SwitchPlugin and BackgroundTaskPlugin with tests (#3137)

frontend/ui/plugin/background_task_plugin.lua

@@ -0,0 +1,35 @@
+--[[--
+BackgroundTaskPlugin creates a plugin with a switch to enable or disable it and executes a
+background task.
+See spec/unit/background_task_plugin_spec.lua for the usage.
+]]
+
+local PluginShare = require("pluginshare")
+local SwitchPlugin = require("ui/plugin/switch_plugin")
+
+local BackgroundTaskPlugin = SwitchPlugin:extend()
+
+function BackgroundTaskPlugin:_schedule(settings_id)
+    local enabled = function()
+        if not self.enabled then
+            return false
+        end
+        if settings_id ~= self.settings_id then
+            return false
+        end
+
+        return true
+    end
+
+    table.insert(PluginShare.backgroundJobs, {
+        when = self.when,
+        repeated = enabled,
+        executable = self.executable,
+    })
+end
+
+function BackgroundTaskPlugin:_start()
+    self:_schedule(self.settings_id)
+end
+
+return BackgroundTaskPlugin