Simple .NET/ASP.NET PDF document editor web control SDK

When nothing else works, or if you choose to follow a different approach, you might end up in a situation in which you have to create your own widget. Creating a custom widget consists of implementing an interface of signals and slots as well as a set of applicable event handlers. To show you how this is done, I will guide you through the CircleBar widget (see Figure 6-6). The application shown in the figure has a CircleBar widget over a horizontal slider. Moving the slider changes the value of the circle bar, as does rotating the mouse wheel when hovering over the circle bar widget. The function of the CircleBar widget is to show a value between 0 and 100 by varying the size of the filled circle. A full circle means 100, while a dot in the middle means 0. The user can change the value shown by using the mouse scroll wheel.

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The ListView control is able to support multiple selection. We ve disabled this, but we still have to negotiate an API designed to support it it offers a SelectedIndices collection, providing all the selected items. We just make sure it s not empty, and then use the first index. We remove the object via the binding source so that the data binding system knows what s going on, just as we did when adding a new item. (In fact, it would work if we didn t do this because this example uses a BindingList to hold the model, and that raises change notifications. Unlike AddNew, there s no particular advantage to going via the binding source here, but since we re treating the binding source as the central point through which all changes are handled, it s good to be consistent.) We can now add multiple entries. This reveals a missing piece we have done nothing yet to ensure that when the user selects an item in the list view, the corresponding entry s properties appear in the rest of the form. So we need to add a handler to the list view s SelectedIndexChanged event. That s its default event, so you can just doubleclick the list view in the form designer. Then all we need to do is set the binding source s Position property:

private void entriesListView_SelectedIndexChanged(object sender, EventArgs e) { if (entriesListView.SelectedIndices.Count != 0) { int entryIndex = entriesListView.SelectedIndices[0]; entriesSource.Position = entryIndex; } }

We ve had to jump through the same hoop to get the selected item index. There s really just one line of interest here the one that sets the Position.

We have to do this only because the ListView doesn t do data binding automatically. The ListBox and most data grid controls will automatically synchronize the current data binding position with the selected item.

Figure 6-6. The CircleBar widget and a horizontal slider The main function, shown in Listing 6-18, sets up the slider and the circle bar. The code works by first creating a base widget for the QVBoxLayout that holds the slider and the circle bar. The slider and circle bar are then interconnected, so a valueChanged signal from one of them results in a setValue call to the other one. Then the base widget is shown before the application is started. Listing 6-18. Setting up the CircleBar and the slider int main( int argc, char **argv ) { QApplication app( argc, argv ); QWidget base; QVBoxLayout *layout = new QVBoxLayout( base ); CircleBar *bar = new CircleBar; QSlider *slider = new QSlider( Qt::Horizontal ); layout->addWidget( bar ); layout->addWidget( slider ); QObject::connect( slider, SIGNAL(valueChanged(int)), bar, SLOT(setValue(int)) ); QObject::connect( bar, SIGNAL(valueChanged(int)), slider, SLOT(setValue(int)) ); base.show(); return app.exec(); } From the main function you can see that the CircleBar widget needs a setValue(int) slot and a valueChanged(int) signal. To make the interface complete, you also need to have a value method to read the value.

One little bug remains. When we delete all the items, the text boxes and date picker are bound to nothing. This doesn t crash the program; it just means the user can type in details that go nowhere. There are a couple of ways we could fix this. In the list change notification handler, we could look at the number of entries, and disable everything except the New button to make it clear that there s nothing to edit right now. Or we could handle change notifications in the text boxes text box controls raise a TextChanged event, and we could handle that (as well as changes to the date picker or description) and create a new entry if the user types into an empty list. Since neither of these would illustrate anything you haven t already seen we ll leave this part up to you.