先说下 Retrofit 的简单使用方法。
先创建一个接口：
public interface GithubService {
@GET("users/{user}/repos")
Call<List<Repo>> listRepos(@Path("user") String user);
}
然后是 Retrofit 的创建和使用：
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com/")
.build();
GithubService githubService = retrofit.create(GithubService.class);
Call<List<Repo>> githubCall = githubService.listRepos("clipperl");
githubCall.enqueue(new Callback<List<Repo>>() {
@Override
public void onResponse(Call<List<Repo>> call, Response<List<Repo>> response) {
System.out.println("success! " + response.body());
}
@Override
public void onFailure(Call<List<Repo>> call, Throwable t) {
}
});
我们先尝试从githubCall.enqueue()入手，结果发现进入了一个接口的抽象方法中：
public interface Call<T> extends Cloneable {
void enqueue(Callback<T> callback);
}
其实真正的入口藏的比较深，我们要先从下边这行入手，：
GithubService githubService = retrofit.create(GithubService.class);
查看 create()方法，
public <T> T create(final Class<T> service) {
Utils.validateServiceInterface(service);
if (validateEagerly) {
eagerlyValidateMethods(service);
}
return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
new InvocationHandler() {
private final Platform platform = Platform.get();
@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);
}
ServiceMethod<Object, Object> serviceMethod =
(ServiceMethod<Object, Object>) loadServiceMethod(method);
OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.adapt(okHttpCall);
}
});
}
这里，关键的代码是Proxy.newProxyInstance()，这是一个动态代码，之前已经分析过动态代理在 Retrofit 中的使用，这里就不赘述了。
当我们调用自定义接口的抽象方法githubService.listRepos("clipperl")；
Call<List<Repo>> githubCall = githubService.listRepos("clipperl");
其实执行的是InvocationHandler的invoke()，所以可以得出一个结论：
githubCall 就是 serviceMethod.adapt(okHttpCall) 的返回值；
接着看serviceMethod.adapt(okHttpCall)，
T adapt(Call<R> call) {
return callAdapter.adapt(call);
}
再深入看，发现
public interface CallAdapter<R, T> {
adapt(Call<R> call);
}
我们在ServiceMethod 类中搜索callAdapter，发现它在build()方法中被赋值了，
callAdapter = createCallAdapter();
继续找，
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);
}
}
关键代码是：
return (CallAdapter<T, R>) retrofit.callAdapter(returnType, annotations);
接下来进入Retrofit类，
public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
return nextCallAdapter(null, returnType, annotations);
}
再追踪，
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;
}
}
StringBuilder builder = new StringBuilder("Could not locate call adapter for ")
.append(returnType)
.append(".\n");
if (skipPast != null) {
builder.append(" Skipped:");
for (int i = 0; i < start; i++) {
builder.append("\n * ").append(callAdapterFactories.get(i).getClass().getName());
}
builder.append('\n');
}
builder.append(" Tried:");
for (int i = start, count = callAdapterFactories.size(); i < count; i++) {
builder.append("\n * ").append(callAdapterFactories.get(i).getClass().getName());
}
throw new IllegalArgumentException(builder.toString());
}
关键代码是：
CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
if (adapter != null) {
return adapter;
}
我们发现了一个新的变量callAdapterFactories：
final List<CallAdapter.Factory> callAdapterFactories;
寻找它在哪被赋值，
public Retrofit build() {
if (baseUrl == null) {
throw new IllegalStateException("Base URL required.");
}
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
callFactory = new OkHttpClient();
}
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));
// Make a defensive copy of the converters.
List<Converter.Factory> converterFactories =
new ArrayList<>(1 + this.converterFactories.size());
// Add the built-in converter factory first. This prevents overriding its behavior but also
// ensures correct behavior when using converters that consume all types.
converterFactories.add(new BuiltInConverters());
converterFactories.addAll(this.converterFactories);
return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
}
关键代码是：
// 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));
继续看
platform.defaultCallAdapterFactory(callbackExecutor)
进入Platform类找defaultCallAdapterFactory()方法，
直接找过去是：
CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
if (callbackExecutor != null) {
return new ExecutorCallAdapterFactory(callbackExecutor);
}
return DefaultCallAdapterFactory.INSTANCE;
}
其实我们用的是Platform的子类Android，长这个样子，
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);
}
}
}
我们看到 Android类中的defaultCallAdapterFactory()返回的是一个ExecutorCallAdapterFactory对象；
回忆一下，刚才我们一路在是寻找callAdapterFactories集合的元素的get()方法，
CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
if (adapter != null) {
return adapter;
}
现在找到了，callAdapterFactories集合中的元素是ExecutorCallAdapterFactory对象，调用了ExecutorCallAdapterFactory对象的get()方法，接下看ExecutorCallAdapterFactory类的get()方法，
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);
}
};
}
get()方法返回了CallAdapter对象，这个CallAdapter对象就是前边一直要找的callAdapter，
T adapt(Call<R> call) {
return callAdapter.adapt(call);
}
调用callAdapter的adapt()方法，创建一个ExecutorCallbackCall对象，这个ExecutorCallbackCall对象就是前边我们提到的githubCall，
Call<List<Repo>> githubCall = githubService.listRepos("clipperl");
终于快到头了，githubCall调用的enqueue()方法，就是调用ExecutorCallbackCall的enqueue()方法， ExecutorCallbackCall是ExecutorCallAdapterFactory的内部类，我们来看它的enqueeu()方法，
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);
}
});
}
});
}
}
enqueue(final Callback<T> callback)方法中传入的参数就是我们Retrofit请求成功后的回调接口，
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com/")
.build();
GithubService githubService = retrofit.create(GithubService.class);
Call<List<Repo>> githubCall = githubService.listRepos("clipperl");
githubCall.enqueue(new Callback<List<Repo>>() {
@Override
public void onResponse(Call<List<Repo>> call, Response<List<Repo>> response) {
System.out.println("success! " + response.body());
}
@Override
public void onFailure(Call<List<Repo>> call, Throwable t) {
}
});
只不过ExecutorCallbackCall会把我们的回调接口包装起来，
ExecutorCallbackCall的enqueue()方法
@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);
}
});
}
});
}
我们的回调接口会被包到delegate的enqueue()方法的Callback中，这是第二种Callback，请注意区分，后边还有第三种Callback，请别晕。
那么 delegate 是什么，它是我们从 ExecutorCallAdapterFactory 中传过来的，
final class ExecutorCallAdapterFactory extends CallAdapter.Factory {
final Executor callbackExecutor;
ExecutorCallAdapterFactory(Executor callbackExecutor) {
this.callbackExecutor = callbackExecutor;
}
@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);
}
};
}
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;
}
还记得这个方法不，这个 call 就是 delegate，
T adapt(Call<R> call) {
return callAdapter.adapt(call);
}
那这个 call 是谁呢，它就是 okHttpCall,
ServiceMethod<Object, Object> serviceMethod =
(ServiceMethod<Object, Object>) loadServiceMethod(method);
OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.adapt(okHttpCall);
前边我们知道我们的 Callback 回调被这个 okHttpCall 的 enqueue() 包起来了，我们去看看 okHttpCall
的 enqueue() 方法做了什么，
@Override public void enqueue(final Callback<T> callback) {
checkNotNull(callback, "callback == null");
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 {
call = rawCall = createRawCall();
} catch (Throwable t) {
throwIfFatal(t);
failure = creationFailure = t;
}
}
}
if (failure != null) {
callback.onFailure(this, failure);
return;
}
if (canceled) {
call.cancel();
}
call.enqueue(new okhttp3.Callback() {
@Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
Response<T> response;
try {
response = parseResponse(rawResponse);
} catch (Throwable 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();
}
}
});
}
这里创建了 okhttp3.Call 的 call，我们前边的 第二种 Callback 套的第一种 Callback （我们写的）这里又被第三种 Callback 包起来了，所以现在清楚了，Retrofit 的 Callback 回调，被套了两层，最终是受 okhttp3 的 Callback 调度。
接下来还有一个问题，异步的问题，Retrofit 使用 enqueue 异步请求，我们现在还没看到 Retrofit 怎么让我们的回调函数在主线程执行。
回到ExecutorCallAdapterFactory 的 内部类 ExecutorCallbackCall 的 enqueue() 方法中，有这样一行代码，
callbackExecutor.execute(new Runnable()...
我们的回调接口原来在这里被放到一个线程中执行了，
final class ExecutorCallAdapterFactory extends CallAdapter.Factory {
final Executor callbackExecutor;
ExecutorCallAdapterFactory(Executor callbackExecutor) {
this.callbackExecutor = callbackExecutor;
}
@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);
}
};
}
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);
}
});
}
});
}
}
}
那接下来看下 callbackExecutor 是什么就能知道了，还记得之前那个 callAdapterFactories 吧，在给 赋值之前，callbackExecutor 就已经被赋值了，
public Retrofit build() {
if (baseUrl == null) {
throw new IllegalStateException("Base URL required.");
}
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
callFactory = new OkHttpClient();
}
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));
// Make a defensive copy of the converters.
List<Converter.Factory> converterFactories =
new ArrayList<>(1 + this.converterFactories.size());
// Add the built-in converter factory first. This prevents overriding its behavior but also
// ensures correct behavior when using converters that consume all types.
converterFactories.add(new BuiltInConverters());
converterFactories.addAll(this.converterFactories);
return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
}
是这几行代码，
Executor callbackExecutor = this.callbackExecutor;
if (callbackExecutor == null) {
callbackExecutor = platform.defaultCallbackExecutor();
}
我们还去找 Android 版的 Platform，
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);
}
}
}
callbackExecutor 是一个 MainThreadExecutor， MainThreadExecutor 的 execute() 就是用一个handler 向 UI 线程发消息，让我们的 Callback 回调代码在 UI 线程执行。
