C. Keith Ray

C. Keith Ray is developing software for Sizeography and Upstart Technology. He writes code for multiple platforms and languages, including iOS® and Macintosh®.
Go to Sizeography and Upstart Technology to join our mailing lists and see more about our products. Keith's Résumé (pdf)

Saturday, April 15, 2017

Template Design Pattern in Swift

In the Template Design pattern, you write an abstract class where an algorithm is implemented, but at least one part of the algorithm is not implemented -- it calls an abstract/unimplemented method for some part of the algorithm. The subclass inherits from the abstract class, implementing the abstract method(s) but not changing the overall algorithm. 

I will show two ways to implement this design pattern.

First, using classes:

import Foundation

class TemplateClass { // "Abstract" class
    func doit() {
        before()
        during()
        after()
    }
    func before() {
        print("before")
    }
    
    func during() { // in C++ should be "pure virtual".
        fatalError("subclass must override")
    } // in Java, should be "abstract"
    
    func after() {
        print("after")
    }
}

class TemplateClassExample: TemplateClass {
    
    override func during() {
        print("during")
    }
}

var tx = TemplateClassExample()

tx.doit() // prints "before", "during", "after"

As you can see, Swift doesn't allow us to declare a method "abstract". It's similar to Objective-C and Smalltalk in this way. We can put in an assertion or throw an exception, but we don't have the compiler forcing us to override/implement the abstract method.

Second, using Protocols:

protocol TemplateProtocol {
    func during()
}

extension TemplateProtocol {
    func doit() {
        before()
        during()
        after()
    }
    func before() {
        print("before")
    }
    func after() {
        print("after")
    }
}

class TemplateProtocolExample: TemplateProtocol {

    internal func during() {
        print("during")
    }
}

var tp = TemplateProtocolExample()

tp.doit() // prints "before", "during", "after"

Here the protocol forces us to implement the abstract method, but we can't put the algorithm's concrete methods in the protocol declaration, they have to go in the protocol extension. This seems to be a rather arbitrary restriction, but it's probably reflecting the way Swift evolved. I don't think the earliest versions of Swift allowed extending protocols with extensions containing concrete methods, though obviously Swift 3.x does allow this.


Friday, August 12, 2016

CGRect Extensions in Swift

While I was implementing drawing code, I found I wanted several CGRect functions that did not exist in Swift 2 and Swift 3. This is the Swift 2 code and tests that I implemented.  The following explanation of that code can be copied into a playground if you desire.

/*
  CGRectExtensions.playground

  Created by C. Keith Ray on 2016.08.06.
  Copyright © 2016 by C. Keith Ray. All rights reserved.
  Permission is granted for anyone to use and modify this
  code for any purpose, as long as this copyright message
  is retained.
*/

import Cocoa
import CoreGraphics

/* These are the constructors available for CGRect:

 CGRect()
 CGRect(origin: CGPoint, size: CGSize)
 CGRect(x: CGFloat, y: CGFloat, width: CGFloat, 
     height: CGFloat)
 CGRect(x: Double, y: Double, width: Double, height: Double)
 CGRect(x: Int, y: Int, width: Int, height: Int)

 I'd like to be able to create CGRects given a height 
 and width, and a center-point. Creating a new constructor 
 isn't hard, we just add it to an extension.
*/

extension CGRect {
    public init(center: CGPoint, size: CGSize) {
        // This assumes width and height are positive numbers.
        let origin = CGPoint(x: center.x - size.width/2.0,
            y: center.y - size.height/2.0)
        self.init(origin: origin, size: size)
    }
}

let a = CGRect(center: CGPoint(x:25,y:15), size: CGSize(width:20,height:10))
print("a.origin.x = \(a.origin.x)")
print("a.origin.y = \(a.origin.y)")
print("a.size.width = \(a.size.width)")
print("a.size.height = \(a.size.height)")
/* prints:
a.origin.x = 15.0
a.origin.y = 10.0
a.size.width = 20.0
a.size.height = 10.0
*/

/*
 We'd also like to get the center of the rectangle easily.
*/

extension CGRect {
    public var center: CGPoint {
        get { return CGPoint(x: self.midX, y: self.midY) }
    }
}

print("a.center = \(a.center)")
// a.center = (25.0, 15.0)

/*
 I'd like to be able to get a rectangle that is close to
 an existing rectangle, but is a different size.
 For example, a taller and thinner rectangle at the same
 origin: ("inc" stands for "increment")

 let b = a.incHeightBy(5).incWidthBy(-5)

 Also, I don't want to modify the original rectangle. Using
 existing features we could do this instead:

 let b = a // copy a
 b.size.height += 5
 b.size.width -= 5

 but you can see that takes more lines of code.
*/

extension CGRect {
    public func incWidthBy(deltaX: CGFloat) -> CGRect {
        var result = self.standardized;
        result.size.width += deltaX
        return result
    }
    
    public func incHeightBy(deltaY: CGFloat) -> CGRect {
        var result = self.standardized;
        result.size.height += deltaY
        return result
    }
}

/* 
 We call self.standardized because rectangles can have 
 negative width and height (which puts the origin at the 
 opposite corner than usual) but we don't want to deal 
 with that in our math. The existing CGRect functions
 call self.standardized, too.
*/

let b = a.incHeightBy(5).incWidthBy(-5)
print("a = \(a)") // a = (15.0, 10.0, 20.0, 10.0)
print("b = \(b)") // b = (15.0, 10.0, 15.0, 15.0)

/*
 I want similar functions to change the origin. A 
 rectangle with the same width and height, but a 
 different origin can be gotten by:

 let c = a.incXBy(2).incYBy(3)

 Again, not changing the original rectangle. An 
 alternative would be:

 let c = a // copy a
 a.origin.x += 2
 a.origin.y += 3

 (Note: "incOriginBy(dx:dy:)" is essentially the 
 same as the already-existing function 
 "offsetBy(dx:dy:)")
*/

extension CGRect {
    public func incXBy(deltaX: CGFloat) -> CGRect {
        var result = self.standardized
        result.origin.x += deltaX
        return result
    }
    
    public func incYBy(deltaY: CGFloat) -> CGRect {
        var result = self.standardized
        result.origin.y += deltaY
        return result
    }
}

let c = a.incXBy(2).incYBy(3)
print("a = \(a)") // a = (15.0, 10.0, 20.0, 10.0)
print("c = \(c)") // c = (17.0, 13.0, 20.0, 10.0)

/*
 To be complete, we can also increment origin and size 
 using pairs of floats.
*/

extension CGRect {
    public func incOriginBy(dx deltaX: CGFloat
                            dy deltaY: CGFloat) -> CGRect {
        var result = self.standardized
        result.origin.x += deltaX
        result.origin.y += deltaY
        return result
    }
    public func incSizeBy(dx deltaX: CGFloat
                          dy deltaY: CGFloat) -> CGRect {
        var result = self.standardized;
        result.size.width += deltaX
        result.size.height += deltaY
        return result
    }
}

let d = CGRect(x: 10, y: 20, width: 30, height: 40)
let e = d.incOriginBy(dx: 5, dy: 10)
let f = d.incSizeBy(dx: 15, dy: 20)
print("d = \(d)")  // d = (10.0, 20.0, 30.0, 40.0)
print("e = \(e)")  // e = (15.0, 30.0, 30.0, 40.0)
print("f = \(f)")  // f = (10.0, 20.0, 45.0, 60.0)

/*
 We made center as a getter, but we can also make a 
 function that returns a new rectangle with a different 
 center. I'll call that newCenter.
*/

extension CGRect {
    public func newCenter(cxy: CGPoint) -> CGRect {
        let me = self.standardized
        let result = CGRect(center: cxy, size: me.size)
        return result
    }
}

let g = a.newCenter(CGPoint(x: 50, y: 30))
print("a.center = \(a.center)")
print("a = \(a)")
print("g = \(g)")
print("g.center = \(g.center)")
/* prints
a.center = (25.0, 15.0)
a = (15.0, 10.0, 20.0, 10.0)
g = (40.0, 25.0, 20.0, 10.0)
g.center = (50.0, 30.)
*/

/*
 and, last but not least, an overload of newCenter taking 
 x,y instead of CGPoint.
*/

extension CGRect {
    public func newCenter(newX newX: CGFloat, newY: CGFloat
            -> CGRect {
        let me = self.standardized
        let result = CGRect(center: CGPoint(x: newX, y: newY), 
                              size: me.size)
        return result
    }
}

let h = a.newCenter(newX: 50, newY: 30)
print("a.center = \(a.center)")
print("a = \(a)")
print("h = \(h)")
print("h.center = \(h.center)")
/* prints
a.center = (25.0, 15.0)
a = (15.0, 10.0, 20.0, 10.0)
h = (40.0, 25.0, 20.0, 10.0)
h.center = (50.0, 30.0)
*/

/*
 One CGRect function I don't need to implement is "insetBy",
 which reduces the size of a rectangle, but keeps the same 
 center by altering the origin.
*/

let i = h.insetBy(dx: 3, dy: 3)
print("h = \(h)")
print("i = \(i)")
print("i.center = \(i.center)")
print("h.center = \(h.center)")
/* prints
h = (40.0, 25.0, 20.0, 10.0)
i = (43.0, 28.0, 14.0, 4.0)
i.center = (50.0, 30.0)
h.center = (50.0, 30.0)
*/

//
//  CGRectExtensions.swift
//
//  Created by C. Keith Ray on 2016.08.06.
//  Copyright © 2016 by C. Keith Ray. All rights reserved.
//  Permission is granted for anyone to use and modify this
//  code for any purpose, as long as this copyright message
//  is retained.
//

import Foundation
import CoreGraphics

extension CGRect {
    
    public init(center: CGPoint, size: CGSize) {
        let origin = CGPoint(x: center.x - size.width/2.0,
            y: center.y - size.height/2.0)
        self.init(origin: origin, size: size)
    }
    
    public var center: CGPoint {
        get { return CGPoint(x: self.midX, y: self.midY) }
    }
    
    public func newCenter(cxy: CGPoint) -> CGRect {
        let me = self.standardized
        let result = CGRect(center: cxy, size: me.size)
        return result
    }
    
    public func newCenter(newX newX: CGFloat, newY: CGFloat
            -> CGRect {
        let me = self.standardized
        let result = CGRect(center: CGPoint(x: newX, y: newY), 
                              size: me.size)
        return result
    }
    
    public func incXBy(deltaX: CGFloat) -> CGRect {
        var result = self.standardized
        result.origin.x += deltaX
        return result
    }
    
    public func incYBy(deltaY: CGFloat) -> CGRect {
        var result = self.standardized
        result.origin.y += deltaY
        return result
    }
    
    public func incOriginBy(dx deltaX: CGFloat
                            dy deltaY: CGFloat) -> CGRect {
        var result = self.standardized
        result.origin.x += deltaX
        result.origin.y += deltaY
        return result
    }
    
    public func incWidthBy(deltaX: CGFloat) -> CGRect {
        var result = self.standardized;
        result.size.width += deltaX
        return result
    }
    
    public func incHeightBy(deltaY: CGFloat) -> CGRect {
        var result = self.standardized;
        result.size.height += deltaY
        return result
    }
    
    public func incSizeBy(dx deltaX: CGFloat
                          dy deltaY: CGFloat) -> CGRect {
        var result = self.standardized;
        result.size.width += deltaX
        result.size.height += deltaY
        return result
    }
}

//
//  CGRectExtensionsTests.swift
//
//  Created by C. Keith Ray on 2016.08.06.
//  Permission is granted for anyone to use and modify this
//  code for any purpose, as long as this copyright message
//  is retained.

import XCTest

class CGRectExtensionsTests: XCTestCase {
    
    override func setUp() {
        super.setUp()
    }
    
    override func tearDown() {
        super.tearDown()
    }
    
    func test_init_center() {
        let result = CGRect(center: CGPoint(
            x: 120.0y: 100.0), size: CGSize(width: 
            6.0, height: 4.0))
        let expected = CGRect(x: 120 - 3, y: 100 - 2
            width: 6.0height: 4.0)
        XCTAssertEqual(result, expected)
    }
    
    func test_center() {
        let input = CGRect(x: 10.0, y: 20.0, width: 
            100.0height: 200.0)
        let result = input.center
        let expected = CGPoint(x: 60.0, y: 120.0)
        XCTAssertEqual(result, expected)
    }
    
    func test_incXBy5() {
        let input = CGRect(x: 10.0, y: 20.0, width: 
            100.0height: 200.0)
        let result = input.incXBy(5.0)
        let expected = CGRect(x: 15.0, y: 20.0
            width: 100.0height: 200.0)
        XCTAssertEqual(result, expected)
    }
    
    func test_incXByMinus5() {
        let input = CGRect(x: 10.0, y: 20.0, width: 
            100.0height: 200.0)
        let result = input.incXBy(-5.0)
        let expected = CGRect(x: 5.0, y: 20.0
            width: 100.0height: 200.0)
        XCTAssertEqual(result, expected)
    }
    
    func test_incYBy5() {
        let input = CGRect(x: 10.0, y: 20.0, width: 
            100.0height: 200.0)
        let result = input.incYBy(5.0)
        let expected = CGRect(x: 10.0, y: 25.0
            width: 100.0height: 200.0)
        XCTAssertEqual(result, expected)
    }
    
    func test_incYByMinus5() {
        let input = CGRect(x: 10.0, y: 20.0, width: 
            100.0height: 200.0)
        let result = input.incYBy(-5.0)
        let expected = CGRect(x: 10.0, y: 15.0
            width: 100.0height: 200.0)
        XCTAssertEqual(result, expected)
    }
    
    func test_incOriginBy() {
        let input = CGRect(x: 10.0, y: 20.0, width: 
            100.0height: 200.0)
        let result = input.incOriginBy(dx: 5.0, dy: 6.0)
        let expected = CGRect(x: 15.0, y: 26.0
            width: 100.0height: 200.0)
        XCTAssertEqual(result, expected)
    }
    
    func test_incWidthBy() {
        let input = CGRect(x: 10.0, y: 20.0, width: 
            100.0height: 200.0)
        let result = input.incWidthBy(5.0)
        let expected = CGRect(x: 10.0, y: 20.0
            width: 105.0height: 200.0)
        XCTAssertEqual(result, expected)
    }
    
    func test_incHeightBy() {
        let input = CGRect(x: 10.0, y: 20.0, width: 
            100.0height: 200.0)
        let result = input.incHeightBy(5.0)
        let expected = CGRect(x: 10.0, y: 20.0
            width: 100.0height: 205.0)
        XCTAssertEqual(result, expected)
    }
    
    func test_incSizeBy() {
        let input = CGRect(x: 10.0, y: 20.0, width: 
            100.0height: 200.0)
        let result = input.incSizeBy(dx: 5.0, dy: 4.0)
        let expected = CGRect(x: 10.0, y: 20.0
            width: 105.0height: 204.0)
        XCTAssertEqual(result, expected)
    }
}