1820 Bug - Convert styleFillVariants from slice to func

[edwardfward]

PR Details

Fixes a bug with gradient fill styles that was causing different colored fills to return the same styleID making it impossible to use more than one gradient fill per workbook.

Description

Changed the styleFillVariants variable from []xlsxGradientFill to func() []xlsxGradientFill per @xuri's recommendation.

Related Issue

• Closes Different gradient fill colors return the same style index #1820

Motivation and Context

Customer required several gradient fills in the workbook we were generating for them. This PR allows use to generate those gradient fills.

How Has This Been Tested

• Added unit test to verify gradients did not produce the same styleID
• Executed go test ./...

ok      github.com/xuri/excelize/v2     23.656s

• Executed the following test program to verify the fills we required returned unique styleID

package main

import (
	"fmt"
	"github.com/xuri/excelize/v2"
)

func main() {
	f := excelize.NewFile()

	s1 := excelize.Style{
		Fill: excelize.Fill{
			Type:    "gradient",
			Pattern: 4,
			Shading: 1,
			Color:   []string{"#000000", "#FFFFFF"},
		},
	}

	s2 := excelize.Style{
		Fill: excelize.Fill{
			Type:    "gradient",
			Pattern: 2,
			Shading: 1,
			Color:   []string{"#FFFF00", "#000000"},
		},
	}

	s1Index, err := f.NewStyle(&s1)
	if err != nil {
		fmt.Println(err)
		return

	}
	s2Index, _ := f.NewStyle(&s2)

	fmt.Println("s1Index:", s1Index)
	fmt.Println("s2Index:", s2Index)
}

Result:

s1Index: 1
s2Index: 2

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.20%. Comparing base (ee2ef15) to head (db46bc7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1821   +/-   ##
=======================================
  Coverage   99.20%   99.20%           
=======================================
  Files          32       32           
  Lines       23852    23871   +19     
=======================================
+ Hits        23663    23682   +19     
  Misses        101      101           
  Partials       88       88           

Flag	Coverage Δ
unittests	99.20% <100.00%> (+<0.01%)	⬆️

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[xuri]

LGTM, thanks for your contribution.