WEBVTT 00:00.150 --> 00:00.720 Hello again! 00:01.080 --> 00:03.870 In this video, we are going to add a ball to our game. 00:04.920 --> 00:09.810 We are going to do this by writing a sprite which can move between updates. 00:10.260 --> 00:16.620 So in one frame, it is going to have some position on the screen, and in the next frame, it will have a different 00:16.680 --> 00:17.220 position. 00:20.040 --> 00:23.940 There are going to be more than one of these sprites which can move around. 00:24.240 --> 00:26.580 So let's write a class to represent them. 00:27.480 --> 00:33.930 Moving underscore entity will be a subclass of entity, and it represents entities which can move on 00:33.930 --> 00:34.410 the screen. 00:35.250 --> 00:37.560 It has a member for the velocity. 00:37.860 --> 00:39.840 That is the amount and the direction 00:39.900 --> 00:45.240 that the entity will move between each update. And we will store that in a Vector2f. 00:45.900 --> 00:48.150 And this is also going to be an abstract base class. 00:51.120 --> 00:53.940 So the hierarchy, as we have it at the moment, looks like this. 00:54.510 --> 00:56.160 We have the entity base class. 00:56.910 --> 01:00.690 We have the concrete background class, which inherits from entity. 01:01.650 --> 01:05.040 We have moving_entity, which is also an abstract base class. 01:05.760 --> 01:09.690 And then we have the ball, which is a concrete child of moving_entity. 01:11.990 --> 01:14.300 So here is our entity hierarchy. 01:14.300 --> 01:19.130 We have the base class which we had before with the pure virtual member functions. 01:20.570 --> 01:24.290 We have the moving_entity class which inherits from entity. 01:24.980 --> 01:27.350 It has a member for the velocity. 01:28.250 --> 01:30.980 It does not implement any of the pure virtual functions. 01:31.310 --> 01:36.560 So this is itself an abstract base class. And any class which inherits from this has to implement 01:36.560 --> 01:37.910 the virtual member functions. 01:40.740 --> 01:42.030 In the ball class. 01:42.060 --> 01:49.080 This is quite similar to background, except now we inherit from moving_entity, and not just entity. Se have 01:49.080 --> 01:57.390 a static texture member again. We have a constructor and then we implement the virtual member functions. 02:00.300 --> 02:01.320 In the source file. 02:01.320 --> 02:02.610 The constructor is very similar. 02:03.000 --> 02:05.370 We load the image file into the texture. 02:05.700 --> 02:07.500 Then we load the texture into the sprite. 02:08.160 --> 02:13.740 We set the initial position of the sprite, and then we set the initial velocity. 02:15.070 --> 02:21.910 I have added another constant to the constants file. The game parameters. So that controls the speed 02:21.910 --> 02:22.390 of the ball. 02:22.720 --> 02:29.230 So this means the ball is going to move six pixels to the right, and six pixels downwards, on each update. 02:30.550 --> 02:35.950 Then when we update the position of the ball, we need to add this velocity to the present position. 02:36.700 --> 02:38.740 The sprite has a member function, which will do this. 02:39.010 --> 02:45.730 So we call sprite dot move and pass the velocity, and that will correctly calculate the new position. And 02:45.730 --> 02:48.430 then we draw the ball, in the same way that we did for the background. 02:48.760 --> 02:50.220 We get the window to do it for us. 02:52.330 --> 02:54.030 The main function is very similar. 02:54.040 --> 02:56.080 We include the ball header. 02:56.560 --> 03:01.090 The background code is the same as it was before, so I do not need to go over that again. 03:02.380 --> 03:05.230 We create the background object, then the ball object. 03:05.920 --> 03:08.410 We are going to put this in the middle of the screen, to start with. 03:09.820 --> 03:12.670 Then we have our usual set up and loop. 03:14.170 --> 03:19.570 When we get to the update, we update the background and now we also update the ball as well. 03:20.350 --> 03:24.760 Then we draw the background in the buffer, and we draw the ball in the buffer as well. 03:24.910 --> 03:28.210 Then, once we have finished building up the next frame, we can display it. 03:29.320 --> 03:30.580 So let's see what happens. 03:33.570 --> 03:33.780 Hmm. 03:34.170 --> 03:34.770 What happened there? 03:37.600 --> 03:39.310 (Just move myself over a bit) 03:41.390 --> 03:46.340 So the ball starts off in the middle. And it goes down six, and across six. And it keeps on going. 03:47.690 --> 03:52.430 You may need to slow down the video replay rate on your interface, to see this. 03:54.870 --> 03:55.340 So there it is. 03:55.950 --> 04:00.630 The problem is there is nothing to say that, when the ball reaches the edge of the screen, it must stop, 04:00.630 --> 04:01.980 or come back, or whatever. 04:02.550 --> 04:03.810 So that is something we need to add. 04:04.920 --> 04:05.220 Okay. 04:05.220 --> 04:06.300 So that is it for this video. 04:06.600 --> 04:07.410 I will see you next time. 04:07.410 --> 04:09.390 But until then, keep coding!