祝各位2017年事业辉煌！开年第一篇博客，继续探索Xamarin.Forms…
为什么我做Xamarin开发的时候中意于Prism.Forms框架？本章为你揭晓。
实例代码地址：https://github.com/NewBLife/XamarinDemo/tree/master/TextToSpeechDemo
DependencyService
1、简介
软件开发有一个原则叫【依赖倒置Dependence Inversion Principle 】
A.高层次的模块不应该依赖于低层次的模块，他们都应该依赖于抽象。
B.抽象不应该依赖于具体实现，具体实现应该依赖于抽象。
Xamarin.Forms在面对无法实现的平台特有功能时就是使用以上原则设计一个叫【DependencyService】的功能。DependencyService的目的就是让PCL共通代码可以调用与平台相关的功能，它使Xamarin.Forms能像原生应用一样做任何事情！
2、工作原理
- 接口：定义功能接口在PCL类库或者共享类库
- 接口实现：各个平台实现接口功能
- 注册：各个平台实现接口的类库注册DependencyAttribute属性
- 调用：PCL类库或者共享类库调用DependencyService.Get<接口>（）方法获取平台实例对象
稍微看看原代码了解Xamarin.Forms如何实现依赖注入
DependencyAttribute.cs文件，定义了程序集属性标签：
using System;
namespace Xamarin.Forms
{
[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
public class DependencyAttribute : Attribute
{
public DependencyAttribute(Type implementorType)
{
Implementor = implementorType;
}
internal Type Implementor { get; private set; }
}
}
DependencyService.cs文件的Get方法（实体对象默认是单例形式存在）。
static bool s_initialized;
static readonly List<Type> DependencyTypes = new List<Type>();
static readonly Dictionary<Type, DependencyData> DependencyImplementations = new Dictionary<Type, DependencyData>();
public static T Get<T>(DependencyFetchTarget fetchTarget = DependencyFetchTarget.GlobalInstance) where T : class
{
if (!s_initialized)
Initialize();
Type targetType = typeof(T);
if (!DependencyImplementations.ContainsKey(targetType))
{
Type implementor = FindImplementor(targetType);
DependencyImplementations[targetType] = implementor != null ? new DependencyData { ImplementorType = implementor } : null;
}
DependencyData dependencyImplementation = DependencyImplementations[targetType];
if (dependencyImplementation == null)
return null;
if (fetchTarget == DependencyFetchTarget.GlobalInstance)
{
if (dependencyImplementation.GlobalInstance == null)
{
dependencyImplementation.GlobalInstance = Activator.CreateInstance(dependencyImplementation.ImplementorType);
}
return (T)dependencyImplementation.GlobalInstance;
}
return (T)Activator.CreateInstance(dependencyImplementation.ImplementorType);
}
DependencyService.cs文件的Initialize方法，遍历所有程序集获取标记了DependencyAttribute属性的类型。这是不太好的地方，这样的做法性能会大打折扣，这也是为什么不推荐使用DependencyService的一个方面。
static void Initialize()
{
Assembly[] assemblies = Device.GetAssemblies();
if (Registrar.ExtraAssemblies != null)
{
assemblies = assemblies.Union(Registrar.ExtraAssemblies).ToArray();
}
Type targetAttrType = typeof(DependencyAttribute);
// Don't use LINQ for performance reasons
// Naive implementation can easily take over a second to run
foreach (Assembly assembly in assemblies)
{
Attribute[] attributes = assembly.GetCustomAttributes(targetAttrType).ToArray();
if (attributes.Length == )
continue;
foreach (DependencyAttribute attribute in attributes)
{
if (!DependencyTypes.Contains(attribute.Implementor))
{
DependencyTypes.Add(attribute.Implementor);
}
}
}
s_initialized = true;
}
3，实例使用
使用TextToSpeechDemo(文本语音)实例讲解如何使用DependencyService。
项目结构：
接口定义：
namespace TextToSpeechDemo
{
public interface ITextToSpeech
{
void Speak(string text);
}
}
Android平台实现ITextToSpeech接口：API定义
最重要的[assembly: Dependency(typeof(TextToSpeech_Android))] 这句注册Dependency属性。
using Android.Runtime;
using Android.Speech.Tts;
using System.Collections.Generic;
using TextToSpeechDemo.Droid;
using Xamarin.Forms; [assembly: Dependency(typeof(TextToSpeech_Android))]
namespace TextToSpeechDemo.Droid
{
public class TextToSpeech_Android : Java.Lang.Object, ITextToSpeech, TextToSpeech.IOnInitListener
{
TextToSpeech speaker;
string toSpeak;
public TextToSpeech_Android() { } public void Speak(string text)
{
var ctx = Forms.Context;
toSpeak = text;
if (speaker == null)
{
speaker = new TextToSpeech(ctx, this);
}
else
{
var p = new Dictionary<string, string>();
speaker.Speak(toSpeak, QueueMode.Flush, p);
}
} public void OnInit([GeneratedEnum] OperationResult status)
{
if (status.Equals(OperationResult.Success))
{ System.Diagnostics.Debug.WriteLine("speaker init"); var p = new Dictionary<string, string>(); speaker.Speak(toSpeak, QueueMode.Flush, p); }
else
{ System.Diagnostics.Debug.WriteLine("was quiet"); }
}
}
}
iOS平台实现ITextToSpeech接口：
最重要的[assembly: Dependency(typeof(TextToSpeech_iOS))] 这句注册Dependency属性。
using AVFoundation;
using TextToSpeechDemo.iOS;
using Xamarin.Forms; [assembly: Dependency(typeof(TextToSpeech_iOS))]
namespace TextToSpeechDemo.iOS
{
class TextToSpeech_iOS : ITextToSpeech
{
public void Speak(string text)
{
var speechSynthesizer = new AVSpeechSynthesizer(); var speechUtterance = new AVSpeechUtterance(text)
{
Rate = AVSpeechUtterance.MaximumSpeechRate / ,
Voice = AVSpeechSynthesisVoice.FromLanguage("en-US"),
Volume = 0.5f,
PitchMultiplier = 1.0f
}; speechSynthesizer.SpeakUtterance(speechUtterance);
}
}
}
UWP平台实现ITextToSpeech接口：
最重要的[assembly: Dependency(typeof(TextToSpeech_UWP))] 这句注册Dependency属性。
using System;
using TextToSpeechDemo.UWP;
using Windows.Media.SpeechSynthesis;
using Windows.UI.Xaml.Controls;
using Xamarin.Forms; [assembly: Dependency(typeof(TextToSpeech_UWP))]
namespace TextToSpeechDemo.UWP
{
class TextToSpeech_UWP : ITextToSpeech
{
public async void Speak(string text)
{
MediaElement mediaElement = new MediaElement(); var synth = new SpeechSynthesizer();
var stream = await synth.SynthesizeTextToStreamAsync(text);
mediaElement.SetSource(stream, stream.ContentType);
mediaElement.Play();
}
}
}
调用平台特性的时候通过DependencyService.Get<T>()实现：
public void btnSpeak_Clicked(object sender, EventArgs args)
{
DependencyService.Get<ITextToSpeech>().Speak(txtData.Text.Trim());
}
整体效果：
IPlatformInitializer
1、简介
IPlatformInitializer其实为Prism.Forms共通类库里面的一个接口，代码如下：
namespace Prism
{ public interface IPlatformInitializer<T>
{ void RegisterTypes(T container);
}
}
包含一个注册类型函数（注册实现了平台特性的类型）。至于为什么是泛型接口？这是为了支持多种IOC容器（AutoFac，Unity，DryIoc，Ninject等），主流为Unity。Unity的IPlatformInitializer代码如下：传入了Unity的容器类型IUnityContainer
using Microsoft.Practices.Unity; namespace Prism.Unity
{
public interface IPlatformInitializer : IPlatformInitializer<IUnityContainer>
{
}
}
2、工作原理
- 接口：定义功能接口在PCL类库或者共享类库
- 接口实现：各个平台实现接口功能
- 注册：各个平台实现IPlatformInitializer接口，并在RegisterTypes方法中将实现接口的类注册到IOC容器内
- 调用：ViewModel的构造函数添加接口为参数（Prism.Forms会自动从IOC容器加载）
调用RegisterTypes是在Prism.Forms共通类库里面PrismApplicationBase<T>的构造函数中：
IPlatformInitializer<T> _platformInitializer = null;
protected PrismApplicationBase(IPlatformInitializer<T> initializer = null)
{ base.ModalPopping += PrismApplicationBase_ModalPopping;
base.ModalPopped += PrismApplicationBase_ModalPopped; _platformInitializer = initializer;
InitializeInternal();
}
/// <summary>
/// Run the intialization process.
/// </summary>
void InitializeInternal()
{ ConfigureViewModelLocator();
Initialize();
OnInitialized();
} /// <summary>
/// Run the bootstrapper process.
/// </summary>
public virtual void Initialize()
{
Logger = CreateLogger();
ModuleCatalog = CreateModuleCatalog();
ConfigureModuleCatalog();
Container = CreateContainer();
ConfigureContainer();
NavigationService = CreateNavigationService();
RegisterTypes();
_platformInitializer
?
.RegisterTypes(Container);
InitializeModules();
}
3，实例使用
使用PrismTextToSpeech(文本语音)实例讲解如何使用IPlatformInitializer。
项目结构：
接口定义：
namespacePrismTextToSpeech.Services
{
public interface ITextToSpeech
{
void Speak(string text);
}
}
Android平台实现ITextToSpeech接口：API定义
与DependencyService的区别是没有Dependency属性。
using Android.Runtime;
using Android.Speech.Tts;
using PrismTextToSpeech.Services;
using System.Collections.Generic;
using Xamarin.Forms; namespace PrismTextToSpeech.Droid
{
public class TextToSpeech_Android : Java.Lang.Object, ITextToSpeech, TextToSpeech.IOnInitListener
{
TextToSpeech speaker;
string toSpeak;
public TextToSpeech_Android() { } public void Speak(string text)
{
var ctx = Forms.Context;
toSpeak = text;
if (speaker == null)
{
speaker = new TextToSpeech(ctx, this);
}
else
{
var p = new Dictionary<string, string>();
speaker.Speak(toSpeak, QueueMode.Flush, p);
}
} public void OnInit([GeneratedEnum] OperationResult status)
{
if (status.Equals(OperationResult.Success))
{ System.Diagnostics.Debug.WriteLine("speaker init"); var p = new Dictionary<string, string>(); speaker.Speak(toSpeak, QueueMode.Flush, p); }
else
{ System.Diagnostics.Debug.WriteLine("was quiet"); }
}
}
}
注册类型到IOC容器：
using Android.App;
using Android.Content.PM;
using Android.OS;
using Microsoft.Practices.Unity;
using Prism.Unity;
using PrismTextToSpeech.Services; namespace PrismTextToSpeech.Droid
{
[Activity(Label = "PrismTextToSpeech", Icon = "@drawable/icon", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
{
protected override void OnCreate(Bundle bundle)
{
TabLayoutResource = Resource.Layout.tabs;
ToolbarResource = Resource.Layout.toolbar; base.OnCreate(bundle); global::Xamarin.Forms.Forms.Init(this, bundle);
LoadApplication(new App(new AndroidInitializer()));
}
} public class AndroidInitializer : IPlatformInitializer
{
public void RegisterTypes(IUnityContainer container)
{
container.RegisterType
<ITextToSpeech, TextToSpeech_Android>
();
}
}
}
iOS与UWP的接口实现与DependencyService的一样，唯独就是没有Dependency属性，这里略过。
调用的时候：
using Prism.Commands;
using Prism.Mvvm;
using PrismTextToSpeech.Services; namespace PrismTextToSpeech.ViewModels
{
public class MainPageViewModel : BindableBase
{
private ITextToSpeech _textToSpeech; private string _speakText;
public string SpeakText
{
get { return _speakText; }
set
{
SetProperty(ref _speakText, value);
SpeakCommand.RaiseCanExecuteChanged();
}
} public MainPageViewModel(ITextToSpeech textToSpeech)
{
_textToSpeech
=
textToSpeech;
} public DelegateCommand SpeakCommand => new DelegateCommand(
() =>
{
_textToSpeech.Speak(SpeakText);
},
() => !string.IsNullOrEmpty(SpeakText)).ObservesProperty(() => this.SpeakText);
}
}
Prism就是这么简单，效果更佳：
DependencyAttribute+IPlatformInitializer
1、简介
这种方式是Prism为了兼容DepdencyService而创建的，及Prism内部封装了DependencyService。
namespace Prism.Services
{
/// <summary>
/// A service that provides acess to platform-specific implementations of a specified type
/// </summary>
public class DependencyService : IDependencyService
{
/// <summary>
/// Returns a platform-specific implementation of a type registered with the Xamarin.Forms.DependencyService
/// </summary>
/// <typeparam name="T">The type of class to get</typeparam>
/// <returns>The class instance</returns>
public T Get<T>() where T : class
{
return Xamarin.Forms.DependencyService.Get<T>();
}
}
}
2、使用方法
- 接口：与DependencyService或者IPlatformInitializer实例一样
- 接口实现：与DependencyService实例一样
- 注册：与DependencyService实例一样，各个平台实现接口的类库注册DependencyAttribute属性
- 调用：与IPlatformInitializer实例一样，ViewModel的构造函数添加接口为参数（Prism.Forms会自动从IOC容器加载）
总结
DependencyService其实就是依赖注入的自我实现，而Prism的IPlatformInitializer则巧妙的借助Unity等容器达到同样的目的。不过从应用以后扩展角度也好，性能角度也好还是建议使用IOC容器技术（Prism创始人Brian Lagunas也是这么建议的）。特别是在使用Mvvm模式开发的时候，更加需要依赖IOC容器来管理ViewModel与Service，这也是我选择Prism做Xamarin开发的原因之一。