Class for stdout redirection

REPL.swift

import Foundation
import Combine

class REPL {
    private static let shared = REPL()
    
    public static var publisher: AnyPublisher<String, Never> {
        shared.publisher.eraseToAnyPublisher()
    }
    
    public typealias Publisher = PassthroughSubject<String, Never>
    public let publisher = Publisher()
    
    private let stdOut = Pipe()
    private var buffer = Buffer()
    
    private init() { setup() }
    
    public func setup() {
        printWarning()
        setvbuf(stdout, nil, _IONBF, 0)
        dup2(stdOut.fileHandleForWriting.fileDescriptor, STDOUT_FILENO)
        stdOut.fileHandleForReading.readabilityHandler = { [weak self] handle in
            guard
                let self = self,
                let output = self.buffer.append(handle.availableData)
            else { return }
            self.publisher.send(output)
        }
    }
    
    private func printWarning(
        file: String = #file,
        line: UInt = #line,
        function: String = #function
    ) {
        print("⚠️ WARNING: STDOUT WILL BE OVERRIDEN BY REPL INSTANCE")
        print("file:", file)
        print("line:", line)
        print("function:", function)
        print("✅ It's fine if you expect it to happen.")
        print("Output will be redirected to \(stdOut)")
        print("You can read it from this fileHandle:", stdOut.fileHandleForReading)
    }
}


extension REPL {
    private struct Buffer {
        private var storage = Data()

        mutating func append(_ data: Data) -> String? {
            storage.append(data)
            if let string = String(data: storage, encoding: .utf8),
                string.last?.isNewline == true
            {
                buffer.removeAll()
                return string
            }
            return nil
        }
    }
}

Usage

REPL.publisher.sink { output in 
    // Process value
    someLogger.log(output)
}.store(in: someCancellablesSet)

How does it work

REPL

REPL creates a new pipe for handling console output

private let stdOut = Pipe()

Then sets this pipe as a new stdout

setvbuf(stdout, nil, _IONBF, 0) // make stdout unbuffered
dup2(stdOut.fileHandleForWriting.fileDescriptor, STDOUT_FILENO) // override stdout file descriptor
dup2(stdOut.fileHandleForWriting.fileDescriptor, STDERR_FILENO)  // override stderr file descriptor

Handle output with buffer

self.buffer.append(handle.availableData)

Publish result if present

self.publisher.send(output)

REPL.Buffer

• Uses private storage: Data to be a buffer
• Appends data to private storage
• If the last symbol in data passed to append method was a newline then it returns storage contents and clears it.

Sources:

• objc.io
• stackoverflow
• medium

---

See also:

• LoggingKit

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Removed stderr redirection due to fatal errors, bad access and other system errors, which is inconvenient for the case, when app crashes.