Properties editor design pattern? - c++

Question

Warning: This is super in-depth. I understand if you don't even want to read this, this is mostly for me to sort out my thought process.
Okay, so here's what I'm trying to do. I've got these objects:
When you click on one (or select several) it should display their properties on the right (as shown). When you edit said properties it should update the internal variables immediately.
I'm trying to decide on the best way to do this. I figure the selected objects should be stored as a list of pointers. It's either that, or have an isSelected bool on each object, and then iterate over all of them, ignoring the non-selected ones, which is just inefficient. So we click on one, or select several, and then the selectedObjects list is populated. We then need to display the properties. To keep things simple for the time being, we'll assume that all objects are of the same type (share the same set of properties). Since there aren't any instance-specific properties, I figure we should probably store these properties as static variables inside the Object class. Properties basically just have a name (like "Allow Sleep"). There is one PropertyManager for each type of property (int,bool,double). PropertyManagers store all the values for properties of their respective type (this is all from the Qt API). Unfortunately, because PropertyManagers are required to create Properties I can't really decouple the two. I suppose this means that I have to place the PropertyManagers with the Properties (as static variables). This means we have one set of properties, and one set of property managers to manage all the variables in all the objects. Each property manager can only have one callback. That means this callback has to update all the properties of its respective type, for all objects (a nested loop). This yields something like this (in pseudo-code):
function valueChanged(property, value) {
if(property == xPosProp) {
foreach(selectedObj as obj) {
obj->setXPos(value);
}
} else if(property == ...
Which already bothers me a little bit, because we're using if statements where we shouldn't need them. The way around this would be to create a different property manager for every single property, so that we can have unique callbacks. This also means we need two objects for each property, but it might be a price worth paying for cleaner code (I really don't know what the performance costs are right now, but as I know you'll also say -- optimize when it becomes a problem). So then we end up with a ton of callbacks:
function xPosChanged(property, value) {
foreach(selectedObj as obj) {
obj->setXPos(value);
}
}
Which eliminates the entire if/else garbage but adds a dozen more event-listeners. Let's assume I go with this method. So now we had a wad of static Properties, along with their corresponding static PropertyManagers. Presumably I'd store the list of selected objects as Object::selectedObjects too since they're used in all the event callbacks, which logically belong in the object class. So then we have a wad of static event callbacks too. That's all fine and dandy.
So now when you edit a property, we can update the interal variables for all the selected objects via the event callback. But what happens when the internal variable is updated via some other means, how do we update the property? This happens to be a physics simulator, so all the objects will have many of their variables continuously updated. I can't add callbacks for these because the physics is handled by another 3rd party library. I guess this means I just have to assume all the variables have been changed after each time step. So after each time step, I have to update all the properties for all the selected objects. Fine, I can do that.
Last issue (I hope), is what values should we display when multiple objects are selected an there is an inconsistency? I guess my options are to leave it blank/0 or display a random object's properties. I don't think one option is much better than the other, but hopefully Qt provides a method to highlight such properties so that I can at least notify the user. So how do I figure out which properties to "highlight"? I guess I iterate over all the selected objects, and all their properties, compare them, and as soon as there is a mismatch I can highlight it. So to clarify, upon selected some objects:
add all objects to a selectedObjects list
populate the properties editor
find which properties have identical values and update the editor appropriately
I think I should store the properties in a list too so that I can just push the whole list onto the properties editor rather than adding each property individually. Should allow for more flexibility down the road I think.
I think that about covers it... I'm still not certain how I feel about having so many static variables, and a semi-singleton class (the static variables would be initialized once when the first object is created I guess). But I don't see a better solution.
Please post your thoughts if you actually read this. I guess that's not really a question, so let me rephrase for the haters, What adjustments can I make to my suggested design-pattern to yield cleaner, more understandable, or more efficient code? (or something along those lines).
Looks like I need to clarify. By "property" I mean like "Allow Sleeping", or "Velocity" -- all objects have these properties -- their VALUES however, are unique to each instance. Properties hold the string that needs to be displayed, the valid range for the values, and all the widget info. PropertyManagers are the objects that actually hold the value. They control the callbacks, and the value that's displayed. There is also another copy of the value, that's actually used "internally" by the other 3rd party physics library.
Trying to actually implement this madness now. I have an EditorView (the black area drawing area in the image) which catches the mouseClick event. The mouseClick events then tells the physics simulator to query all the bodies at the cursor. Each physics body stores a reference (a void pointer!) back to my object class. The pointers get casted back to objects get pushed onto a list of selected objects. The EditorView then sends out a signal. The EditorWindow then catches this signal and passes it over to the PropertiesWindow along with the selected objects. Now the PropertiesWindow needs to query the objects for a list of properties to display... and that's as far as I've gotten so far. Mind boggling!
The Solution
/*
* File: PropertyBrowser.cpp
* Author: mark
*
* Created on August 23, 2009, 10:29 PM
*/
#include <QtCore/QMetaProperty>
#include "PropertyBrowser.h"
PropertyBrowser::PropertyBrowser(QWidget* parent)
: QtTreePropertyBrowser(parent), m_variantManager(new QtVariantPropertyManager(this)) {
setHeaderVisible(false);
setPropertiesWithoutValueMarked(true);
setIndentation(10);
setResizeMode(ResizeToContents);
setFactoryForManager(m_variantManager, new QtVariantEditorFactory);
setAlternatingRowColors(false);
}
void PropertyBrowser::valueChanged(QtProperty *property, const QVariant &value) {
if(m_propertyMap.find(property) != m_propertyMap.end()) {
foreach(QObject *obj, m_selectedObjects) {
obj->setProperty(m_propertyMap[property], value);
}
}
}
QString PropertyBrowser::humanize(QString str) const {
return str.at(0).toUpper() + str.mid(1).replace(QRegExp("([a-z])([A-Z])"), "\\1 \\2");
}
void PropertyBrowser::setSelectedObjects(QList<QObject*> objs) {
foreach(QObject *obj, m_selectedObjects) {
obj->disconnect(this);
}
clear();
m_variantManager->clear();
m_selectedObjects = objs;
m_propertyMap.clear();
if(objs.isEmpty()) {
return;
}
for(int i = 0; i < objs.first()->metaObject()->propertyCount(); ++i) {
QMetaProperty metaProperty(objs.first()->metaObject()->property(i));
QtProperty * const property
= m_variantManager->addProperty(metaProperty.type(), humanize(metaProperty.name()));
property->setEnabled(metaProperty.isWritable());
m_propertyMap[property] = metaProperty.name();
addProperty(property);
}
foreach(QObject *obj, m_selectedObjects) {
connect(obj, SIGNAL(propertyChanged()), SLOT(objectUpdated()));
}
objectUpdated();
}
void PropertyBrowser::objectUpdated() {
if(m_selectedObjects.isEmpty()) {
return;
}
disconnect(m_variantManager, SIGNAL(valueChanged(QtProperty*, QVariant)),
this, SLOT(valueChanged(QtProperty*, QVariant)));
QMapIterator<QtProperty*, QByteArray> i(m_propertyMap);
bool diff;
while(i.hasNext()) {
i.next();
diff = false;
for(int j = 1; j < m_selectedObjects.size(); ++j) {
if(m_selectedObjects.at(j)->property(i.value()) != m_selectedObjects.at(j - 1)->property(i.value())) {
diff = true;
break;
}
}
if(diff) setBackgroundColor(topLevelItem(i.key()), QColor(0xFF,0xFE,0xA9));
else setBackgroundColor(topLevelItem(i.key()), Qt::white);
m_variantManager->setValue(i.key(), m_selectedObjects.first()->property(i.value()));
}
connect(m_variantManager, SIGNAL(valueChanged(QtProperty*, QVariant)),
this, SLOT(valueChanged(QtProperty*, QVariant)));
}
With a big thanks to TimW

Answer 1

Did you have a look at Qt's (dynamic) property system?
bool QObject::setProperty ( const char * name, const QVariant & value );
QVariant QObject::property ( const char * name ) const
QList<QByteArray> QObject::dynamicPropertyNames () const;
//Changing the value of a dynamic property causes a
//QDynamicPropertyChangeEvent to be sent to the object.
function valueChanged(property, value) {
foreach(selectedObj as obj) {
obj->setProperty(property, value);
}
}
Example
This is an incomplete example to give you my idea about the property system.
I guess SelectableItem * selectedItem must be replaced with a list of items in your case.
class SelectableItem : public QObject
{
Q_OBJECT
Q_PROPERTY(QString name READ name WRITE setName );
Q_PROPERTY(int velocity READ velocity WRITE setVelocity);
public:
QString name() const { return m_name; }
int velocity() const {return m_velocity; }
public slots:
void setName(const QString& name)
{
if(name!=m_name)
{
m_name = name;
emit update();
}
}
void setVelocity(int value)
{
if(value!=m_velocity)
{
m_velocity = value;
emit update();
}
}
signals:
void update();
private:
QString m_name;
int m_velocity;
};
class MyPropertyWatcher : public QObject
{
Q_OBJECT
public:
MyPropertyWatcher(QObject *parent)
: QObject(parent),
m_variantManager(new QtVariantPropertyManager(this)),
m_propertyMap(),
m_selectedItem(),
!m_updatingValues(false)
{
connect(m_variantManager, SIGNAL(valueChanged(QtProperty*, QVariant)), SLOT(valueChanged(QtProperty*,QVariant)));
m_propertyMap[m_variantManager->addProperty(QVariant::String, tr("Name"))] = "name";
m_propertyMap[m_variantManager->addProperty(QVariant::Int, tr("Velocity"))] = "velocity";
// Add mim, max ... to the property
// you could also add all the existing properties of a SelectableItem
// SelectableItem item;
// for(int i=0 ; i!=item.metaObject()->propertyCount(); ++i)
// {
// QMetaProperty metaProperty(item.metaObject()->property(i));
// QtProperty *const property
// = m_variantManager->addProperty(metaProperty.type(), metaProperty.name());
// m_propertyMap[property] = metaProperty.name()
// }
}
void setSelectedItem(SelectableItem * selectedItem)
{
if(m_selectedItem)
{
m_selectedItem->disconnect( this );
}
if(selectedItem)
{
connect(selectedItem, SIGNAL(update()), SLOT(itemUpdated()));
itemUpdated();
}
m_selectedItem = selectedItem;
}
private slots:
void valueChanged(QtProperty *property, const QVariant &value)
{
if(m_updatingValues)
{
return;
}
if(m_selectedItem && m_map)
{
QMap<QtProperty*, QByteArray>::const_iterator i = m_propertyMap.find(property);
if(i!=m_propertyMap.end())
m_selectedItem->setProperty(m_propertyMap[property], value);
}
}
void itemUpdated()
{
m_updatingValues = true;
QMapIterator<QtProperty*, QByteArray> i(m_propertyMap);
while(i.hasNext())
{
m_variantManager->next();
m_variantManager->setValue(
i.key(),
m_selectedItem->property(i.value()));
}
m_updatingValues = false;
}
private:
QtVariantPropertyManager *const m_variantManager;
QMap<QtProperty*, QByteArray> m_propertyMap;
QPointer<SelectableItem> m_selectedItem;
bool m_updatingValues;
};

Answer 2

Calm down, your code has not O(n^2) complextity. You have a nested loop, but only one counts to N (the number of objects), the other counts to a fixed number of properties, which is not related to N. So you have O(N).
For the static variables, you write "there aren't any instance-specific properties", later you write about updates of the individual properties of your objects, which are exactly instance-specific properties. Maybe you are confusing the "class Properties" (which is of course shared among all properties) with the individual properties? So I think you don't need static members at all.
Do you want to display changes to the objects only if they appear, or do you want a continuos display? If your hardware is able to handle the latter, I would recommend going that way. In that case, you have to iterate over all objects anyway and update them along the way.
Edit: The difference is that in the former (update on change) the drawing is initiated by the operation of changing the values, for example a object movement. For the latter, a continuos display, you would add a QTimer, which fires say 60 times a second and calls a SLOT(render()) which does the actual rendering of all objects. Depending on the rate of changes this may actually be faster. And it is probably easier to implement.
Another possibilty is let Qt handle the whole drawing, using a Graphics View, which handles the objects-to-draw internally in a very efficient tree structure. Take a look at
http://doc.trolltech.com/4.5/graphicsview.html
If you want to display only the changes, you could use individual callbacks for each properties value. Each time the value of a property is changed (in this case making the properties vlaues private and using setSomeThing(value)), you call the update function with an emit(update()). If you are absolutly concernd about emit being slow, you could use "real" callbacks via function pointers, but I don't recommend that, Qt's connect/signal/slot is so much easier to use. And the overhead is in most cases really neglible.