Add html datasources

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	<div class='klayout-generated-doc'><p class="navigator"><b>KLayout Documentation (Qt 5): </b><a href="../index.html">Main Index</a> » <a href="index.html">Programming scripts</a> » Events And Callbacks</p><h1>Events And Callbacks</h1><a name="k_1"></a><a name="k_2"></a><ul><li><a href="#h2-12">Introduction</a></li><li><a href="#h2-51">Reimplementation (Strategy Pattern)</a></li><li><a href="#h2-126">Events</a></li></ul><a name="h2-12"></a><h2>Introduction</h2><p>
	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.
	</p><p>
	There are basically two ways to attach specific code to a callback:
	</p><ul><li><b>Reimplementation</b>: some API classes provide "virtual" methods. "virtual" is a C++ term and
	means a method that can be overridden in a derived class. This technique is employed for example in the
	"Strategy" design pattern. In strictly typed C++ this is quite a common pattern which allows definition of interfaces
	and concrete implementations based on those interfaces. Ruby as a dynamic language doesn't care much about
	classes and their relationship: an object either has a method or it hasn't. So, reimplementation is just a matter
	of providing the right method. An examples for the strategy pattern is the BrowserSource class (<a href="../code/class_BrowserSource.html">BrowserSource</a>).
	</li><li><b>Events</b>: events allow attaching a piece of code to an event. In Ruby, such a block is "Proc" object, in "Python" it is a
	"callable" object ("lambda function" is a term used in both languages for this kind of concept).
	In case the event is triggered, this attached code is executed. Multiple lambda functions can be attached to the same event and
	removed from the latter. Events can be cleared of attached code, where only the blocks attached from one language can be cleared
	together - code attached from Python cannot be cleared from Ruby.
	An example for events is the Action class (<a href="../code/class_Action.html">Action</a>) which provides both the reimplementation interface
	("triggered" method) and the event interface ("on_triggered"). By the way, Qt signals are mapped to events in KLayout's Qt
	binding (<a href="qt_binding.html">The Qt Binding</a>).
	</li></ul><p>
	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.
	</p><a name="h2-51"></a><h2>Reimplementation (Strategy Pattern)</h2><p>
	The BrowserSource (<a href="../code/class_BrowserSource.html">BrowserSource</a>) class is a nice example for the Strategy pattern.
	It is used by the BrowserDialog class (<a href="../code/class_BrowserDialog.html">BrowserDialog</a>) 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.
	</p><p>
	Here is the code.
	This example demonstrates how the "get" method is reimplemented to deliver the actual text.
	</p><pre>
	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
	</pre>

	The Python version is this:

	<pre>
	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_()
	</pre><p>
	Ruby even allows reimplementation of a method without deriving a new class, because it allows
	defining methods per instance:
	</p><pre>
	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
	</pre><a name="h2-126"></a><h2>Events</h2><p>
	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.
	</p><p>
	Here is a simple example that uses the parameterless "on_triggered" event of the Action
	class (<a href="../code/class_Action.html">Action</a>). It puts a new entry into the tool bar and if it is
	clicked, it displays a message box:
	</p><pre>
	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
	</pre><p>
	The Python version is:
	</p><pre>
	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)
	</pre><p>
	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:
	</p><pre>
	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
	</pre><p>
	Synonyms for the <tt>+=</tt> operator are <tt>add</tt> and <tt>connect</tt>. The latter makes code more familiar for PyQt users.
	In the same way, synonyms for the <tt>-=</tt> operator are <tt>remove</tt> and <tt>disconnect</tt>.
	</p><p>
	If the Qt binding is available (see <a href="qt_binding.html">The Qt Binding</a>), 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:
	</p><pre>
	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
	</pre><p>
	The Python version is:
	</p><pre>
	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_()
	</pre><p>
	Using the += operator on the event, multiple handlers can be added to a signal:
	</p><pre>
	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
	</pre><p>
	with the Python version:
	</p><pre>
	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_()
	</pre></div>
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