# Working with Task Queues Using JSVM-API

## Introduction

JSVM-API provides APIs for processing and dispatching the tasks that are queued up for execution. You can use the APIs to start the running of a task queue in a JSVM and check whether there are micro tasks waiting in the queue. The task queue can be executed cyclically by external events.

## Basic Concepts

- Task queue: a mechanism used to manage the scheduling and execution of asynchronous tasks to ensure that tasks are processed in sequence.
- Micro task: a small task that needs to be executed as soon as possible. Micro tasks usually have a higher priority.

## Available APIs

| API| Description|
| -------- | -------- |
|OH_JSVM_PumpMessageLoop| Starts running a task queue.|
|OH_JSVM_PerformMicrotaskCheckpoint| Executes micro tasks in a task queue.|
## Example

If you are just starting out with JSVM-API, see [JSVM-API Development Process](use-jsvm-process.md). The following demonstrates only the C++ code involved in task queue development.

### OH_JSVM_PumpMessageLoop and OH_JSVM_PerformMicrotaskCheckpoint

Use **OH_JSVM_PumpMessageLoop** to start running a task queue.

CPP code:

```cpp
#include "ark_runtime/jsvm.h"
#include <cassert>
#include <string.h>
#include "hilog/log.h"
#include <unistd.h>
#undef  LOG_TAG
#define LOG_TAG "log"
#undef  LOG_DOMAIN
#define LOG_DOMAIN 0x1

// JS code to be executed.
static const char *STR_TASK = R"JS( 
    // Wasm bytecode (using the add module as an example)
    // The following is the text format of the Wasm bytecode corresponding to wasmBuffer, which contains only the add function.
    // (module
    //   (func $add (param $lhs i32) (param $rhs i32) (result i32)
    //     local.get $lhs
    //     local.get $rhs
    //     i32.add
    //   )
    //   (export "add" (func $add))
    // )
    var wasmBytes = new Uint8Array([0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x07, 0x01,
                                       0x60, 0x02, 0x7f, 0x7f, 0x01, 0x7f, 0x03, 0x02, 0x01, 0x00, 0x07,
                                       0x07, 0x01, 0x03, 0x61, 0x64, 0x64, 0x00, 0x00, 0x0a, 0x09, 0x01,
                                       0x07, 0x00, 0x20, 0x00, 0x20, 0x01, 0x6a, 0x0b]);    

    var p = WebAssembly.instantiate(wasmBytes, {});
    p.then((result) => {
        consoleinfo("Called with instance " + result);
    });
    p.finally(() => {
       consoleinfo("Called Finally");
    });
)JS";

// Ensure normal printing of the JS code information.
static JSVM_Value ConsoleInfo(JSVM_Env env, JSVM_CallbackInfo info) {
    size_t argc = 1;
    JSVM_Value args[1];
    char log[256] = "";
    size_t logLength;
    JSVM_CALL(OH_JSVM_GetCbInfo(env, info, &argc, args, NULL, NULL));

    OH_JSVM_GetValueStringUtf8(env, args[0], log, 255, &logLength);
    log[255] = 0;
    OH_LOG_INFO(LOG_APP, "JSVM API TEST: %{public}s", log);
    return nullptr;
}

// Register the consoleinfo callback.
JSVM_CallbackStruct param[] = {
    {.data = nullptr, .callback = ConsoleInfo},
};
JSVM_PropertyDescriptor descriptor[] = {
    {"consoleinfo", NULL, &param[0], NULL, NULL, NULL, JSVM_DEFAULT},
};

static int32_t TestJSVM() {
    JSVM_InitOptions init_options;
    memset(&init_options, 0, sizeof(init_options));
    if (g_aa == 0) {
        OH_JSVM_Init(&init_options);
        g_aa++;
    }
    // Create a JSVM instance and open the VM scope.
    JSVM_VM vm;
    JSVM_CreateVMOptions options;
    memset(&options, 0, sizeof(options));
    CHECK(OH_JSVM_CreateVM(&options, &vm) == JSVM_OK);
    JSVM_VMScope vm_scope;
    CHECK(OH_JSVM_OpenVMScope(vm, &vm_scope) == JSVM_OK);
    
    JSVM_Env env;
    CHECK(OH_JSVM_CreateEnv(vm, sizeof(descriptor) / sizeof(descriptor[0]), descriptor, &env) == JSVM_OK);
    JSVM_EnvScope envScope;
    CHECK_RET(OH_JSVM_OpenEnvScope(env, &envScope));
    JSVM_HandleScope handlescope;
    CHECK_RET(OH_JSVM_OpenHandleScope(env, &handlescope));
    JSVM_Value sourcecodevalue;
    CHECK_RET(OH_JSVM_CreateStringUtf8(env, STR_TASK, strlen(STR_TASK), &sourcecodevalue));
    JSVM_Script script;
    CHECK_RET(OH_JSVM_CompileScript(env, sourcecodevalue, nullptr, 0, true, nullptr, &script));
    JSVM_Value result;
    CHECK_RET(OH_JSVM_RunScript(env, script, &result));
    bool rst = false;
    auto start = std::chrono::system_clock::now();
    while (true) {
        // If no task is started in the task queue, set rst to false.
        CHECK_RET(OH_JSVM_PumpMessageLoop(vm, &rst)); 
        CHECK_RET(OH_JSVM_PerformMicrotaskCheckpoint(vm));
        // Exit at the scheduled time.
        auto now = std::chrono::system_clock::now();
        auto cost = std::chrono::duration_cast<std::chrono::milliseconds>(now - start).count();
        if (cost > 100) {
            break;
        }
    }

    // Close and destroy the environment and the VM.
    CHECK_RET(OH_JSVM_CloseHandleScope(env, handlescope));
    CHECK_RET(OH_JSVM_CloseEnvScope(env, envScope));
    CHECK(OH_JSVM_DestroyEnv(env) == JSVM_OK);
    CHECK(OH_JSVM_CloseVMScope(vm, vm_scope) == JSVM_OK);
    CHECK(OH_JSVM_DestroyVM(vm) == JSVM_OK);
    return 0;
}

```