KLayout 0.29.8 (2024-10-28 63dd591e5) [master]

Events And Callbacks

Introduction

In some places, the API requires to attach code to an event. An event could be a menu item which is selected or a change of some status which might require some action. The API allows implementation of specific code which is called in that case. This enables us to implement the functionality behind a menu item. In this text we will refer to such functionality by the general term "callback". In general a callback is custom code that is called from the API in contrast to API code that is called from the custom code.

There are basically two ways to attach specific code to a callback:

The "Observer" class which was there prior to KLayout 0.25 has been dropped in favour of the more flexible events. It is no longer supported.

Reimplementation (Strategy Pattern)

The BrowserSource (BrowserSource) class is a nice example for the Strategy pattern. It is used by the BrowserDialog class (BrowserDialog) as a kind of internal HTML server which handles URL's starting with "int:". For this, a script has to provide a class that reimplements the "get(url)" method. In the following example, a BrowserSource is created that takes an URL with an integer index number and delivers a HTML text with a link to the URL with the next index.

Here is the code. This example demonstrates how the "get" method is reimplemented to deliver the actual text.

module MyMacro
  
  include RBA
  
  class MyBrowserSource < BrowserSource
    def get(url)
      next_url = url.sub(/\d+/) { |num| (num.to_i+1).to_s }
      "This is #{url}. <a href='#{next_url}'>Goto next (#{next_url})</a>"
    end
  end
  
  dialog = BrowserDialog::new
  dialog.source = MyBrowserSource::new
  dialog.home = "int:0"
  dialog.exec

end
The Python version is this:
from pya import BrowserSource, BrowserDialog

class MyBrowserSource(BrowserSource):
  def get(self, url):
    next_url = "int:" + str(int(url.split(":")[1]) + 1)
    return f"This is {url}. <a href='{next_url}'>Goto next ({next_url})</a>>"

dialog = BrowserDialog()
dialog.home = "int:0"
dialog.source = MyBrowserSource()
dialog.exec_()

Ruby even allows reimplementation of a method without deriving a new class, because it allows defining methods per instance:

module MyMacro
  
  include RBA
  
  source = BrowserSource::new
  def source.get(url)
    next_url = url.sub(/\d+/) { |num| (num.to_i+1).to_s }
    "This is #{url}. <a href='#{next_url}'>Goto next (#{next_url})</a>"
  end
  
  dialog = BrowserDialog::new
  dialog.source = source
  dialog.home = "int:0"
  dialog.exec

end

Events

Events are the callback variant which is the easiest one to use. Using an event it is possible to directly attach a block of code to a callback. An event has a specific signature, i.e. the parameters it provides. The block can obtain this parameters by listing them in its argument list.

Here is a simple example that uses the parameterless "on_triggered" event of the Action class (Action). It puts a new entry into the tool bar and if it is clicked, it displays a message box:

module MyMacro
  
  include RBA
  
  action = Action::new
  action.on_triggered do
    MessageBox::info("A message", "The action was triggered", MessageBox::Ok)
  end
  action.title = "My Action"
  
  Application::instance.main_window.menu.insert_item("@toolbar.end", "my_action", action)
  
end

The Python version is:

from pya import Action, MessageBox, Application

def on_triggered():
  MessageBox.info("A message", "The action was triggered", MessageBox.Ok)

action = Action()
action.on_triggered = on_triggered
action.title = "My Action"

Application.instance().main_window().menu().insert_item("@toolbar.end", "my_action", action)

Specifying a block to an event will make the event only execute that block. A more flexible way of controlling the code attached to events is available through the += and -= operators:

module MyMacro

  include RBA

  code = lambda do 
    MessageBox::info("A message", "The action was triggered", MessageBox::Ok)
  end

  action = Action::new
  action.on_triggered += code

  ...
  
  # to remove the code from the event, use:
  action.on_triggered -= code

  # to replace all event handlers by the one given by "code":
  action.on_triggered = code

  # to clear all event handlers use:
  action.on_triggered.clear

Synonyms for the += operator are add and connect. The latter makes code more familiar for PyQt users. In the same way, synonyms for the -= operator are remove and disconnect.

If the Qt binding is available (see The Qt Binding), Qt signals are implemented as events. This way it's very simple to create a Qt dialog. In following example, the "textChanged" signal of QLineEdit is attached a code block which copies the text of the input field to the label below:

module MyMacro
  
  include RBA
  
  dialog = QDialog::new(Application::instance.main_window)
  layout = QVBoxLayout::new(dialog)
  input = QLineEdit::new(dialog)
  label = QLabel::new(dialog)
  layout.addWidget(input)
  layout.addWidget(label)

  # implement the textChanged signal as event:
  input.textChanged { |text| label.text = text }

  dialog.exec

end

The Python version is:

from pya import QDialog, QVBoxLayout, QLineEdit, QLabel, Application

dialog = QDialog(Application.instance().main_window())
layout = QVBoxLayout(dialog)
input = QLineEdit(dialog)
label = QLabel(dialog)
layout.addWidget(input)
layout.addWidget(label)

def text_changed(text):
  label.text = text

# implement the textChanged signal as event:
input.textChanged = text_changed

dialog.exec_()

Using the += operator on the event, multiple handlers can be added to a signal:

module MyMacro
  
  include RBA
  
  dialog = QDialog::new(Application::instance.main_window)
  layout = QVBoxLayout::new(dialog)
  input = QLineEdit::new(dialog)
  label1 = QLabel::new(dialog)
  label2 = QLabel::new(dialog)
  layout.addWidget(input)
  layout.addWidget(label1)
  layout.addWidget(label2)

  # two signal consumers:
  input.textChanged += lambda { |text| label1.text = text }
  input.textChanged += lambda { |text| label2.text = text.reverse }
  
  dialog.exec

end

with the Python version:

from pya import QDialog, QVBoxLayout, QLineEdit, QLabel, Application

dialog = QDialog(Application.instance().main_window())
layout = QVBoxLayout(dialog)
input = QLineEdit(dialog)
label1 = QLabel(dialog)
label2 = QLabel(dialog)
layout.addWidget(input)
layout.addWidget(label1)
layout.addWidget(label2)

def text_changed1(text):
  label1.text = text

def text_changed2(text):
  label2.text = text[::-1]

# two signal consumers:
input.textChanged += text_changed1
input.textChanged += text_changed2

dialog.exec_()