# core/json/stream/streamers

This module provides a bunch of classes to stream object or array elements.
Also, the module provides an abstract class to create your own streamers

## Usage

```js
import { convertIfDate } from 'core/json';

import Parser from 'core/json/stream/parser';
import { ArrayStreamer } from 'core/json/stream/assembler';

const
  parser = Parser.from('[1, {"a": true}, true, -0.234]', new ArrayStreamer({reviver: convertIfDate}));

for await (const val of parser) {
  // {index: 0, value: 1}
  // {index: 1, value: {a: true}}
  // {index: 2, value: true}
  // {index: 3, value: -0.234}
  console.log(val);
}
```

## Streamers

### ObjectStreamer

An instance of the ObjectStreamer class takes an iterable object of parsed tokens of some object and
yields assembled elements of this object.

```js
import Parser from 'core/json/stream/parser';
import { ObjectStreamer } from 'core/json/stream/assembler';

const
  parser = Parser.from('{"a": 1, "b": true}', new ObjectStreamer());

for await (const val of parser) {
  // {key: 'a', value: 1}
  // {key: 'b', value: true}
  console.log(val);
}
```

#### ArrayStreamer

An instance of ArrayStreamer ObjectStreamer class takes an iterable object of parsed tokens of some array and
yields assembled elements of this array.

```js
import Parser from 'core/json/stream/parser';
import { ArrayStreamer } from 'core/json/stream/assembler';

const
  parser = Parser.from('[1, {"a": true}, true, -0.234]', new ArrayStreamer());

for await (const val of parser) {
  // {index: 0, value: 1}
  // {index: 1, value: {a: true}}
  // {index: 2, value: true}
  // {index: 3, value: -0.234}
  console.log(val);
}
```

#### AbstractStreamer

When creating a new streamer class, extend it from the `AbstractStreamer` and implement the `checkToken` and `push` methods.

```typescript
import type { Token } from 'core/json/stream/parser';
import type { AssemblerOptions } from 'core/json/stream/assembler';

import Streamer, { StreamedArray } from 'core/json/stream/streamers/interface';

export default class ArrayStreamer<T = unknown> extends Streamer<StreamedArray<T>> {
  /**
   * Index of the current streamed array element
   */
  protected index: number = 0;

  public constructor(opts?: AssemblerOptions) {
    super(opts);
  }

  /** @inheritDoc */
  protected checkToken(chunk: Token): boolean {
    if (chunk.name !== 'startArray') {
      throw new TypeError('The top-level object should be an array');
    }

    return true;
  }

  /** @inheritDoc */
  protected*push(): Generator<StreamedArray<T>> {
    const
      {value} = this.assembler;

    if (Object.isArray(value) && value.length > 0) {
      yield {
        index: this.index++,
        value: Object.cast(value.pop())
      };
    }
  }
}
```

#### API

##### constructor

The instance constructor takes one optional parameter.
It is an object with parameters for a token [[Assembler]].

##### processToken

Processes the passed JSON token and yields values.

```js
import Parser from 'core/json/stream/parser';
import { ArrayStreamer } from 'core/json/stream/streamers';

const
  streamer = new Streamer(),
  values = [];

for (const token of new Parser().processChunk('["foo"]')) {
  values.push(...streamer.processToken(token));
}

// ['foo']
console.log(values);
```