Fix issue 4: Fix background color of solid pattern

For a solid fill, Excel reverses the role of foreground and
background colours

examples/xlsx/style/main.cpp

@@ -46,6 +46,10 @@ int main()
     format5->setNumberFormat(22);
     xlsx.write("A5", QDate(2013, 8, 29), format5);
 
+    QXlsx::Format *format6 = xlsx.createFormat();
+    format6->setPatternBackgroundColor(QColor(Qt::gray));
+    xlsx.write("A6", "Background color: green", format6);
+
     xlsx.saveAs(DATA_PATH"TestStyle.xlsx");
     return 0;
 }

src/xlsx/xlsxstyles.cpp

@@ -311,12 +311,13 @@ void Styles::writeFill(XmlStreamWriter &writer, FillData *fill)
     writer.writeStartElement(QStringLiteral("fill"));
     writer.writeStartElement(QStringLiteral("patternFill"));
     writer.writeAttribute(QStringLiteral("patternType"), patternStrings[fill->pattern]);
+    // For a solid fill, Excel reverses the role of foreground and background colours
     if (fill->fgColor.isValid()) {
-        writer.writeEmptyElement(QStringLiteral("fgColor"));
+        writer.writeEmptyElement(fill->pattern == Format::PatternSolid ? QStringLiteral("bgColor") : QStringLiteral("fgColor"));
         writer.writeAttribute(QStringLiteral("rgb"), QStringLiteral("FF")+fill->fgColor.name().mid(1));
     }
     if (fill->bgColor.isValid()) {
-        writer.writeEmptyElement(QStringLiteral("bgColor"));
+        writer.writeEmptyElement(fill->pattern == Format::PatternSolid ? QStringLiteral("fgColor") : QStringLiteral("bgColor"));
         writer.writeAttribute(QStringLiteral("rgb"), QStringLiteral("FF")+fill->bgColor.name().mid(1));
     }
 

tests/auto/styles/tst_stylestest.cpp

@@ -13,6 +13,7 @@ public:
 private Q_SLOTS:
     void testEmptyStyle();
     void testAddFormat();
+    void testSolidFillBackgroundColor();
 };
 
 StylesTest::StylesTest()
@@ -41,6 +42,19 @@ void StylesTest::testAddFormat()
     QVERIFY2(xmlData.contains("<cellXfs count=\"2\">"), ""); //Note we have a default one
 }
 
+// For a solid fill, Excel reverses the role of foreground and background colours
+void StylesTest::testSolidFillBackgroundColor()
+{
+    QXlsx::Styles styles;
+    QXlsx::Format *format = styles.createFormat();
+    format->setPatternBackgroundColor(QColor(Qt::red));
+    styles.addFormat(format);
+
+    QByteArray xmlData = styles.saveToXmlData();
+
+    QVERIFY(xmlData.contains("<patternFill patternType=\"solid\"><fgColor rgb=\"FFff0000\"/>"));
+}
+
 QTEST_APPLESS_MAIN(StylesTest)
 
 #include "tst_stylestest.moc"