iOS 实现

你定义的接口需要在每个目标平台中实现。对于 iOS，这是通过 AVFoundation 框架完成的。ITextToSpeech 接口的以下实现处理用英语说出给定文本。

using AVFoundation;

public class TextToSpeechiOS : ITextToSpeech
{
    public TextToSpeechiOS () {}

    public void Speak (string whatToSay)
    {
        var speechSynthesizer = new AVSpeechSynthesizer ();

        var speechUtterance = new AVSpeechUtterance (whatToSay) {
            Rate = AVSpeechUtterance.MaximumSpeechRate/4,
            Voice = AVSpeechSynthesisVoice.FromLanguage ("en-US"),
            Volume = 0.5f,
            PitchMultiplier = 1.0f
        };

        speechSynthesizer.SpeakUtterance (speechUtterance);
    }
}

创建类后，你需要启用 DependencyService 以在运行时发现它。这是通过在类定义之上添加 [assembly] 属性并在任何命名空间定义之外添加来完成的。

using AVFoundation;
using DependencyServiceSample.iOS;

[assembly: Xamarin.Forms.Dependency (typeof (TextToSpeechiOS))]
namespace DependencyServiceSample.iOS {
    public class TextToSpeechiOS : ITextToSpeech
...

此属性使用 DependencyService 注册类，因此可在需要 ITextToSpeech 接口的实例时使用。