0 1 00:00:00,300 --> 00:00:05,730 So, now that we've had to look at what's the code inside our AppDelegate, this is a good time to go back 1 2 00:00:05,730 --> 00:00:11,250 into our TodoListvView Controller and fix all of these errors that we've got by switching over from a 2 3 00:00:11,250 --> 00:00:16,700 custom class that saved using plist to our Core Data data model. 3 4 00:00:16,830 --> 00:00:22,340 And in the process, we'll learn about how to create or rather save data using Core Data. 4 5 00:00:22,350 --> 00:00:28,500 So the first thing that we need to change is that instead of creating a newItem as a new object of 5 6 00:00:28,500 --> 00:00:33,710 item, we actually need to initialize our Core Data object a little bit differently. 6 7 00:00:33,780 --> 00:00:40,790 So if we say let newItem = item, which if you remember is our Core Data class, 7 8 00:00:40,860 --> 00:00:46,740 so if you hold down option and click on it, you can see it's an i object of the type 8 9 00:00:46,740 --> 00:00:47,550 NSManagedObject. 9 10 00:00:47,850 --> 00:00:55,830 And if we open a round bracket to initialize it, then we get to specify the context where this item is 10 11 00:00:55,830 --> 00:00:56,960 going to exist. 11 12 00:00:56,970 --> 00:00:58,380 So what is that context? 12 13 00:00:58,380 --> 00:01:03,550 Well, it's basically going to be the viewContext of our persistentContainer. 13 14 00:01:03,660 --> 00:01:09,510 So while we're inside the same class, it's easy to access this variable called persistentContainer. 14 15 00:01:09,690 --> 00:01:13,510 But if we're inside TodoListViewController, how do we create that context? 15 16 00:01:13,530 --> 00:01:26,550 We can just say let context = AppDelegate.persistentContainer.viewContext. And the 16 17 00:01:26,550 --> 00:01:30,240 reason why this doesn't work is because AppDelegate is a class, 17 18 00:01:30,300 --> 00:01:33,210 it's not the object of AppDelegate. 18 19 00:01:33,210 --> 00:01:39,050 So remember, when we refer to the TodoListViewController, for example, we won't try to call its 19 20 00:01:39,060 --> 00:01:42,670 methods or grab its properties by calling the class. 20 21 00:01:42,720 --> 00:01:46,300 Instead, we need to get the object of AppDelegate. 21 22 00:01:46,350 --> 00:01:51,420 So this would be the blueprint and we need to get the object that's created from the blueprint. 22 23 00:01:51,420 --> 00:01:59,580 So this, again, is where our handy singletons come into use. And instead of calling App Delegate, I'm instead 23 24 00:01:59,580 --> 00:02:04,190 going to tap into UIApplication.shared, 24 25 00:02:04,440 --> 00:02:07,030 and this is a singleton app instance. 25 26 00:02:07,050 --> 00:02:13,410 And at the time point when our app is running live inside the user's iPhone, then the shared UIApplication 26 27 00:02:13,470 --> 00:02:16,870 will correspond to our live application object. 27 28 00:02:16,900 --> 00:02:25,410 And inside this shared UIApplication object is something called delegate, and this is the delegate of 28 29 00:02:25,410 --> 00:02:26,610 the app object, 29 30 00:02:26,760 --> 00:02:29,190 alternatively known as the AppDelegate, 30 31 00:02:29,190 --> 00:02:33,600 and we're going to downcast it as our class AppDelegate. 31 32 00:02:33,600 --> 00:02:38,480 So let's just give you a bit more space to see that whole line as it is. 32 33 00:02:38,640 --> 00:02:41,790 So we're tapping into the UIApplication class. 33 34 00:02:41,790 --> 00:02:47,970 We're getting the shared singleton object which corresponds to the current app as an object, tapping 34 35 00:02:47,970 --> 00:02:53,410 into its delegate which has the data type of an optional UIApplicationDelegate. 35 36 00:02:53,430 --> 00:02:58,970 We're casting it into our class AppDelegate, because they both inherit from the same superclass 36 37 00:02:58,980 --> 00:03:00,350 UIApplicationDelegate. 37 38 00:03:00,390 --> 00:03:07,950 This is perfectly valid and we now have access to our AppDelegate as an object. 38 39 00:03:07,950 --> 00:03:13,650 So we're able to tap into its property code persistentContainer and we're going to grab the viewContext 39 40 00:03:14,160 --> 00:03:19,830 of that persistentContainer. So we can now say our context is this context. 40 41 00:03:19,830 --> 00:03:27,810 Now, the next error that we have is inside saveItems. And because we're no longer using the encoder, 41 42 00:03:27,810 --> 00:03:28,870 we can delete that, 42 43 00:03:29,220 --> 00:03:32,700 and we have to delete all of this as well. 43 44 00:03:32,730 --> 00:03:40,620 So what we ultimately need to do inside our saveItems method is we need to be able to commit our context 44 45 00:03:40,620 --> 00:03:43,790 to permanent storage inside our persistentContainer. 45 46 00:03:44,010 --> 00:03:51,210 And in order to do that, we need to call rye context.save. And that basically transfers what's currently 46 47 00:03:51,210 --> 00:03:53,770 inside our staging area or our scratchpad 47 48 00:03:53,790 --> 00:03:57,690 that's the context to our permanent data stores. 48 49 00:03:57,720 --> 00:04:04,130 So inside the do block, we need to be able to say context.save. 49 50 00:04:04,260 --> 00:04:07,060 Now, where are we going to get our context? 50 51 00:04:07,080 --> 00:04:11,840 Well, it's going to be exactly the same as this. 51 52 00:04:11,980 --> 00:04:18,250 So we can either copy and paste it over here or whenever you're copying and pasting the exact same pieces 52 53 00:04:18,250 --> 00:04:21,220 of code, then it's a good time point to evaluate. 53 54 00:04:21,220 --> 00:04:25,240 Maybe we should make it a shared constant up here. 54 55 00:04:25,320 --> 00:04:32,730 So, now instead of having this long line of code here, we can simply say let newItem = item 55 56 00:04:32,730 --> 00:04:37,630 the context is going to be self.context because we're inside a closure. 56 57 00:04:37,830 --> 00:04:42,960 And down here, we can simply say context.save and mark it with the "try" keyword. 57 58 00:04:43,110 --> 00:04:51,930 And inside the catch block, we're going to print and say "Error saving context" with this error message. 58 59 00:04:52,080 --> 00:05:00,210 So all that we've done so far is to set up the code to use Core Data for saving our new items that have 59 60 00:05:00,210 --> 00:05:03,150 been added using the UIAlert. 60 61 00:05:03,150 --> 00:05:09,330 So I'm going to comment out the load items function and I'm also going to comment out the part where 61 62 00:05:09,330 --> 00:05:14,930 we call that function. And if we want to be able to use item which, if you remember, is an NSManagedObject, 62 63 00:05:14,970 --> 00:05:18,320 so it's something that only exists in the Core Data framework. 63 64 00:05:18,360 --> 00:05:21,590 Then, of course, we also need to import Core Data, 64 65 00:05:21,810 --> 00:05:24,540 and now those errors will go away. 65 66 00:05:24,540 --> 00:05:28,600 So if I run the app now, you're going to see that we're going to come into an interesting bug. 66 67 00:05:28,650 --> 00:05:31,470 So when I hit Add, let's create a new item. 67 68 00:05:35,030 --> 00:05:41,060 And once I hit an add item, you can see it show up inside the table view, but that's only because it's being 68 69 00:05:41,120 --> 00:05:42,820 added to an array. 69 70 00:05:43,010 --> 00:05:51,170 And if we drag up our debug console, we'll see the reason. And it gives us an error with a code, and it says, 70 71 00:05:51,350 --> 00:05:53,030 "Error saving context." 71 72 00:05:53,120 --> 00:06:00,080 So that, of course, refers to this line here. And this is the error that we're catching because we wrote 72 73 00:06:00,080 --> 00:06:02,020 this "Error saving context," remember? 73 74 00:06:02,270 --> 00:06:10,220 And the error message is "The operation couldn't be completed" because if we have a look at our data, we've 74 75 00:06:10,220 --> 00:06:13,250 got this item called done and it's equal to nil. 75 76 00:06:13,250 --> 00:06:16,910 So because our item done is not optional, 76 77 00:06:16,910 --> 00:06:21,830 we have to provide it with a value when we create our newItem. 77 78 00:06:21,860 --> 00:06:29,690 So over here, in addition to adding a title, we're going to set the done to false for every single new 78 79 00:06:29,690 --> 00:06:32,110 item that we add into our table view 79 80 00:06:32,150 --> 00:06:35,630 which makes sense, right? Because everything starts off being not done. 80 81 00:06:35,900 --> 00:06:39,220 So let's run our app again and see if this now works. 81 82 00:06:39,230 --> 00:06:44,850 So let's add a new item. 82 83 00:06:45,030 --> 00:06:52,200 And now you can see that the item has been added to the table view and we don't get any errors in our 83 84 00:06:52,200 --> 00:06:54,580 debug console from that catch block. 84 85 00:06:54,850 --> 00:07:01,830 So the problem, though, is that right now, we haven't yet got code that loads our items from our Core Data 85 86 00:07:01,840 --> 00:07:02,640 data model. 86 87 00:07:02,830 --> 00:07:10,150 And if I terminate this app and I reopen it, you can see that we have no items in here. But we did save 87 88 00:07:10,150 --> 00:07:13,770 our data using Core Data and we didn't get any errors, 88 89 00:07:13,780 --> 00:07:15,330 so it succeeded. 89 90 00:07:15,640 --> 00:07:17,160 So how can I prove this to you? 90 91 00:07:17,260 --> 00:07:23,620 Well, in the next lesson, I'm going to show you how you can view your SQLite database and see the 91 92 00:07:23,620 --> 00:07:25,720 items that get saved into it. 92 93 00:07:26,140 --> 00:07:28,890 So for all of that and more, see you on the next lesson.