swift - 设置完成后，如何自动调用 NSSpeechSynthesizer Delegate 协议(protocol)方法？

Question

我正在学习使用 NSSpeechSynthesizer 及其两个 NSSpeechSynthesizerDelegate 协议(protocol)方法的教程。在我的 ViewController 中，我没有显式调用协议(protocol)方法，所以我很好奇我需要研究什么才能理解这些方法在运行时是如何调用的？委托(delegate)方法按预期工作，但我想知道它们是如何被调用的，这使得这成为可能？

import Cocoa



class MainWindowController: NSWindowController, NSSpeechSynthesizerDelegate, NSWindowDelegate {
    //Now MainWindowController is more powerful by having its own KITT being able to delegate powerful functionality and do less work.  The delegate will do all the heavy lifting and return the results to MainWindowController instances.
    // MARK: - Properties
    @IBOutlet weak var textField: NSTextField!
    @IBOutlet weak var speakButton: NSButton!
    @IBOutlet weak var stopButton: NSButton!
    let speechSynth = NSSpeechSynthesizer.init(voice: NSSpeechSynthesizer.VoiceName.init(rawValue: "com.apple.speech.synthesis.voice.Victoria"))

    var isSpeaking: Bool = false {
        didSet {
            updateButtons()
        }
    }
    // MARK: - Overriden Properties
    override var windowNibName: NSNib.Name? {
        return NSNib.Name("MainWindowController")
    }
    // MARK: - Overidden Methods
    override func windowDidLoad() {
        super.windowDidLoad()
        updateButtons()
        speechSynth?.delegate = self
    }

    // MARK: - UI methods
    @IBAction func speakIt(sender: NSButton) {
        //Get tuype-in text as a string
        let string = textField.stringValue
        if string.isEmpty {
            print("string from \(textField) is empty")
        } else {
            speechSynth?.startSpeaking(string)
            isSpeaking = true
        }
    }

    @IBAction func stopIt(sender: NSButton) {
        speechSynth?.stopSpeaking()
    }

    func updateButtons(){
        if isSpeaking {
            speakButton.isEnabled = false
            stopButton.isEnabled = true
        } else {
            speakButton.isEnabled = true
            stopButton.isEnabled = false
        }
    }

    // MARK: - NSSpeechSynthesizerDelegate Methods
    //this functionality is considered more powerful and is made possible due to the speechSynthesizer.delegate = self
    //the delegate is doing the work and reporting that completed work to the MainWindowController instance
    //so kinda like the delegate is providing the signature and its up to us as the developers based on what we do with those parameters inside the function in order  for us to add our own creativity.
    func speechSynthesizer(_ sender: NSSpeechSynthesizer, didFinishSpeaking finishedSpeaking: Bool) {
        //by setting this variable to FALSE, it will fire off the didSet computed property which this variable has both storage and behavior.
        isSpeaking = false
    }

    // MARK: - NSWindowDelegate Methods
    func windowShouldClose(_ sender: NSWindow) -> Bool {
        return !isSpeaking
    }
}

Answer 1

您的 windowDidLoad 方法包含这一行:

    speechSynth?.delegate = self

这意味着语音合成器对象有一个返回到您的 MainWindowController 的引用，因此语音合成器对象可以向您的 MainWindowController 发送消息。

NSSpeechSynthesizer 中的简化实现在 Swift 中看起来像这样:

class NSSpeechSynthesizer: NSSoundDelegate {

    weak var delegate: NSSpeechSynthesizerDelegate?

    func startSpeaking(_ string: String) {
        guard
            let audioData = audioData(for: string),
            let sound = NSSound(data: audioData)
            else { return }
        sound.delegate = self
        sound.play()
    }

    // Part of NSSoundDelegate
    func sound(_ sound: NSSound, didFinishPlaying finished: Bool) {
        // The first ? means Swift only sends the message if
        // delegate is not nil.
        // The second ? means Swift only sends the message if delegate
        // implements speechSynthesizer(_:didFinishSpeaking:).
        delegate?.speechSynthesizer?(self, didFinishSpeaking: finished)
    }

}

但它实际上是在 Objective-C 中实现的，您必须更加详细地检查委托(delegate)是否处理消息:

- (void)sound:(NSSound *)sound didFinishPlaying:(BOOL)finished {
    if ([delegate respondsToSelector:@selector(speechSynthesizer:didFinishSpeaking:)]) {
        [delegate speechSynthesizer:self didFinishSpeaking:finished];
    }
}