Saturday, February 13, 2021

How to run an Xcode project on an iOS device without payment for the US$99 Apple Developer account (2021)

To run an Xcode project on an iPhone/iPad without paying US$99 for Apple Developer account, this error message may happen:

Your development team has reached maximum number of registered iPhone devices.

According to StackOverflow, I got this warning because I have a previous paid account but I cannot configure it at this stage. So I register a new and free Apple Developer account. and add it to Xcode with Preferences->Accounts.

The project with Xcode 11.6 is configured as below:

After selecting the new Apple Developer account as the team and making sure that the bundle identifier is new, the deployment becomes successful!


Development team has reached maximum number of registered iPhone devices (StackOverflow)

How to Test iOS App without Developer Account?

Run an Xcode project on an iOS device without US$99 Apple Developer account (Configuration for iPhone) (StudySwift)

Saturday, February 6, 2021

Xcode 11.6 (Swift 5.2.4) - Hello World and how to set the background color programmatically


import SwiftUI

struct ContentView: View {

    let screenWidth = UIScreen.main.bounds.width

    var body: some View {

        ZStack {



            Text("Hello, World!")


            Text("Study Swift")

            .position(CGPoint(x: screenWidth/2, y: 400))





struct ContentView_Previews: PreviewProvider {

    static var previews: some View {






How to set a background color for the viewController in swiftUI? (StackOverflow)

Wednesday, September 30, 2020

Thursday, August 27, 2020

Mac Technique: Ctrl + Alt + Del command for Windows on Mac

For using the Ctrl + Alt + Del command for Windows with a Mac keyboard, hold the following keys:

Control + option (alt) + fn + delete


Ctrl + Alt + Del on a Mac with Windows (superuser)

Mac Technique: Delete Key for Windows on Mac

Thursday, June 7, 2018

Complex numbers - simple calculations

This example is tested with Xcode 9.2 playground in Swift 4.0.3. Basic complex number calculations such as the magnitude are includes as below:

class ComplexNum {
    var real : Float
    var imag : Float
    init (real: Float, imag: Float){
        self.real = real
        self.imag = imag
    func show()->String {
        let plusChar = (imag>0) ? "+" : "-"
        return "\(real) \(plusChar) \(abs(imag))i"
    func mag()->Float {
        return sqrt(self.real*self.real+self.imag*self.imag)
    //Quick calculation with (i)
    func multiplyPlusI(num:ComplexNum) -> ComplexNum{
        return ComplexNum(real: -num.imag, imag: num.real)
    //Quick calculation with (-i)
    func multiplyMinusI(num:ComplexNum) -> ComplexNum{
        return ComplexNum(real: num.imag, imag: -num.real)
func addComplex(a: ComplexNum, b: ComplexNum)->ComplexNum {
    return ComplexNum(real: a.real+b.real, imag: a.imag+b.imag)
func multiplyComplex(a: ComplexNum, b: ComplexNum)->ComplexNum {
    return ComplexNum(real: a.real*b.real-a.imag*b.imag, imag: a.real*b.imag+a.imag*b.real)

let x = ComplexNum(real: 3, imag: 1)
print("x = \(")
print("|x| = \(x.mag())")
print("x(j) = \(x.multiplyPlusI(num: x).show())")
print("x(-j) = \(x.multiplyMinusI(num: x).show())")

let y = ComplexNum(real: -1, imag: -2)
print("y = \(")
print("|y| = \(y.mag())")
print("y(j) = \(x.multiplyPlusI(num: y).show())")
print("y(-j) = \(x.multiplyMinusI(num: y).show())")

print("x + y = \(addComplex(a: x, b: y).show())")
print("x * y = \(multiplyComplex(a: x, b: y).show())")


x = 3.0 + 1.0i
|x| = 3.16228
x(j) = -1.0 + 3.0i
x(-j) = 1.0 - 3.0i
y = -1.0 - 2.0i
|y| = 2.23607
y(j) = 2.0 - 1.0i
y(-j) = -2.0 + 1.0i
x + y = 2.0 - 1.0i
x * y = -1.0 - 7.0i

Related Information:
Matlab: Real and imaginary parts of complex number and multiplication with imaginary unit j

Friday, April 6, 2018

Mac Technique: How to Enable the Hidden Security & Privacy Option to Allow Apps from Unidentified Developers

With macOS High Sierra 10.13.4, there is no option by default to allow apps from unidentified developers in  -> System Preferences -> Security & Privacy -> General:

Close the window and type the following command in terminal:

sudo spctl --master-disable

and enter your password. Now go back to and the previously hidden Anywhere option is displayed:

Note that if you change a more secure option, the Anywhere option is hidden again when you turn on the Security & Privacy window next time.

Monday, April 2, 2018

Mac Technique: Dr. Eye Chinese-English Bilingual Dictionary with macOS 10.13.4 譯典通英漢雙向字典

Dr. Eye (譯典通), a famous Chinese-English bidirectional dictionary, is now available free with macOS High Sierra 10.13.4.

After installing the latest version of macOS, select Dictionary -> Preferences and 譯典通英漢雙向字典(Traditional Chinese-English).

Now both English and Chinese words may be looked up in the dictionary:

It is interesting that the Bopomofo (ㄅㄆㄇㄈ) Zhuyin Mandarin Phonetic Symbols (注音符號) used in Taiwan are displayed in this dictionary.

The Dr. Eye dictionary is also available with iOS 11.3.