[Feature] Add Row Factory Support for Flexible Row Generation

[mrwangshipei]

🚀 [Feature] Add Row Factory Support for Flexible Row Generation

📌 Summary

This PR introduces a Row Factory API to MiniExcel, allowing users to dynamically generate rows with fine-grained control, addressing limitations of the current template-based injection mechanism.

🎯 中文简介：本 PR 引入 行工厂模式（Row Factory），允许开发者以编程方式控制每一行的结构、内容和样式，解决现有模板模式在多表头、重复列名、隐藏行等场景下的局限性。

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✅ Motivation

Currently, MiniExcel relies on traversing List or Dictionary to inject data into templates. This is simple and fast, but not suitable for:

• Creating multiple header rows with partial visibility (e.g., hiding index row)
• Handling duplicate column names (Dictionary keys must be unique)
• Implementing complex row layout, such as merged cells or conditional formatting
• Supporting streaming large datasets with lazy row generation

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🧩 Proposed API

A new overload for SaveAs or similar method:

MiniExcel.SaveAs("file.xlsx", rowFactory, config);

Where rowFactory is defined as:

Func<int, RowContext, MiniExcelRow> rowFactory = (rowIndex, ctx) =>
{
    return new MiniExcelRow
    {
        Cells = new[] { "Index" + rowIndex, "Name", "Age" },
        IsHidden = rowIndex == 0
    };
};

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🌟 Advantages

Feature	Benefit
🎭 Dynamic content	Vary structure and style per row
💨 Streaming	Factory pattern supports deferred execution
🎨 Style control	Apply conditional formats, custom fonts, merges, etc.

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🛠️ Implementation Notes

• MiniExcelRow is a new internal DTO containing:
  • object[] Cells
  • bool IsHidden
  • (Optionally) CellStyle[] Styles, int MergeAcross, etc.
• Existing injection logic remains unchanged
• Backward compatible — opt-in feature

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🧪 Example Use Case

Generate hidden index row + repeated column names:

Hidden Row	A	B	C
Index Row	1	2	3
Header Row	姓名	姓名	年龄

Func<int, RowContext, MiniExcelRow> rowFactory = (i, ctx) =>
{
    if (i == 0) return new MiniExcelRow { Cells = new[] { "1", "2", "3" }, IsHidden = true };
    if (i == 1) return new MiniExcelRow { Cells = new[] { "姓名", "姓名", "年龄" } };
    return new MiniExcelRow { Cells = new[] { "张三", "李四", 18 } };
};

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🔄 Compatibility

• ✅ No breaking changes
• ✅ Can co-exist with template and dictionary modes
• ✅ Easily unit-testable and extensible

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📎 References

• [Issue: Multiple Header Rows with Hidden Index Mapping](https://github.com/mini-software/MiniExcel/issues/831)
• 表格自动向下填充新行 https://gitee.com/dotnetchina/MiniExcel/issues/ICJV59

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Thanks for reviewing!
If you think this should live under a separate namespace or extension method set, happy to adapt the structure.

[michelebastione]

Thank you for the very detailed useful proposal. We are in the process of restructuring the codebase but we'll take this into consideration as soon as we can!