Dynamic Form Models

A collection of questions and blanks to fill in the answers is called a form. Quite common in GUI applications, a form is used whenever a series of questions needs to be asked of the user. There are many ways to design and implement forms. In this section, we will make a model for the form, and then a view for it.

Example 17.1. src/libs/forms/testform.cpp

[ . . . . ]
class BridgeKeeper : public FormModel {

BridgeKeeper::BridgeKeeper() {
 FormFactory ff;
 *this << ff.newQuestion("name", "What is your name?");
 *this << ff.newQuestion("quest", "What is your quest?");
 QStringList colors;
 colors << "red" << "blue" << "green" << "orange";
 *this << ff.newQuestion("color",
 "What is your favorite color?",colors);
 *this << ff.newQuestion("speed",
 "What is the mean air speed of an unladen swallow?",

The model created in Example 17.1 represents a form containing questions of different "types," where the first two are simple string inputs, but the last two are constrained to a set of possible values. The main program creates a model and a view, and hooks them together. In Example 17.2, you can think of main as the controller.

Example 17.2. src/libs/forms/testform.cpp

[ . . . . ]

int main(int argc, char** argv) {
 QApplication a(argc, argv);
 QMainWindow mw;
 BridgeKeeper keeper;
 qDebug() << keeper.toString();
 FormDialog fv(&keeper);
 fv.setWindowTitle("I am the keeper of the Bridge of Death.");
 int retval = a.exec();
 QVariant speed = keeper.property("speed");
 QVariant color = keeper.property("color");
 QVariant quest = keeper.property("quest");
 QVariant name = keeper.property("name");
 if (color.toString() == "blue") {
 qDebug() << "no, I mean red! aaaaaaahhhhhhhhhhh!" ;
 else {
 qDebug() << "My name is " << name.toString()
 << ", and I " << quest.toString()
 << ". My favorite color is " << color.toString()
 << ". The speed is " << speed.toInt();
 return retval;

In Example 17.2, you can think of main as the controller.

The FormDialog below is automatically generated from the model above, even though it does not depend on the specific model.

The dialog embeds a FormView, which contains the actual input widgets.

  1. View classes access Questions only through the public polymorphic interface.
  2. Model classes emit signals to communicate information to views, instead of invoking objects directly through references or pointers passed around as function parameters.
  3. The code that does depend on both model and view (or on specific types of Question) is kept separate as controller code.

A FormModel wraps a collection of Questions, which are model classes. A FormView wraps a collection of InputFields, which are views (because they derive from QWidget), but encapsulate complex input widgets.

In the Doxygen collaboration diagram in Figure 17.2, there is a 1:1 correspondence between InputField and Question, but the classes are strictly decoupled.[1]

[1] "Strictly decoupled" means that they know nothing about each other.

Figure 17.2. Forms

A Question models (ideally) all of the information needed by FormFactory to create an appropriate InputField. A FormView is a grouping of input widgets. An input widget serves as a proxy, or delegate, between Qt input widgets and Question-derived models.

The Strategy pattern encapsulates each member of a family of algorithms so that they can be selected independently by clients. Each encapsulated algorithm is called a strategy.

Figure 17.3 shows that an InputField can be a variety of things. Because Qt input widgets[2] do not have a common QVariant-based interface for getting and setting data, the InputField serves as an adaptor, that provides a property-like interface. InputField uses the Strategy pattern to organize the getters and setters for different types as virtual functions.

[2] QLineEdit, QComboBox, QDateEdit, QSpinBox

Figure 17.3. Input fields

With a hierarchy of views, we end up with an extensible framework for adding other kinds of InputFields later.

Exercises: Dynamic Form Models


Add a DoubleInputField class, derived from InputField, and update the FormFactory to return it as needed.


Write a testcase with a QVariant Question in it, and verify that a QDoubleSpinBox shows up.

17.2.1. Form Models

In addition to different kinds of input widgets (for the different data types), there can also be different kinds of Question models for getting/setting the data in different places.

The FormModel and the Question classes, shown in Figure 17.4, are two adjoining layers in the model. Because they are models, they are meant to be very simple classes, holding data but containing no GUI or controller code.

Figure 17.4. FormModel and Question

FormModel provides a simple operation, setValues(), for updating all of its Question's values, shown in Example 17.3.

Example 17.3. src/libs/forms/formmodel.cpp

[ . . . . ]

bool FormModel::setValues(QList list) {
 bool retval = true;
 for (int i=0; isetValue(str) && retval;
 emit modelChanged();
 return retval;

Question encapsulates all the things that are needed for an interaction with a user, including the type and value of the expected answer. The constructors, declared in Example 17.4, are protected because we will use a factory to create Question objects.

Example 17.4. src/libs/forms/question.h

[ . . . . ]
class Question : public QObject {
 Question(QString name, QString label = QString(),
 QVariant::Type type=QVariant::String);
 Question(QString name, QString label,
 QStringList choices, bool open=false);
 Question() {}
 virtual Qt::ItemFlags flags() const;
 virtual QString toString() const;
 virtual QVariant value() const;

 virtual QVariant::Type type() const ;
 QStringList choices() const ;
 QString label() const {return m_Label;}
 virtual ~Question() {}
 public slots:
 virtual bool setValue(QVariant newValue);
 void valueChanged();
 void setType(QVariant::Type type) ;
 void setLabel(QString label) ;
 QString m_Label;
 QVariant m_Value;
 QStringList m_Choices;
 QVariant::Type m_Type;
[ . . . . ]

When we need a Question instance, FormFactory creates it by using one of the protected constructors, defined in Example 17.5.

Example 17.5. src/libs/forms/question.cpp

[ . . . . ]

Question::Question( QString name, QString label, QVariant::Type t):
 m_Label(label) {
 if (m_Label == QString())
 m_Label = name;
 m_Value = QVariant(t);
 m_Type = m_Value.type();

Question::Question( QString name, QString label, QStringList choices,
 bool) : m_Label(label), m_Choices(choices) {
 if (m_Label == QString())
 m_Label = name;
 m_Type = QVariant::StringList;

17.2.2. Form Views

The view classes, shown in Figure 17.5, are separated into three layers.

  1. FormDialog includes the buttons and actions plus some management/ controller code.
  2. FormView is solely responsible for holding layouts for labels and input widgets.
  3. InputField is responsible for a single input widget.

Figure 17.5. Form views

FormView can be automatically created from a FormModel without any knowledge of the individual InputField or Question types. This is thanks to the createEditor Factory method, used in Example 17.6, which returns polymorphic objects.

Example 17.6. src/libs/forms/formfactory.cpp

[ . . . . ]

FormView* FormFactory::formView(FormModel* mod) {
 FormView* retval = new FormView();
 retval->m_Model = mod;
 retval->m_LabelLayout = new QVBoxLayout();
 retval->m_EditLayout = new QVBoxLayout();
 foreach (Question* q, mod->questions()) {
 QLabel* label = new QLabel(q->label());
 InputField* editField = createEditor(q); <-- 1
 retval->m_Fields += editField;
 QWidget* labels = new QWidget();
 QWidget* edits = new QWidget();
 return retval;

(1)This is a factory method that returns polymorphic concrete instances.

The specific InputField types that get created depend on the type of the Question passed in, and that is determined in FormFactory, shown in Example 17.7.

Example 17.7. src/libs/forms/formfactory.cpp

[ . . . . ]

InputField* FormFactory::createEditor(Question* q) {
 QVariant::Type type = q->type();
 InputField* retval = 0;
 switch(type) {
 case QVariant::StringList:
 retval = new ChoiceInputField(
 q->objectName(), q->choices());

 case QVariant::String:
 retval = new StringInputField(q->objectName());
 case QVariant::Int:
 retval = new IntInputField(q->objectName());
 case Variant::Dir:
 retval = new DirInputField(q->objectName());
 retval=new StringInputField(q->objectName(), 0);
 qDebug() << QString("Unknown property type %1").arg(type);
 if (q->flags() != Qt::ItemIsEditable) {
 return retval;

In Example 17.7, notice the switch statement, which is normally to be avoided in object-oriented code. We have it here to map polymorphically from the QVariant::Type (an enumerated value) to an InputField class. This makes it possible for us to use the Strategy pattern on InputField (which provides input and output in various ways, on various types).

By default, createEditor() returns a StringInputField, shown in Example 17.8. It has a simple QLineEdit as its input widget.

Example 17.8. src/libs/forms/inputfields.h

[ . . . . ]

class StringInputField : public InputField {
 StringInputField(QString name, QWidget* parent = 0);
 QVariant value() const ;
 QWidget* widget() const ;
public slots:
 void setReadOnly(bool v);
 void setView(QVariant qv);
 void clearView();
 QLineEdit *qle;

17.2.3. Unforseen Types

It is possible there will be other "types" of data that correspond to different kinds of input widgets, but are not among those defined in QVariant. In Example 17.9, we introduce user-defined enumerated values above QVariant (127) that will not share a value with those already predefined in QVariant::Type.

Example 17.9. src/libs/dataobjects/variant.h

#ifndef VARIANT_H
#define VARIANT_H

namespace Variant {
 const QVariant::Type File = static_cast

A directory can be encoded and decoded as a QString quite naturally, but a Question with a Variant::Directory as its type gives a hint to the FormFactory that the input widget it creates should be a QFileDialog that is already in "directory-chooser" mode.

Example 17.10. src/libs/forms/dirinputfield.h

[ . . . . ]
class DirInputField : public StringInputField {
 DirInputField(QString name);
 QWidget* widget() const;
 void clearView();
 static void setFileDialog(QFileDialog* fd) {
 sFileDialog = fd;
 public slots:
 void browse();
 QHBoxLayout *m_Layout;
 QPushButton *m_Button;
 QWidget *m_Widget;
 static QFileDialog* sFileDialog;
[ . . . . ]

The DirInputField, defined in Example 17.10, extends the StringInputField and still has the QLineEdit for accepting a string from the user. In addition, there is a Browse button, which when clicked will pop up a QFileDialog pre-set to accept only a directory as a valid selection.

17.2.4. Controlling Actions

In this section, we discuss issues of synchronizing data between the model and the view. Since these methods depend on both model and view, we are going to isolate them from both, in their own controller classes. In Example 17.11, we derived two custom QAction classes, each responsible for synchronizing in one direction.

Example 17.11. src/libs/forms/formactions.h

[ . . . . ]
class OkAction : public QAction {
 OkAction(FormModel* model, FormView* view);
 public slots:
 void ok();
 FormModel *m_Model;
 FormView *m_View;

class CancelAction : public QAction {
 public slots:
 void cancel();
[ . . . . ]

OkAction (or apply) should send the data from the view to the model. CancelAction, in the case where the dialog is not to be closed afterwards, should do the opposite (send the data from the model back to the view, to restore old or set default values). Their definitions are in Example 17.12.

Example 17.12. src/libs/forms/formactions.cpp

#include "formactions.h"
#include "formmodel.h"
#include "formview.h"
#include "inputfield.h"
#include "question.h"
OkAction::OkAction(FormModel* model, FormView* view) :
 QAction( tr("&Ok"), view), m_Model(model), m_View(view) {
 connect (this, SIGNAL(triggered()), this, SLOT(ok()));

void OkAction::ok() {
 qDebug() << "OK()" << endl;
 QList values;
 InputList fields = m_View->fields();
 foreach (InputField* field, fields) {
 QVariant v = field->value();
 qDebug() << "submitting value: " << v.toString();
 values += v;
 qDebug() << m_Model->toString();

CancelAction::CancelAction(FormModel* model, FormView* view) :
 QAction( tr("&Cancel"), view), m_Model(model), m_View(view) {
 connect (this, SIGNAL(triggered()), this, SLOT(cancel()));

void CancelAction::cancel() {
 qDebug() << "Cancel() " << endl;
 QList qlist = m_Model->questions();
 InputList fields = m_View->fields();
 for (int i=qlist.size()-1; i>-1; --i) {
 Question* q = qlist.at(i);
 InputField* f = fields.at(i);
 qDebug() << QString(" name: %1 val: %2")

These actions are in fact delegates, and perform a similar function to Qt's QItemDelegate.

17.2.5. DataObject Form Model

In Example 17.1, we extended FormModel, and in the constructor we created and added Question objects to compose a custom form. The FormModel itself can be used in other ways, including those listed below.

  1. Creating a FormModel from a DataObject (one Question per property)
  2. Connecting fields of a FormModel to QSettings values, to give persistence
  3. Importing and exporting in XML[3]

    [3] This is how Designer recreates its GUIs.

  4. Importing and exporting in HTML[4]

    [4] Basically, this is similar to importing and exporting in XML, except that the tags we use, <form> and <input>, are specified by the W3C [w3c]. By using this format, we can create XHTML forms and load them in as FormModels.

  5. Can you think of others?

We wish to create a FormModel from a DataObject, so this means that another function goes into the ModelFactory. We extended Question, the basic FormModel building block, so that it would get/set values from/to a DataObject property instead of its own m_Value.

PropQuestion, shown in Figure 17.6, serves as a proxy or delegate between a DataObject property and an InputField widget. We see this in Example 17.13: Most of the methods simply pass on the request to the underlying DataObject, mDest.

Figure 17.6. Rephrasing the question


Example 17.13. src/libs/forms/propquestion.cpp

#include "propquestion.h"

PropQuestion::PropQuestion(QString name, DataObject* dest):
 m_Dest(dest) {
 m_Prop = m_Dest->metaProperty(name);

Qt::ItemFlags PropQuestion::flags() const {
 if (m_Prop.isWritable()) return Qt::ItemIsEditable;
 else return Qt::ItemIsSelectable;

QVariant PropQuestion::value() const {
 return m_Dest->property(objectName());

bool PropQuestion::setValue(QVariant newValue) {
 return m_Dest->setProperty(objectName(), newValue);

Example 17.14 uses the DataObject model applied to the FileTagger class to auto-generate a form, which looks like Figure 17.7.

Figure 17.7. Auto-generated FileTagger form


Example 17.14. src/libs/forms/testform2.cpp

#include "formfactory.h"
#include "formdialog.h"
#include "formmodel.h"
#include "filetagger.h"

int main(int argc, char** argv) {
 QApplication a(argc, argv);
 QMainWindow mw;
 FileTagger ft;
 FormModel* mod = FormFactory::newForm(&ft);
 FormDialog dialog(mod);
 return a.exec();

The newForm Factory method defined in Example 17.15 simply returns a PropQuestion instead of a Question when it is creating the FormModel from a DataObject.

Example 17.15. src/libs/forms/formfactory.cpp

[ . . . . ]

FormModel* FormFactory::newForm(DataObject* dobj) {
 QStringList props = dobj->propertyNames();
 FormModel *mod = new FormModel(dobj->className());
 foreach (QString prop, props) {
 if (prop == "objectName")
 PropQuestion *pq = new PropQuestion(prop, dobj);
 *mod << pq;
 return mod;

Exercises: DataObject Form Model

I am the keeper of the bridge of death


Example 17.16 is an XHTML [w3c] fragment that contains three different kinds of input widgets and roughly represents the form we've seen earlier.

Example 17.16. src/modelview/html/bridgekeeper.html

Answer these questions three and you can proceed over the bridge.

What is your name? What is your quest? What is your favorite color? bluegreenorangeburgundycrimson

It is possible to preview it in a browser by opening it as a file. It doesn't look fancy without any css styling, but you can use it as a sanity check for your files.

Write a FormReader class that can read an XML file of the above format. (Do not worry about handling XHTML elements or formats that are not shown in Example 17.16.)


Write a FormWriter class that can write a FormModel to an XML file in the same format.


Write a Mad Libs game that asks the user for a bunch of nouns, verbs, adjectives, and adverbs such that when the form is submitted, it sticks the strings into a paragraph and shows the result to the user. The passage of text should be at least two paragraphs long, and contain at least ten blanks to be filled in.

Part I: Introduction to C++ and Qt 4

C++ Introduction


Introduction to Qt



Inheritance and Polymorphism

Part II: Higher-Level Programming


Introduction to Design Patterns


Generics and Containers

Qt GUI Widgets


Validation and Regular Expressions

Parsing XML

Meta Objects, Properties, and Reflective Programming

More Design Patterns

Models and Views

Qt SQL Classes

Part III: C++ Language Reference

Types and Expressions

Scope and Storage Class

Statements and Control Structures

Memory Access

Chapter Summary

Inheritance in Detail

Miscellaneous Topics

Part IV: Programming Assignments

MP3 Jukebox Assignments

Part V: Appendices

MP3 Jukebox Assignments


MP3 Jukebox Assignments

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An Introduction to Design Patterns in C++ with Qt 4
An Introduction to Design Patterns in C++ with Qt 4
ISBN: 0131879057
EAN: 2147483647
Year: 2004
Pages: 268
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