Add the draft for the upload pipeline

See #438

docs/php/api/file_uploads.md

@@ -0,0 +1,162 @@
+# File Uploads
+
+Uploading files is handled through the unified upload pipeline introduced in WoltLab Suite 6.1.
+The API covers the upload process, storage, thumbnail generation and serving of files to the browser.
+Attachments are implemented as an extra layer on top of the upload pipeline.
+
+The most common use cases are the attachment system that relies on the WYSIWYG editor as well as the `FileProcessorFormField`.
+
+# Provide the `IFileProcessor`
+
+At the very core of each uploadable type is an implementation of `IFileProcessor` that handles the validation and handling of new files.
+
+It is strongly recommended to derive from `AbstractFileProcessor` that makes it easy to opt out of some extra features and will provide backwards compatibility with new features.
+
+## Important Methods and Features
+
+Please refer to the documentation in `IFileProcessor` for an explanation of methods that are not explained here.
+
+### Thumbnails
+
+It is possible to automatically generate thumbnails for any uploaded file.
+The number of thumbnail variants is not limited but each thumbnail must have an identifier that is unique for your file processor.
+
+```php
+#[\Override]
+public function getThumbnailFormats(): array
+{
+    return [
+        new ThumbnailFormat(
+            '', // An empty string is a valid identifier.
+            800,
+            600,
+            true, // Retaining the dimensions can cause _one_ of the sides to underflow the configured width or height.
+        ),
+        new ThumbnailFormat(
+            'square',
+            100,
+            100,
+            false, // This will generate a 100x100 thumbnail where the longest size will be cropped from the center.
+        ),
+    ];
+}
+```
+
+The abstract implementation returns an empty array which will disable thumbnails entirely.
+Changes to the thumbnail configuration, for example, updating the dimensions or adding new thumbnail formats are not applied to existing files automatically.
+
+The system tracks the configuration used to generate a thumbnail.
+The existing rebuild data worker for files will check the existing thumbnails against the formats provided by your processor and regenerate any thumbnail when possible.
+
+The identifier must be stable because it is used to verify if a specific thumbnail still matches the configured settings.
+
+### Resizing Images Before Uploading
+
+You can opt-in to the resize feature by returning a custom `ResizeConfiguration` from `getResizeConfiguration`, otherwise images of arbitrary size will be accepted.
+
+```php
+#[\Override]
+public function getResizeConfiguration(): ResizeConfiguration
+{
+    if (!\ATTACHMENT_IMAGE_AUTOSCALE) {
+        // The resizing has been disabled through the options.
+        return ResizeConfiguration::unbounded();
+    }
+
+    return new ResizeConfiguration(
+        \ATTACHMENT_IMAGE_AUTOSCALE_MAX_WIDTH,
+        \ATTACHMENT_IMAGE_AUTOSCALE_MAX_HEIGHT,
+        ResizeFileType::fromString(\ATTACHMENT_IMAGE_AUTOSCALE_FILE_TYPE),
+        \ATTACHMENT_IMAGE_AUTOSCALE_QUALITY
+    );
+}
+```
+
+The `ResizeFileType` controls the output format of the resized image.
+The available options are `jpeg` and `webp` but it is also possible to keep the existing image format.
+
+### Adopting Files and Thumbnails
+
+Files are associated with your object type after being uploaded but you possibly want to store the file id in your database table.
+This is where `adopt(File $file, array $context): void` comes into play which notifies you of the successful upload of a file while providing the context that is used to upload the file in the first place.
+
+Thumbnails are generated in a separate request for performance reasons and you are being notified through `adoptThumbnail(FileThumbnail $thumbnail): void`.
+This is meant to allow you to track the thumbnail id in the database.
+
+### Tracking Downloads
+
+File downloads are handled through the `FileDownloadAction` which validates the requested file and permissions to download it.
+Every time a file is being downloaded, `trackDownload(File $file): void` is invoked to allow you to update any counters.
+
+Static images are served directly by the web server for performance reasons and it is not possible to track those accesses.
+
+## Registering the File Processor
+
+The file processor is registered as an object type for `com.woltlab.wcf.file` through the `objectType.xml`:
+
+```xml
+<type>
+	<name>com.woltlab.wcf.attachment</name>
+	<definitionname>com.woltlab.wcf.file</definitionname>
+	<classname>wcf\system\file\processor\AttachmentFileProcessor</classname>
+</type>
+```
+
+# `FileProcessorFormField`
+
+It is highly recommended that you take advantage of the existing form builder field `FileProcessorFormField`.
+The integration with the form builder enables you to focus on the file processing and does not require you to manually handle the integration of the upload field.
+
+TODO: Explain the usage.
+
+# Implementing an Unmanaged File Upload
+
+If you cannot use or want to use the existing form builder implementation you can still implement the UI yourself following this guide.
+
+## Creating the Context for New Files
+
+The HTML element for the file upload is generated through the helper method `FileProcessor::getHtmlElement()` that expects a reference to your `IFileProcessor` as well as a context for new files.
+
+The context is an array with arbitrary values that will be provided to your `IFileProcessor` when processing uploaded files.
+You can provide anything you need in order to recognize what the file belongs to, for example, an identifier, object ids, et cetera.
+
+```php
+final class ExampleFileProcessor extends AbstractFileProcessor {
+    public function toHtmlElement(string $someIdentifier, int $someObjectID): string {
+        return FileProcessor::getInstance()->getHtmlElement(
+            $this,
+            [
+                'identifier' => $someIdentifier,
+                'objectID' => $someObjectID,
+            ],
+        );
+    }
+}
+```
+
+This code will generate a `<woltlab-core-file-upload>` HTML element that can be inserted anywhere on the page.
+You do not need to initialize any extra JavaScript to make the element work, it will be initialized dynamically.
+
+## Lifecycle of Uploaded Files
+
+Any file that passes the pre-upload validation will be uploaded to the server.
+This will trigger the `uploadStart` event on the `<woltlab-core-file-upload>` element, exposing a `<woltlab-core-file>` element as the only detail.
+
+It is your responsibility to insert this element at a location of your choice on the page, it represents the upload progress, reports any errors when uploading the file and shows the uploaded file on completion.
+
+The `<woltlab-core-file>` element exposes the `.ready` property that is a Promise representing the state of the upload.
+A successful upload will resolve the promise without any value, you can then access the file id through the `.fileId` property.
+
+A failed upload will reject the `.ready` promise without any value, instead you can retrieve the error through the `.apiError` property if you want to further process it.
+The UI is automatically updated with the error message, you only need to handle the `.apiError` property if you need to inspect the root cause.
+
+## Deleting a File
+
+You can delete a file from the UI by invoking `deleteFile()` from `WoltLabSuite/Core/Api/Files` which takes the value of `.fileId`.
+
+## Render a Previously Uploaded File
+
+You can render the `<woltlab-core-file>` element through `File::toHtmlElement()`.
+This method accepts an optional list of meta data that is serialized to JSON and exposed on the `data-meta-data` property.
+
+The `.ready` promise exists for these files too and will resolve immediately.

mkdocs.yml

@@ -32,6 +32,7 @@ nav:
           - 'Comments': 'php/api/comments.md'
           - 'Cronjobs': 'php/api/cronjobs.md'
           - 'Events': 'php/api/events.md'
+          - 'File Uploads': 'php/api/file_uploads.md'
           - 'Form Builder':
               - 'Overview': 'php/api/form_builder/overview.md'
               - 'Structure': 'php/api/form_builder/structure.md'