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Wt: A Web Toolkit


Server-Side Event Handling

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Wt's signals and slots are implemented on top of the Boost.Signals package, which is part of the C++ Boost template libraries. All classes that inherit from WObject can use signals and slots with automatic care for connections when objects are destructed, because WObject inherits from boost::trackable.

Wt makes all user interaction with HTML DOM elements available by emitting the corresponding Wt::Signal. These events include keyboard events (keyWentDown, keyWentUp, keyPressed), mouse events (clicked, doubleClicked, mouseMoved, and so on) and many others. The event is only propagated if the corresponding signal has any connections, and therefore avoids unnecessary communication. Unfortunately, without availability of JavaScript, the application may only react to click events. Wt is designed to be functional even when the client has no JavaScript support, and since the availability of JavaScript is indicated using WEnvironment::javaScript(), the absence of richer interaction may be worked around if required, by providing alternative methods for essential operations; for example, by adding extra buttons.

Listing One illustrates a common Wt construct that instantiates a widget and reacts to some of its events (for brevity, we mix the implementation in the class declaration). In this example, a button is created as part of a new composite widget. When the button is clicked, the method doFumble() gets called, which in this case, disables the button (to prevent the user from clicking twice), and then performs some business logic.

Listing One

class MyWidget : public WCompositeWidget
{
public:
  MyWidget(WContainerWidget *parent = 0)
    : WcompositeWidget(parent),
      ...
   {
      ...

      fumbleButton_ = new WPushButton("Fumble");
      fumbleButton_->clicked.connect(SLOT(this, MyWidget::doFumble));
      ...
   }
private:
   WpushButton *fumbleButton_;

   void doFumble()
   {
      fumbleButton_->disable();
      
      fumbleSome(...);
   }
};


In this example, there is an obvious drawback—the button is only disabled after at least a client-server roundtrip, but also after all the fumbling has been done! This can easily be avoided by using client-side event handling.


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