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CHAPTER 2 RAPID APPLICATION DEVELOPMENT USING QT
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Note There are more ways to use a UI created in Designer from a QDialog object than the method
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shown here. The method used here is called the single inheritance approach. In the Designer user manual, two alternate methods are described: the multiple inheritance method (inheriting both QDialog and Ui classes) and the direct method (creating a QDialog and a Ui from the method using the dialog). I prefer using the single inheritance approach and will use it throughout this book. It keeps the generated code separated from the manually written source code through the ui object something that helps making changes more controllable. Feel free to consult the Designer user manual and try the alternatives if you want to.
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Listing 2-4 shows the implementation of the addItem slot. The function looks very simple and uses the EditDialog class (which has not been discussed yet). Before continuing with it, let s see how a dialog is used. First, the dlg variable is created. The this pointer passed on to the EditDialog sets the parent of the dlg to the list dialog. Then you call the exec method of the dialog, which shows the dialog in an application modal state. That a dialog is application modal means that no other dialog or window of the application can get UI focus until the dialog is closed forcing the user to use or close the shown dialog. The exec method returns a status from the dialog, where Qt::Accepted means that the OK button was clicked last (or that the accept slot was called to close the dialog). The other possible result is Qt::Rejected, meaning that the dialog was closed from the title bar or cancelled. When the dialog has been shown using exec, and the result is Qt::Accepted, a new item is added to the list widget: ui.list. The new item is built using the name and number getter members from the editing dialog (you will have a look at them later on in this chapter). Listing 2-4. Adding a new item to the list void ListDialog::addItem() { EditDialog dlg( this ); if( dlg.exec() == Qt::Accepted ) ui.list->addItem( dlg.name() + " -- " + dlg.number() ); } The opposite of adding a new item is shown in Listing 2-5. Deleting a list widget item is just a matter of calling delete on it. The currently selected item is returned from the currentItem method, so just delete whatever that method returns. If no item is selected, the return value is 0 (zero, a null pointer), but that is not a problem when used in a call to delete it is simply ignored. Listing 2-5. Deleting an item of the list void ListDialog::deleteItem() { delete ui.list->currentItem(); }
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CHAPTER 2 RAPID APPLICATION DEVELOPMENT USING QT
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When trying to edit the current item, it is important to ensure that the currentItem is a valid pointer, which is why the editItem slot in Listing 2-6 starts by checking it. If the returned pointer is a null pointer, the slot returns without doing anything. If a valid pointer is encountered, the text of the current list widget item is split into a name and a number using the split method. They are used to set up an editing dialog. When setting the name and the number, the parts of the split text are trimmed, which means removing all additional white space from the ends of the string (white space consists of all characters that take up space without showing). Examples of white space are spaces, tabs, line-feeds, newlines, and so on. As soon as the editing dialog has been set up, the code looks very much like the addItem slot, just that the current item s text is changed instead of adding a new item to the list widget. Listing 2-6. Editing an item of the list void ListDialog::editItem() { if( !ui.list->currentItem() ) return; QStringList parts = ui.list->currentItem()->text().split( "--" ); EditDialog dlg( this ); dlg.setName( parts[0].trimmed() ); dlg.setNumber( parts[1].trimmed() ); if( dlg.exec() == Qt::Accepted ) ui.list->currentItem()->setText( dlg.name() + " -- " + dlg.number() ); } You have used the editing dialog twice now, so it is time to have a look at it. In Listing 2-7, you can see the class declaration. The EditDialog class inherits QDialog and has a private variable called ui containing the generated code for the user interface. This is very much like the ListDialog class. The class then contains getters and setters for two properties: name and number. Because the dialog is specially made for the application and not at all likely to be reused in other circumstances, I have taken the liberty to avoid the policies for getters and setters. The setters are not slots, nor are there any signals that are emitted when a property is changed. When it is obvious that a class will not be reused, there is no point in overdesigning it to make it reusable. Because there are no signals or slots, the Q_OBJECT macro is omitted, so the class does not have a meta-object. This saves memory at run-time and makes compilation slightly quicker. Listing 2-7. Editing dialog class class EditDialog : public QDialog { public: EditDialog( QWidget *parent=0 );
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