ARouter主要是用于组件化开发中的组件之间的通信。

从ARouter的用法透析组件通信原理

// 需要接收消息的组件，要注册Router注解
@Route(group = "app", path = "/app/Component")
public class ComponentActivity extends AppCompatActivity {

// 通过Router直接发送事件

// 构造参数
Bundle bundle = new Bundle();
        bundle.putString("data", "data from other module");
        // 发送事件
        ARouter.getInstance()
                .build("/app/Component") // 跳转页面路由地址
                .with(bundle) // 传参
                .navigation();

ARouter的实现原理

在代码里加入的@Route注解，会在编译时期通过apt生成一些存储path和activity.class映射关系的类文件，然后app进程启动的时候会加载这些类文件，把保存这些映射关系的数据读到内存里(保存在map里)，然后在进行路由跳转的时候，通过build()方法传入要到达页面的路由地址，ARouter会通过它自己存储的路由表找到路由地址对应的Activity.class(activity.class = map.get(path))，然后new Intent(context, activity.Class)，当调用ARouter的withString()方法它的内部会调用intent.putExtra(String name, String value)，调用navigation()方法，它的内部会调用startActivity(intent)进行跳转，这样便可以实现两个相互没有依赖的module顺利的启动对方的Activity了。

1.ARouter.init(this);的执行过程

init()方法源码如下：

# ARouter.java
/**
     * Init, it must be call before used router.
     */
    public static void init(Application application) {
        if (!hasInit) {
            logger = _ARouter.logger;
            _ARouter.logger.info(Consts.TAG, "ARouter init start.");
            hasInit = _ARouter.init(application);

            if (hasInit) {
                _ARouter.afterInit();
            }

            _ARouter.logger.info(Consts.TAG, "ARouter init over.");
        }
    }

其中调用了_ARouter的init方法,最终调用到LogisticsCenter的init()

# _ARouter
protected static synchronized boolean init(Application application) {
        mContext = application;
        LogisticsCenter.init(mContext, executor);
        logger.info(Consts.TAG, "ARouter init success!");
        hasInit = true;

        // It's not a good idea.
        // if (Build.VERSION.SDK_INT > Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
        //     application.registerActivityLifecycleCallbacks(new AutowiredLifecycleCallback());
        // }
        return true;
    }

#LogisticsCenter
public synchronized static void init(Context context, ThreadPoolExecutor tpe) throws HandlerException {
        mContext = context;
        executor = tpe;

        try {
            long startInit = System.currentTimeMillis();
            //billy.qi modified at 2017-12-06
            //load by plugin first
            loadRouterMap();
            if (registerByPlugin) {
                logger.info(TAG, "Load router map by arouter-auto-register plugin.");
            } else {
                Set<String> routerMap;

                // It will rebuild router map every times when debuggable.
                if (ARouter.debuggable() || PackageUtils.isNewVersion(context)) {
                    logger.info(TAG, "Run with debug mode or new install, rebuild router map.");
                    // These class was generated by arouter-compiler.
                    routerMap = ClassUtils.getFileNameByPackageName(mContext, ROUTE_ROOT_PAKCAGE);
                    if (!routerMap.isEmpty()) {
                        context.getSharedPreferences(AROUTER_SP_CACHE_KEY, Context.MODE_PRIVATE).edit().putStringSet(AROUTER_SP_KEY_MAP, routerMap).apply();
                    }

                    PackageUtils.updateVersion(context);    // Save new version name when router map update finishes.
                } else {
                    logger.info(TAG, "Load router map from cache.");
                    routerMap = new HashSet<>(context.getSharedPreferences(AROUTER_SP_CACHE_KEY, Context.MODE_PRIVATE).getStringSet(AROUTER_SP_KEY_MAP, new HashSet<String>()));
                }

                logger.info(TAG, "Find router map finished, map size = " + routerMap.size() + ", cost " + (System.currentTimeMillis() - startInit) + " ms.");
                startInit = System.currentTimeMillis();

                for (String className : routerMap) {
                    if (className.startsWith(ROUTE_ROOT_PAKCAGE + DOT + SDK_NAME + SEPARATOR + SUFFIX_ROOT)) {
                        // This one of root elements, load root.
                        ((IRouteRoot) (Class.forName(className).getConstructor().newInstance())).loadInto(Warehouse.groupsIndex);
                    } else if (className.startsWith(ROUTE_ROOT_PAKCAGE + DOT + SDK_NAME + SEPARATOR + SUFFIX_INTERCEPTORS)) {
                        // Load interceptorMeta
                        ((IInterceptorGroup) (Class.forName(className).getConstructor().newInstance())).loadInto(Warehouse.interceptorsIndex);
                    } else if (className.startsWith(ROUTE_ROOT_PAKCAGE + DOT + SDK_NAME + SEPARATOR + SUFFIX_PROVIDERS)) {
                        // Load providerIndex
                        ((IProviderGroup) (Class.forName(className).getConstructor().newInstance())).loadInto(Warehouse.providersIndex);
                    }
                }
            }

            logger.info(TAG, "Load root element finished, cost " + (System.currentTimeMillis() - startInit) + " ms.");

            if (Warehouse.groupsIndex.size() == 0) {
                logger.error(TAG, "No mapping files were found, check your configuration please!");
            }

            if (ARouter.debuggable()) {
                logger.debug(TAG, String.format(Locale.getDefault(), "LogisticsCenter has already been loaded, GroupIndex[%d], InterceptorIndex[%d], ProviderIndex[%d]", Warehouse.groupsIndex.size(), Warehouse.interceptorsIndex.size(), Warehouse.providersIndex.size()));
            }
        } catch (Exception e) {
            throw new HandlerException(TAG + "ARouter init logistics center exception! [" + e.getMessage() + "]");
        }
    }

在该方法中只做了一件事，导入生成的路由表。

ARouter.getInstance().build(path)

在build的时候，传入要跳转的路由地址，build()方法会返回一个Postcard对象，我们称之为跳卡。Postcard里面保存着跳转的信息。

public final class Postcard extends RouteMeta {
    // Base
    private Uri uri;
    private Object tag;             // A tag prepare for some thing wrong.
    private Bundle mBundle;         // Data to transform
    private int flags = -1;         // Flags of route
    private int timeout = 300;      // Navigation timeout, TimeUnit.Second
    private IProvider provider;     // It will be set value, if this postcard was provider.
    private boolean greenChannel;
    private SerializationService serializationService;

}

navigation

Postcard 的 navigation() 方法又会调用到 _ARouter 的以下方法来完成 Activity 的跳转。

final class _ARouter {

    /**
     * Use router navigation.
     *
     * @param context     Activity or null.
     * @param postcard    Route metas
     * @param requestCode RequestCode
     * @param callback    cb
     */
    protected Object navigation(final Context context, final Postcard postcard, final int requestCode, final NavigationCallback callback) {
        PretreatmentService pretreatmentService = ARouter.getInstance().navigation(PretreatmentService.class);
        if (null != pretreatmentService && !pretreatmentService.onPretreatment(context, postcard)) {
            // Pretreatment failed, navigation canceled.
            //用于执行跳转前的预处理操作，可以通过 onPretreatment 方法的返回值决定是否取消跳转
            return null;
        }

        try {
            LogisticsCenter.completion(postcard);
        } catch (NoRouteFoundException ex) {
            //没有找到匹配的目标类
            //下面就执行一些提示操作和事件回调通知

            logger.warning(Consts.TAG, ex.getMessage());
            if (debuggable()) {
                // Show friendly tips for user.
                runInMainThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(mContext, "There's no route matched!\n" +
                                " Path = [" + postcard.getPath() + "]\n" +
                                " Group = [" + postcard.getGroup() + "]", Toast.LENGTH_LONG).show();
                    }
                });
            }
            if (null != callback) {
                callback.onLost(postcard);
            } else {
                // No callback for this invoke, then we use the global degrade service.
                DegradeService degradeService = ARouter.getInstance().navigation(DegradeService.class);
                if (null != degradeService) {
                    degradeService.onLost(context, postcard);
                }
            }
            return null;
        }

        if (null != callback) {
            //找到了匹配的目标类
            callback.onFound(postcard);
        }

        if (!postcard.isGreenChannel()) {   // It must be run in async thread, maybe interceptor cost too mush time made ANR.
            //没有开启绿色通道，那么就还需要执行所有拦截器
            //外部可以通过拦截器实现：控制是否允许跳转、更改跳转参数等逻辑

            interceptorService.doInterceptions(postcard, new InterceptorCallback() {
                /**
                 * Continue process
                 *
                 * @param postcard route meta
                 */
                @Override
                public void onContinue(Postcard postcard) {
                    //拦截器允许跳转
                    _navigation(context, postcard, requestCode, callback);
                }

                /**
                 * Interrupt process, pipeline will be destory when this method called.
                 *
                 * @param exception Reson of interrupt.
                 */
                @Override
                public void onInterrupt(Throwable exception) {
                    if (null != callback) {
                        callback.onInterrupt(postcard);
                    }

                    logger.info(Consts.TAG, "Navigation failed, termination by interceptor : " + exception.getMessage());
                }
            });
        } else {
            //开启了绿色通道，直接跳转，不需要遍历拦截器
            return _navigation(context, postcard, requestCode, callback);
        }

        return null;
    }

    //由于本例子的目标页面是 Activity，所以只看 ACTIVITY 即可
    private Object _navigation(final Context context, final Postcard postcard, final int requestCode, final NavigationCallback callback) {
        final Context currentContext = null == context ? mContext : context;
        switch (postcard.getType()) {
            case ACTIVITY:
                // Build intent
                //Destination 就是指向目标 Activity 的 class 对象
                final Intent intent = new Intent(currentContext, postcard.getDestination());
                //塞入携带的参数
                intent.putExtras(postcard.getExtras());

                // Set flags.
                int flags = postcard.getFlags();
                if (-1 != flags) {
                    intent.setFlags(flags);
                } else if (!(currentContext instanceof Activity)) {    // Non activity, need less one flag.
                    intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                }

                // Set Actions
                String action = postcard.getAction();
                if (!TextUtils.isEmpty(action)) {
                    intent.setAction(action);
                }

                // Navigation in main looper.
                //最终在主线程完成跳转
                runInMainThread(new Runnable() {
                    @Override
                    public void run() {
                        startActivity(requestCode, currentContext, intent, postcard, callback);
                    }
                });

                break;
            ··· //省略其它类型判断
        }

        return null;
    }

}
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navigation 方法的重点在于 LogisticsCenter.completion(postcard) 这一句代码。它反射调用 ARouteraccount 的 loadInto 方法，获取详细的路由对应信息。

completion 方法就是用来获取详细的路由对应信息的。该方法会通过 postcard 携带的 path 和 group 信息从 Warehouse 取值，如果值不为 null 的话就将信息保存到 postcard 中，如果值为 null 的话就抛出 NoRouteFoundException。

    /**
     * Completion the postcard by route metas
     *
     * @param postcard Incomplete postcard, should complete by this method.
     */
    public synchronized static void completion(Postcard postcard) {
        if (null == postcard) {
            throw new NoRouteFoundException(TAG + "No postcard!");
        }

        RouteMeta routeMeta = Warehouse.routes.get(postcard.getPath());
        if (null == routeMeta) {    //为 null 说明目标类不存在或者是该 group 还未加载过
            Class<? extends IRouteGroup> groupMeta = Warehouse.groupsIndex.get(postcard.getGroup());  // Load route meta.
            if (null == groupMeta) {
                //groupMeta 为 null，说明 postcard 的 path 对应的 group 不存在，抛出异常
                throw new NoRouteFoundException(TAG + "There is no route match the path [" + postcard.getPath() + "], in group [" + postcard.getGroup() + "]");
            } else {
                // Load route and cache it into memory, then delete from metas.
                try {
                    if (ARouter.debuggable()) {
                        logger.debug(TAG, String.format(Locale.getDefault(), "The group [%s] starts loading, trigger by [%s]", postcard.getGroup(), postcard.getPath()));
                    }
                  //会执行到这里，说明此 group 还未加载过，那么就来反射加载 group 对应的所有 path 信息
                   //获取后就保存到 Warehouse.routes
                    IRouteGroup iGroupInstance = groupMeta.getConstructor().newInstance();
                    iGroupInstance.loadInto(Warehouse.routes);

                    //移除此 group
                    Warehouse.groupsIndex.remove(postcard.getGroup());

                    if (ARouter.debuggable()) {
                        logger.debug(TAG, String.format(Locale.getDefault(), "The group [%s] has already been loaded, trigger by [%s]", postcard.getGroup(), postcard.getPath()));
                    }
                } catch (Exception e) {
                    throw new HandlerException(TAG + "Fatal exception when loading group meta. [" + e.getMessage() + "]");
                }

                //重新执行一遍
                completion(postcard);   // Reload
            }
        } else {
            //拿到详细的路由信息了，将这些信息存到 postcard 中

            postcard.setDestination(routeMeta.getDestination());
            postcard.setType(routeMeta.getType());
            postcard.setPriority(routeMeta.getPriority());
            postcard.setExtra(routeMeta.getExtra());

            //省略一些和本例子无关的代码
            ···
        }
    }
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回到_ARouter的navigation()方法来，可以清楚的看到最终人仍然是通过Intent(context,MainActivity.class)的方式来实现跳转的。