qtwebkit-bridge.qdoc [plain text]
/*!
\inmodule QtWebKit
\page qtwebkit-bridge.html
\title The QtWebKit Bridge
\contentspage QtWebKit
\section1 Overview
\section2 The technology
The QtWebKit bridge is a mechanism that extends WebKit's JavaScript environment to access native
objects represented as \l{QObject}s. It takes advantage of the \l{QObject} introspection,
a part of the \l{Object Model}, which makes it easy to integrate with the dynamic JavaScript environment.
For example \l{QObject} properties map directly to JavaScript properties.
\section2 Use Cases
There are two main use cases for the QtWebKit bridge: web content in native applications and thin clients.
\section3 Web Content in Native Applications
This is a common use case in classic Qt application, and a design pattern used by several modern
applications like an application that contains a media-player, playlist manager, and music store.
The playlist manager is usually best authored as a classic desktop application,
with the native-looking robust \l{QWidget}s as the application's backbone.
The media-player control usually has a custom look and feel and is best written using the \l{Graphics View framework}
or \l{QtDeclarative}. The music store, which shows dynamic content
from the Internet and gets modified rapidly, is best authored in HTML and maintained on the server.
With the QtWebKit bridge, the music store component can interact with native parts of the application,
for example, when a file needs to be saved to a specific location.
\section3 Thin Clients
The use case uses Qt as a native backend of a full web application,
a so-called thin client. In this use case, the entire UI is driven by
HTML, JavaScript and CSS. Additionally, it uses Qt-based components to
access native features usually not exposed to the web, or to enable helper
components that are best written in C++.
An example for such a client is a UI for a video-on-demand service on a TV. The entire content and
UI can be kept on the server, served dynamically through HTTP and rendered with WebKit. Additional
native components are used to access hardware-specific features like extracting a list of images
out of a video stream.
\section2 Difference from Other Bridge Technologies
Of course, QtWebKit is not the only bridge technology out there. NPAPI, for example,
is a long-time standard for web-native bridging. Due to Qt's meta-object system, full applications
leveraging web technologies are much easier to develop with the QtWebKit bridge than with NPAPI. NPAPI, however, is better
for cross-browser plugins, due to it being an accepted standard.
When developing a plugin for a browser, NPAPI is recommended. When developing a full application
utilizing HTML-rendering, the QtWebKit bridge is recommended.
\section2 Relationship with QtScript
The QtWebKit bridge is similar to \l{QtScript}, especially for some of the features described in the
\l{Making Applications Scriptable} page. However, Qt 4.7 does not provide the full QtScript API for web applications.
Full support is planned for future versions. You might notice that some of the features
described here are an exact copy of the ones described in the \l{Making Applications Scriptable} page. That is because
the QtWebKit bridge is a subset of that functionality, and this page tries to capture the full
capabilities available through the QtWebKit bridge specifically.
\section1 Accessing QObjects
\section2 Making QObjects known to JavaScript via QWebFrame
By default, no QObjects are accessible through the web environment, for security reasons.
When a web application wants to access a native QObject, it must explicitly grant access
to this QObject, using the following call:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 0
See \l{QWebFrame::addToJavaScriptWindowObject()} for more information.
\section2 Using Signals and Slots
The QtWebKit bridge adapts Qt's central \l{Signals and Slots} feature for
scripting. There are three principal ways to use signals and slots
with the QtWebKit bridge:
\list
\i \bold{Hybrid C++/script}: C++ application code connects a
signal to a script function. This approach is useful if you have
a QObject but don't want to expose the object itself to the scripting
environment. You just want to define how the script responds to a
signal and leave it up to the C++ side of your application to establish
the connection between the C++ signal and the JavaScript slot.
\i \bold{Hybrid script/C++}: A script can connect signals and slots
to establish connections between pre-defined objects that the
application exposes to the scripting environment. In this scenario,
the slots themselves are still written in C++, but the definition of
the connections is fully dynamic (script-defined).
\i \bold{Purely script-defined}: A script can both define signal
handler functions (effectively "slots written in JavaScript"),
\e{and} set up the connections that utilize those handlers. For
example, a script can define a function that will handle the
QLineEdit::returnPressed() signal, and then connect that signal to the
script function.
\endlist
Note that QtScript functions such as qScriptConnect are unavilable in the web environment.
\section3 Signal to Function Connections
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 7
The call to \c{connect()} establishes a connection between the signal
\c{somethingChanged} and the slot \c{myInterestingScriptFunction}.
Whenever the object \c{myObject} emits the signal \c{somethingChanged},
the slot \c{myInterestingScriptFunction} gets called automatically.
The argument of \c{connect()} can be any JavaScript function as in the above
example or a slot of a QObject as in the following example:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 8
When the argument is a slot of a QObject, the argument types of the
signal and the slot do not have to be compatible. If possible, the QtWebKit
bridge converts the signal arguments such that they match the slot argument.
To disconnect a slot from a signal, you call the signal's
\c{disconnect()} function with the slot as its argument:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 9
When a script function is invoked in response to a signal, the
\c this object will be the Global Object.
\section3 Signal to Member Function Connections
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 10
The call to \c{connect() establishes a connection between the signal
\c{somethingChanged} and the slot \c{function}. Whenever the object
\c{myObject} emits the signal \c{somethingChanged}, the slot \c{function}
of the object \c{thisObject} gets called automatically. Let's illustrate
this with an example.
If you have a push button in a form, you typically want the form
to do something in response to the button's \c{clicked} signal. The
call to \c{connect()} makes sure that the function \c{onClicked()} is
called whenever you click on the push button, that is, whenever the
the signal \c{clicked()} is emitted by \c{myButton}. The slot \c{onClicked()}
prints the value of \c{x} as stored in the \c{form}.
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 11
To disconnect a slot from a signal, you pass the same arguments to
\c{disconnect()} as you passed to \c{connect()}. In general, this looks
as follows:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 12
\section3 Signal to Named Member Function Connections
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 14
This form of the \c{connect()} function requires that the first argument \c{thisObject} is
the object that will be bound to \c{this} when the function \c{functionName} is
invoked in response to the signal \c{somethingChanged}. The second argument \c{functionName} specifies the
name of a function that is connected to the signal. It refers to a
member function of the object \c{thisObject}.
Note that the function is resolved when the connection is made, not
when the signal is emitted.
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 15
To disconnect from the signal, pass the same arguments to \c{disconnect()}
as you passed to \c{connect}:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 17
\section3 Error Handling
When \c{connect()} or \c{disconnect()} succeeds, the function will
return \c{undefined}; otherwise, it will throw a script exception.
You can obtain an error message from the resulting \c{Error} object.
Example:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 18
\section3 Emitting Signals from Scripts
To emit a signal from script code, you simply invoke the signal
function, passing the relevant arguments:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 19
It is currently not possible to define a new signal in a script;
i.e., all signals must be defined by C++ classes.
\section3 Overloaded Signals and Slots
When a signal or slot is overloaded, the QtWebKit bridge will attempt to
pick the right overload based on the actual types of the QScriptValue arguments
involved in the function invocation. For example, if your class has slots
\c{myOverloadedSlot(int)} and \c{myOverloadedSlot(QString)}, the following
script code will behave reasonably:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 20
You can specify a particular overload by using array-style property access
with the \l{QMetaObject::normalizedSignature()}{normalized signature} of
the C++ function as the property name:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 21
If the overloads have different number of arguments, the QtWebKit bridge will
pick the overload with the argument count that best matches the
actual number of arguments passed to the slot.
For overloaded signals, JavaScript will throw an error if you try to connect
to the signal by name; you have to refer to the signal with the full
normalized signature of the particular overload you want to connect to.
\section3 Invokable Methods
Both slots and signals are invokable from scripts by default. In addition, it is also
possible to define a method that is invokable from scripts, although the method is neither a signal nor a slot.
This is especially useful for functions with return types, as slots normally do not return anything
(it would be meaningless to return a value from a slot, as the connected signals cannot handle return values).
To make a non-slot method invokable, simply add the Q_INVOKABLE macro before its definition:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 22
\section2 Accessing Properties
The properties of a QObject are available as properties
of the corresponding JavaScript object. When you manipulate
a property in script code, the C++ get/set method for that
property will automatically be invoked. For example, if your
C++ class has a property declared as follows:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 23
then script code can do things like the following:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 24
\section2 Accessing Child QObjects
Every named child of a QObject (that is, every child for which
QObject::objectName() does not return the empty string) is by default available as
a property of the JavaScript wrapper object. For example,
if you have a QDialog with a child widget whose \c{objectName} property is
\c{"okButton"}, you can access this object in script code through
the expression
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 25
Because \c{objectName} is itself a Q_PROPERTY, you can manipulate
the name in script code to rename an object. For example:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 26
\section2 Data types
When calling slots, receiving signals or accessing properties, usually some payload is involved.
For example, a property "text" might return a \l{QString} parameter.
The QtWebKit bridge does the job of converting between a given JavaScript data-type, and the
expected or given Qt type. Each Qt type has a coresponding set of rules of how JavaScript treats it.
The data type conversions are also applicable for the data returned from non-void invokable methods.
\section3 Numbers
All Qt numeric data types are converted to or from a JavaScript number. These include int, short, float,
double, and the portable Qt types (qreal, qint etc). A special case is \l{QChar}.
If a slot expects a QChar, the QtWebKit bridge uses the Unicode value in case of a number and the first character in case of a string.
Note that non-standard (typedef'ed) number types are not automatically converted to
or from a JavaScript number - we suggest to use standard number types for signals, slots
and properties.
When a non-number is passed as an argument to a method or property that expects a number,
the appropriate JavaScript conversion function (parseInt / parseFloat) is used.
\section3 Strings
When JavaScript accesses methods or properties that expect a \l{QString}, the QtWebKit bridge
will automatically convert the value to a string (if it is not already a string), using the
built-in JavaScript toString method.
When a QString is passed to JavaScript from a signal or a property, the QtWebKit bridge
converts it into a JavaScript string.
\section3 Date & Time
Both \l{QDate}, \l{QTime} and \l{QDateTime} are automatically translated to or from the JavaScript
Date object. If a number is passed as an argument to a method that expects one of the date/time
types, the QtWebKit bridge treats it as a timestamp. If a sting is passed, QtWebKit
tries the different Qt date parsing functions to perform the right translation.
\section3 Regular Expressions
The QtWebKit bridge automatically converts a JavaScript RegEx object to a \l{QRegExp}.
If a string is passed to a method expecting a \l{QRegExp}, the string is converted
to a \l{QRegExp}.
\section3 Lists
The QtWebKit bridge treats several types of lists in a special way: \l{QVariantList}, \l{QStringList},
\l{QObjectList} and \l{QList}<int>. When a slot or property expects one of those list types,
the QtWebKit bridge tries to convert a JavaScript array into that type, converting each of
the array's elements to the single-element type of the list.
The most useful type of list is \l{QVariantList}, which can be converted to and from any
JavaScript array.
\section3 Compound (JSON) objects
JavaScript compound objects, also known as JSON objects, are variables that hold a list
of key-value pairs, where all the keys are strings and the values can have any type.
This translates very well to \l{QVariantMap}, which is nothing more than a \l{QMap} from \l{QString}
to \l{QVariant}.
The seamless conversion between JSON objects and \l{QVariantMap} allows for a very convenient
way of passing arbitrary structured data between C++ and the JavaScript environment. If the native \l{QObject} makes sure that compound values are converted to \l{QVariantMap}s and \l{QVariantList}s, JavaScript is
guaranteed to receive them in a meaningful way.
Note that types that are not supported by JSON, such as JavaScript functions and getters/setters,
are not converted.
\section3 QVariants
When a slot or property accepts a \l{QVariant}, the QtWebKit bridge creates a \l{QVariant} that best
matches the argument passed by JavaScript. A string, for example, becomes a \l{QVariant} holding a \l{QString},
a normal JSON object becomes a \l{QVariantMap}, and a JavaScript array becomes a \l{QVariantList}.
Using \l{QVariant}s generously in C++ in that way makes C++ programming feel a bit more like JavaScript programming,
as it adds another level of indirection. Passing \l{QVariant}s around like this is very flexible. The program can figure out
the type of argument at runtime just like JavaScript would do. But doing so also takes away the type safety and robustness of C++.
We recommended to use \l{QVariant}s only for high-level functions, and to keep most of your
\l{QObject}s type-safe.
\section3 QObjects
Pointers to a \l{QObject} or a \l{QWidget} can be used in signals, slots and properties. This object
can then be used like an object that is exposed directly. Its slots can be invoked, its signals connected to, etc.
However, this functionality is fairly limited - the type used has to be \l{QObject}* or \l{QWidget}*. If the type
specified is a pointer to a non-\l{QWidget} subclass of \l{QObject}, the QtWebKit bridge does not recognize it as
a \l{QObject}.
In general its advised to use care when passing \l{QObject}s as arguments, as those objects don't become owned by
the JavaScript engine; That means that the application developer has to be extra careful not to try to access
\l{QObject}s that have already been deleted by the native environment.
\section3 Pixmaps and Images
\since 4.7
The QtWebKit bridge handles \l{QPixmap}s and \l{QImage}s in a special way. Since QtWebKit stores \l{QPixmap}s to
represent HTML images, \l{QPixmap}s coming from the native environment can be used directly inside WebKit.
A \l{QImage} or a \l{QPixmap} coming from Qt is converted to an intermediate JavaScript object,
which can be represented like this:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 1
The JavaScript environment can then use the pixmap from Qt and display it inside the HTML environment,
by assigning it to an existing \c{<img>} element with \c{assignToHTMLImageElement()}. It can also use the \c{toDataURL()} function,
which allows using the pixmap as the \c{src} attribute of an image or as a \c{background-image} URL. Note that the \c{toDataURL()}
function is costly and should be used with caution.
Example code:
C++:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 2
HTML:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 3
When a Qt object expects a \l{QImage} or a \l{QPixmap} as input, and the argument passed is an HTML image element,
the QtWebKit bridge would convert the pixmap assigned to that image element into a \l{QPixmap} or a \l{QImage}.
\since 4.7
\section3 QWebElement
A signal, slot or property that expects or returns a \l{QWebElement} can work seamlessly with JavaScript references
to DOM elements. The JavaScript environment can select DOM elements, keep them in variables, then pass them to Qt as
a \l{QWebElement}, and receive them back. Example:
C++:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 4
HTML:
\snippet webkitsnippets/qtwebkit_bridge_snippets.cpp 5
This is specifically useful to create custom renderers or extensions to the web environment. Instead of forcing Qt
to select the element, the web environment selects the element and then sends the selected element directly to Qt.
Note that \l{QWebElement}s are not thread safe - an object handling them has to live in the UI thread.
\section1 Architecture Issues
\section2 Limiting the Scope of the Hybrid Layer
When using QtWebKit's hybrid features, it is a common pitfall to make the API exposed to JavaScript very rich and
use all its features. This, however, leads to complexity and can create bugs that are hard to find.
Instead, it is advisable to keep the hybrid layer small and manageable: create a gate only when
there's an actual need for it, i.e. there's a new native enabler that requires a direct interface
to the application layer. Sometimes new functionality is better handled internally in the native layer
or in the web layer; simplicity is your friend.
This usually becomes more apparent when the hybrid layer can create or destroy objects, or uses
signals, slots or properties with a \l{QObject}* argument. It is advised to be very careful and to treat
an exposed \l{QObject} as a system - with careful attention to memory management and object ownership.
\section2 Internet Security
When exposing native objects to an open web environment, it is important to understand the security
implications. Think whether the exposed object enables the web environment access things that
shouldn't be open, and whether the web content loaded by that web page comes from a trusted source. In general, when
exposing native QObjects that give the web environment access to private information or to functionality
that's potentially harmful to the client, such exposure should be balanced by limiting the web page's
access to trusted URLs only with HTTPS, and by utilizing other measures as part of a security strategy.
*/