COMP327 Mobile Computing Session:

Transcription

1 COMP327 Mobile Computing Session: Lecture Set 5a - + Comments on Lab Work & Assignments [ last updated: 22 October 2018 ] 1

2 In these Slides... We will cover... Additional Swift 4 features Any and AnyObject Casting Closures Assertions Yet More Swift These slides cover some additional Swift features. 2

3 Any and AnyObject Originally mainly for interfacing with Objective C Not used so much now* AnyObject - can only refer to a Class Swift is strongly typed - you ll have to cast an object retrieved via one of these to an actual Type. *e.g. used now with UserDefaults when retrieving a small data object 3

4 Casting Swift is strongly typed. Sometimes its necessary to cast one type as another. e.g. Array as NSArray to get at the extended functionality available for NSArray This isn t the same sort of thing as e.g. let z = String(5) // Int into a string compared with let y = [5,19,7,23] let x = y as NSArray 4

5 Closures Self-contained blocks of code that can be passed around and used in your code. Can capture and store references to constants or variables from the context in which they re defined (aka closing over those variables) Three kinds: global functions - have a name & cannot capture any values nested functions - have a name & can capture values from their enclosing functions closure expressions - don t have a name & can capture values from their context 5

6 Closure Expression Syntax { ( parameters ) -> return type in } statements { ( parameters ) in } statements No return value? Can leave out that part of the closure parameters can be in-out (but can t have a default value) We can potentially use a variable number of arguments as long as they are named. Tuples can be used as parameters and as return types. 6

7 Example of a Closure Expression The Swift standard library features a method called sorted(by:) It returns a new sorted array of the same type and size as the input array (which itself isn t modified) It accepts a closure that takes two arguments of the same type as the array s contents, and which returns a boolean indicating if item 1 > item 2 7

8 Closure example: Reverse sort of a string array let names = ["Katie", "Boris","Frans","Terry","Valli"] func backward(_ s1: String, _ s2: String) -> Bool { return s1 > s2 } var reversednames = names.sorted(by: backward) print(reversednames) ["Valli", "Terry", "Katie", "Frans", "Boris"] 8

9 Closure example: Reverse sort of a string array { ( parameters ) -> return type in statements } let names = ["Katie", "Boris","Frans","Terry","Valli"] var reversednames = names.sorted(by: { (s1: String, s2: String) -> Bool in return s1 > s2 }) print(reversednames) ["Valli", "Terry", "Katie", "Frans", "Boris"] Note that the parameters and return type are written inside the curly brackets, not outside of them 9

10 Closure example: Reverse sort of a string array { ( parameters ) -> return type in statements } let names = ["Katie", "Boris","Frans","Terry","Valli"] var reversednames = names.sorted(by: { s1, $0 > s2 $1 in }) return s1 > s2 s1 }) > s2 }) print(reversednames) ["Valli", "Terry", "Katie", "Frans", "Boris"] 10

11 Trailing Closures If you need to pass a long closure expression to a function as it s final argument, you can write it as a trailing closure. func somefunctionthattakesaclosureasaparameter(closure: () -> Void) { // function body goes here } // Here's how you call this function without using a trailing closure: somefunctionthattakesaclosureasaparameter(closure: { // closure's body goes here }) // Here's how you call this function with a trailing closure instead: somefunctionthattakesaclosureasaparameter() { // trailing closure's body goes here } 11

12 Trailing Closure example: Reverse sort of a string array let names = ["Katie", "Boris","Frans","Terry","Valli"] var reversednames = names.sorted() { { $0 $0 > > $1 $1 } } print(reversednames) ["Valli", "Terry", "Katie", "Frans", "Boris"] 12

13 Trailing Closure longer example: implementing map(_: ) let digitnames = [ 0: "Zero", 1: "One", 2: "Two", 3: "Three", 4: "Four", 5: "Five", 6: "Six", 7: "Seven", 8: "Eight", 9: "Nine" ] let numbers = [16, 58, 510] let strings = numbers.map { (number) -> String in var number = number var output = "" repeat { output = digitnames[number % 10]! + output number /= 10 } while number > 0 return output } print(strings) ["OneSix", "FiveEight", "FiveOneZero"] 13

14 Assertions assert(() -> Bool, message ) Executes the closure - if that returns true, your program exits. Used to debug code If you leave them in your code and compiler for the App Store they are deactivated 14

15 Comments on Lab Work Think about efficiency when writing your App Don t perform unnecessary calculations etc. e.g. in CellForRowAt IndexPath Don t cache too much information (though remember that network connectivity may not be instantaneous). Any unnecessary work wastes (precious) battery Delegation!! 15

16 Your ViewController is the delegate of the Table View. It adopted the protocols and therefore receives messages to deal with certain events on it s behalf e.g. to supply the contents of a cell when a table view is scrolled 16

17 Initially, you will only be required to provide content for the visible cells. If other cells become visible (e.g. through scrolling), then you will be asked at that point to provide their contents Cells that are not visible are not requested from you. It is up to you to decide how much work to do in advance, to support those cells potentially becoming visible. 17

18 Comments on Lab Work Use playgrounds to test out code / algorithms By all means use examples etc. from stackoverflow.com, but try if possible to understand what the code is actually doing When the compiler spots an error and offers to fix it, try to understand what it thinks the error is. It may not be correct (and it s fix might make things worse). 18