1 00:00:03,670 --> 00:00:05,900 Hello, everyone, and welcome back. 2 00:00:05,920 --> 00:00:11,890 In this lesson, we are going to continue to dive deeper into angular on pushing detection and see how 3 00:00:11,890 --> 00:00:15,330 this change detection works with observable streams. 4 00:00:15,460 --> 00:00:21,170 Now, in the previous example we were using here the courses file containing here's some data we were 5 00:00:21,170 --> 00:00:23,960 applying here, the data directly on the template. 6 00:00:24,070 --> 00:00:28,010 Let's now refactor this and fetch the data from the back end. 7 00:00:28,210 --> 00:00:30,800 Again, this will give us an observable data stream. 8 00:00:30,820 --> 00:00:36,770 We're going to pass it to the template and see how angular on pushchairs detection handles that. 9 00:00:36,910 --> 00:00:38,470 So let's remove here. 10 00:00:38,470 --> 00:00:41,230 This course is variable and let's define here. 11 00:00:41,380 --> 00:00:47,560 The course is observable like before, which is an observable, of course, array, meaning that the 12 00:00:47,560 --> 00:00:51,580 value emitted by these observable is an array of courses. 13 00:00:51,610 --> 00:00:55,940 Now let's populate the courses variable here on engy on init. 14 00:00:55,990 --> 00:01:02,440 We are going to assign the value returned here by the course service whenever we call the low with courses 15 00:01:02,440 --> 00:01:03,040 function. 16 00:01:03,190 --> 00:01:05,800 Now we want to display this data on the screen. 17 00:01:05,800 --> 00:01:11,260 Let's for that first remove here the implementation of unpadded course because we are getting here a 18 00:01:11,260 --> 00:01:15,930 compilation error and we are going to see how can we display this data on the screen. 19 00:01:15,940 --> 00:01:21,000 Let's take the courses observable and let's use it here directly in the template. 20 00:01:21,010 --> 00:01:27,160 So just like we have done before, we are going to apply here NGF and we are going to take the courses 21 00:01:27,160 --> 00:01:30,280 observable and we are going to apply it to the async pipe. 22 00:01:30,460 --> 00:01:35,560 We are going to take the result of evaluating this expression and we are going to define this as being 23 00:01:35,560 --> 00:01:38,160 the courses local template variable. 24 00:01:38,210 --> 00:01:39,070 Let's try this out. 25 00:01:39,100 --> 00:01:42,040 If we were lower the application, we are going to see that again. 26 00:01:42,040 --> 00:01:45,320 We have here our data working as expected. 27 00:01:45,370 --> 00:01:51,250 Let's remember that in these implementation, we are using here normal change detection here at the 28 00:01:51,250 --> 00:01:53,000 level of the application component. 29 00:01:53,140 --> 00:01:58,720 Let's see if we would still get this list of courses on the screen, if we would use, for example, 30 00:01:58,720 --> 00:02:00,530 on push change detection. 31 00:02:00,550 --> 00:02:06,010 So if we try this out, we are going to see that we still have here the data being displayed correctly 32 00:02:06,010 --> 00:02:06,790 on the screen. 33 00:02:06,820 --> 00:02:13,930 This means that unposed change detection will look not only at the inputs of each component to determine 34 00:02:13,930 --> 00:02:17,620 if something has changed and the component needs to be rendered. 35 00:02:17,620 --> 00:02:24,430 But on pushchairs, detection will also look at any observable streams that are registered in the template 36 00:02:24,430 --> 00:02:30,520 using the async pipe and see if a new value got emitted by the registered observable. 37 00:02:30,700 --> 00:02:36,370 This means that with on push change detection, there are two ways to push data to a component, either 38 00:02:36,370 --> 00:02:42,880 via input properties of the component or via observable streams via the async pipe. 39 00:02:42,880 --> 00:02:48,400 Both ways of pushing data to the component will trigger the rendering of the component. 40 00:02:48,460 --> 00:02:54,320 Let's now see what will happen in the case of push change detection if we would not be using here via 41 00:02:54,320 --> 00:02:55,200 a sync pipe. 42 00:02:55,210 --> 00:03:00,970 So let's see that instead of having here, this course is observable directly being passed to the template 43 00:03:00,990 --> 00:03:07,640 that instead we will have here at course's member variable D member variable is of course, array. 44 00:03:07,660 --> 00:03:10,860 So this is the variable that we are going to be passing to the template. 45 00:03:10,990 --> 00:03:16,960 Now, instead of assigning this call to the courses observable, let's instead take the return, the 46 00:03:16,960 --> 00:03:19,740 observable and we we're going to subscribe to it directly. 47 00:03:19,900 --> 00:03:25,330 We are going to receive here the courses value each time that the observable emits a value. 48 00:03:25,330 --> 00:03:31,490 And we are going to take this value and we are going to assign it to our new member variable courses. 49 00:03:31,510 --> 00:03:35,070 So in order to avoid any confusion, let's comment out here. 50 00:03:35,080 --> 00:03:37,240 The course is observable member variable. 51 00:03:37,240 --> 00:03:42,310 So we have just refactor our components so that we don't have here and observable member variable. 52 00:03:42,310 --> 00:03:45,310 Instead, we have a plain member variable. 53 00:03:45,310 --> 00:03:50,230 It's just an array of courses and we're going to try to pass these to the component, just like we were 54 00:03:50,230 --> 00:03:51,020 doing before. 55 00:03:51,130 --> 00:03:57,580 So instead of using here the async pipe, we are simply going to come out this expression and we are 56 00:03:57,580 --> 00:04:03,370 going to see that the courses member variable is already available here at the level of the component. 57 00:04:03,520 --> 00:04:07,480 This means that if we try this out, we could expect to see the same result. 58 00:04:07,480 --> 00:04:09,310 But let's see what happens. 59 00:04:09,520 --> 00:04:15,280 So if we reload here, the application, we can see that we have no data getting filled in on the screen, 60 00:04:15,400 --> 00:04:18,839 even though we are populating here, the courses member variable. 61 00:04:18,940 --> 00:04:20,490 So what is going on here? 62 00:04:20,500 --> 00:04:23,980 Why is the data not getting displayed this time around? 63 00:04:24,010 --> 00:04:28,760 Well, the problem is that this component is using on push change detection. 64 00:04:28,810 --> 00:04:34,540 Again, we fall into a situation where even though it's more performant to use on push, we run into 65 00:04:34,540 --> 00:04:37,630 situations where the view does not display the data. 66 00:04:37,640 --> 00:04:42,910 So we are going to comment out here on push change detection or you're going to see if this would work 67 00:04:42,910 --> 00:04:44,710 with default change detection. 68 00:04:44,710 --> 00:04:47,290 And indeed, it does work correctly. 69 00:04:47,440 --> 00:04:53,200 So the way that default change detection works is every expression here on the template is going to 70 00:04:53,200 --> 00:04:55,990 get evaluated, see if there were any changes. 71 00:04:56,200 --> 00:05:00,200 This includes here the expression path to energy for which is the. 72 00:05:00,410 --> 00:05:06,470 Racecourses, because whenever the data arrives from the weekend, we assign it here to the cast member 73 00:05:06,470 --> 00:05:12,800 variable, we are going to do a switch of the value of these variable from undefined to a value containing 74 00:05:12,800 --> 00:05:18,200 an array of courses that just arrived from the backend that is going to trigger change detection, which 75 00:05:18,200 --> 00:05:22,490 means that the ANGULAR is going to render the template of this component. 76 00:05:22,490 --> 00:05:25,430 And then we see the data on the screen again. 77 00:05:25,430 --> 00:05:32,390 We can see the default change detection just works in the larger variety of scenarios without us having 78 00:05:32,390 --> 00:05:34,640 to reason about the mechanism itself. 79 00:05:34,730 --> 00:05:38,760 Let's not turn on on push again at the level of the application component. 80 00:05:38,840 --> 00:05:40,650 So again, the data is gone. 81 00:05:40,760 --> 00:05:47,600 What happens here is that the component is not getting notified in any way that it needs to be rendered. 82 00:05:47,630 --> 00:05:51,160 So we on push that can happen in three different ways. 83 00:05:51,170 --> 00:05:56,660 This could happen if an event would occur at the level of the application, either a DOM event that 84 00:05:56,660 --> 00:06:01,010 we have registered using the angular template syntax or a custom event. 85 00:06:01,070 --> 00:06:06,020 Another way that we could notify the application component that it needs to be rendered would be if 86 00:06:06,020 --> 00:06:08,020 some input data has changed. 87 00:06:08,030 --> 00:06:14,500 But this component here has no input member variables such as, for example, the cost curve component, 88 00:06:14,610 --> 00:06:20,300 the first way that the application component could be notified that it needs to be updated when using 89 00:06:20,300 --> 00:06:26,260 on Bush would be if unobservable registered with the sync pipe would trigger a new value. 90 00:06:26,360 --> 00:06:32,820 And these observable here that we are using is not being passed to the template using the sync pipe. 91 00:06:32,840 --> 00:06:39,440 So in summary, there is no way for the application component using unpunched change detection to know 92 00:06:39,440 --> 00:06:42,620 that it needs to be rendered with on Bush. 93 00:06:42,620 --> 00:06:47,290 We are no longer evaluating all the expressions here in the template. 94 00:06:47,300 --> 00:06:53,440 Instead, we are pushing data to the component, either via inputs or via the async by. 95 00:06:53,490 --> 00:06:57,080 So the component does not get updated in this case. 96 00:06:57,080 --> 00:07:04,100 If we know refactor our implementation to use again the async pipe by removing here the call to subscribe 97 00:07:04,250 --> 00:07:10,640 and then applying here the previous NGF expression with the use of the async pipe to the template. 98 00:07:10,640 --> 00:07:15,710 Then this time around, after reloading the application, we are going to see that we get the data again 99 00:07:15,710 --> 00:07:18,240 and that is because we are using the async pipe. 100 00:07:18,260 --> 00:07:25,370 So in summary, if you were working with observable data streams and you are using on change detection 101 00:07:25,370 --> 00:07:30,170 in some of your components, make sure to use the sync pipe as much as possible. 102 00:07:30,320 --> 00:07:36,710 This is one of the multiple benefits of using the async pipe it makes on push that much easier to reason 103 00:07:36,710 --> 00:07:37,130 about. 104 00:07:37,280 --> 00:07:43,880 And of course, the sync by will also take care of and registering from any observables when the component 105 00:07:43,880 --> 00:07:44,750 gets destroyed. 106 00:07:44,930 --> 00:07:48,070 So there are many advantages for using the async pipe. 107 00:07:48,260 --> 00:07:53,480 Let's now quickly summarize everything that we have learned about on push change detection. 108 00:07:53,570 --> 00:07:57,890 As we have seen in these examples on change detection is faster. 109 00:07:57,890 --> 00:08:01,490 It's more performance, but it's also trickier to use default. 110 00:08:01,490 --> 00:08:03,440 Change detection just works. 111 00:08:03,440 --> 00:08:05,360 We don't have to think about it. 112 00:08:05,360 --> 00:08:11,270 It simply keeps the data in sync with the view at all times in a transparent way. 113 00:08:11,420 --> 00:08:17,810 If by some reason you have to display a lot of data on the screen and you are running into performance 114 00:08:17,810 --> 00:08:24,170 issues while doing so, then you might consider using on push to solve that problem if by some reason 115 00:08:24,170 --> 00:08:26,040 you decide to use on push. 116 00:08:26,060 --> 00:08:28,850 Here are a couple of important recommendations. 117 00:08:28,880 --> 00:08:33,909 When working with observable data streams, use the sink pipe as much as possible. 118 00:08:33,950 --> 00:08:39,440 This is going to reduce the number of cases that you will run into where the view is not getting updated 119 00:08:39,440 --> 00:08:42,049 to reflect the latest changes in the data. 120 00:08:42,200 --> 00:08:48,650 Also, if you are using on PUSH, it's important to avoid to mutate the data directly at the level of 121 00:08:48,650 --> 00:08:54,520 the view, which is a good idea anyway, because it leads to programs that are easier to reason about. 122 00:08:54,530 --> 00:09:00,920 So you could consider making your data objects immutable either by freezing them before passing them 123 00:09:00,920 --> 00:09:01,520 to The View. 124 00:09:01,520 --> 00:09:04,220 So this is standard JavaScript functionality. 125 00:09:04,310 --> 00:09:07,950 You just make all the existing properties only. 126 00:09:08,000 --> 00:09:14,480 Alternatively, you can consider using a library such as Immutable G.S. or IMOD for the vast majority 127 00:09:14,480 --> 00:09:15,230 of use cases. 128 00:09:15,230 --> 00:09:21,500 I would recommend to you to use default angular change detection and to not use on push. 129 00:09:21,530 --> 00:09:25,570 But if you ever run into the need of using unpunched now you know how to do it. 130 00:09:25,580 --> 00:09:30,410 Let's not talk about another performance optimization that we have available in ANGULAR. 131 00:09:30,410 --> 00:09:32,570 It's the attribute decorator.