Introduction

This guide explains tlmdgrid customsource in plain language. It is meant for Delphi or C++ Builder developers who use the LMD GridPack. I write short sentences and clear steps. You will learn what a custom source is, why you might need one, and how to implement one safely. I include real examples, debugging tips, and maintenance advice. The tone is friendly and practical. If you are new to TLMDGrid or you maintain legacy VCL code, this article helps you get confident. Read the headings to jump to parts you need. If you want, I can also produce a ready-to-paste code sample tailored to your project.

What is a TLMDGrid custom source?

A tlmdgrid customsource is a bridge between external data and the TLMDGrid visual control. TLMDGrid normally manages its own internal storage for rows and columns. When you want your grid to show data stored elsewhere, you descend from the TLMDGridCustomSource class and implement required methods. The custom source answers questions like row count, column values, and cell edits. It gives you full control over how the grid reads and writes data. Many teams use a custom source to keep data in business objects, in-memory caches, or non-Delphi backends while still leveraging the TLMDGrid UI features.

Why use a custom source instead of the internal grid store?

There are many reasons to use a tlmdgrid customsource. First, you may already have complex business objects you must display without copying them into the grid. Second, you might stream data on demand to save memory for very large sets. Third, a custom source lets you map non-standard storage, like nested lists or remote data. Finally, custom sources make it easier to enforce business rules at read or write time. Using a custom source can reduce duplication and keep the model authoritative. That said, custom sources are more work than using the grid’s internal store. Plan your API and tests before writing one.

Core methods and lifecycle of TLMDGridCustomSource

A tlmdgrid customsource must implement several core methods. These include functions to return the number of rows and columns, routines to get and set cell values, and optional notifications for insertions or deletions. The grid calls these methods on paint and during user actions. Implementations typically include: GetRowCount, GetColCount, GetCellValue, SetCellValue, and NotifyChange. You must handle concurrency if your data can change outside the grid. Life-cycle events include creation, attaching to the grid via the grid’s CustomSource property, and eventual freeing. Properly calling Invalidate or Refresh on the grid after changes ensures the UI stays in sync.

A simple example: exposing a list of records

A common use case for tlmdgrid customsource is exposing a TList<TMyRecord> to the grid. Implement GetRowCount to return the list count and GetCellValue to read fields by column index. When users edit a cell, SetCellValue writes back to the record and marks the list item dirty. For adding rows, implement an InsertRow method that adds an item to the list and notifies the grid about the change. Remember to manage object lifetimes: if your list contains owned objects, free them when removed. This simple pattern keeps business data natural while using TLMDGrid features like column editors and sorting.

Handling edits and validations in the custom source

When you implement tlmdgrid customsource, validation is your responsibility. The SetCellValue method should validate new values and either accept them or reject with a clear outcome. You can use grid events to show error messages or to prevent commits. For complex validation, perform checks against other rows or external rules. If an edit fails, restore the previous value or prompt the user. Always consider transaction boundaries: if a cell change should be part of a larger update, buffer changes and commit them together. This pattern prevents partial updates and keeps data integrity intact.

Virtual mode and on-demand loading patterns

A tlmdgrid customsource makes virtual mode easier. Virtual mode means the grid asks for values only for visible rows and columns. For very large datasets, implement paging or lazy loading in GetCellValue. When the grid asks for a row not in memory, fetch it from disk or a remote service. Cache recently used rows to reduce latency. Use row count hints if the total is known, or return an estimated size and update later. Carefully trigger grid invalidation after data arrives. On-demand loading keeps memory low while giving users fast interactions.

Sorting and searching with a custom source

Sorting and searching usually run in the custom source layer when using tlmdgrid customsource. Instead of letting the grid reorder an internal buffer, implement sort logic that reorders your data structure or returns values in sorted order. For large sets, use indexes or external database queries to avoid moving large memory blocks. Searching can be done by scanning the data or using pre-built indexes for speed. Expose capabilities to the grid so it shows sort glyphs and responds to header clicks. Coordinating sort state between the grid and the custom source is key to a consistent user experience.

Column mapping and dynamic schema support

If your data model uses dynamic fields, a tlmdgrid customsource can expose a dynamic schema. Implement GetColCount and provide FieldName or ColumnCaption based on your data model. The grid will use these values for column creation and headers. Dynamic schema support is useful when columns vary by dataset or when the grid displays user-defined columns. Remember to notify the grid and call RebuildColumns when the schema changes. Keep metadata about column types for proper cell editors and formatting so the grid renders and validates correctly.

Integrating with databases and data-aware patterns

For database-backed scenarios, a tlmdgrid customsource can be a lightweight adapter over queries or datasets. Instead of using TLMDDBGrid, some teams prefer a custom source to apply business logic or to fetch joined data not available as a single dataset. Implement read-through caching and transaction-aware writes to coordinate with your DB. When committing changes, use parameterized queries or prepared statements to avoid injection and ensure correctness. Be mindful of connection lifetimes and use connection pooling. For multi-user systems, consider optimistic concurrency checks and clear conflict resolution strategies.

Threading and concurrency concerns

When your tlmdgrid customsource reads data from background threads, take care with thread safety. TLMDGrid methods will be called on the UI thread. If a background fetch completes, synchronize updates back to the main thread using TThread.Synchronize or TThread.Queue. Do not call the grid directly from worker threads. Protect shared data structures with appropriate locks, but keep locks short to avoid UI freezes. Use message queues or event-driven updates to keep the UI responsive. Well-structured synchronization prevents race conditions and ensures the grid shows consistent data.

Performance tuning tips for large datasets

Large datasets demand careful design in a tlmdgrid customsource. Use efficient data formats and avoid per-cell memory allocations. Batch updates are faster than many single-row notifications. Use virtual mode to limit the number of objects in memory and apply caching strategies. Reduce grid repaint calls during bulk operations by disabling updates or using a BeginUpdate/EndUpdate pattern if available. Profile GetCellValue hotspots and optimize common paths. Avoid expensive conversions in hot code. These small changes can dramatically speed UI interactions for very large grids.

Editing controls and custom editors

TLMDGrid supports different editor types. When you use a tlmdgrid customsource, tell the grid which editor to use for each column by exposing column type metadata or by handling editor events. You can supply dropdown lists, calendars, numeric editors, and custom dialogs. For complex editors, pass context data through the custom source so the editor can validate or show related information. This collaboration between grid and custom source yields rich, user-friendly editing experiences while keeping the underlying data model intact.

Persistence strategies and saving changes

Decide how the tlmdgrid customsource persists changes. Options include immediate writes on SetCellValue, buffered commits, or periodic saves. Buffered commits allow undo-like behavior and batch database updates. For critical systems, add transaction semantics and rollback paths. Always handle errors during save by reporting feedback to the user and offering retry options. Log changes for auditing and debugging. Persistence strategies should match business needs: high-throughput systems may prefer streaming writes, while editors may prefer staged commits with review steps.

Debugging and common pitfalls

Debugging a tlmdgrid customsource requires checking method calls and lifecycle events. Common pitfalls include returning inconsistent row counts, forgetting to notify the grid after data changes, and mishandling object lifetimes leading to access violations. Use logging in core methods to trace calls during user actions like sorting or editing. When you see UI glitches, verify that GetCellValue and SetCellValue behave deterministically. Always test edge cases: empty datasets, single-row datasets, and maximal sizes. Good logging and tests help find subtle bugs quickly.

Migration tips: moving from internal store to custom source

If you refactor an app to use a tlmdgrid customsource, plan the migration. Start by keeping both systems working side-by-side. Create an adapter that presents the old internal store through the custom source API. Gradually switch features over and run integration tests. Document the custom source API and add tests for each grid operation. A staged migration reduces the chance of regressions and helps the team learn the new pattern. I once migrated a complex form with dozens of grid behaviors using this staged approach and avoided a big release rollback.

Testing strategies and automated checks

Unit tests for a tlmdgrid customsource focus on data logic: row counts, value mapping, sorting, and persistence. GUI integration tests exercise real workflows using a test harness or automated UI frameworks. Use mock data to simulate remote backends and test fallback behavior. Add regression tests for previously reported bugs. Continuous integration that runs tests regularly catches regressions early. Automated tests make refactors safer and reduce reliance on manual QA for grid behavior.

Security and validation best practices

A tlmdgrid customsource that accepts user edits must validate inputs tightly. Sanitize strings, enforce numeric ranges, and check permissions before allowing writes. For database commits, use parameterized queries and proper escaping. Avoid exposing internal memory addresses or sensitive data in grid columns. For multi-user systems, implement access checks and log changes. Security is part of design: validate at the source and never trust UI-only checks.

FAQs — practical answers to common questions

Q1: What class do I descend to build a tlmdgrid customsource?
A: Use the TLMDGridCustomSource base class provided by the LMD GridPack. Implement core methods for row count and cell access.

Q2: How do I attach the custom source to the grid?
A: Set the grid’s CustomSource property to your custom source instance. The grid will begin calling your implemented methods.

Q3: Can I use the custom source with C++ Builder as well as Delphi?
A: Yes. LMD GridPack supports both Delphi and C++ Builder. The custom source pattern works similarly in either environment.

Q4: How do I notify the grid about data changes?
A: Call the grid’s Invalidate, Refresh, or a specific notification method if available after you update the underlying data.

Q5: Is virtual mode automatic with custom source?
A: The grid will query values on demand. Implement caching and lazy loading within your custom source to support virtual-mode scenarios.

Q6: What are frequent causes of crashes with custom sources?
A: Common causes include invalid memory access from freed objects, inconsistent row counts, and calling UI methods from background threads. Use careful lifecycle management and synchronization.

Conclusion

A tlmdgrid customsource gives you powerful control over how TLMDGrid shows and edits data. It fits well when you need to present existing models, stream large datasets, or add business logic at the data boundary. The trade-off is extra coding and careful testing. Start small: expose a single read-only list first. Add editing, sorting, and persistence later. If you want a starter code sample for your project, tell me your Delphi or C++ Builder version and your data model. I will create a ready-to-paste custom source with comments and basic tests to help you ship faster.