async_worker.md

AsyncWorker

AsyncWorker is an abstract class that you can subclass to remove many of the tedious tasks of moving data between the event loop and worker threads. This class internally handles all the details of creating and executing an asynchronous operation.

Once created, execution is requested by calling Queue. When a thread is available for execution the Execute method will be invoked. Once Execute complets either OnOK or OnError will be invoked. Once the OnOK or OnError methods are complete the AsyncWorker instance is destructed.

For the most basic use, only the Execute method must be implemented in a subclass.

Methods

Env

Requests the environment in which the async worker has been initially created.

Env Env() const;

Returns the environment in which the async worker has been created.

Queue

Requests that the work be queued for execution.

void Queue();

Cancel

Cancels queued work if it has not yet been started. If it has already started executing, it cannot be cancelled. If cancelled successfully neither OnOK nor OnError will be called.

void Cancel();

Receiver

ObjectReference& Receiver();

Returns the persistent object reference of the receiver object set when the async worker was created.

Callback

FunctionReference& Callback();

Returns the persistent function reference of the callback set when the async worker was created. The returned function reference will receive the results of the computation that happened in the Execute method, unless the default implementation of OnOK or OnError is overridden.

SetError

Sets the error message for the error that happened during the execution. Setting an error message will cause the OnError method to be invoked instead of OnOK once the Execute method completes.

void SetError(const std::string& error);

• [in] error: The reference to the string that represent the message of the error.

Execute

This method is used to execute some tasks out of the event loop on a libuv worker thread. Subclasses must implement this method and the method is run on a thread other than that running the main event loop. As the method is not running on the main event loop, it must avoid calling any methods from node-addon-api or running any code that might invoke JavaScript. Instead once this method is complete any interaction through node-addon-api with JavaScript should be implemented in the OnOK method which runs on the main thread and is invoked when the Execute method completes.

virtual void Execute() = 0;

OnOK

This method is invoked when the computation in the Excecute method ends. The default implementation runs the Callback provided when the AsyncWorker class was created.

virtual void OnOK();

OnError

This method is invoked afer Execute() completes if an error occurs while Execute is running and C++ exceptions are enabled or if an error was set through a call to SetError. The default implementation calls the callback provided when the AsyncWorker class was created, passing in the error as the first parameter.

virtual void OnError(const Error& e);

Constructor

Creates a new AsyncWorker.

explicit AsyncWorker(const Function& callback);

• [in] callback: The function which will be called when an asynchronous operations ends. The given function is called from the main event loop thread.

Returns an AsyncWork instance which can later be queued for execution by calling Queue.

Constructor

Creates a new AsyncWorker.

explicit AsyncWorker(const Function& callback, const char* resource_name);

• [in] callback: The function which will be called when an asynchronous operations ends. The given function is called from the main event loop thread.
• [in] resource_name: Null-terminated strings that represents the identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.

Returns an AsyncWork instance which can later be queued for execution by calling Queue.

Constructor

Creates a new AsyncWorker.

explicit AsyncWorker(const Function& callback, const char* resource_name, const Object& resource);

• [in] callback: The function which will be called when an asynchronous operations ends. The given function is called from the main event loop thread.
• [in] resource_name: Null-terminated strings that represents the identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
• [in] resource: Object associated with the asynchronous operation that will be passed to possible async_hooks.

Returns an AsyncWork instance which can later be queued for execution by calling Queue.

Constructor

Creates a new AsyncWorker.

explicit AsyncWorker(const Object& receiver, const Function& callback);

• [in] receiver: The this object passed to the called function.
• [in] callback: The function which will be called when an asynchronous operations ends. The given function is called from the main event loop thread.

Returns an AsyncWork instance which can later be queued for execution by calling Queue.

Constructor

Creates a new AsyncWorker.

explicit AsyncWorker(const Object& receiver, const Function& callback,const char* resource_name);

• [in] receiver: The this object passed to the called function.
• [in] callback: The function which will be called when an asynchronous operations ends. The given function is called from the main event loop thread.
• [in] resource_name: Null-terminated strings that represents the identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.

Returns an AsyncWork instance which can later be queued for execution by calling Queue.

Constructor

Creates a new AsyncWorker.

explicit AsyncWorker(const Object& receiver, const Function& callback, const char* resource_name, const Object& resource);

• [in] receiver: The this object passed to the called function.
• [in] callback: The function which will be called when an asynchronous operations ends. The given function is called from the main event loop thread.
• [in] resource_name: Null-terminated strings that represents the identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
• [in] resource: Object associated with the asynchronous operation that will be passed to possible async_hooks.

Returns an AsyncWork instance which can later be queued for execution by calling Queue.

Destructor

Deletes the created work object that is used to execute logic asynchronously.

virtual ~AsyncWorker();

Operator

operator napi_async_work() const;

Returns the N-API napi_async_work wrapped by the AsyncWorker object. This can be used to mix usage of the C N-API and node-addon-api.

Example

The first step to use the AsyncWorker class is to create a new class that inherit from it and implement the Execute abstract method. Typically input to your worker will be saved within class' fields generally passed in through its constructor.

When the Execute method completes without errors the OnOK function callback will be invoked. In this function the results of the computation will be reassembled and returned back to the initial JavaScript context.

AsyncWorker ensures that all the code in the Execute function runs in the background out of the event loop thread and at the end the OnOK or OnError function will be called and are executed as part of the event loop.

The code below show a basic example of AsyncWorker the implementation:

#include<napi.h>

#include <chrono>
#include <thread>

use namespace Napi;

class EchoWorker : public AsyncWorker {
    public:
        EchoWorker(Function& callback, std::string& echo)
        : AsyncWorker(callback), echo(echo) {}

        ~EchoWorker() {}
    // This code will be executed on the worker thread
    void Execute() {
        // Need to simulate cpu heavy task
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }

    void OnOK() {
        HandleScope scope(Env());
        Callback().Call({Env().Null(), String::New(Env(), echo)});
    }

    private:
        std::string echo;
};

The EchoWorker's contructor calls the base class' constructor to pass in the callback that the AsyncWorker base class will store persistently. When the work on the Execute method is done the OnOk method is called and the results return back to JavaScript invoking the stored callback with its associated environment.

The following code shows an example on how to create and and use an AsyncWorker

Value Echo(const CallbackInfo& info) {
    // You need to check the input data here
    Function cb = info[1].As<Function>();
    std::string in = info[0].As<String>();
    EchoWorker* wk = new EchoWorker(cb, in);
    wk->Queue();
    return info.Env().Undefined();

Using the implementation of an AsyncWorker is straight forward. You need only create a new instance and pass to its constructor the callback you want to execute when your asynchronous task ends and other data you need for your computation. Once created the only other action you have to do is to call the Queue method that will that will queue the created worker for execution.