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+# CLI and App
+
+`App` is a singleton **instance** of an [Astal.Application](https://aylur.github.io/libastal/astal3/class.Application.html).
+
+Depending on gtk version require paths will differ
+
+<!--TODO: remove gtk4 notice when its available-->
+
+```lua
+local App = require("astal.gtk3.app")
+
+local App = require("astal.gtk4.app") -- not yet available
+```
+
+## Entry point
+
+`App:start` is a wrapper function for `App:run`
+that will only run the application if there is no
+instance already running with the specified name.
+
+:::code-group
+
+```lua [init.lua]
+App:start({
+    instance_name = "my-instance", -- defaults to "astal"
+    main = function()
+        -- setup anything
+        -- instantiate widgets
+    end,
+})
+```
+
+:::
+
+## Messaging from CLI
+
+If you want to interact with an instance from the CLI,
+you can do so by sending a request.
+
+```lua
+App:start({
+    main = function() end,
+
+    ---@param request string
+    ---@param res fun(response: any): nil
+    request_handler = function(request, res)
+        if request == "say hi" then
+            return res("hi cli")
+        end
+        res("unknown command")
+    end
+})
+```
+
+```sh
+astal say hi
+# hi cli
+```
+
+## Toggling Windows by their name
+
+In order for Astal to know about your windows, you have to register them.
+You can do this by specifying a **unique** `name` and calling `App:add_window`.
+
+```lua
+local App = require("astal.gtk3.app")
+
+local function Bar()
+    return Widget.Window({
+        name = "Bar",
+        setup = function(self)
+            App:add_window(self)
+        end,
+        Widget.Box()
+    })
+end
+```
+
+You can also invoke `App:add_window` by simply passing the `App` to the `application` prop.
+
+```lua
+local App = require("astal.gtk3.app")
+
+local function Bar()
+    return Widget.Window({
+        name = "Bar",
+        application = App,
+        Widget.Box()
+    })
+end
+```
+
+```sh
+astal -t Bar
+```
+
+:::warning
+When assigning the `application` prop make sure `name` comes before.
+Props are set sequentially and if name is applied after application it won't work.
+:::
+
+## Client instances
+
+The first time `App:start` is invoked the `main` function gets called.
+While that instance is running any subsequent execution of the script will call
+the `client` function.
+
+:::code-group
+
+```lua [init.lua]
+App:start({
+    -- main instance
+    main = function(...)
+        local args = { ... }
+        print(string.format("{%s}", table.concat(args, ", ")))
+    end,
+
+    -- client instance
+    client = function(request, ...)
+        local res = request("you can send a request to the main instance")
+        print(res)
+    end,
+
+    -- this runs in the main instance
+    request_handler = function(request, res)
+        res("response from main instance")
+    end
+})
+```
+
+:::