## Asynchronous work helpers

`Nan::AsyncWorker` and `Nan::AsyncProgressWorker` are helper classes that make working with asynchronous code easier.

 - Nan::AsyncWorker
 - Nan::AsyncProgressWorker
 - Nan::AsyncQueueWorker

### Nan::AsyncWorker

`Nan::AsyncWorker` is an _abstract_ class that you can subclass to have much of the annoying asynchronous queuing and handling taken care of for you. It can even store arbitrary V8 objects for you and have them persist while the asynchronous work is in progress.

Definition:

```c++
class AsyncWorker {
 public:
  explicit AsyncWorker(Callback *callback_);

  virtual ~AsyncWorker();

  virtual void WorkComplete();

  void SaveToPersistent(const char *key, const v8::Local<v8::Value> &value);

  void SaveToPersistent(const v8::Local<v8::String> &key,
                        const v8::Local<v8::Value> &value);

  void SaveToPersistent(uint32_t index,
                        const v8::Local<v8::Value> &value);

  v8::Local<v8::Value> GetFromPersistent(const char *key) const;

  v8::Local<v8::Value> GetFromPersistent(const v8::Local<v8::String> &key) const;

  v8::Local<v8::Value> GetFromPersistent(uint32_t index) const;

  virtual void Execute() = 0;

  uv_work_t request;

  virtual void Destroy();

 protected:
  Persistent<v8::Object> persistentHandle;

  Callback *callback;

  virtual void HandleOKCallback();

  virtual void HandleErrorCallback();

  void SetErrorMessage(const char *msg);

  const char* ErrorMessage();
};
```


### Nan::AsyncProgressWorker

`Nan::AsyncProgressWorker` is an _abstract_ class that extends `Nan::AsyncWorker` and adds additional progress reporting callbacks that can be used during the asynchronous work execution to provide progress data back to JavaScript.

Definition:

```c++
class AsyncProgressWorker : public AsyncWorker {
 public:
  explicit AsyncProgressWorker(Callback *callback_);

  virtual ~AsyncProgressWorker();

  void WorkProgress();

  class ExecutionProgress {
   public:
    void Send(const char* data, size_t size) const;
  };

  virtual void Execute(const ExecutionProgress& progress) = 0;

  virtual void HandleProgressCallback(const char *data, size_t size) = 0;

  virtual void Destroy();
```


### Nan::AsyncQueueWorker

`Nan::AsyncQueueWorker` will run a `Nan::AsyncWorker` asynchronously via libuv. Both the `execute` and `after_work` steps are taken care of for you. Most of the logic for this is embedded in `Nan::AsyncWorker`.

Definition:

```c++
void AsyncQueueWorker(AsyncWorker *);
```
