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Scripting Your Qt Application
09/25/09

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About Me (Kent Hansen)
• Working on Qt since 2005
• QtScript
• Qt State Machine framework
• Plays harmonica and Irish whistle
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Goals For This Talk
• Give overview of QtScript and things related to it
• Show how to embed QtScript into your application
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Agenda
• What is application scripting?
• QtScript tour (the essentials)
• Debugger
• Misc.
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“Scripting Your Application”: What?
• Premise: Core functionality implemented in C++
• Interface(s) exported to scripting environment
• Plugin-based, interactive (console), ...
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Golden Example: The Web Browser
• Script loaded as part of web page
• Browser exports tree-based API to page (DOM)
• Script manipulates the page
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What is QtScript?
• A Qt module
• A scripting language
• An “engine” for evaluating scripts
• An API for embedding into your application
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The QtScript Language
• Based on ECMA-262 standard
–“JavaScript minus DOM”
• Object-oriented, dynamically typed
• Java-like syntax
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Disclaimer
This is not a JavaScript tutorial!
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The QtScript Engine
• Executes scripts
• Performs C++ <--> script bridging
• It's re-entrant
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The QtScript API
• Embed QtScript engine in your application
• Expose C++ objects to script environment
• Work with script values from C++
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Why should you use QtScript?
• Easy to embed into any Qt application
• Tight integration with rest of Qt
• Easy-to-use API
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Agenda
• What is application scripting?
• QtScript tour (the essentials)
• Debugger
• Misc.
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QtScript tour (the essentials)
• Basic embedding (example)
• Working with QScriptValues
• Configuring the script environment
• Qt meta-object system integration
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Example: Meaning of Life Calculator
“Our objective: Write an application that uses
QtScript to calculate the meaning of life”
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QScriptEngine object
QScriptEngine engine;
• “Vanilla” environment
• Built-in ECMA objects
–Array, Math, RegExp, Error, ...
–parseInt(), parseFloat(), ...
• No Qt-specific script APIs
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Mutating the Environment (I)
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Mutating the Environment (II)
• Evaluate scripts
• Operate on QtScript values in C++
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QtScript tour (the essentials)
• Basic embedding (example)
• Working with QScriptValues
• Configuring the script environment
• Qt meta-object system integration
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QScriptValue class
• Represents a QtScript (JavaScript) value
• Undefined
• Null
• Boolean
• Number
• String
• Object
–Functions, arrays, regexps, errors, ...
• Invalid (no value)
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QScriptValue type checks & conversion
• isXXX(): Tests if value is of a certain type
• toXXX(): Converts to Qt/C++ type
• qscriptvalue_cast(): Converts to C++ type
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QScriptValue construction
• Constructors for standard Qt/C++ types
• qScriptValueFromValue()
• Counterpart to qscriptvalue_cast()
• QScriptEngine::newXXX()
• Object, QObject, Array, RegExp
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QScriptValue property access
JS: myObject.foo = 123;
myObject['foo'] = 123;
C++: myObject.setProperty(“foo”, 123);
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QScriptValue holds reference to object (I)
QScriptValue object = engine.evaluate(
“{
foo: 123,
toString: function() {
return 'MyObject(foo=' + this.foo + ')';
}
}”);
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QScriptValue holds reference to object (II)
QScriptValue same = object;
object.setProperty(“foo”, 456);
qDebug() << same.toString();
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QScriptValue called as function
JS: myArray.sort(); or
myArray['sort'].call(myArray);
C++: myArray.property(“sort”)
.call(myArray);
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Calling a function with arguments (I)
JS: myObject.myFunction(123, “foo”);
C++: myFunction.call(
myObject,
QScriptValueList()
<< 123 << “foo”);
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Calling a function with arguments (II)
QScriptValue adder = engine.evaluate(
“(function(a, b) { return a+b; })”);
QScriptValue result = adder.call(
/*thisObject=*/QScriptValue(),
QScriptValueList() << 41 << 1);
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Calling a function as constructor (I)
JS: function Point(x, y) {
this.x = x;
this.y = y;
}
...
new Point(10, 20);
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Calling a function as constructor (II)
C++:
QScriptValue p = Point.construct(
QScriptValueList() << 10 << 20);
qDebug() << p.property(“x”).toNumber();
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QScriptValueIterator class
QScriptValueIterator it(myObject);
while (it.hasNext()) {
it.next(); qDebug() << it.name();
}
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Working with JavaScript Arrays (I)
JS:
var a = [10, 20, 30];
print(a[0], a['0']); // same property
a.foo = 123;
a.push(40);
a[a.length] = 'appended';
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Working with JavaScript Arrays (II)
C++:
QScriptValue a = engine.evaluate(
“[10, 20, 30]”); // array literal
qDebug() << a.property(“length”).toInt32();
qDebug() << a.property(“0”).toNumber();
// for convenience + speed
qDebug() << a.property(0).toNumber();
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Error Handling (I)
// Uh-oh, this script will throw an error!
QScriptValue result = engine.evaluate(
“noSuchVariable”);
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Error Handling (II)
if (result.isError()) {
QString message = result.toString();
// Notify user about the problem ...
}
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QScriptValue API Summary
• isXXX(), toXXX()
• property() and setProperty()
• call(), construct()
• QScriptValueIterator for introspection
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QtScript tour (the essentials)
• Basic embedding (example)
• Working with QScriptValues
• Configuring the script environment
• Qt meta-object system integration
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The Global Object
• A built-in script object
• Every QScriptEngine has one
• QScriptEngine::globalObject()
• Initially contains properties defined by ECMA-262
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Tailoring Scripting to Your Application
• Design “contract” of C++ <--> script interaction
• Set properties of Global Object
• Evaluate scripts that use your API
• (Optional: Call back into script objects as
appropriate)
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Example: Meaning of Life Environment
C++:
QScriptEngine engine;
QScriptValue global = engine.globalObject();
global.setProperty(“meaningOfLife”, 42,
QScriptValue::ReadOnly);
JS:
var myMeaningOfLife = Math.sqrt(meaningOfLife);
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Example: Fake Navigator Object
C++:
QScriptValue navigator = engine.newObject();
navigator.setProperty(“appVersion”, 1);
navigator.setProperty(“appMinorVersion”, 1);
navigator.setProperty(“cookieEnabled”, false);
navigator.setProperty(“browserLanguage”, “en_US”);
navigator.setProperty(“platform”, “No Such OS”);
navigator.setProperty(“userAgent”, “007”);
global.setProperty(“navigator”, navigator);
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QtScript tour (the essentials)
• Basic embedding (example)
• Working with QScriptValues
• Configuring the script environment
• Qt meta-object system integration
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Qt Meta-Object System
• In Qt, QObject-based classes can be introspected
at runtime
• Properties
• Signals & slots
• QObject::connect()
• Qt Designer property editor
• ...
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Relax! Leave it to magic
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QtScript <--> QObject Integration (I)
• QScriptEngine::newQObject(QObject*)
• Returns a script object that acts as proxy
• Proxy delegates property access to
QObject::property() and QObject::setProperty()
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QtScript <--> QObject Integration (II)
• Proxy provides signals and slots as properties
• Proxy provides named child objects as properties
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QtScript <--> QObject Integration (III)
• Ownership control
• Customize proxy behavior
• Don't expose child objects
• Don't expose super class contents
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Example: Scripting a QLineEdit
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qScriptConnect() function
• Connect a signal to a script function
• Let scripts define signal handlers
• Application does the “plumbing”
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Summary of “API essentials”
• QScriptValue
• QScriptEngine
• evaluate()
• globalObject()
• newQObject()
• qScriptConnect()
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Agenda
• What is application scripting?
• QtScript tour (the essentials)
• Debugger
• Misc.
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The QtScript Debugger
• Graphical debugger
• Introduced in Qt 4.5
• To make the API available in your application:
QT += scripttools
#include <QtScriptTools>
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Debugger Demo
Look at the pretty little demo!
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The QtScript Debugger API (I)
One class in public API: QScriptEngineDebugger
QScriptEngine engine;
QScriptEngineDebugger debugger;
debugger.attachTo(&engine);
// evaluate scripts ...
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The QtScript Debugger API (II)
• Debugger actions can be programmatically
triggered
• Individual debugger widgets are accessible
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Invoking the Debugger from a Script
• Use the debugger keyword
• Becomes no-op if no debugger is attached
JS:
var a = Math.random();
debugger; // synchronously run debugger
var b = Math.random();
debugger;
return a + b;
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Agenda
• What is application scripting?
• QtScript tour (the essentials)
• Debugger
• Misc.
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Prototypes...?
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Prototype-based Inheritance (I)
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Prototype-based Inheritance: Why care?
• Not every QObject member is a property / signal /
slot
• Not everything is a QObject
• Not everything should/needs to be a QObject
• JS developers are familiar with prototypes
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Prototype-based Inheritance: How?
• Create a prototype object for the C++ type
• Register the type with Qt's meta-type system
• Associate prototype object with meta-type ID
• Create QScriptValues from the C++ type
(qScriptValueFromValue())
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Bindings for Qt's APIs (I)
• Available as stand-alone project
• qtscriptgenerator on Qt Labs
(http://labs.qt.nokia.com)
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Bindings for Qt's APIs (II)
• Creates plugins that can be imported into a script
engine
• Provides bindings for most of Qt's classes
• Makes it possible to write pure script applications
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Summary
• It's easy to make your Qt application scriptable
with QtScript
• There's a QtScript debugger (also part of Qt)
• You can achieve a lot by mastering a small API
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Thank You!
Questions?
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