Retrofit 结合 Lifecycle, 将 Http 生命周期管理到极致

作者：xcheng_ 链接：https://www.jianshu.com/p/07fe489a53f2 声明：本文已获xcheng_投稿发表，转发等请联系原作者授权

http请求一直是开发中无法避免的存在，生命周期的管理也一直是开发者的痛点，稍不注意就在回调是抛出异常，如NullPointerException,showDialog导致的WindowLeaked等。

Google 最新推荐的 Lifecycle 架构就是可以让你自己的类拥有像 activity 或 fragment 一样生命周期的功能。

于是我决定采用lifecycle结合retrofit 将http请求和Activity或Fragment的生命周期相结合

本文将从以下几个方面一步步实现功能

• 多线程分发Lifecycle. Event
• retrofit如何与其关联?自定义 CallAdapter.Factory
• 异步请求如何关联
• 同步请求如何关联

如果有不熟悉lifecycle的可以自行学习，这里不做介绍了

lifecycle官方文档地址：

https://developer.android.com/topic/libraries/architecture/lifecycle

为什么要使用lifecycle？

activity 和fragment 是有声明周期的，有时候，我们的很多操作需要写在声明周期的方法中，比如，下载，文件操作等，这样很多情况下回导致，我们在activity中的声明周期方法中写越来越多的代码，activity或者fragment 越来越臃肿，代码维护越来越困难。使用lifecycle就可以很好的解决这类问题。 lifecycle代码简洁，我们可以通过实现LifecycleObserver 接口，来监听声明周期，然后我们在activity和fragment中去注册监听。

一 、分发lifecycle event

对于event的分发我们采用观察者模式，需要支持多线程环境，因为http请求可能在任意线程中发起。

首先定义一个 LifecycleProvider类,如下

/**
 * 统一分发Activity和 Fragment的生命周期时间.
 */
public interface LifecycleProvider {
    /**
     * Adds an observer to the list. The observer cannot be null and it must not already
     * be registered.
     *
     * @param observer the observer to register
     * @throws IllegalArgumentException the observer is null
     */
    void observe(Observer observer);

    /**
     * Removes a previously registered observer. The observer must not be null and it
     * must already have been registered.
     *
     * @param observer the observer to unregister
     * @throws IllegalArgumentException the observer is null
     */
    void removeObserver(Observer observer);

    /**
     * A simple callback that can receive from {@link android.arch.lifecycle.Lifecycle}
     */
    interface Observer {
        /**
         * Called when the event is changed.
         *
         * @param event The new event
         */
        void onChanged(@NonNull Lifecycle.Event event);
    }
}

实现类 为AndroidLifecycle 继承了LifecycleObserver接口监听Lifecycle event

public final class AndroidLifecycle implements LifecycleProvider, LifecycleObserver {
    private final Object mLock = new Object();

    @GuardedBy("mLock")
    private final ArrayList<Observer> mObservers = new ArrayList<>();
    /**
     * 缓存当前的Event事件
     */
    @GuardedBy("mLock")
    @Nullable
    private Lifecycle.Event mEvent;

    @MainThread
    public static LifecycleProvider createLifecycleProvider(LifecycleOwner owner) {
        return new AndroidLifecycle(owner);
    }

    private AndroidLifecycle(LifecycleOwner owner) {
        owner.getLifecycle().addObserver(this);
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_ANY)
    void onEvent(LifecycleOwner owner, Lifecycle.Event event) {
        synchronized (mLock) {
            //保证线程的可见性
            mEvent = event;
            for (int i = mObservers.size() - 1; i >= 0; i--) {
                mObservers.get(i).onChanged(event);
            }
        }
        if (event == Lifecycle.Event.ON_DESTROY) {
            owner.getLifecycle().removeObserver(this);
        }
    }

    @Override
    public void observe(Observer observer) {
        if (observer == null) {
            throw new IllegalArgumentException("The observer is null.");
        }
        synchronized (mLock) {
            if (mObservers.contains(observer)) {
                return;
            }
            mObservers.add(observer);
            if (mEvent != null) {
                observer.onChanged(mEvent);
            }
        }
    }

    @Override
    public void removeObserver(Observer observer) {
        if (observer == null) {
            throw new IllegalArgumentException("The observer is null.");
        }
        synchronized (mLock) {
            int index = mObservers.indexOf(observer);
            if (index == -1) {
                return;
            }
            mObservers.remove(index);
        }
    }
}

使用时只要在onChanged方法中就可以处理对应的事件,使用如下

    LifecycleProvider provider = AndroidLifecycle.createLifecycleProvider(this);
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        provider.observe(new LifecycleProvider.Observer() {
            @Override
            public void onChanged(@NonNull Lifecycle.Event event) {
                //do...
            }
        });
    }

observe方法不依赖于主线程，可以在任何地方调用。这样二次分发设计的目的有两个

• owner.getLifecycle().addObserver(LifecycleObserver)方法是线程不安全的，需要依赖主线程
• 可以缓存最新的Lifecycle.Event

二、retrofit 关联生命周期

retrofit 如何才能关联生命周期呢，通用的做法肯定是自定义CallAdapter.Factory，我们可以返回我们想要的自定义Call，在Call接口添加bindToLifecycle方法于LifecycleProvider相关联

• 自定义Call接口如下，添加了绑定生命周期的方法，这里只展示核心方法 public interface Call<T> extends Callable<T>, Cloneable { //忽略其他代码 /** * 绑定生命周期 * * @param provider LifecycleProvider * @param event {@link Lifecycle.Event}, {@link Lifecycle.Event#ON_ANY} is not allowed * @return LifeCall */ LifeCall<T> bindToLifecycle(LifecycleProvider provider, Lifecycle.Event event); /** * default event is {@link Lifecycle.Event#ON_DESTROY} * * @param provider LifecycleProvider * @return LifeCall * @see Call#bindToLifecycle(LifecycleProvider, Lifecycle.Event) */ LifeCall<T> bindUntilDestroy(LifecycleProvider provider); }
• 且看如何实现此接口 RealCall final class RealCall<T> implements Call<T> { //忽略其他代码 @Override public LifeCall<T> bindToLifecycle(LifecycleProvider provider, Lifecycle.Event event) { Utils.checkNotNull(provider, "provider==null"); Utils.checkNotNull(event, "event==null"); if (event == Lifecycle.Event.ON_ANY) { throw new IllegalArgumentException("ON_ANY event is not allowed."); } return new RealLifeCall<>(clone(), event, provider); } @Override public LifeCall<T> bindUntilDestroy(LifecycleProvider provider) { return bindToLifecycle(provider, Lifecycle.Event.ON_DESTROY); } }
• LifeCall 生命周期管理的接口类,它继承了LifecycleProvider.Observer,因此可以在onChanged方法接收分发的Lifecycle.Event public interface LifeCall<T> extends Callable<T>, LifecycleProvider.Observer { /** * Returns true if this call has been disposed. * * @return true if this call has been disposed */ boolean isDisposed(); /** * The method may be called concurrently from multiple * threads; the method must be thread safe. Calling this method multiple * times has no effect. * <p> * like {@code Observable#doOnDispose(Action)},{@code SingleSubject#onSuccess(Object)} * <p> * you can invoke with {@link Lifecycle.Event#ON_ANY} to dispose from outside immediately. */ @Override void onChanged(@NonNull Lifecycle.Event event); }
• 且看如何实现此接口RealLifeCall 在onChanged中判断，当event参数为指定的event时取消请求，并且标记为disposed，从provider中移除RealLifeCall观察对象。注意的是可以手动调用LifeCall.onChanged(LifeCycle.Event.ON_ANY)取消请求用于你想处理的任何场景，如果isDisposed()返回为true，在异步Callback调用的情况下是不会回调的。 final class RealLifeCall<T> implements LifeCall<T> { private final Call<T> delegate; private final Lifecycle.Event event; private final LifecycleProvider provider; private final AtomicBoolean once = new AtomicBoolean(); RealLifeCall(Call<T> delegate, Lifecycle.Event event, LifecycleProvider provider) { this.delegate = delegate; this.event = event; this.provider = provider; provider.observe(this); } //忽略其他代码 @Override public void onChanged(@NonNull Lifecycle.Event event) { if (this.event == event || event == Lifecycle.Event.ON_DESTROY //Activity和Fragment的生命周期是不会传入 {@code Lifecycle.Event.ON_ANY}, //可以手动调用此方法传入 {@code Lifecycle.Event.ON_ANY},用于区分是否为手动调用 || event == Lifecycle.Event.ON_ANY) { if (once.compareAndSet(false, true)/*保证原子性*/) { delegate.cancel(); provider.removeObserver(this); } } } @Override public boolean isDisposed() { return once.get(); } }
• 如何返回Call ?自定义CallAdapter.Factory retrofit的解耦灵活我们可以做很多自定义的配置，自定义Factory返回我们的Call接口对象，只需在创建retrofit对象是调用addCallAdapterFactory(CallAdapterFactory.INSTANCE)添加进去即可。 注：executor默认为Android主线程调度使用，Callback回调函数会在对应线程执行。详情可以看retrofit2.Platform.Android.defaultCallbackExecutor()方法 public final class CallAdapterFactory extends CallAdapter.Factory { private static final String RETURN_TYPE = Call.class.getSimpleName(); public static final CallAdapter.Factory INSTANCE = new CallAdapterFactory(); private static final Executor OPTIONAL_NULL_EXECUTOR = new Executor() { @Override public void execute(@NonNull Runnable command) { command.run(); } }; private CallAdapterFactory() { } @Override public CallAdapter<?, ?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) { if (getRawType(returnType) != Call.class) { return null; } if (!(returnType instanceof ParameterizedType)) { throw new IllegalArgumentException( String.format("%s return type must be parameterized as %s<Foo> or %s<? extends Foo>", RETURN_TYPE, RETURN_TYPE, RETURN_TYPE)); } final Type responseType = getParameterUpperBound(0, (ParameterizedType) returnType); final Executor executor = Utils.isAnnotationPresent(annotations, SkipCallbackExecutor.class) ? null : retrofit.callbackExecutor(); return new CallAdapter<Object, Call<?>>() { @Override public Type responseType() { return responseType; } @Override public Call<Object> adapt(retrofit2.Call<Object> call) { if (executor != null) { return new RealCall<>(executor, call); } return new RealCall<>(OPTIONAL_NULL_EXECUTOR, call); } }; } }
• 丰富的Callback接口 支持开始、结束、成功、失败、异常统一解析、简单的数据二次处理操作，HttpError统一包装异常信息 public interface Callback<T> { /** * @param call The {@code Call} that was started */ void onStart(Call<T> call); @NonNull HttpError parseThrowable(Call<T> call, Throwable t); /** * 过滤一次数据,如剔除List中的null等,默认可以返回t */ @NonNull T transform(Call<T> call, T t); void onError(Call<T> call, HttpError error); void onSuccess(Call<T> call, T t); /** * @param t 请求失败的错误信息 */ void onCompleted(Call<T> call, @Nullable Throwable t); }
• 异步调用 定义接口 @FormUrlEncoded @POST("user/login") Call<LoginInfo> getLogin(@Field("username") String username, @Field("password") String password); 安全的异步发起请求： public class MainActivity extends AppCompatActivity { LifecycleProvider provider = AndroidLifecycle.createLifecycleProvider(this); @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); RetrofitFactory.create(ApiService.class) .getLogin("loginName", "password") //.bindUntilDestroy(provider) .bindToLifecycle(provider, Lifecycle.Event.ON_STOP) .enqueue(new DefaultCallback<LoginInfo>() { @Override public void onStart(Call<LoginInfo> call) { showLoading(); } @Override public void onError(Call<LoginInfo> call, HttpError error) { Toast.makeText(MainActivity.this, error.msg, Toast.LENGTH_SHORT).show(); } @Override public void onSuccess(Call<LoginInfo> call, LoginInfo loginInfo) { Toast.makeText(MainActivity.this, "登录成功", Toast.LENGTH_SHORT).show(); } @Override public void onCompleted(Call<LoginInfo> call, @Nullable Throwable t){ hideLoading(); } }); } }
• 如何同步调用 一般同步调用的场景不多，一些连续且相互依赖的请求可以使用同步请求减少逻辑复杂性 如：注册成功后直接登录，如果采用异步的方式实现，回调接口缠绕在一起，代码不好维护。采用同步的方式实现更为方便。 @FormUrlEncoded @POST("user/register") Call<RegisterInfo> register(@Field("username") String username, @Field("password") String password); @FormUrlEncoded @POST("user/login") Call<LoginInfo> getLogin(@Field("username") String username, @Field("password") String password); new Thread(){ @Override public void run() { super.run(); try { RegisterInfo registerInfo=RetrofitFactory.create(ApiService.class) .register("loginName", "password") .bindToLifecycle(provider, Lifecycle.Event.ON_STOP) .execute(); //注册成功开始登录 LoginInfo loginInfo=RetrofitFactory.create(ApiService.class) .getLogin("loginName", "password") .bindToLifecycle(provider, Lifecycle.Event.ON_STOP) .execute(); //登录成功 } catch (Throwable throwable) { //异常处理 throwable.printStackTrace(); } } }.start(); 这里涉及二个问题
• 关于Thread，可以自行用线程池实现，这里制作演示
• 线程调度，成功和失败的结果需要回调到主线程中，android中回调主线程采用的Handler.post(Runnable)或者postDelayed(Runnable, long)方法实现，当主线程调度执行run方法是可能Activity或者Fragment已经被销毁。那么怎样才能安全的回调到主线程呢？ 调度方法和生命周期关联，在主线程执行时再次做判断。NetTaskExecutor 是做的Handler的封装 public final class ToMainThread implements LifecycleProvider.Observer { @Nullable private volatile Lifecycle.Event mEvent; private final LifecycleProvider provider; public ToMainThread(LifecycleProvider provider) { this.provider = provider; provider.observe(this); } public void to(@NonNull final Runnable runnable, final Lifecycle.Event event) { NetTaskExecutor.getInstance().postToMainThread(new Runnable() { @Override public void run() { if (mEvent == event || mEvent == Lifecycle.Event.ON_DESTROY) return; runnable.run(); } }); } public void toDelayed(@NonNull final Runnable runnable, final Lifecycle.Event event, long delayMillis) { NetTaskExecutor.getInstance().postToMainThreadDelayed(new Runnable() { @Override public void run() { if (mEvent == event || mEvent == Lifecycle.Event.ON_DESTROY) return; runnable.run(); } }, delayMillis); } @Override public void onChanged(@NonNull Lifecycle.Event event) { this.mEvent = event; if (event == Lifecycle.Event.ON_DESTROY) { provider.removeObserver(this); } } } 完整的同步执行代码如下，这样处理完全关联了生命周期。不会出任何问题 new Thread() { @Override public void run() { super.run(); try { RegisterInfo registerInfo = RetrofitFactory.create(ApiService.class) .register("loginName", "password") .bindToLifecycle(provider, Lifecycle.Event.ON_STOP) .execute(); //注册成功开始登录 LoginInfo loginInfo = RetrofitFactory.create(ApiService.class) .getLogin("loginName", "password") .bindToLifecycle(provider, Lifecycle.Event.ON_STOP) .execute(); //登录成功 toMainThread.to(new Runnable() { @Override public void run() { Toast.makeText(MainActivity.this, "登录成功", Toast.LENGTH_SHORT).show(); } }, Lifecycle.Event.ON_STOP); } catch (Throwable throwable) { //异常处理,这里只做简单的演示 //你可以对throwable做解析分类 toMainThread.to(new Runnable() { @Override public void run() { Toast.makeText(MainActivity.this, "服务异常", Toast.LENGTH_SHORT).show(); } }, Lifecycle.Event.ON_STOP); } } };
• 结束

github 地址： https://github.com/xchengDroid/retrofit-helper

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