add high level guide for using the filesystem api

Signed-off-by: Robin Appelman <robin@icewind.nl>

lib/public/Files/README.md

@@ -0,0 +1,79 @@
+# Nextcloud filesystem API
+
+High level guide to using the Nextcloud filesystem API
+
+## Node API
+
+The "Node API" is the primary api for apps to access the Nextcloud filesystem, each item in the filesystem is
+represented as either a File or Folder node with each node providing access to the relevant filesystem information
+and actions for the node.
+
+### Getting access
+
+Access to the filesystem is provided by the `IRootFolder` which can be injected into your class.
+From the root folder you can either access a user's home folder or access a file or folder by its absolute path.
+
+```php
+use OCP\Files\IRootFolder;
+use OCP\IUserSession;
+
+class FileCreator {
+	/** @var IUserSession */
+	private $userSession;
+	/** @var IRootFolder */
+	private $rootFolder;
+	
+	public function __constructor(IUserSession $userSession, IRootFolder $rootFolder) {
+		$this->userSession = $userSession;
+		$this->rootFolder = $rootFolder;
+	}
+	
+	/**
+	 * Create a new file with specified content in the home folder of the current user
+	 * returning the size of the resulting file. 
+	 */
+	public function createUserFile(string $path, string $content): int {
+		$user = $this->userSession->getUser();
+		if ($user !== null) {
+			// the "user folder" corresponds to the root of the user visible files
+			$userFolder = $this->rootFolder->getUserFolder($user->getUID());
+			// paths passed to a folder method are relative to that folder
+			$file = $userFolder->newFile($path, $content);
+			return $file->getSize();
+		} else {
+			return 0;
+		}
+	}
+}
+```
+
+For details on the specific methods provided by file and folder nodes see the method documentation from the `OCP\Files\File` and `OCP\Files\Folder` interfaces.
+
+## Direct storage access
+
+While it should be generally avoided in favor of the higher level apis,
+sometimes an app needs to talk directly to the storage implementation of it's metadata cache.
+
+You can get access to the underlying storage of a file or folder by calling `getStorage` on the node or first getting
+the mountpoint by calling `getMountPoint` and getting the storage from there.
+
+Once you have the storage instance you can use the storage api from `OCP\Files\Storage\IStorage`, note however that
+all paths used in the storage api are internal to the storage, the `IMountPoint` returned from `getMountPoint` provides
+methods for translating between absolute filesystem paths and internal storage paths.
+
+If you need to query the cached metadata directory you can get the `OCP\Files\Cache\ICache` from the storage by calling `getCache`.
+
+## Implementing a storage
+
+The recommended way for implementing a storage backend is by sub-classing `OC\Files\Storage\Common` which provides
+fallback implementations for various methods, reducing the amount of work required to implement the full storage api.
+Note however that various of these fallback implementations are likely to be significantly less efficient than an
+implementation of the method optimized for the abilities of the storage backend.
+
+## Adding mounts to the filesystem
+
+The recommended way of adding your own mounts to the filesystem from an app is implementing `OCP\Files\Config\IMountProvider`
+and registering the provider using `OCP\Files\Config\IMountProviderCollection::registerProvider`.
+
+Once registered, your provider will be called every time the filesystem is being setup for a user and your mount provider
+can return a list of mounts to add for that user.