Android主流三方庫源碼分析(2、深刻理解Retrofit源碼)

前言

成爲一名優秀的Android開發,須要一份完備的知識體系,在這裏,讓咱們一塊兒成長爲本身所想的那樣~。

前篇咱們詳細地分析了OKHttp的核心源碼,若是對OKHttp內部機制不瞭解的能夠看看Android主流三方庫源碼分析(1、深刻理解OKHttp源碼)。這篇,將會來深刻地分析下目前Android最優秀的網絡封裝框架Retrofit的源碼流程。java

1、基本使用流程

一、定義HTTP API,用於描述請求

public interface GitHubService {

     @GET("users/{user}/repos")
     Call<List<Repo>> listRepos(@Path("user") String user);
}
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二、建立Retrofit並生成API的實現(注意:方法上面的註解表示請求的接口部分,返回類型是請求的返回值類型,方法的參數便是請求的參數)

// 1.Retrofit構建過程
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com/")
.build();

// 2.建立網絡請求接口類實例過程
GitHubService service = retrofit.create(GitHubService.class);
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三、調用API方法,生成Call,執行請求

// 3.生成並執行請求過程
Call<List<Repo>> repos = service.listRepos("octocat");
repos.execute() or repos.enqueue()
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Retrofit的基本使用流程很簡潔,可是簡潔並不表明簡單,Retrofit爲了實現這種簡潔的使用流程,內部使用了優秀的架構設計和大量的設計模式,在我分析過Retrofit最新版的源碼和大量優秀的Retrofit源碼分析文章後,我發現,要想真正理解Retrofit內部的核心源碼流程和設計思想,首先,須要對這九大設計模式有必定的瞭解,以下:android

1.Retrofit構建過程 
建造者模式、工廠方法模式

2.建立網絡請求接口實例過程
外觀模式、代理模式、單例模式、策略模式、裝飾模式(建造者模式)

3.生成並執行請求過程
適配器模式(代理模式、裝飾模式)
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其次,須要對OKHttp源碼有必定的瞭解,若是不瞭解的能夠看看這篇Android主流三方庫源碼分析(1、深刻理解OKHttp源碼)。最後,讓咱們按以上流程去深刻Retrofit源碼內部,領悟它帶給咱們的設計之美git

2、Retrofit構建過程

一、Retrofit核心對象解析

首先Retrofit中有一個全局變量很是關鍵,在V2.5以前的版本,使用的是LinkedHashMap(),它是一個網絡請求配置對象,是由網絡請求接口中方法註解進行解析後獲得的。github

public final class Retrofit {

    // 網絡請求配置對象,存儲網絡請求相關的配置,如網絡請求的方法、數據轉換器、網絡請求適配器、網絡請求工廠、基地址等
    private final Map<Method, ServiceMethod<?>> serviceMethodCache = new ConcurrentHashMap<>();
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Retrofit使用了建造者模式經過內部類Builder類創建一個Retrofit實例,以下:json

public static final class Builder {

    // 平臺類型對象(Platform -> Android)
    private final Platform platform;
    // 網絡請求工廠,默認使用OkHttpCall(工廠方法模式)
    private @Nullable okhttp3.Call.Factory callFactory;
    // 網絡請求的url地址
    private @Nullable HttpUrl baseUrl;
    // 數據轉換器工廠的集合
    private final List<Converter.Factory> converterFactories = new ArrayList<>();
    // 網絡請求適配器工廠的集合,默認是ExecutorCallAdapterFactory
    private final List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>();
    // 回調方法執行器,在 Android 上默認是封裝了 handler 的 MainThreadExecutor, 默認做用是:切換線程(子線程 -> 主線程)
    private @Nullable Executor callbackExecutor;
    // 一個開關,爲true則會緩存建立的ServiceMethod
    private boolean validateEagerly;
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二、Builder內部構造

下面看看Builder內部構造作了什麼。設計模式

public static final class Builder {

    ...
    
    Builder(Platform platform) {
        this.platform = platform;
    }

    
    public Builder() {
        this(Platform.get());
    }
    
    ...
    
}


class Platform {

    private static final Platform PLATFORM = findPlatform();
    
    static Platform get() {
      return PLATFORM;
    }
    
    private static Platform findPlatform() {
      try {
        // 使用JVM加載類的方式判斷是不是Android平臺
        Class.forName("android.os.Build");
        if (Build.VERSION.SDK_INT != 0) {
          return new Android();
        }
      } catch (ClassNotFoundException ignored) {
      }
      try {
        // 同時支持Java平臺
        Class.forName("java.util.Optional");
        return new Java8();
      } catch (ClassNotFoundException ignored) {
      }
      return new Platform();
    }

static class Android extends Platform {

    ...

    
    @Override public Executor defaultCallbackExecutor() {
        //切換線程(子線程 -> 主線程)
        return new MainThreadExecutor();
    }

    // 建立默認的網絡請求適配器工廠,若是是Android7.0或Java8上,則使
    // 用了併發包中的CompletableFuture保證了回調的同步
    // 在Retrofit中提供了四種CallAdapterFactory(策略模式):
    // ExecutorCallAdapterFactory(默認)、GuavaCallAdapterFactory、
    // va8CallAdapterFactory、RxJavaCallAdapterFactory
    @Override List<? extends CallAdapter.Factory> defaultCallAdapterFactories(
        @Nullable Executor callbackExecutor) {
      if (callbackExecutor == null) throw new AssertionError();
      ExecutorCallAdapterFactory executorFactory = new   ExecutorCallAdapterFactory(callbackExecutor);
      return Build.VERSION.SDK_INT >= 24
        ? asList(CompletableFutureCallAdapterFactory.INSTANCE, executorFactory)
        : singletonList(executorFactory);
    }
    
    ...

    @Override List<? extends Converter.Factory> defaultConverterFactories() {
      return Build.VERSION.SDK_INT >= 24
          ? singletonList(OptionalConverterFactory.INSTANCE)
          : Collections.<Converter.Factory>emptyList();
    }

    ...
    
    static class MainThreadExecutor implements Executor {
    
        // 獲取Android 主線程的Handler 
        private final Handler handler = new Handler(Looper.getMainLooper());

        @Override public void execute(Runnable r) {
        
            // 在UI線程對網絡請求返回數據處理
            handler.post(r);
        }
    }
}
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能夠看到,在Builder內部構造時設置了默認Platform、callAdapterFactories和callbackExecutor。api

三、添加baseUrl

很簡單,就是將String類型的url轉換爲OkHttp的HttpUrl過程以下:緩存

/**
 * Set the API base URL.
 *
 * @see #baseUrl(HttpUrl)
 */
public Builder baseUrl(String baseUrl) {
    checkNotNull(baseUrl, "baseUrl == null");
    return baseUrl(HttpUrl.get(baseUrl));
}

public Builder baseUrl(HttpUrl baseUrl) {
    checkNotNull(baseUrl, "baseUrl == null");
    List<String> pathSegments = baseUrl.pathSegments();
    if (!"".equals(pathSegments.get(pathSegments.size() - 1))) {
      throw new IllegalArgumentException("baseUrl must end in /: " + baseUrl);
    }
    this.baseUrl = baseUrl;
    return this;
}
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四、添加GsonConverterFactory

首先,看到GsonConverterFactory.creat()的源碼。微信

public final class GsonConverterFactory extends Converter.Factory {
 
    public static GsonConverterFactory create() {
        return create(new Gson());
    }
    
    
    public static GsonConverterFactory create(Gson gson) {
        if (gson == null) throw new NullPointerException("gson ==   null");
        return new GsonConverterFactory(gson);
    }
    
    private final Gson gson;
    
    // 建立了一個含有Gson對象實例的GsonConverterFactory
    private GsonConverterFactory(Gson gson) {
        this.gson = gson;
    }
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而後,看看addConverterFactory()方法內部。網絡

public Builder addConverterFactory(Converter.Factory factory) {
    converterFactories.add(checkNotNull(factory, "factory null"));
    return this;
}
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可知,這一步是將一個含有Gson對象實例的GsonConverterFactory放入到了數據轉換器工廠converterFactories裏。

五、build過程

public Retrofit build() {

    if (baseUrl == null) {
      throw new IllegalStateException("Base URL required.");
    }
    
    okhttp3.Call.Factory callFactory = this.callFactory;
    if (callFactory == null) {
        // 默認使用okhttp
         callFactory = new OkHttpClient();
    }
    
    Executor callbackExecutor = this.callbackExecutor;
    if (callbackExecutor == null) {
        // Android默認的callbackExecutor
        callbackExecutor = platform.defaultCallbackExecutor();
    }
    
    // Make a defensive copy of the adapters and add the defaultCall adapter.
    List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
    // 添加默認適配器工廠在集合尾部
    callAdapterFactories.addAll(platform.defaultCallAdapterFactorisca  llbackExecutor));
    
    // Make a defensive copy of the converters.
    List<Converter.Factory> converterFactories = new ArrayList<>(
        1 + this.converterFactories.size() + platform.defaultConverterFactoriesSize());
    // Add the built-in converter factory first. This prevents overriding its behavior but also
    // ensures correct behavior when using converters thatconsumeall types.
    converterFactories.add(new BuiltInConverters());
    converterFactories.addAll(this.converterFactories);
    converterFactories.addAll(platform.defaultConverterFactories();
    
    return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
        unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
        
}
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能夠看到,最終咱們在Builder類中看到的6大核心對象都已經配置到Retrofit對象中了。

3、建立網絡請求接口實例過程

retrofit.create()使用了外觀模式和代理模式建立了網絡請求的接口實例,咱們分析下create方法。

public <T> T create(final Class<T> service) {
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
        // 判斷是否須要提早緩存ServiceMethod對象
        eagerlyValidateMethods(service);
    }
    
    // 使用動態代理拿到請求接口全部註解配置後,建立網絡請求接口實例
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new  Class<?>[] { service },
        new InvocationHandler() {
          private final Platform platform = Platform.get();
          private final Object[] emptyArgs = new Object[0];

          @Override public Object invoke(Object proxy, Method method, @Nullable Object[] args)
              throws Throwable {
            // If the method is a method from Object then defer to normal invocation.
            if (method.getDeclaringClass() == Object.class) {
              return method.invoke(this, args);
            }
            if (platform.isDefaultMethod(method)) {
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
          }
    });
 }

private void eagerlyValidateMethods(Class<?> service) {

  Platform platform = Platform.get();
  for (Method method : service.getDeclaredMethods()) {
    if (!platform.isDefaultMethod(method)) {
      loadServiceMethod(method);
    }
  }
}
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繼續看看loadServiceMethod的內部流程

ServiceMethod<?> loadServiceMethod(Method method) {

    ServiceMethod<?> result = serviceMethodCache.get(method);
    if (result != null) return result;

    synchronized (serviceMethodCache) {
      result = serviceMethodCache.get(method);
      if (result == null) {
            // 解析註解配置獲得了ServiceMethod
            result = ServiceMethod.parseAnnotations(this, method);
            // 能夠看到,最終加入到ConcurrentHashMap緩存中
            serviceMethodCache.put(method, result);
      }
    }
    return result;
}


abstract class ServiceMethod<T> {
  static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method   method) {
        // 經過RequestFactory解析註解配置(工廠模式、內部使用了建造者模式)
        RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);
    
        Type returnType = method.getGenericReturnType();
        if (Utils.hasUnresolvableType(returnType)) {
          throw methodError(method,
              "Method return type must not include a type variable or wildcard: %s", returnType);
        }
        if (returnType == void.class) {
          throw methodError(method, "Service methods cannot return void.");
        }
    
        // 最終是經過HttpServiceMethod構建的請求方法
        return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
    }

    abstract T invoke(Object[] args);
}
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如下爲請求構造核心流程

根據RequestFactory#Builder構造方法和parseAnnotations方法的源碼,可知的它的做用就是用來解析註解配置的。

Builder(Retrofit retrofit, Method method) {
    this.retrofit = retrofit;
    this.method = method;
    // 獲取網絡請求接口方法裏的註釋
    this.methodAnnotations = method.getAnnotations();
    // 獲取網絡請求接口方法裏的參數類型       
    this.parameterTypes = method.getGenericParameterTypes();
    // 獲取網絡請求接口方法裏的註解內容    
    this.parameterAnnotationsArray = method.getParameterAnnotations();
}
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接着看HttpServiceMethod.parseAnnotations()的內部流程。

static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
        
    //1.根據網絡請求接口方法的返回值和註解類型,
    // 從Retrofit對象中獲取對應的網絡請求適配器
    CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit,method);
    
    // 獲得響應類型
    Type responseType = callAdapter.responseType();
    
    ...

    //2.根據網絡請求接口方法的返回值和註解類型從Retrofit對象中獲取對應的數據轉換器 
    Converter<ResponseBody, ResponseT>responseConverter =
        createResponseConverter(retrofit,method, responseType);

    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    
    return newHttpServiceMethod<>(requestFactory, callFactory, callAdapter,responseConverter);
}
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1.createCallAdapter(retrofit, method)
private static <ResponseT, ReturnT> CallAdapter<ResponseT, ReturnT>     createCallAdapter(
      Retrofit retrofit, Method method) {
      
    // 獲取網絡請求接口裏方法的返回值類型
    Type returnType = method.getGenericReturnType();
    
    // 獲取網絡請求接口接口裏的註解
    Annotation[] annotations = method.getAnnotations();
    try {
      //noinspection unchecked
      return (CallAdapter<ResponseT, ReturnT>)  retrofit.callAdapter(returnType, annotations);
    } catch (RuntimeException e) { // Wide exception range because factories are user code.
      throw methodError(method, e, "Unable to create call adapter for %s", returnType);
    }
}
  
public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
    return nextCallAdapter(null, returnType, annotations);
}

public CallAdapter<?, ?> nextCallAdapter(@Nullable CallAdapter.Factory skipPast, Type returnType,
  Annotation[] annotations) {
    ...

    int start = callAdapterFactories.indexOf(skipPast) + 1;
    // 遍歷 CallAdapter.Factory 集合尋找合適的工廠
    for (int i = start, count = callAdapterFactories.size(); i <count; i++) {
        CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
        if (adapter != null) {
          return adapter;
        }
    }
}
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2.createResponseConverter(Retrofit retrofit, Method method, Type responseType)
private static <ResponseT> Converter<ResponseBody, ResponseT>  createResponseConverter(
     Retrofit retrofit, Method method, Type responseType) {
   Annotation[] annotations = method.getAnnotations();
   try {
     return retrofit.responseBodyConverter(responseType,annotations);
   } catch (RuntimeException e) { // Wide exception range because    factories are user code.
     throw methodError(method, e, "Unable to create converter for%s",   responseType);
   }
}

public <T> Converter<ResponseBody, T> responseBodyConverter(Type type, Annotation[] annotations) {
    return nextResponseBodyConverter(null, type, annotations);
}

public <T> Converter<ResponseBody, T> nextResponseBodyConverter(
  @Nullable Converter.Factory skipPast, Type type, Annotation[] annotations) {
...

int start = converterFactories.indexOf(skipPast) + 1;
// 遍歷 Converter.Factory 集合並尋找合適的工廠, 這裏是GsonResponseBodyConverter
for (int i = start, count = converterFactories.size(); i < count; i++) {
  Converter<ResponseBody, ?> converter =
      converterFactories.get(i).responseBodyConverter(type, annotations, this);
  if (converter != null) {
    //noinspection unchecked
    return (Converter<ResponseBody, T>) converter;
  }
}
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最終,執行HttpServiceMethod的invoke方法

@Override ReturnT invoke(Object[] args) {
    return callAdapter.adapt(
        new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
}
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最終在adapt中建立了一個ExecutorCallbackCall對象,它是一個裝飾者,而在它內部真正去執行網絡請求的仍是OkHttpCall。

4、建立網絡請求接口類實例並執行請求過程

一、service.listRepos()

一、Call<List<Repo>> repos = service.listRepos("octocat");
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service對象是動態代理對象Proxy.newProxyInstance(),當調用getCall()時會被 它攔截,而後調用自身的InvocationHandler#invoke(),獲得最終的Call對象。

二、同步執行流程 repos.execute()

@Override public Response<T> execute() throws IOException {
    okhttp3.Call call;

    synchronized (this) {
      if (executed) throw new IllegalStateException("Already executed.");
      executed = true;

      if (creationFailure != null) {
        if (creationFailure instanceof IOException) {
          throw (IOException) creationFailure;
        } else if (creationFailure instanceof RuntimeException) {
          throw (RuntimeException) creationFailure;
        } else {
          throw (Error) creationFailure;
        }
      }

      call = rawCall;
      if (call == null) {
        try {
          // 建立一個OkHttp的Request對象請求
          call = rawCall = createRawCall();
        } catch (IOException | RuntimeException | Error e) {
          throwIfFatal(e); //  Do not assign a fatal error to     creationFailure.
          creationFailure = e;
          throw e;
        }
      }
    }

    if (canceled) {
      call.cancel();
    }

    // 調用OkHttpCall的execute()發送網絡請求(同步),
    // 並解析網絡請求返回的數據
    return parseResponse(call.execute());
}


private okhttp3.Call createRawCall() throws IOException {
    // 建立 一個okhttp3.Request
    okhttp3.Call call =
    callFactory.newCall(requestFactory.create(args));
    if (call == null) {
      throw new NullPointerException("Call.Factory returned null.");
    }
    return call;
}


Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException {
    ResponseBody rawBody = rawResponse.body(); 
    
    // Remove the body's source (the only stateful object) so we can   pass the response along.
    rawResponse = rawResponse.newBuilder()
        .body(new NoContentResponseBody(rawBody.contentType(), rawBody.contentLength()))
        .build();    
    
    // 根據響應返回的狀態碼進行處理    
    int code = rawResponse.code();
    if (code < 200 || code >= 300) {
      try {
        // Buffer the entire body to avoid future I/O.
        ResponseBody bufferedBody = Utils.buffer(rawBody);
        return Response.error(bufferedBody, rawResponse);
      } finally {
        rawBody.close();
      }
    }    
    if (code == 204 || code == 205) {
      rawBody.close();
      return Response.success(null, rawResponse);
    }    
    
    
    ExceptionCatchingResponseBody catchingBody = new ExceptionCatchingResponseBody(rawBody);
    try {
      // 將響應體轉爲Java對象
      T body = responseConverter.convert(catchingBody);
      
      return Response.success(body, rawResponse);
    } catch (RuntimeException e) {
      // If the underlying source threw an exception, propagate that     rather than indicating it was
      // a runtime exception.
      catchingBody.throwIfCaught();
      throw e;
    }
}
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三、異步請求流程 reponse.enqueque

@Override 
public void enqueue(final Callback<T> callback) {

    // 使用靜態代理 delegate進行異步請求 
    delegate.enqueue(new Callback<T>() {

      @Override 
      public void onResponse(Call<T> call, finalResponse<T>response) {
        // 線程切換,在主線程顯示結果
        callbackExecutor.execute(new Runnable() {
            @Override 
             public void run() {
            if (delegate.isCanceled()) {
              callback.onFailure(ExecutorCallbackCall.this, newIOException("Canceled"));
            } else {
              callback.onResponse(ExecutorCallbackCall.this,respons);
            }
          }
        });
      }
      @Override 
      public void onFailure(Call<T> call, final Throwable t) {
        callbackExecutor.execute(new Runnable() {
          @Override public void run() {
            callback.onFailure(ExecutorCallbackCall.this, t);
          }
        });
      }
    });
}
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看看 delegate.enqueue 內部流程。

@Override 
public void enqueue(final Callback<T> callback) {
   
    okhttp3.Call call;
    Throwable failure;

    synchronized (this) {
      if (executed) throw new IllegalStateException("Already executed.");
      executed = true;

      call = rawCall;
      failure = creationFailure;
      if (call == null && failure == null) {
        try {
          // 建立OkHttp的Request對象,再封裝成OkHttp.call
          // 方法同發送同步請求,此處上面已分析
          call = rawCall = createRawCall(); 
        } catch (Throwable t) {
          failure = creationFailure = t;
        }
      }

@Override public void enqueue(final Callback<T> callback) {
  checkNotNull(callback, "callback == null");

  okhttp3.Call call;
  Throwable failure;

  ...

  call.enqueue(new okhttp3.Callback() {
    @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
      Response<T> response;
      try {
        // 此處上面已分析
        response = parseResponse(rawResponse);
      } catch (Throwable e) {
        throwIfFatal(e);
        callFailure(e);
        return;
      }

      try {
        callback.onResponse(OkHttpCall.this, response);
      } catch (Throwable t) {
        t.printStackTrace();
      }
    }

    @Override public void onFailure(okhttp3.Call call, IOException e) {
      callFailure(e);
    }

    private void callFailure(Throwable e) {
      try {
        callback.onFailure(OkHttpCall.this, e);
      } catch (Throwable t) {
        t.printStackTrace();
      }
    }
  });
}
複製代碼

若是你看到這裏的話,恭喜你,你已經對Retrofit已經有一個比較深刻的瞭解了,可是,筆者仍是要建議你們本身主動配合着Retrofit最新版的源碼一步步去完全地認識它,只有這樣,你才能看到它真實的心裏,最後附上一張Stay的Retrofit源碼流程圖,要注意的是,這是V2.5以前版本的流程,可是,在看完上面的源碼分析後,咱們知道,主體流程是沒有變化的。

image

5、總結

從本質上來講,Retrofit雖然只是一個RESTful 的HTTP 網絡請求框架的封裝庫。可是,它內部經過 大量的設計模式 封裝了 OkHttp,讓使用者感到它很是簡潔、易懂。它內部主要是用動態代理的方式,動態將網絡請求接口的註解解析成HTTP請求,最後執行請求的過程。好了,至此,咱們的Android主流三方庫源碼分析的網絡庫分析部分已經完畢。接下來,將爲你們帶來最流行的圖片加載框架Glide的源碼分析,敬請期待~

參考連接:

一、Retrofit V2.5.0源碼

二、Android進階之光

三、Android:手把手帶你 深刻讀懂 Retrofit 2.0 源碼

四、Retrofit分析-漂亮的解耦套路

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