Retrofit源碼之請(qǐng)求對(duì)象的轉(zhuǎn)換筆記

更新時(shí)間：2018年05月14日 09:17:06 作者：低情商的大仙

這篇文章主要介紹了Retrofit源碼之請(qǐng)求對(duì)象的轉(zhuǎn)換筆記，小編覺(jué)得挺不錯(cuò)的，現(xiàn)在分享給大家，也給大家做個(gè)參考。一起跟隨小編過(guò)來(lái)看看吧

之前在Retrofit源碼初探一文中我們提出了三個(gè)問(wèn)題：

什么時(shí)候開始將注解中參數(shù)拼裝成http請(qǐng)求的信息的？
如何產(chǎn)生發(fā)起http請(qǐng)求對(duì)象的？
如何將對(duì)象轉(zhuǎn)換成我們?cè)诮涌谥兄付ǖ姆祷刂档模?/li>

其中前兩個(gè)問(wèn)題在前幾篇文章已經(jīng)做了解答，今天我們探究下最后一個(gè)問(wèn)題：

我們定義接口時(shí)，有這樣的：

@GET("hello/world")
Call<News> getNews(@Query("num") String num,@Query("page")String page);

也有這樣的：

@GET("book/search")
 Observable<Book> getSearchBook(@Query("q") String name,
                  @Query("tag") String tag, @Query("start") int start,
                  @Query("count") int count);

可以看到接口的返回值是不一樣的，現(xiàn)在我們就來(lái)分析下，一個(gè)OkHttpCall對(duì)象是如何轉(zhuǎn)換成對(duì)應(yīng)的返回值的。

核心代碼是這句：

return serviceMethod.adapt(okHttpCall);

進(jìn)到adapt中去：

T adapt(Call<R> call) {
  return callAdapter.adapt(call);
 }

可以看到是調(diào)用了callAdapter.adapt方法，此處的callAdapter是一個(gè)接口類型，所以想要看它的adapt方法的具體實(shí)現(xiàn)就得看這個(gè)callAdapter具體怎么生成的。

經(jīng)過(guò)搜索，發(fā)現(xiàn)它的生成方式如下：

 ServiceMethod(Builder<R, T> builder) {
   //………………
  this.callAdapter = builder.callAdapter;
   //………………
  }

而這個(gè)構(gòu)造方法是在ServiceMethod.Builder的build方法中調(diào)用的：

 public ServiceMethod build() {
   callAdapter = createCallAdapter();
   //…………
   return new ServiceMethod<>(this);
   }

所以繼續(xù)跟進(jìn)createCallAdapter()中去：

 private CallAdapter<T, R> createCallAdapter() {
   Type returnType = method.getGenericReturnType();
   if (Utils.hasUnresolvableType(returnType)) {
    throw methodError(
      "Method return type must not include a type variable or wildcard: %s", returnType);
   }
   if (returnType == void.class) {
    throw methodError("Service methods cannot return void.");
   }
   Annotation[] annotations = method.getAnnotations();
   try {
    //noinspection unchecked
    return (CallAdapter<T, R>) retrofit.callAdapter(returnType, annotations);
   } catch (RuntimeException e) { // Wide exception range because factories are user code.
    throw methodError(e, "Unable to create call adapter for %s", returnType);
   }
  }

可以看到，這里的主要作用就是獲取方法級(jí)別的注解以及返回值，然后傳入到retrofit.callAdapter中去獲取正真的CallAdapter，所以繼續(xù)跟到retrofit.callAdatper中去：

 public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
  return nextCallAdapter(null, returnType, annotations);
 }

繼續(xù)進(jìn)到nextCallAdapter中：

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

  int start = callAdapterFactories.indexOf(skipPast) + 1;
  for (int i = start, count = callAdapterFactories.size(); i < count; i++) {
   CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
   if (adapter != null) {
    return adapter;
   }
  }
  //省略一些不重要代碼
 }

這里主要就是遍歷Retrofit的所有CallAdapter，然后找到能夠處理該返回類型以及方法注解的那個(gè)直接返回。
對(duì)于默認(rèn)返回類型的處理CallAdapter，其實(shí)是在Retrofit生成時(shí)默認(rèn)加上的：

 public Retrofit build() {
   //省略部分代碼
   Executor callbackExecutor = this.callbackExecutor;
   if (callbackExecutor == null) {
    callbackExecutor = platform.defaultCallbackExecutor();
   }

   // Make a defensive copy of the adapters and add the default Call adapter.
   List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);   
   callAdapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
   //省略部分代碼
   return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
     unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
 }

這里有一點(diǎn)要事先說(shuō)下，所有的CalllAdapter對(duì)象其實(shí)都是通過(guò)CallAdapter.Factory對(duì)象調(diào)用get()方法生成的。
所以這里利用platform.defaultCallAdapterFactory()生成了一個(gè)對(duì)應(yīng)的CallAdapter.Factory對(duì)象，但生成這個(gè)對(duì)象首先生成了一個(gè)callbackExecutor,我們先看下它是怎么回事：

@Nullable Executor defaultCallbackExecutor() {
  return null;
 }

咦，為什么是返回null的？別慌，Retrofit的build中的platform根據(jù)不同的情況會(huì)是不同的子類，并不一定是Platform的實(shí)例，而是它的子類：

 static class Android extends Platform {
  @Override public Executor defaultCallbackExecutor() {
   return new MainThreadExecutor();
  }

  @Override CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
   if (callbackExecutor == null) throw new AssertionError();
   return new ExecutorCallAdapterFactory(callbackExecutor);
  }

  static class MainThreadExecutor implements Executor {
   private final Handler handler = new Handler(Looper.getMainLooper());

   @Override public void execute(Runnable r) {
    handler.post(r);
   }
  }
 }

我們重點(diǎn)關(guān)注Android平臺(tái)的，可以看到這里生成的callbackExecutor的execute()方法主要就是用來(lái)將操作發(fā)送到主線程執(zhí)行。

ok,callbackExecutor我們弄清楚了，那么接下來(lái)我們繼續(xù)看platform.defaultCallAdapterFactory()方法生成了什么樣的CallAdapter.Factory對(duì)象:

 CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
  if (callbackExecutor != null) {
   return new ExecutorCallAdapterFactory(callbackExecutor);
  }
  return DefaultCallAdapterFactory.INSTANCE;
 }

對(duì)于Android平臺(tái)來(lái)說(shuō)，我們之前生成了一個(gè)對(duì)應(yīng)的callbackExecutor,所以我們繼續(xù)跟進(jìn)if中的語(yǔ)句,發(fā)現(xiàn)最終生成了一個(gè)ExecutorCallAdapterFactory()對(duì)象，當(dāng)然，我們主要是看它的get()方法能得到什么樣的CallAdapter對(duì)象：

 @Override
 public CallAdapter<?, ?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
  if (getRawType(returnType) != Call.class) {
   return null;
  }
  final Type responseType = Utils.getCallResponseType(returnType);
  return new CallAdapter<Object, Call<?>>() {
   @Override public Type responseType() {
    return responseType;
   }

   @Override public Call<Object> adapt(Call<Object> call) {
    return new ExecutorCallbackCall<>(callbackExecutor, call);
   }
  };
 }

這個(gè)get()方法生成了一個(gè)匿名的CallAdapter對(duì)象，所以:

serviceMethod.adapt(okHttpCall)最終就是調(diào)用這個(gè)匿名對(duì)象的adapt方法

可以看到adapt方法最終就是將OkHttpCall對(duì)象轉(zhuǎn)換成了ExecutorCallbackCall對(duì)象。那這個(gè)對(duì)象能干什么？

 static final class ExecutorCallbackCall<T> implements Call<T> {
  final Executor callbackExecutor;
  final Call<T> delegate;

  ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
   this.callbackExecutor = callbackExecutor;
   this.delegate = delegate;
  }

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

   delegate.enqueue(new Callback<T>() {
    @Override public void onResponse(Call<T> call, final Response<T> response) {
     callbackExecutor.execute(new Runnable() {
      @Override public void run() {
       if (delegate.isCanceled()) {
        // Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.
        callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
       } else {
        callback.onResponse(ExecutorCallbackCall.this, response);
       }
      }
     });
    }

    @Override public void onFailure(Call<T> call, final Throwable t) {
     callbackExecutor.execute(new Runnable() {
      @Override public void run() {
       callback.onFailure(ExecutorCallbackCall.this, t);
      }
     });
    }
   });
  }

  @Override public boolean isExecuted() {
   return delegate.isExecuted();
  }

  @Override public Response<T> execute() throws IOException {
   return delegate.execute();
  }

  @Override public void cancel() {
   delegate.cancel();
  }

  @Override public boolean isCanceled() {
   return delegate.isCanceled();
  }

  @SuppressWarnings("CloneDoesntCallSuperClone") // Performing deep clone.
  @Override public Call<T> clone() {
   return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone());
  }

  @Override public Request request() {
   return delegate.request();
  }
 }

可以明顯看到這個(gè)方法就是對(duì)OkHttpCall對(duì)象的一個(gè)包裝，不同的是對(duì)它的enque()方法重寫了，重寫的目的很簡(jiǎn)單，就是為了將異步結(jié)果交給MainThreadExecutor，最終轉(zhuǎn)換到主線程執(zhí)行回調(diào)。

總結(jié)

上面源碼分析了很多，有點(diǎn)雜亂，這里我們統(tǒng)一總結(jié)下OkHttpCall到接口定義的返回類型（這里以Call<ResponseBody>為例,）的轉(zhuǎn)換過(guò)程：

通過(guò)platform(在Android平臺(tái)上是它的子類Android) 生成一個(gè)Executor對(duì)象，在Android上就是MainThreadExecutor對(duì)象。
通過(guò)platform生成一個(gè)CallAdapterFactory對(duì)象，在Android上就是ExecutorCallAdapterFactory對(duì)象，該對(duì)象能通過(guò)get()方法生成一個(gè)CallAdapter對(duì)象，來(lái)將OkHttpCall對(duì)象轉(zhuǎn)成ExecutorCallbackCall對(duì)象。
將上面提到的CallAdapterFactory對(duì)象塞到Retrofit對(duì)象中，最終在ServiceMethod的adapt()方法中調(diào)用，將OkHttpCall轉(zhuǎn)成ExecutorCallback,然后就可以正常的調(diào)用enque()方法發(fā)起請(qǐng)求了。

以上就是本文的全部?jī)?nèi)容，希望對(duì)大家的學(xué)習(xí)有所幫助，也希望大家多多支持腳本之家。

您可能感興趣的文章: