Improve UICheckBox in different pixel densities.

Improve stylesheet units printing.
Added `dpr` as device-pixel rounded stylesheet length unit.
Fixes in 7GUIs Cells.

src/examples/7guis/cells/cells.cpp

@@ -26,12 +26,20 @@ EE_MAIN_FUNC int main( int, char** ) {
 	table->setColumnsWidth( PixelDensity::dpToPx( 80 ) );
 	table->setRowHeaderWidth( PixelDensity::dpToPx( 32 ) );
 	table->setSelectionType( UITableView::SelectionType::Cell );
+	table->setRowSearchByName( false );
 	table->setEditable( true );
 	table->setEditTriggers( UIAbstractView::EditTrigger::DoubleClicked |
 							UIAbstractTableView::EditTrigger::EditKeyPressed );
 	table->setEditShortcuts( { { KEY_F2 }, { KEY_RETURN }, { KEY_KP_ENTER } } );
 	table->setSelection( model->index( 0, 0 ) );
 	table->setFocus();
+	table->on( Event::KeyDown, [table, model]( const Event* event ) {
+		if ( table->getSelection().first().isValid() &&
+			 event->asKeyEvent()->getKeyCode() == KEY_DELETE ) {
+			model->cell( table->getSelection().first() ).clear();
+			model->invalidate( Model::UpdateFlag::DontInvalidateIndexes );
+		}
+	} );
 	table->onCreateEditingDelegate = [table,
 									  model]( const ModelIndex& index ) -> ModelEditingDelegate* {
 		auto ret = StringModelEditingDelegate::New();

src/examples/7guis/cells/formula.cpp

@@ -54,6 +54,8 @@ void SheetFunction::initOpTable() {
 							 tot += val;
 						 return vals.empty() ? 0 : tot / vals.size();
 					 } } );
+	opTable.insert( { String::hash( "COUNT" ),
+					  []( const std::vector<double>& vals ) { return vals.size(); } } );
 }
 
 std::vector<double> SheetFunction::evalList( const std::vector<std::shared_ptr<Formula>>& args,

src/examples/7guis/cells/spreadsheet.cpp

@@ -16,20 +16,41 @@ void Cell::parseFormula( const std::string& formulaStr ) {
 
 Cell::Cell( std::string val, Spreadsheet& sheet ) : value( std::move( val ) ), sheet( sheet ) {}
 
-void Cell::setData( std::string&& data ) {
+Cell::~Cell() {}
+
+bool Cell::subscribeToObservers() {
+	if ( formula ) {
+		for ( const auto& ref : formula->getReferences( sheet ) ) {
+			if ( this == ref || hasObserver( ref ) ) {
+				unsubscribeFromObservers();
+				return false;
+			}
+			ref->addObserver( this );
+		}
+	}
+	return true;
+}
+
+void Cell::unsubscribeFromObservers() {
 	if ( formula ) {
 		for ( const auto& ref : formula->getReferences( sheet ) )
 			ref->deleteObserver( this );
 	}
+}
+
+void Cell::setData( std::string&& data ) {
+	unsubscribeFromObservers();
 	value = std::move( data );
 	parseFormula( value );
 	calc();
-	if ( formula ) {
-		for ( const auto& ref : formula->getReferences( sheet ) )
-			ref->addObserver( this );
+	bool circular = !subscribeToObservers();
+	if ( circular ) {
+		displayValue = "!CIRCULAR";
+		formulaContainsErrors = true;
+	} else {
+		setChanged();
+		notifyObservers();
 	}
-	setChanged();
-	notifyObservers();
 }
 
 std::optional<double> Cell::eval() const {
@@ -72,6 +93,12 @@ void Cell::calc() {
 		setChanged();
 }
 
+void Cell::clear() {
+	value = displayValue = "";
+	formula = {};
+	formulaContainsErrors = false;
+}
+
 void Cell::update() {
 	calc();
 	notifyObservers();

src/examples/7guis/cells/spreadsheet.hpp

@@ -32,6 +32,8 @@ class Observable {
 
 	void notifyObservers();
 
+	bool hasObserver( Observer* observer ) { return mObservers.count( observer ); }
+
   protected:
 	std::unordered_set<Observer*> mObservers;
 	std::atomic<bool> mChanged{ false };
@@ -46,7 +48,7 @@ class Cell : public Observer, public Observable {
   public:
 	Cell( std::string val, Spreadsheet& sheet );
 
-	virtual ~Cell() {}
+	virtual ~Cell();
 
 	void setData( std::string&& data );
 
@@ -58,6 +60,8 @@ class Cell : public Observer, public Observable {
 
 	void calc();
 
+	void clear();
+
 	void update();
 
 	bool hasFormula() const;
@@ -74,6 +78,10 @@ class Cell : public Observer, public Observable {
 	bool formulaContainsErrors{ false };
 
 	void parseFormula( const std::string& formulaStr );
+
+	bool subscribeToObservers();
+
+	void unsubscribeFromObservers();
 };
 
 class Spreadsheet : public Model {