主要参考文章：
Android Retrofit 2.0 的详细 使用攻略（含实例讲解）
Android：手把手带你 深入读懂 Retrofit 2.0 源码
主要过程：
1、建造者模式- 初始化所需 变量
2、使用retrofit.create(AccessApi.class) 生成动态代理对象，调用方法时，解析方法上的 注解+参数+参数的注解 生成ServiceMethod 并缓存
3、ServiceMethod 中能获取到retrofit初始化的变量，ServiceMethod.invoke(...) 中使用 这些变量发起请求callAdapter.adapt(new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
4、以rxjava为例，ServiceMethod.invoke(...) 返回来Observable， 订阅者->callAdapter->OkHttpCall->okhttp3.Call->网络请求->okhttp3.Call->OkHttpCall->converter数据解析->callAdapter->订阅者
1、初始化
主要是初始化以下 Retrofit 变量
| 变量 | 说明 |
|---|---|
| callFactory | OkHttpClient 调用newCall(...) 生成 原始请求 |
| CallbackExecutor | MainThreadExecutor使用handler切换到主线程 |
| CallAdapterFactories | 对OkHttpCall在一次包装，可以理解为CallAdapter(OkHttpCall(okhttp3.Call)) |
| ConverterFactories | 在OkHttpCall中 对 返回的原始数据进行 解析 |
HttpLoggingInterceptor loggerInterceptor = new HttpLoggingInterceptor();//打印 信息
loggerInterceptor.setLevel(HttpLoggingInterceptor.Level.BODY);
OkHttpClient client = new OkHttpClient.Builder().addNetworkInterceptor(loggerInterceptor).build();
// 初始化Retrofit
mRetrofit = new Retrofit.Builder()
.client(client)
.baseUrl(baseUrl)
.addConverterFactory(GsonConverterFactory.create(buildGson()))
.addCallAdapterFactory(RxJava2CallAdapterFactory.create())
.build();
1.1 Retrofit.Builder() 变量
private final Platform platform; //平台类型
private @Nullable okhttp3.Call.Factory callFactory;//OkHttpClient
private @Nullable HttpUrl baseUrl;
//自己添加的 GsonConverterFactory
private final List converterFactories = new ArrayList<>();
//自己添加的 RxJava2CallAdapterFactory
private final List callAdapterFactories = new ArrayList<>();
//没赋值 就是用 platform.defaultCallbackExecutor() = MainThreadExecutor
private @Nullable Executor callbackExecutor;
private boolean validateEagerly;
平台类型 有 Android， java8；这里主要看android
| 平台类型 | CallbackExecutor | CallAdapterFactories | ConverterFactories |
|---|---|---|---|
| Android | MainThreadExecutor | CompletableFutureCallAdapterFactory (api 24以上才有) ExecutorCallAdapterFactory |
OptionalConverterFactory (api 24以上才有) |
| java8 | 。。。 | 。。。 | 。。。 |
1.2 Retrofit.Builder.build() 得到Retrofit 实例
Retrofit(okhttp3.Call.Factory callFactory, HttpUrl baseUrl,
List converterFactories, List callAdapterFactories,
@Nullable Executor callbackExecutor, boolean validateEagerly) {
this.callFactory = callFactory;//OkHttpClient
this.baseUrl = baseUrl;
this.converterFactories = converterFactories; // Copy+unmodifiable at call site.
//BuiltInConverters + 自己添加的 + android平台
//BuiltInConverters、GsonConverterFactory、 OptionalConverterFactory（api24以上有）
this.callAdapterFactories = callAdapterFactories; // Copy+unmodifiable at call site.
//自己添加的+android平台 RxJava2CallAdapterFactory,CompletableFutureCallAdapterFactory（>api24）、ExecutorCallAdapterFactory
this.callbackExecutor = callbackExecutor;//android 平台来说就是 MainThreadExecutor
this.validateEagerly = validateEagerly;//作用：是否提前对业务接口中的注解进行验证转换的标志位
}
| -- | CallbackExecutor | CallAdapterFactories | ConverterFactories |
|---|---|---|---|
| Retrofit.Builder | MainThreadExecutor | CompletableFutureCallAdapterFactory(>api24) ExecutorCallAdapterFactory |
OptionalConverterFactory (>api24) |
| Retrofit | MainThreadExecutor | RxJava2CallAdapterFactory CompletableFutureCallAdapterFactory（>api24） ExecutorCallAdapterFactory |
BuiltInConverters GsonConverterFactory OptionalConverterFactory （>api24） |
2、使用
构建用了动态代理， 最终是通过okhttp
2.1 ServiceMethod 构建
public class JavaBean {}
public interface AccessApi {
@GET("openapi.do?keyfrom=Yanzhikai&key=2032414398&type=data&doctype=json&version=1.1&q=car")
Call getCall();
}
AccessApi NetService = retrofit.create(AccessApi.class);
Call call = NetService.getCall();
retrofit.create(AccessApi.class) 生成 代理对象
public T create(final Class service) {
Utils.validateServiceInterface(service);
if (validateEagerly) {
eagerlyValidateMethods(service);//提前解析所有方法，加入缓存
}
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 {//返回 接口方法的返回参数，即方法返回的结果
// If the method is a method from Object then defer to normal invocation.
if (method.getDeclaringClass() == Object.class) {
return method.invoke(this, args);
}
//1.8之后接口可以有默认的实现
if (platform.isDefaultMethod(method)) {
return platform.invokeDefaultMethod(method, service, proxy, args);
}
return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
}
});
}
loadServiceMethod(…) —> ServiceMethod.parseAnnotations(this, method); 根据方法的注解 生成 ServiceMethod实例
static ServiceMethod parseAnnotations(Retrofit retrofit, Method method) {
RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);
//解析方法的注解，方法参数的注解，加上各种校验注解使用的正确性，构造出RequestFactory，待新建请求时使用
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.");
}
return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}
HttpServiceMethod
返回了 new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter)
参1、requestFactory = RequestFactory.parseAnnotations(retrofit, method);
参2、callFactory = retrofit.callFactory//OkHttpClient
参3、callAdapter 从 retrofit.callAdapterFactories 中 根据返参和注解，查找并生成 合适的CallAdapter；
参4、responseConverter 从 retrofit.converterFactories中 根据返参和注解，查找并生成 合适的Converter
2.2 ServiceMethod 方法的调用
@Override ReturnT invoke(Object[] args) {
return callAdapter.adapt(
new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
}
OkHttpCall中会新建 原始请求，okhttp3 的请求
private okhttp3.Call createRawCall() throws IOException {
okhttp3.Call call = callFactory.newCall(requestFactory.create(args));
if (call == null) {
throw new NullPointerException("Call.Factory returned null.");
}
return call;
}
OkHttpCall.enqueue(final Callback
结果返回后通过 responseConverter.convert(...) 将okhttp3 返回的数据 解析 为 想要的返回值（如Gson 转型）最后是一层一层回调出去。