Xamarin+Prism开发详解六:DependencyService与IPlatformInitializer的关系

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

Xamarin+Prism开发详解六:DependencyService与IPlatformInitializer的关系

  • 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:

Xamarin+Prism开发详解六:DependencyService与IPlatformInitializer的关系

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:

Xamarin+Prism开发详解六:DependencyService与IPlatformInitializer的关系

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:

    Xamarin+Prism开发详解六:DependencyService与IPlatformInitializer的关系

    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:

    Xamarin+Prism开发详解六:DependencyService与IPlatformInitializer的关系

    Xamarin+Prism开发详解六:DependencyService与IPlatformInitializer的关系

    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。

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