 ‫go ahead and play. 109 00:08:41,160 --> 00:08:48,000 ‫Now have a look at our output log and they go, we've got a yellow warning that no physics handle was 110 00:08:48,000 --> 00:08:48,840 ‫found. 111 00:08:48,840 --> 00:08:50,040 ‫So let's stop play. 112 00:08:50,070 --> 00:08:55,860 ‫Let's go ahead and add the physics handle back because we are going to want it and we'll save the player. 113 00:08:55,860 --> 00:08:57,090 ‫Beep Great stuff. 114 00:08:57,090 --> 00:09:03,570 ‫So we've figured out how we can get hold of other components from within a component and we've learnt 115 00:09:03,570 --> 00:09:08,820 ‫how to deal with a null pointers and why we need to be careful. 116 00:09:08,850 --> 00:09:10,500 ‫I'll see you in the next lecture.