How to do non-blocking keyboard input on console app using Swift? - swift

Question

I want to make a simple console game using Swift 5. I need to read keyboard input without blocking the game animation (using emojis). The game keeps on going while there's no keyboard input, but will react accordingly if there are ones.
I've seen some example how to do it in other languages such as C and Python. I knew Swift has Darwin module that provide many POSIX functions. However, those C codes seem incompatible with Swift 5.
For example, how to convert the C code below into Swift? There's no FD_ZERO nor FD_SET in Darwin module.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/select.h>
#include <termios.h>
struct termios orig_termios;
void reset_terminal_mode()
{
tcsetattr(0, TCSANOW, &orig_termios);
}
void set_conio_terminal_mode()
{
struct termios new_termios;
/* take two copies - one for now, one for later */
tcgetattr(0, &orig_termios);
memcpy(&new_termios, &orig_termios, sizeof(new_termios));
/* register cleanup handler, and set the new terminal mode */
atexit(reset_terminal_mode);
cfmakeraw(&new_termios);
tcsetattr(0, TCSANOW, &new_termios);
}
int kbhit()
{
struct timeval tv = { 0L, 0L };
fd_set fds;
FD_ZERO(&fds);
FD_SET(0, &fds);
return select(1, &fds, NULL, NULL, &tv);
}
int getch()
{
int r;
unsigned char c;
if ((r = read(0, &c, sizeof(c))) < 0) {
return r;
} else {
return c;
}
}
int main(int argc, char *argv[])
{
int key;
printf("press a key: ");
fflush(stdout);
set_conio_terminal_mode();
while (1) {
if (kbhit()) {
key = getch();
if (key == 13) {
printf("\n\r");
break;
} else if (key >= 20) {
printf("%c, ", key);
fflush(stdout);
}
}
else {
/* do some work */
printf(".");
usleep(10);
printf(".");
usleep(10);
printf(".");
usleep(10);
printf("\e[3D");
usleep(10);
}
}
reset_terminal_mode();
}
I expect swifty code to do the same thing in Swift.

Answer 1

The termios functions translate almost one-to-one to Swift:
#if os(Linux)
import Glibc
#else
import Darwin
#endif
var orig_termios = termios()
func reset_terminal_mode() {
tcsetattr(0, TCSANOW, &orig_termios)
}
func set_conio_terminal_mode() {
tcgetattr(0, &orig_termios)
var new_termios = orig_termios
atexit(reset_terminal_mode)
cfmakeraw(&new_termios)
tcsetattr(0, TCSANOW, &new_termios)
}
set_conio_terminal_mode()
The problem with select() is that FD_ZERO etc are “non-trivial” macros and not imported into Swift. But you can use poll() instead:
func kbhit() -> Bool {
var fds = [ pollfd(fd: STDIN_FILENO, events: Int16(POLLIN), revents: 0) ]
let res = poll(&fds, 1, 0)
return res > 0
}
An alternative is to use the Dispatch framework. Here is a simple example which might help you get started. A dispatch source is used to wait asynchronously for available input, which is then appended to an array, from where it is retrieved in the getch() function. A serial queue is used to synchronize access to the array.
import Dispatch
let stdinQueue = DispatchQueue(label: "my.serial.queue")
var inputCharacters: [CChar] = []
let stdinSource = DispatchSource.makeReadSource(fileDescriptor: STDIN_FILENO, queue: stdinQueue)
stdinSource.setEventHandler(handler: {
var c = CChar()
if read(STDIN_FILENO, &c, 1) == 1 {
inputCharacters.append(c)
}
})
stdinSource.resume()
// Return next input character, or `nil` if there is none.
func getch() -> CChar? {
return stdinQueue.sync {
inputCharacters.isEmpty ? nil : inputCharacters.remove(at: 0)
}
}