byte-stream/lib/OutputBuffer.php

<?php

namespace Amp\ByteStream;

use Amp\Deferred;
use Amp\Promise;
use Amp\Success;

class OutputBuffer implements OutputStream, Promise {
    /** @var \Amp\Deferred|null */
    private $deferred;

    /** @var string */
    private $contents;

    private $closed = false;

    public function __construct() {
        $this->deferred = new Deferred;
    }

    public function write(string $data): Promise {
        if ($this->closed) {
            throw new ClosedException("The stream has already been closed.");
        }

        $this->contents .= $data;

        return new Success(\strlen($data));
    }

    public function end(string $finalData = ""): Promise {
        if ($this->closed) {
            throw new ClosedException("The stream has already been closed.");
        }

        $this->contents .= $finalData;
        $this->closed = true;

        $this->deferred->resolve($this->contents);
        $this->contents = "";

        return new Success(\strlen($finalData));
    }

    public function onResolve(callable $onResolved) {
        $this->deferred->promise()->onResolve($onResolved);
    }
}