Managing Gates

For the gate logic, we're going to create a separate type entirely 0:01 as a nested object in control tower just like runway and terminal. 0:04 Here's what our code needs to do. 0:09 We have a set of gates for 0:11 each terminal that we need to query every time a plane requests a space. 0:12 The information about terminals is encoded in the terminal type. 0:17 So we could associate the terminal's gates with each enum member. 0:21 Now one way we can do this is to use computed properties, so for 0:26 example, we could have a computed property that returns the list of gates for 0:30 terminal A, B, C, and so on. 0:34 But unfortunately, that has its limits. 0:37 A computed property has no storage in memory and either returns a static value 0:41 every time or one that is computed from stored properties. 0:46 We cannot use a computed property because we can't return the same gates every time. 0:51 Let's say Terminal A had three gates, 1 through 3. 0:57 An airline requests to park, and so we say go ahead and park at A1. 1:02 If we use a computed property for the gates, 1:07 we're going to return the gates 1 through 3 every time. 1:09 Enums can't contain stored properties either. 1:14 So, to get around this, 1:17 we're going to encode the gate information in a different type altogether. 1:18 There are other ways to do this, but because I want to show you certain 1:23 things about protocols and enums, we'll go down this round. 1:27 The nice thing though is the way we're about to implement gate logic, 1:31 we're going to learn a few more things about Swift in the process. 1:34 So, right after the terminal enum, let's add a new type, 1:39 and we'll name this GateManager. 1:43 And this is going to be a class. 1:46 We're going to add four stored properties to this class to keep track of the status 1:51 of various gates in the terminal. 1:56 I'm going to copy paste this in, and 1:59 as always, you can grab the code from the link in the teacher's notes as well. 2:01 For these properties, we're using good old dictionaries and 2:06 arrays to keep track of what gates are available. 2:09 The first property, for example, gates for 2:13 terminal A keeps track of the gates for terminal A obviously. 2:15 The property contains a dictionary on the outside with two keys, occupied and empty. 2:21 Using each key gets you an array of integers 2:30 pertaining to the different gates available. 2:33 Now there are ways we can improve this rather than using nested collection types, 2:36 but I'm going to set that as an exercise for you. 2:41 Get that brain of yours thinking. 2:44 So the logic here is pretty simple. 2:47 When Delta Airlines requests landing, 2:49 we know it's Terminal A based on the logic we wrote earlier. 2:52 So we queried the gatesForTerminalA property. 2:56 Using the key empty, that's this one here, we check if there are any empty gates. 3:00 If yes, we return the value. 3:06 Otherwise, we return nil. 3:08 When an empty gate is assigned to a flight, 3:10 we need to update the stored property. 3:12 So let's say we assign gate 9 for the flight to land at or park at. 3:14 Well, now 9 isn't empty anymore, so 3:19 we need to move 9 over into the occupied list. 3:21 So to start, in this class, let's write two methods to get this work done. 3:24 So first, we'll write a method to update the gate. 3:29 So we'll say func update. 3:34 This method is going to take a property that lists the gates. 3:38 So now this type is this thing here, string, 3:43 a dictionary with strings as keys and then array of integers as a value. 3:47 And then it will also take a gate as an integer. 3:54 Given a particular date, we want to switch it from the empty to the occupied array. 3:59 So let's say we call this method update. 4:05 And for gates, we pass in gates for terminal A, and 4:07 the gate number as number 9. 4:11 We want to remove the gate 9. 4:14 We want to remove the value 9 from the empty array and 4:17 add it to the occupied array. 4:20 So the first thing we'll do in our method is get the list of occupied gates for 4:23 a particular terminal. 4:27 So we'll say if var occupiedGates = gates, 4:29 and we'll use the key occupied. 4:35 The gate's dictionary here represents the collection of gates for a terminal. 4:44 And using the occupied key, we can get all the occupied gates. 4:50 You'll notice that we're doing something we've never done before, and 4:54 that's specifying if var instead of if let. 4:57 if var creates a variable that can be modified, 5:01 whereas let creates a constant value. 5:04 The reason we want a variable is now that we have an array, 5:07 we're going to append the gate value. 5:11 So this occupied gates is now this list or this array of gates, 5:13 and to that, we're going to append the new gate. 5:19 That's this thing right here, saying append(gate). 5:24 Now we've taken the gate and assigned it to the occupied list. 5:28 You'll notice we didn't modify the array contained within the dictionary directly. 5:32 We created a copy by assigning it to a local constant or 5:37 local variable when we modify that variable. 5:41 So now, now that we've modified it, we're going to use updateValue forKey 5:44 on the gate's dictionary and reassign this array as a value. 5:50 So we'll say gates.updateValue forKey. 5:54 And for the value, we are going to pass in the occupiedGates, and for the key, 6:00 we'll say occupied. 6:04 This does not work however, and the reason for that is the gate's value. 6:09 This right here is a function argument as you can see here. 6:14 And function arguments are immutable within the body of a function. 6:18 We can work around this by specifying var in front of the parameter, 6:23 but this is a temporary solution. 6:27 If you click on this, it says that var parameters are deprecated and 6:30 will be removed in Swift three. 6:34 So let's delete that. 6:36 To future proof our code, what we can do is pass by reference. 6:40 And we do this using the inout keyword in front of the parameter. 6:45 Now, previously, before we added this, if we call this method, update, and 6:51 passed in gatesForTerminalA, what we actually pass in is a copy of this value. 6:56 Remember that Swift dictionaries are value types. 7:02 By adding the inout keyword in front of our parameter, 7:06 now when we call the method, we are actually passing by reference. 7:09 So again, if we call this method and pass in gatesForTerminalA as the argument, 7:14 by modifying gates inside the body of the method, we're modifying gates for 7:19 Terminal A directly. 7:23 Okay, so now this works. 7:26 Now on the flip side, now that we've added the gate to the occupied list, 7:28 we also need to remove it from the empty list. 7:31 Now this code is pretty straightforward. 7:34 First we get the list of empty gates using the empty key. 7:36 Then we get the index of the value we're trying to remove. 7:45 So we'll chain this and say, 7:49 let index = emptyGates.indexOf and we'll pass in the gate. 7:51 Now once we have these two values, we can easily remove the gate by saying, 7:59 emptyGates.removeAtIndex, and pass in the index that we just obtained. 8:04 Now our emptyGates actually reflects the updated status of the gates. 8:12 So we can assign this back to the gates property by saying 8:16 updateValue forKey and using the relevant key value pair. 8:21 So far so good. 8:30 While this method doesn't actually determine if a gate is empty, 8:32 it ensures we're not assigning the same gate to two flights. 8:35 In the next video, let's implement the actual gate sorting logic and 8:39 finish off the gate manager. 8:43