Added Formula support

src/Spout/Writer/XLSX/Formula.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Box\Spout\Writer\XLSX;
+
+/**
+ * Class Formula
+ *
+ * @package Box\Spout\Reader\XLSX
+ */
+class Formula
+{
+
+
+    /**
+     * @var string
+     */
+    protected $formula;
+
+    /**
+     * @var string
+     */
+    protected $value = '0';
+
+    /**
+     * Formula constructor.
+     * @param string $formula
+     * @param string $value
+     */
+    public function __construct($formula, $value = '0')
+    {
+        $this->formula = $formula;
+        $this->value = $value;
+    }
+
+    public function getXml()
+    {
+        return '><f>' . $this->formula . '</f><v>' . $this->value . '</v></c>';
+    }
+
+}

src/Spout/Writer/XLSX/Internal/Worksheet.php

@@ -5,6 +5,7 @@ namespace Box\Spout\Writer\XLSX\Internal;
 use Box\Spout\Common\Exception\InvalidArgumentException;
 use Box\Spout\Common\Exception\IOException;
 use Box\Spout\Common\Helper\StringHelper;
+use Box\Spout\Writer\XLSX\Formula;
 use Box\Spout\Writer\Common\Helper\CellHelper;
 use Box\Spout\Writer\Common\Internal\WorksheetInterface;
 
@@ -184,7 +185,7 @@ EOD;
 
         $rowXML = '<row r="' . $rowIndex . '" spans="1:' . $numCells . '">';
 
-        foreach($dataRow as $cellValue) {
+        foreach ($dataRow as $cellValue) {
             $rowXML .= $this->getCellXML($rowIndex, $cellNumber, $cellValue, $style->getId());
             $cellNumber++;
         }
@@ -227,6 +228,8 @@ EOD;
                 // NOTE: not appending to $cellXML is the right behavior!!
                 $cellXML = '';
             }
+        } elseif ($cellValue instanceof Formula) {
+            $cellXML .= $cellValue->getXml();
         } else {
             throw new InvalidArgumentException('Trying to add a value with an unsupported type: ' . gettype($cellValue));
         }

tests/Spout/Writer/XLSX/FormulaTest.php

@@ -0,0 +1,60 @@
+<?php
+
+namespace Box\Spout\Writer\XLSX;
+
+
+use Box\Spout\Common\Type;
+use Box\Spout\Reader\ReaderFactory;
+use Box\Spout\Reader\XLSX\Sheet;
+use Box\Spout\TestUsingResource;
+use Box\Spout\Writer\WriterFactory;
+
+class FormulaTest extends \PHPUnit_Framework_TestCase
+{
+
+    use TestUsingResource;
+
+    /**
+     * @throws \Box\Spout\Common\Exception\IOException
+     * @throws \Box\Spout\Common\Exception\UnsupportedTypeException
+     * @throws \Box\Spout\Writer\Exception\WriterNotOpenedException
+     * @throws \Box\Spout\Common\Exception\InvalidArgumentException
+     * @throws \Box\Spout\Reader\Exception\ReaderNotOpenedException
+     */
+    public function testGetXml()
+    {
+        $fileName = 'test_formula.xlsx';
+        $this->createGeneratedFolderIfNeeded($fileName);
+        $resourcePath = $this->getGeneratedResourcePath($fileName);
+
+        $writer = WriterFactory::create(Type::XLSX);
+        $writer->openToFile($resourcePath);
+
+        $writer->addRows([
+            [1],
+            [1],
+            [1],
+            [1],
+        ]);
+
+        $writer->addRow([
+            new Formula('SUM(A1:A4)', 4)
+        ]);
+        $writer->close();
+
+
+        $reader = ReaderFactory::create(Type::XLSX);
+        $reader->open($resourcePath);
+
+        $i = 0;
+        /** @var Sheet $sheet */
+        foreach ($reader->getSheetIterator() as $sheet) {
+            foreach ($sheet->getRowIterator() as $item) {
+                $i++;
+                if ($i == 5) {
+                    static::assertEquals(4, $item[0]);
+                }
+            }
+        }
+    }
+}