要求与规范🔗

Flatpak deliberately imposes very few requirements on applications. However, a small number of standard Linux desktop conventions are expected, primarily to ensure that applications integrate with Linux desktops and app centers. Developers might also encounter a small number of Linux technical conventions.

Information on additional desktop integration options can be found in 桌面集成.

应当遵循的标准🔗

Applications using Flatpak are generally expected to comply with the following standards. Applications that have previously targeted the Linux desktop will typically need to make very few (if any) changes to meet these requirements.

应用ID🔗

As described in 使用Flatpak, Flatpak requires each application to have a unique identifier, which follows a format such as org.example.app.

The format is in reverse-DNS style, meaning the first section should be a domain controlled by the project, while the trailing section represents the specific project. There are exceptions to this, such as extensions, which use the base application-id of the project they extend rather than their own.

As will be seen below and in future sections, this ID is expected to be used in a number of places. Developers must follow the standard D-Bus naming conventions for bus names when creating their own IDs. This format is already recommended by the Desktop File specification and also the Appstream specification.

Here are some practical examples of poor ID choices:

  • org.example.desktop

    This ID is problematic because the Appstream standard, for legacy reasons, treats IDs ending with .desktop as a special case, leading to inconsistency. For the same reason, .Desktop suffixes should not be used for newly named applications. Don't hesitate to repeat the application name even if it is already part of the domain name section of the identifier (e.g. org.example.Example).

  • io.github.foo

    This is problematic because while foo.github.io may be unique to your project, it does not include a project-specific identifier. For instance, if another project creates io.github.foo-bar, issues can arise since the two IDs would then be treated as similar by flatpak (even though they deserve to be treated as different namespaces). A better ID would be io.github.foo.foo, even if the second foo seems redundant.

  • org.example-site.foo

    This ID is not valid according to the D-Bus specification, as the dash - isn't allowed except in the last component. You should replace the dash with an underscore _, and thus use org.example_site.Foo instead.

  • com.github.foo.bar or com.gitlab.foo.bar

    While a project may be hosted on GitHub or GitLab, it does not have control over the github.com or gitlab.com domains. Instead, you should use io.github or io.gitlab as shown above.

  • Org.Example.App

    The domain portion of the ID must be in lowercase, and while not required, the application portion is recommended to be in lowercase as well. Therefore, you should use org.example.app.

MetaInfo files🔗

MetaInfo files provide metadata about applications, which is used by application stores (such as Flathub, GNOME Software and KDE Discover).

The Freedesktop AppStream specification provides a complete reference for providing MetaInfo. You can use the online AppStream MetaInfo Creator to generate a basic file.

MetaInfo files should be named using the application ID, must end with the .metainfo.xml file extension, and must be placed in /app/share/metainfo/. For example:

/app/share/metainfo/org.gnome.Dictionary.metainfo.xml

A legacy convention allows for placing the .appdata.xml file in /app/share/appdata. flatpak-builder will check either directory for its respective extension.

The appstreamcli validate --explain command can be used to check MetaInfo files for errors.

应用图标🔗

Applications are expected to provide an application icon, which is used for their application launcher. These icons should be provided in accordance with the Freedesktop icon specification.

Icons must be named using the application's ID, be in either PNG or SVG format, and must be placed in the standard location:

/app/share/icons/hicolor/$size/apps/

举个例子，128x128px版本的GNOME Dictionary的图标的路径应该是：

/app/share/icons/hicolor/128x128/apps/org.gnome.Dictionary.png

Icons must be square-shaped, meaning their width and height must be the same. The maximum size allowed by the specification is 512x512px. SVG icons are of size scalable:

/app/share/icons/hicolor/scalable/apps/org.gnome.Dictionary.svg

Flatpak will export the following icon name patterns: $FLATPAK_ID, $FLATPAK_ID.foo, $FLATPAK_ID-foo. They may end with an extension suffix like .png or .svg. Exported icons can be found in the icons subfolder of either $HOME/.local/share/flatpak/exports/share or /var/lib/flatpak/exports/share, depending on whether it's a system or user install.

The distribution usually appends those paths to $XDG_DATA_DIRS on the host when installing the flatpak package. Unless an icon is exported by Flatpak, host applications cannot access it.

desktop文件🔗

Desktop files are used to provide the desktop environment with information about each application. The Freedesktop specification provides a complete reference for writing desktop files.

Desktop files must be named using the application's ID, followed by the .desktop file extension, and must be placed in /app/share/applications/. For example:

/app/share/applications/org.gnome.Dictionary.desktop

一个desktop文件应当至少包含以下字段： name*（应用程序名）、*exec*（启动命令）、*type*（类型）、*icon*（图标名）和 *categories （所属分类）：

[Desktop Entry]
Name=Gnome Dictionary
Exec=gnome-dictionary
Type=Application
Icon=org.gnome.Dictionary
Categories=Office;Dictionary;

desktop-file-validate 命令可以用来检测desktop文件的错误。

The Exec key of the desktop files is rewritten by Flatpak when installing an app. The original value of the key becomes the value of the --command argument like so:

Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=gnome-dictionary org.gnome.Dictionary

Flatpak will export the following desktop filename patterns: $FLATPAK_ID.desktop, $FLATPAK_ID.foo.desktop, $FLATPAK_ID-foo.desktop. Exported desktop files can be found in the applications subfolder of either $HOME/.local/share/flatpak/exports/share or /var/lib/flatpak/exports/share, depending on whether it's a system or user install.

The distribution usually appends those paths to $XDG_DATA_DIRS on the host when installing the flatpak package. Unless a desktop file is exported by Flatpak, host applications cannot access it.

The metainfo file along with the other metadata such as icon and desktop files is composed into a catalogue by appstream. Since 1.3.4, Flatpak Builder by default uses appstreamcli from libappstream to compose this and executes the following command:

$ appstreamcli compose --no-partial-urls --prefix=/ --origin=${FLATPAK_ID} --media-baseurl=<repo-media-url> --media-dir=${FLATPAK_DEST}/share/app-info/media --result-root=${FLATPAK_DEST} --data-dir=${FLATPAK_DEST}/share/app-info/xmls --icons-dir=${FLATPAK_DEST}/share/app-info/icons/flatpak --components=${FLATPAK_ID} ${FLATPAK_DEST}

${FLATPAK_ID} is the Flatpak application ID and ${FLATPAK_DEST} is set to /app for apps and /usr for runtimes. The media-baseurl controls the base URL used for mirroring screenshots. Usually each app store will have their own base URL for this. On Flathub this is set to https://dl.flathub.org/media/.

技术规范🔗

Flatpak和多数Linux桌面都遵循下列标准技术规范。有Linux使用经验的用户对此可能已有了解。首次使用Linux的开发者同样可以在这里找到有用的信息。

D-Bus🔗

D-Bus是Linux桌面使用的标准IPC（进程间通讯，译者注）框架。大多数应用程序并不会用到D-Bus；但如果需要，Flatpak也同样提供D-Bus支持。

D-Bus可用于启动应用程序，以及系统服务之间的交互。应用程序也可以实现它们自己的D-Bus服务（此时，D-Bus服务的名称必须与应用程序的ID一致）。

Filesystem layout（文件系统布局，译者注）🔗

每个Flatpak沙箱，一个应用运行的环境，包含这个应用的运行时的文件系统。这个准守 standard Linux filesystem conventions （标准Linux文件系统约定，译者注）。

For example, the root of the sandbox contains the /etc directory for configuration files and /usr for multi-user utilities and applications. In addition to these, each sandbox contains a top-level /app directory, which is where the application's own files are located.

XDG基本目录🔗

XDG base directories are standard locations for user-specific application data. Popular toolkits provide convenience functions for accessing XDG base directories. These include:

  • Electron: XDG base directories can be accessed with app.getPath.

  • Glib: XDG base directories can be accessed through the g_get_user_cache_dir (), g_get_user_data_dir (), g_get_user_config_dir () functions.

  • Qt: XDG base directories can be accessed with the QStandardPaths Class.

如果应用没有使用上述工具包，也可以通过下面的方式找到XDG基本目录：

基本目录

用途

默认位置

XDG_CONFIG_HOME

具体用户配置文件

~/.var/app/<app-id>/config

XDG_DATA_HOME

具体用户数据

~/.var/app/<app-id>/data

XDG_CACHE_HOME

Non-essential user-specific data（非基本具体用户数据，译者注）

~/.var/app/<app-id>/cache

XDG_STATE_HOME

State data such as undo history

~/.var/app/<app-id>/.local/state

举个例子，GNOME Dictionary会把具体用户数据存放在:

~/.var/app/org.gnome.Dictionary/data/gnome-dictionary

These environment variables are always set by Flatpak and override any host values. However, if access to directories on the host is needed, the $HOST_XDG_CONFIG_HOME, $HOST_XDG_DATA_HOME, $HOST_XDG_CACHE_HOME, and $HOST_XDG_STATE_HOME environment variables will be set if they are also available on the host.

Note that $XDG_STATE_HOME and $HOST_XDG_STATE_HOME are only supported by Flatpak 1.13 and later. If your application needs to work on earlier versions of Flatpak, you can use the --persist=.local/state and --unset-env=XDG_STATE_HOME finish args, ensuring the app will use the correct directory, even after Flatpak is later upgraded to version >1.13.

Also, note that applications can be configured to use non-default base directory locations (see 沙箱权限).