I am able to use the PHP Symfony bundle Vich-Uploader to upload files to a private bucket in Amazon S3, via the php sdk Flysystem adapter.

How can I get a signed/presigned URL for a key, after the upload?

I can load the plugin into Flysystem, but Vich won't use the $filesystem->getPresignedUrl('/tmp/some/target') method.

Is this supported by the bundle in any way? Or what class should be extended and registered in order to call the presignedURL method correctly?

There is not a direct way into VichUploader, but it is possible to tell Symfony to use a custom Storage and add the plugin logic inside there.

Install presigned-URLs plugin for Flysystem

composer require sistemi-etime/flysystem-plugin-aws-s3-v3

Use the plugin with the Flysystem, via the bundle oneup_flysystem.yaml config:

oneup_flysystem:
    filesystems:
        aws_s3:
            ...
            ...
            plugins:
                - s3.plugin.presigned_url

Define your own Storage

<?php

use Vich\UploaderBundle\Storage\FlysystemStorage;

class CustomFlysystemStorage extends FlysystemStorage
{
    // Copy the parent method code but then return the presigned URL
    public function resolveUri($obj, ?string $fieldName = null, ?string $className = null)
    {
        [$mapping, $filename] = $this->getFilename($obj, $fieldName, $className);

        if (empty($filename)) {
            return null;
        }

        $dir = $mapping->getUploadDir($obj);
        $path = !empty($dir) ? $dir.'/'.$filename : $filename;

        $fs = $this->getFilesystem($mapping, $obj, $fieldName, $className);

        return $fs->getPresignedUrl($path);
    }
}

Make use of it inside services.yaml:

# alias the custom storage to be used instead of the one defined by VichUploader
vich_uploader.storage.flysystem: '@Your\Own\Namespace\CustomFlysystemStorage'

---

Fix autowiring (why? 🤷‍♂️)

[Edit: that's why]

Additionally I had to explicitly alias the arguments required by the FlysystemStorage in order for Symfony Autowire to initialise the custom storage:

Vich\UploaderBundle\Mapping\PropertyMappingFactory: '@vich_uploader.property_mapping_factory'
League\Flysystem\MountManager: '@oneup_flysystem.mount_manager'

Although I am not sure why this was necessary.

I would expect that either vich_uploader.property_mapping_factory and oneup_flysystem.mount_manager are already defined and aliased by the configs from VichUploaderBundle and OneupFlysystemBundle

If anybody knows how to skip this fix, please comment.