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+# Plugin system
+
+## Developer API
+
+### What can I do with plugins?
+
+The plugin system lets you:
+
+- insert content into specific places across templates.
+- alter data before templates rendering.
+- alter data before saving new links.
+
+
+### How can I create a plugin for Shaarli?
+
+First, chose a plugin name, such as `demo_plugin`.
+
+Under `plugin` folder, create a folder named with your plugin name. Then create a <plugin_name>.meta file and a <plugin_name>.php file in that folder.
+
+You should have the following tree view:
+
+```
+| index.php
+| plugins/
+|---| demo_plugin/
+|   |---| demo_plugin.meta
+|   |---| demo_plugin.php
+```
+
+
+### Plugin initialization
+
+At the beginning of Shaarli execution, all enabled plugins are loaded. At this point, the plugin system looks for an `init()` function in the <plugin_name>.php to execute and run it if it exists. This function must be named this way, and takes the `ConfigManager` as parameter.
+
+    <plugin_name>_init($conf)
+
+This function can be used to create initial data, load default settings, etc. But also to set *plugin errors*. If the initialization function returns an array of strings, they will be understand as errors, and displayed in the header to logged in users.
+
+The plugin system also looks for a `description` variable in the <plugin_name>.meta file, to be displayed in the plugin administration page.
+
+    description="The plugin does this and that."
+
+### Understanding hooks
+
+A plugin is a set of functions. Each function will be triggered by the plugin system at certain point in Shaarli execution.
+
+These functions need to be named with this pattern:
+
+```
+hook_<plugin_name>_<hook_name>($data, $conf)
+```
+
+Parameters:
+
+- data: see [$data section](https://shaarli.readthedocs.io/en/master/Plugin-System/#plugins-data)
+- conf: the `ConfigManager` instance.
+
+For example, if my plugin want to add data to the header, this function is needed:
+
+    hook_demo_plugin_render_header
+
+If this function is declared, and the plugin enabled, it will be called every time Shaarli is rendering the header.
+
+
+### Plugin's data
+
+#### Parameters
+
+Every hook function has a `$data` parameter. Its content differs for each hooks.
+
+**This parameter needs to be returned every time**, otherwise data is lost.
+
+    return $data;
+
+#### Special data
+
+Special additional data are passed to every hook through the
+`$data` parameter to give you access to additional context, and services.
+
+Complete list:
+
+  * `_PAGE_` (string): if the current hook is used to render a template, its name is passed through this additional parameter.
+  * `_LOGGEDIN_` (bool): whether the user is logged in or not.
+  * `_BASE_PATH_` (string): if Shaarli instance is hosted under a subfolder, contains the subfolder path to `index.php` (e.g. `https://domain.tld/shaarli/` -> `/shaarli/`).
+  * `_BOOKMARK_SERVICE_` (`BookmarkServiceInterface`): bookmark service instance, for advanced usage.
+
+Example:
+
+```php
+if ($data['_PAGE_'] === TemplatePage::LINKLIST && $data['LOGGEDIN'] === true) {
+    // Do something for logged in users when the link list is rendered
+}
+```
+
+#### Filling templates placeholder
+
+Template placeholders are displayed in template in specific places.
+
+RainTPL displays every element contained in the placeholder's array. These element can be added by plugins.
+
+For example, let's add a value in the placeholder `top_placeholder` which is displayed at the top of my page:
+
+```php
+$data['top_placeholder'][] = 'My content';
+# OR
+array_push($data['top_placeholder'], 'My', 'content');
+
+return $data;
+```
+
+
+#### Data manipulation
+
+When a page is displayed, every variable send to the template engine is passed to plugins before that in `$data`.
+
+The data contained by this array can be altered before template rendering.
+
+For example, in linklist, it is possible to alter every title:
+
+```php
+// mind the reference if you want $data to be altered
+foreach ($data['links'] as &$value) {
+    // String reverse every title.
+    $value['title'] = strrev($value['title']);
+}
+
+return $data;
+```
+
+### Metadata
+
+Every plugin needs a `<plugin_name>.meta` file, which is in fact an `.ini` file (`KEY="VALUE"`), to be listed in plugin administration.
+
+Each file contain two keys:
+
+- `description`: plugin description
+- `parameters`: user parameter names, separated by a `;`.
+- `parameter.<PARAMETER_NAME>`: add a text description the specified parameter.
+
+> Note: In PHP, `parse_ini_file()` seems to want strings to be between by quotes `"` in the ini file.
+
+### Understanding relative paths
+
+Because Shaarli is a self-hosted tool, an instance can either be installed at the root directory, or under a subfolder.
+This means that you can *never* use absolute paths (eg `/plugins/mything/file.png`).
+
+If a file needs to be included in server end, use simple relative path:
+`PluginManager::$PLUGINS_PATH . '/mything/template.html'`.
+
+If it needs to be included in front end side (e.g. an image),
+the relative path must be prefixed with special data `_BASE_PATH_`:
+`($data['_BASE_PATH_'] ?? '') . '/' . PluginManager::$PLUGINS_PATH . '/mything/picture.png`.
+
+Note that special placeholders for CSS and JS files (respectively `css_files` and `js_files`) are already prefixed
+with the base path in template files.
+
+### It's not working!
+
+Use `demo_plugin` as a functional example. It covers most of the plugin system features.
+
+If it's still not working, please [open an issue](https://github.com/shaarli/Shaarli/issues/new).
+
+
+### Hooks
+
+| Hooks         | Description   |
+| ------------- |:-------------:|
+| [render_header](#render_header) | Allow plugin to add content in page headers. |
+| [render_includes](#render_includes) | Allow plugin to include their own CSS files. |
+| [render_footer](#render_footer) | Allow plugin to add content in page footer and include their own JS files. |
+| [render_linklist](#render_linklist) | It allows to add content at the begining and end of the page, after every link displayed and to alter link data. |
+| [render_editlink](#render_editlink) |  Allow to add fields in the form, or display elements. |
+| [render_tools](#render_tools) |  Allow to add content at the end of the page. |
+| [render_picwall](#render_picwall) |  Allow to add content at the top and bottom of the page. |
+| [render_tagcloud](#render_tagcloud) |  Allow to add content at the top and bottom of the page, and after all tags. |
+| [render_taglist](#render_taglist) |  Allow to add content at the top and bottom of the page, and after all tags. |
+| [render_daily](#render_daily) |  Allow to add content at the top and bottom of the page, the bottom of each link and to alter data. |
+| [render_feed](#render_feed) | Allow to do add tags in RSS and ATOM feeds. |
+| [save_link](#save_link) | Allow to alter the link being saved in the datastore. |
+| [delete_link](#delete_link) | Allow to do an action before a link is deleted from the datastore. |
+| [save_plugin_parameters](#save_plugin_parameters) | Allow to manipulate plugin parameters before they're saved. |
+
+
+#### render_header
+
+Triggered on every page - allows plugins to add content in page headers.
+
+
+##### Data
+
+`$data` is an array containing:
+
+  - [Special data](#special-data)
+
+##### Template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `buttons_toolbar`: after the list of buttons in the header.
+
+![buttons_toolbar_example](http://i.imgur.com/ssJUOrt.png)
+
+- `fields_toolbar`: after search fields in the header.
+
+> Note: This will only be called in linklist.
+
+![fields_toolbar_example](http://i.imgur.com/3GMifI2.png)
+
+
+#### render_includes
+
+Triggered on every page - allows plugins to include their own CSS files.
+
+##### data
+
+`$data` is an array containing:
+
+  - [Special data](#special-data)
+
+##### Template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `css_files`: called after loading default CSS.
+
+> Note: only add the path of the CSS file. E.g: `plugins/demo_plugin/custom_demo.css`.
+
+
+#### render_footer
+
+Triggered on every page.
+
+Allow plugin to add content in page footer and include their own JS files.
+
+##### data
+
+`$data` is an array containing:
+
+  - [Special data](#special-data)
+
+##### Template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `text`: called after the end of the footer text.
+- `endofpage`: called at the end of the page.
+
+![text_example](http://i.imgur.com/L5S2YEH.png)
+
+- `js_files`: called at the end of the page, to include custom JS scripts.
+
+> Note: only add the path of the JS file. E.g: `plugins/demo_plugin/custom_demo.js`.
+
+
+#### render_linklist
+
+Triggered when `linklist` is displayed (list of links, permalink, search, tag filtered, etc.).
+
+It allows to add content at the begining and end of the page, after every link displayed and to alter link data.
+
+##### data
+
+`$data` is an array containing:
+
+  - All templates data, including links.
+  - [Special data](#special-data)
+
+##### template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `action_plugin`: next to the button "private only" at the top and bottom of the page.
+
+![action_plugin_example](http://i.imgur.com/Q12PWg0.png)
+
+- `link_plugin`: for every link, between permalink and link URL.
+
+![link_plugin_example](http://i.imgur.com/3oDPhWx.png)
+
+- `plugin_start_zone`: before displaying the template content.
+
+![plugin_start_zone_example](http://i.imgur.com/OVBkGy3.png)
+
+- `plugin_end_zone`: after displaying the template content.
+
+![plugin_end_zone_example](http://i.imgur.com/6IoRuop.png)
+
+
+#### render_editlink
+
+Triggered when the link edition form is displayed.
+
+Allow to add fields in the form, or display elements.
+
+##### data
+
+`$data` is an array containing:
+
+  - All templates data.
+  - [Special data](#special-data)
+
+##### template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `edit_link_plugin`: after tags field.
+
+![edit_link_plugin_example](http://i.imgur.com/5u17Ens.png)
+
+
+#### render_tools
+
+Triggered when the "tools" page is displayed.
+
+Allow to add content at the end of the page.
+
+##### data
+
+`$data` is an array containing:
+
+  - All templates data.
+  - [Special data](#special-data)
+
+##### template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `tools_plugin`: at the end of the page.
+
+![tools_plugin_example](http://i.imgur.com/Bqhu9oQ.png)
+
+
+#### render_picwall
+
+Triggered when picwall is displayed.
+
+Allow to add content at the top and bottom of the page.
+
+##### data
+
+`$data` is an array containing:
+
+  - All templates data.
+  - [Special data](#special-data)
+
+##### template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `plugin_start_zone`: before displaying the template content.
+- `plugin_end_zone`: after displaying the template content.
+
+![plugin_start_end_zone_example](http://i.imgur.com/tVTQFER.png)
+
+
+#### render_tagcloud
+
+Triggered when tagcloud is displayed.
+
+Allow to add content at the top and bottom of the page.
+
+##### data
+
+`$data` is an array containing:
+
+  - All templates data.
+  - [Special data](#special-data)
+
+##### Template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `plugin_start_zone`: before displaying the template content.
+- `plugin_end_zone`: after displaying the template content.
+
+For each tag, the following placeholder can be used:
+
+- `tag_plugin`: after each tag
+
+![plugin_start_end_zone_example](http://i.imgur.com/vHmyT3a.png)
+
+
+#### render_taglist
+
+Triggered when taglist is displayed - allows to add content at the top and bottom of the page.
+
+##### data
+
+`$data` is an array containing:
+
+  - All templates data.
+  - [Special data](#special-data)
+
+##### Template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `plugin_start_zone`: before displaying the template content.
+- `plugin_end_zone`: after displaying the template content.
+
+For each tag, the following placeholder can be used:
+
+- `tag_plugin`: after each tag
+
+#### render_daily
+
+Triggered when tagcloud is displayed.
+
+Allow to add content at the top and bottom of the page, the bottom of each link and to alter data.
+
+
+##### data
+
+`$data` is an array containing:
+
+  - All templates data, including links.
+  - [Special data](#special-data)
+
+##### Template placeholders
+
+Items can be displayed in templates by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `link_plugin`: used at bottom of each link.
+
+![link_plugin_example](http://i.imgur.com/hzhMfSZ.png)
+
+- `plugin_start_zone`: before displaying the template content.
+- `plugin_end_zone`: after displaying the template content.
+
+
+#### render_feed
+
+Triggered when the ATOM or RSS feed is displayed.
+
+Allow to add tags in the feed, either in the header or for each items. Items (links) can also be altered before being rendered.
+
+##### data
+
+`$data` is an array containing:
+
+  - All templates data, including links.
+  - [Special data](#special-data)
+
+##### Template placeholders
+
+Tags can be added in feeds by adding an entry in `$data['<placeholder>']` array.
+
+List of placeholders:
+
+- `feed_plugins_header`: used as a header tag in the feed.
+
+For each links:
+
+- `feed_plugins`: additional tag for every link entry.
+
+
+#### save_link
+
+Triggered when a link is save (new link or edit).
+
+Allow to alter the link being saved in the datastore.
+
+##### data
+
+`$data` is an array containing the link being saved:
+
+- id
+- title
+- url
+- shorturl
+- description
+- private
+- tags
+- created
+- updated
+
+Also [special data](#special-data).
+
+
+#### delete_link
+
+Triggered when a link is deleted.
+
+Allow to execute any action before the link is actually removed from the datastore
+
+##### data
+
+`$data` is an array containing the link being deleted:
+
+- id
+- title
+- url
+- shorturl
+- description
+- private
+- tags
+- created
+- updated
+
+Also [special data](#special-data).
+
+#### save_plugin_parameters
+
+Triggered when the plugin parameters are saved from the plugin administration page.
+
+Plugins can perform an action every times their settings are updated.
+For example it is used to update the CSS file of the `default_colors` plugins.
+
+##### data
+
+`$data` input contains the `$_POST` array.
+
+So if the plugin has a parameter called `MYPLUGIN_PARAMETER`,
+the array will contain an entry with `MYPLUGIN_PARAMETER` as a key.
+
+Also [special data](#special-data).
+
+## Guide for template designers
+
+### Plugin administration
+
+Your theme must include a plugin administration page: `pluginsadmin.html`.
+
+> Note: repo's template link needs to be added when the PR is merged.
+
+Use the default one as an example.
+
+Aside from classic RainTPL loops, plugins order is handle by JavaScript. You can just include `plugin_admin.js`, only if:
+
+- you're using a table.
+- you call orderUp() and orderUp() onclick on arrows.
+- you add data-line and data-order to your rows.
+
+Otherwise, you can use your own JS as long as this field is send by the form:
+
+<input type="hidden" name="order_{$key}" value="{$counter}">
+
+### Placeholder system
+
+In order to make plugins work with every custom themes, you need to add variable placeholder in your templates.
+
+It's a RainTPL loop like this:
+
+    {loop="$plugin_variable"}
+        {$value}
+    {/loop}
+
+You should enable `demo_plugin` for testing purpose, since it uses every placeholder available.
+
+### List of placeholders
+
+**page.header.html**
+
+At the end of the menu:
+
+    {loop="$plugins_header.buttons_toolbar"}
+        {$value}
+    {/loop}
+
+At the end of file, before clearing floating blocks:
+
+    {if="!empty($plugin_errors) && $is_logged_in"}
+        <ul class="errors">
+            {loop="plugin_errors"}
+                <li>{$value}</li>
+            {/loop}
+        </ul>
+    {/if}
+
+**includes.html**
+
+At the end of the file:
+
+```html
+{loop="$plugins_includes.css_files"}
+<link type="text/css" rel="stylesheet" href="{$value}#"/>
+{/loop}
+```
+
+**page.footer.html**
+
+At the end of your footer notes:
+
+```html
+{loop="$plugins_footer.text"}
+     {$value}
+{/loop}
+```
+
+At the end of file:
+
+```html
+{loop="$plugins_footer.js_files"}
+     <script src="{$value}#"></script>
+{/loop}
+```
+
+**linklist.html**
+
+After search fields:
+
+```html
+{loop="$plugins_header.fields_toolbar"}
+     {$value}
+{/loop}
+```
+
+Before displaying the link list (after paging):
+
+```html
+{loop="$plugin_start_zone"}
+     {$value}
+{/loop}
+```
+
+For every links (icons):
+
+```html
+{loop="$value.link_plugin"}
+    <span>{$value}</span>
+{/loop}
+```
+
+Before end paging:
+
+```html
+{loop="$plugin_end_zone"}
+     {$value}
+{/loop}
+```
+
+**linklist.paging.html**
+
+After the "private only" icon:
+
+```html
+{loop="$action_plugin"}
+     {$value}
+{/loop}
+```
+
+**editlink.html**
+
+After tags field:
+
+```html
+{loop="$edit_link_plugin"}
+     {$value}
+{/loop}
+```
+
+**tools.html**
+
+After the last tool:
+
+```html
+{loop="$tools_plugin"}
+     {$value}
+{/loop}
+```
+
+**picwall.html**
+
+Top:
+
+```html
+<div id="plugin_zone_start_picwall" class="plugin_zone">
+    {loop="$plugin_start_zone"}
+        {$value}
+    {/loop}
+</div>
+```
+
+Bottom:
+
+```html
+<div id="plugin_zone_end_picwall" class="plugin_zone">
+    {loop="$plugin_end_zone"}
+        {$value}
+    {/loop}
+</div>
+```
+
+**tagcloud.html**
+
+Top:
+
+```html
+   <div id="plugin_zone_start_tagcloud" class="plugin_zone">
+        {loop="$plugin_start_zone"}
+            {$value}
+        {/loop}
+    </div>
+```
+
+Bottom:
+
+```html
+    <div id="plugin_zone_end_tagcloud" class="plugin_zone">
+        {loop="$plugin_end_zone"}
+            {$value}
+        {/loop}
+    </div>
+```
+
+**daily.html**
+
+Top:
+
+```html
+<div id="plugin_zone_start_picwall" class="plugin_zone">
+     {loop="$plugin_start_zone"}
+         {$value}
+     {/loop}
+</div>
+```
+
+After every link:
+
+```html
+<div class="dailyEntryFooter">
+     {loop="$link.link_plugin"}
+          {$value}
+     {/loop}
+</div>
+```
+
+Bottom:
+
+```html
+<div id="plugin_zone_end_picwall" class="plugin_zone">
+    {loop="$plugin_end_zone"}
+        {$value}
+    {/loop}
+</div>
+```
+
+**feed.atom.xml** and **feed.rss.xml**:
+
+In headers tags section:
+```xml
+{loop="$feed_plugins_header"}
+  {$value}
+{/loop}
+```
+
+After each entry:
+```xml
+{loop="$value.feed_plugins"}
+  {$value}
+{/loop}
+```