[partition] Only create drop-down if there is something to select

- Swap choices may be 0 (then choose none), 1 (choose that one)
   or more (currently undecided)

src/modules/partition/gui/ChoicePage.cpp

@@ -65,6 +65,14 @@
 
 using PartitionActions::Choices::SwapChoice;
 
+SwapChoice pickOne( const SwapChoiceSet& s )
+{
+    if ( s.count() == 1 )
+        for ( auto i = s.begin(); i != s.end(); ++i )
+            return *i;  // That's the only element
+    return SwapChoice::NoSwap;
+}
+
 /**
  * @brief ChoicePage::ChoicePage is the default constructor. Called on startup as part of
  *      the module loading code path.
@@ -90,7 +98,8 @@ ChoicePage::ChoicePage( const SwapChoiceSet& swapChoices, QWidget* parent )
     , m_bootloaderComboBox( nullptr )
     , m_lastSelectedDeviceIndex( -1 )
     , m_enableEncryptionWidget( true )
-    , m_swapChoices( swapChoices )
+    , m_availableSwapChoices( swapChoices )
+    , m_eraseSwapChoice( pickOne( swapChoices ) )
 {
     setupUi( this );
 
@@ -258,8 +267,11 @@ ChoicePage::setupChoices()
 
     // Fill up swap options
     // .. TODO: only if enabled in the config
-    m_eraseSwapChoices = createCombo( m_swapChoices );
-    m_eraseButton->addOptionsComboBox( m_eraseSwapChoices );
+    if ( m_availableSwapChoices.count() > 1 )
+    {
+        m_eraseSwapChoices = createCombo( m_availableSwapChoices );
+        m_eraseButton->addOptionsComboBox( m_eraseSwapChoices );
+    }
 
     m_itemsLayout->addWidget( m_alongsideButton );
     m_itemsLayout->addWidget( m_replaceButton );

src/modules/partition/gui/ChoicePage.h

@@ -45,7 +45,7 @@ class DeviceInfoWidget;
 
 class Device;
 
-using SwapChoiceSet = QSet< PartitionActions::Choices::SwapChoice>;
+using SwapChoiceSet = QSet< PartitionActions::Choices::SwapChoice >;
 
 /**
  * @brief The ChoicePage class is the first page of the partitioning interface.
@@ -151,7 +151,7 @@ private:
     PrettyRadioButton* m_eraseButton;
     PrettyRadioButton* m_replaceButton;
     PrettyRadioButton* m_somethingElseButton;
-    QComboBox* m_eraseSwapChoices;
+    QComboBox* m_eraseSwapChoices;  // UI, see also m_swapChoices
 
     DeviceInfoWidget* m_deviceInfoWidget;
 
@@ -168,7 +168,8 @@ private:
 
     QString m_defaultFsType;
     bool m_enableEncryptionWidget;
-    SwapChoiceSet m_swapChoices;
+    SwapChoiceSet m_availableSwapChoices;  // What is available
+    PartitionActions::Choices::SwapChoice m_eraseSwapChoice;  // what is selected
 
     QMutex m_coreMutex;
 };