fix some parameters types and implement set_voice

We have an ilegal hardware instruction in
backend::av_foundation::voices::AVSpeechSynthesisVoice::new(identifier)
when sending voiceWithIdentifier. Is it because the runLoop is not
runing when it's called?

examples/hello_world.rs

@@ -47,6 +47,20 @@ fn main() -> Result<(), Error> {
         tts.speak("This is normal volume.", false)?;
         tts.set_volume(original_volume)?;
     }
+    let Features { voices, .. } = tts.supported_features();
+    if voices {
+        let original_voice = tts.voice()?;
+        let voices_list = tts.list_voices();
+        println!("Available voices:\n===");
+        for v in voices_list.iter() {
+            println!("{}",v);
+            tts.set_voice(v)?;
+            println!("voice set");
+            println!("{}", tts.voice()?);
+            tts.speak(v,false)?;
+        }
+        tts.set_voice(original_voice)?;
+    }
     tts.speak("Goodbye.", false)?;
     let mut _input = String::new();
     #[cfg(target_os = "macos")]

src/backends/appkit.rs

@@ -204,7 +204,7 @@ impl Backend for AppKit {
         unimplemented!()
     }
 
-    fn set_voice(&mut self, voice: String) -> Result<(),Error> {
+    fn set_voice(&mut self, voice: &str) -> Result<(),Error> {
         unimplemented!()
     }
 }

src/backends/av_foundation.rs

@@ -150,7 +150,7 @@ impl Backend for AvFoundation {
         AVSpeechSynthesisVoice::list().iter().map(|v| {v.identifier()}).collect()
     }
 
-    fn set_voice(&mut self, voice: String) -> Result<(),Error> {
+    fn set_voice(&mut self, voice: &str) -> Result<(),Error> {
         self.voice = AVSpeechSynthesisVoice::new(voice);
         Ok(())
     }

src/backends/av_foundation/voices.rs

@@ -2,16 +2,19 @@
 use objc::runtime::*;
 use objc::*;
 use core_foundation::array::CFArray;
+use cocoa_foundation::foundation::NSString;
+use cocoa_foundation::base::{nil,id};
 use core_foundation::string::CFString;
 
 #[derive(Copy,Clone)]
 pub struct AVSpeechSynthesisVoice(*const Object);
 
 impl AVSpeechSynthesisVoice {
-    pub fn new(identifier: String) -> Self {
-        let i: CFString = CFString::from(identifier.as_str());
-        let voice: *const Object = unsafe{msg_send![class!(AVSpeechSynthesisVoice).metaclass(), voiceWithIdentifier: i]};
-        AVSpeechSynthesisVoice{0: voice}
+    pub fn new(identifier: &str) -> Self {
+        unsafe{
+            let i: id = NSString::alloc(nil).init_str(identifier);
+            msg_send![class!(AVSpeechSynthesisVoice), voiceWithIdentifier:i]
+        }
     }
 
     pub fn default() -> Self {

src/lib.rs

@@ -102,7 +102,7 @@ pub trait Backend {
     fn is_speaking(&self) -> Result<bool, Error>;
     fn voice(&self) -> Result<String, Error>;
     fn list_voices(&self) -> Vec<String>;
-    fn set_voice(&mut self, voice: String) -> Result<(),Error>;
+    fn set_voice(&mut self, voice: &str) -> Result<(),Error>;
 }
 
 pub struct TTS(Box<dyn Backend>);
@@ -399,13 +399,13 @@ impl TTS {
     /**
      * Set speaking voice.
      */
-    pub fn set_voice(&mut self, voice: String) -> Result<(),Error> {
+    pub fn set_voice<S: Into<String>>(&mut self, voice: S) -> Result<(),Error> {
         let Features {
             voices: voices_feature,
             ..
         } = self.0.supported_features();
         if voices_feature {
-            self.0.set_voice(voice)
+            self.0.set_voice(voice.into().as_str())
         } else {
             Err(Error::UnsupportedFeature)
         }