# Subscribing to Main Thread Jank Events (C/C++) ## Available APIs For details about how to use the APIs (such as parameter usage restrictions and value ranges), see [HiAppEvent](../reference/apis-performance-analysis-kit/_hi_app_event.md#hiappevent). **Subscription APIs** | API | Description | | ------------------------------------------------------------ | -------------------------------------------- | | int OH_HiAppEvent_AddWatcher(HiAppEvent_Watcher *watcher) | Adds a watcher to listen for application events.| | int OH_HiAppEvent_RemoveWatcher(HiAppEvent_Watcher *watcher)| Removes a watcher to unsubscribe from application events.| ## How to Develop 1. Create a native C++ project and import the **jsoncpp** file to the project. The directory structure is as follows: ```yml entry: src: main: cpp: - json: - json.h - json-forwards.h - types: libentry: - index.d.ts - CMakeLists.txt - napi_init.cpp - jsoncpp.cpp ets: - entryability: - EntryAbility.ets - pages: - Index.ets ``` 2. In the **CMakeLists.txt** file, add the source file and dynamic libraries. ```cmake # Add the jsoncpp.cpp file, which is used to parse the JSON strings in the subscription events. add_library(entry SHARED napi_init.cpp jsoncpp.cpp) # Add libhiappevent_ndk.z.so and libhilog_ndk.z.so (log output). target_link_libraries(entry PUBLIC libace_napi.z.so libhilog_ndk.z.so libhiappevent_ndk.z.so) ``` 3. Import the dependencies to the **napi_init.cpp** file, and define **LOG_TAG**. ```c++ #include "napi/native_api.h" #include "json/json.h" #include "hilog/log.h" #include "hiappevent/hiappevent.h" #include "hiappevent/hiappevent_event.h" #undef LOG_TAG #define LOG_TAG "testTag" ``` 4. Subscribe to system events. - Watcher of the onReceive type. In the **napi_init.cpp** file, define the methods related to the watcher of the onReceive type. ```c++ // Define a variable to cache the pointer to the created watcher. static HiAppEvent_Watcher *systemEventWatcher; static void OnReceive(const char *domain, const struct HiAppEvent_AppEventGroup *appEventGroups, uint32_t groupLen) { for (int i = 0; i < groupLen; ++i) { for (int j = 0; j < appEventGroups[i].infoLen; ++j) { OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.domain=%{public}s", appEventGroups[i].appEventInfos[j].domain); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.name=%{public}s", appEventGroups[i].appEventInfos[j].name); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.eventType=%{public}d", appEventGroups[i].appEventInfos[j].type); if (strcmp(appEventGroups[i].appEventInfos[j].domain, DOMAIN_OS) == 0 && strcmp(appEventGroups[i].appEventInfos[j].name, EVENT_MAIN_THREAD_JANK) == 0) { Json::Value params; Json::Reader reader(Json::Features::strictMode()); Json::FastWriter writer; if (reader.parse(appEventGroups[i].appEventInfos[j].params, params)) { auto time = params["time"].asInt64(); auto pid = params["pid"].asInt(); auto uid = params["uid"].asInt(); auto bundleName = params["bundle_name"].asString(); auto bundleVersion = params["bundle_version"].asString(); auto beginTime = params["begin_time"].asInt64(); auto endTime = params["end_time"].asInt64(); auto externalLog = writer.write(params["external_log"]); auto logOverLimit = params["logOverLimit"].asBool(); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.params.time=%{public}lld", time); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.params.pid=%{public}d", pid); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.params.uid=%{public}d", uid); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.params.bundle_name=%{public}s", bundleName.c_str()); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.params.bundle_version=%{public}s", bundleVersion.c_str()); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.params.begin_time=%{public}lld", beginTime); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.params.end_time=%{public}lld", endTime); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.params.external_log=%{public}s", externalLog.c_str()); OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent eventInfo.params.log_over_limit=%{public}d", logOverLimit); } } } } } static napi_value RegisterWatcher(napi_env env, napi_callback_info info) { OH_LOG_INFO(LogType::LOG_APP, "HiAppEvent RegisterWatcher"); // Set the watcher name. The system identifies different watchers based on their names. systemEventWatcher = OH_HiAppEvent_CreateWatcher("onReceiverWatcher"); // Set the event to subscribe to EVENT_MAIN_THREAD_JANK. const char *names[] = {EVENT_MAIN_THREAD_JANK}; // Add the events to watch, for example, system events. OH_HiAppEvent_SetAppEventFilter(systemEventWatcher, DOMAIN_OS, 0, names, 1); // Set the implemented callback. After receiving the event, the watcher immediately triggers the OnReceive callback. OH_HiAppEvent_SetWatcherOnReceive(systemEventWatcher, OnReceive); // Add a watcher to listen for the specified event. OH_HiAppEvent_AddWatcher(systemEventWatcher); return {}; } ``` 5. Register **RegisterWatcher** as an ArkTS API. In the **napi_init.cpp** file, register **RegisterWatcher** as an ArkTS API. ```c++ static napi_value Init(napi_env env, napi_value exports) { napi_property_descriptor desc[] = { { "registerWatcher", nullptr, RegisterWatcher, nullptr, nullptr, nullptr, napi_default, nullptr } }; napi_define_properties(env, exports, sizeof(desc) / sizeof(desc[0]), desc); return exports; } ``` In the **index.d.ts** file, define the ArkTS API. ```typescript export const registerWatcher: () => void; ``` 6. In the **entry/src/main/ets/entryability/EntryAbility.ets** file, add the following interface invocation to **onCreate()**. ```typescript // Import the dependent module. import testNapi from 'libentry.so' // Add the interface invocation to onCreate(). // Register the system event watcher at startup. testNapi.registerWatcher(); ``` 7. In the **entry/src/main/ets/pages/Index.ets** file, add the **timeOut500** button with **onClick()** to trigger a main thread jank event when the button is clicked. The sample code is as follows: ```typescript Button("timeOut350") .fontSize(50) .fontWeight(FontWeight.Bold) .onClick(() => { let t = Date.now(); while (Date.now() - t <= 350) {} }) ``` 8. If the nolog version is used and the developer mode is disabled, the main thread checker will collect tracing data when a task times out. 9. In DevEco Studio, click the **Run** button to run the application project. Click the **timeOut350** button twice consecutively to trigger a main thread jank event. 10. After the main thread jank event is reported, you can view the following event information in the **Log** window. ```text HiAppEvent eventInfo.domain=OS HiAppEvent eventInfo.name=MAIN_THREAD_JANK HiAppEvent eventInfo.eventType=1 HiAppEvent eventInfo.params.time=1717597063727 HiAppEvent eventInfo.params.pid=45572 HiAppEvent eventInfo.params.uid=20020151 HiAppEvent eventInfo.params.bundle_name=com.example.nativemainthread HiAppEvent eventInfo.params.bundle_version=1.0.0 HiAppEvent eventInfo.params.begin_time=1717597063225 HiAppEvent eventInfo.params.end_time=1717597063727 HiAppEvent eventInfo.params.external_log=["/data/storage/el2/log/watchdog/MAIN_THREAD_JANK_20240613221239_45572.txt"] HiAppEvent eventInfo.params.log_over_limit=0 ``` > **NOTE** > For details, see [Main Thread Jank Event Time Specifications](./hiappevent-watcher-mainthreadjank-events-arkts.md#main-thread-jank-event-time-specifications) and [Main Thread Jank Event Specifications](./hiappevent-watcher-mainthreadjank-events-arkts.md#main-thread-jank-event-specifications). 11. Remove the event watcher. ```c++ static napi_value RemoveWatcher(napi_env env, napi_callback_info info) { // Remove the watcher. OH_HiAppEvent_RemoveWatcher(systemEventWatcher); return {}; } ``` 12. Destroy the event watcher. ```c++ static napi_value DestroyWatcher(napi_env env, napi_callback_info info) { // Destroy the created watcher and set systemEventWatcher to nullptr. OH_HiAppEvent_DestroyWatcher(systemEventWatcher); systemEventWatcher = nullptr; return {}; } ```