Development/Tutorials/Plasma4/Ruby/SimplePasteApplet

From KDE TechBase

Abstract

This tutorial will explain how to make simple KDE Plasma applet using Ruby. The applet will be a simple version of the paste applet. It will allow the user to put a bit of text on the clipboard.

Getting started

Before you get started you need to make sure to have the following installed on your computer.

  • KDE 4.2 or later
  • Ruby 1.8
  • KDE Ruby bindings and Plasma scriptengine

These packages can usually be installed through your distributions package manager.

Package layout

Plasma applets written in Ruby can be distributed as a plasmoid package. A minimal Ruby plasmoid package has the following structure:

  • contents/
    • code/
      • main.rb
  • metadata.desktop

The Ruby code for the plasmoid should be put in a file called main.rb in the contents/code folder of your package. The metadata.desktop file holds metadata about your plasmoid. This includes your name, name of your applet and a description. The metadata.desktop file is in the common .desktop file format, which looks a lot like an old INI file.

A minimal metadata.desktop file looks like this: 

[Desktop Entry]
Name=Simple Ruby applet
Comment=This is a simple applet written in Ruby
Icon=chronometer
Type=Service
ServiceTypes=Plasma/Applet

X-Plasma-API=ruby-script
X-Plasma-MainScript=code/main.rb

X-KDE-PluginInfo-Author=Me
X-KDE-PluginInfo-Email=[email protected]
X-KDE-PluginInfo-Name=ruby-test
X-KDE-PluginInfo-Version=0.1
X-KDE-PluginInfo-Website=http://plasma.kde.org/
X-KDE-PluginInfo-Category=Examples
X-KDE-PluginInfo-Depends=
X-KDE-PluginInfo-License=GPL
X-KDE-PluginInfo-EnabledByDefault=true

Code

Let's first start with the most basic Ruby applet and work from there. Every Plasma applet you're going to make in Ruby has to have a class which inherits the PlasmaScripting::Applet class. You can think of this as your main class for your applet. It will always have at least two methods, initialize and init. While these two seem very similar, there is an important difference. The initialize method is Ruby's default constructor. It will be called by the Ruby interpreter when an object of your class gets initialized (a.k.a. constructed). The init method gets called by Plasma. Plasma calls this method after the applet has been loaded. You can therefore assume that everything is set up when init is called, while with initialize, you only know your applet class is ready.

Start your code file with requiring 'plasma_applet' and opening a module. A minimal Ruby Plasma applet looks like this: 

require 'plasma_applet'

module RubyTest
  class Main < PlasmaScripting::Applet
    def initialize parent
      super parent
    end

    def init
      set_minimum_size 150, 150
    end
  end
end

The module name must match the name you specify in metadata.desktop on the X-KDE-PluginInfo-Name line. The name ruby-test translates to a module name of RubyTest.

The only line in the init method now is set_minimum_size. This, as the name suggests, sets the minimum size for the applet.

To run your applet, you have two options. You can install it first, and then run it. Or, if you're running KDE 4.3 or later, just run it.

Installing your appliet

This is where plasmapkg comes in. It's a small tool which installs and upgrades Plasma packages. To put your code in a package, create a package structure as described in the above section. Outside the package folder run plasmapkg -i <package folder name>, plasmapkg will now install your applet. 

mkdir ruby-test-applet
cd ruby-test-applet
# Copy or create metadata.desktop
mkdir contents
cd contents
mkdir code
cd code
# Copy or create main.rb
cd ../../..
plasmapkg -i ruby-test-applet

plasmapkg can install and upgrade Plasma packages. Since you've now installed your applet, you need to upgrade it after making changes. This isn't much different then installing, just use the -u commandline switch instead of -i.

Running your applet

You can run your applet using a tool called plasmoidviewer.

KDE 4.2

In KDE 4.2 plasmoidviewer takes the name of an installed applet as a parameter. The name of your applet is specified with the X-KDE-PluginInfo-Name line in your metadata.desktop file. You should be able to view your applet after installing it. 

plasmoidviewer ruby-test

KDE 4.3

In KDE 4.3 plasmoidviewer still accepts the name of an installed applet as a parameter. But it can also run without a parameter. It will then try to run a plasmoid from the current directory. In order for this to work, the current directory should have the metadata.desktop file, the contents folder and the name of the directory should exactly match the name of the applet as specified in metadata.desktop. 

plasmoidviewer

A label on an applet

When you have a very basic applet running, you can go two ways. You can put QWidgets on your applet, or draw the applet yourself by implementing the paintInterface method. I like to use standard widgets most of the time, so we're going to place some QWidgets on the plasma applet.

Plasma has a couple of themed widgets. A list can be found in the Plasma API. To place these widgets on your applet, you need a layout. The layout you'll be using most of the time will be the GraphicsLinearLayout, which basically puts your widgets in a horizontal or vertical line.

When putting a Plasma::Label on an applet in a GraphicsLinearLayout, you'll get the following code: 

require 'plasma_applet'

module RubyTest
  class Main < PlasmaScripting::Applet
    def initialize parent
      super parent
    end

    def init
      set_minimum_size 150, 150

      label = Plasma::Label.new self
      label.text = 'This is a label on a plasmoid, hello Plasma!'

      layout = Qt::GraphicsLinearLayout.new self
      self.layout = layout
      layout.add_item label
    end
  end
end

In the init method we create a Plasma::Label and assign it a text. The first argument to the constructor of Plasma::Label (the new) is the parent of the label, in this case our applet. Next we create a GraphicsLinearLayout, assign it to the applet and put our label on it.

Tip
In Ruby, instance variables always begin with an '@' character. All variables without an '@' are local variables. Variables with two @ signs are class (a.k.a. static) variables.


Tip
The Ruby keyword self is a reference to the current instance of the class you're in. You can compare it with the this keyword in other languages like C++ and Java.


To view your applet, run plasmapkg -u <folder name> and plasmoidviewer <plasmoid name>. 

plasmapkg -u ruby-test-applet
plasmoidviewer ruby-test

This will show you a nice and simple Hello World in Plasma.

A line edit and a button

Let's continue by adding a line edit and a button with some functionality.

To add a line edit widget we can use the Plasma::LineEdit class. This class is a basic KLineEdit (a text field with one line) themed for Plasma. To add it to our applet, we do the same as with the label. But since we want the line edit to be empty, we don't set a text.

Next is a push button. We can use the Plasma::PushButton class for that. Adding it is exactly the same as adding the label.

With a line edit and button in place, we should let the button do something when it's clicked. Qt, the library on which KDE is based, uses a mechanism called 'signals and slots'. You could compare it to action or event handlers in other languages or frameworks. Normally, one would connect a certain signal (event) to a certain slot (event handler method). Since we're using Ruby, we can't only do that, we can connect a signal to a code block. The syntax for this is easy: 

button.connect(SIGNAL(:clicked)) do
  # do something
end

Now, when the button gets clicked, the code in the block gets executed.

Tip
The Ruby language has the concept of code blocks. These are anonymous functions which you can supply to a method. A code block gets executed when the receiving method calls it.


We will do something simple when the button gets pressed. Qt makes it very easy to put some text on the clipboard, so let's do that. To put text on the clipboard we need Qt::Application.clipboard. This object has the very convenient method text= which puts text on the clipboard.

Putting it all together, we have to following code: 

require 'plasma_applet'

module RubyTest
  class Main < PlasmaScripting::Applet
    def initialize parent
      super parent
    end

    def init
      set_minimum_size 150, 150

      layout = Qt::GraphicsLinearLayout.new Qt::Vertical, self
      self.layout = layout

      label = Plasma::Label.new self
      label.text = 'This plasmoid will copy the text you enter below to the clipboard.'
      layout.add_item label

      line_edit = Plasma::LineEdit.new self
      layout.add_item line_edit

      button = Plasma::PushButton.new self
      button.text = 'Copy to clipboard'
      layout.add_item button

      button.connect(SIGNAL(:clicked)) do
        Qt::Application.clipboard.text = line_edit.text
      end
    end

  end
end

Reading API reference documentation

Now that you have this simple but working Ruby Plasma applet, you can expand it. You can start by trying out some of the other Plasma widgets available. The official Plasma API is unfortunately written for C++. But with a little imagination and some logic you should be able to make use of it.

Still reading this tutorial? Let's take it step by step then.

Look through the Plasma API hierachy and find the LineEdit class. At the top of its own page is a listing of the available Signals. Duplicate the code for the button signal in your text editor: 

button.connect(SIGNAL(:clicked)) do
  Qt::Application.clipboard.text = line_edit.text
end

button.connect(SIGNAL(:clicked)) do
  Qt::Application.clipboard.text = line_edit.text
end

And now reuse that code block to make your line edit variable also paste the typed content to the clipboard when the return key on the keyboard is being pressed. Save and run your code (in KDE 4.2 you must not forget to update manualy, in KDE 4.3 you can run 'Plasmoidviewer' when Konsole is pointing to the folder / directory of your Applet). My clipboard missed a few pastes but both the button and pressing return worked. Look a bit lower on the Class page for LineEdit: the C++ code to signal an action looks different then the Ruby signal you copied and the Ruby signal you composed yourself, but it does list in which KDE version the signal has become available.

Now you know how to let your Plasma Applet react to any signal and you know how to find all available signals in Plasma.

Enabling Public Member Functions

The second list at the top of the lineEdit class page of the API reference documentation names all the Public Member Functions. These are the extra features which we can use for the line_edit class. A nice looking feature is the clear button, which can appear when some text is typed and when clicked with the mouse pointer clears the text, so the user of your widget can type something different. The page mentions two functions related to the clear button for C++ programmers. In Ruby however the functions are cleanly united by the Ruby Qt bindings. So remember you must drop both is and set from the function to make it a Ruby method. The class page also says that this function uses only a true or false boolean parameter, indicated on the class page by the word "bool". To enable the clear button feature we must insert the parameter true with following code: 

line_edit.clear_button_shown = true

Notice that the Ruby Qt bindings allows us to use underscores which increases the readability of all methods for non-native English speakers. So "setClearButtonShown (bool show)" becomes either "clear_button_shown = true" or clear_button_shown = false". Also remember that when scrolling down the class page it states that this feature only works in Plasma version 4.3 or later. This means that on earlier versions your plasmoid application will suddenly not run at all and give an error. For optional features not available in earlier versions of Plasma we should always tell the applet not to run the code when it gives an error. So now we will place the line inside a rescue operation which does nothing but skip the code when it gives an error: 

begin
  line_edit.clear_button_shown = true
rescue
end
Tip
Ruby exception handling allows you to do something else when a block of code gives an error or just skip the code block completely. It works like this: 
begin
  # the piece of code which may error 
  # when the program is used
rescue
  # the optional piece of code which you 
  # want to run when an error has occured
end


Also don't forget to place some comments in your code so you (and others) can understand why code blocks are needed and what they are used for. You will not be able to remember why you wrote most code blocks, but you will be able to see the logic when each reason is given. Hopefully your expanded code works as expected. If you can't find out why it doesn't run then compare it to this finished and commented applet code.

Learning all about Ruby

Now that you know how to set up a basic plasmoid in Ruby and learned how to research features seen in other plasmoids and expand your own plasmoid, it is time to learn all about Ruby itself.

For this you can use other tutorials which teach you nothing but Ruby. These will explain why you have been doing things the way they are described above. They will have example applications using text input, and it is your task to make such examples into plasmoids with line_edit widgets and buttons which reset, advance or do other stuff with your sample code. Only when you can rewrite your code yourself will you have become a programmer. My favorite Ruby tutorial is: Learn To Program.

You could also try looking at some Ruby Plasma examples. These are written a bit different then the example described above, but they should still be useful.

If you have any questions about Plasma development there are several ways to ask for help. First of all there is the Plasma mailinglist. Secondly you can hop by on IRC, #plasma on irc.freenode.org. As a third option you could try asking you question on the KDE forums.

Good luck, and don't forget to publish your Plasma applet on kde-look.org!

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