{"slug": "i-fixed-a-flutter-streaming-bug-by-comparing-logs", "title": "I Fixed a Flutter Streaming Bug by Comparing Logs", "summary": "A developer fixed a Flutter streaming bug in the tkstock project where the AI's typewriter effect froze mid-sentence due to flawed state append logic. By comparing logs before and after each data chunk, the engineer identified that new chunks were being overwritten instead of appended, and applied a minimal fix to restore correct concatenation.", "body_md": "In the Flutter chat interface for our tkstock project, a ghost was haunting the UI. The AI's typewriter effect would freeze mid-sentence. New data wasn't appearing, but the backend hadn't stopped sending it.\n\nWas it a network blip? A crashed thread? No—it was a silent logic error in how we handled state updates.\n\nStreaming UI bugs are tricky because of the \"asynchronous illusion.\" When the interface freezes, it's hard to tell immediately if the SSE (Server-Sent Events) stream broke or if the frontend state merge logic failed.\n\nIf we didn't fix this thoroughly, users would face truncated content during critical information retrieval. Worse, an over-aggressive fix (like showing only the first line) would break the feature entirely.\n\n**1. Flawed State Append Logic**\n\nWhen streaming data arrived, the state update logic failed to correctly concatenate the new string with the old one. New chunks were either discarded or overwritten.\n\n**2. Boundary Misjudgment**\n\nWhen processing text streams containing newlines (`\\n`\n\n), the string slicing or regex matching logic deviated, triggering an incorrect truncation branch.\n\n**3. The Regression Trap**\n\nOur first fix missed the core issue. We introduced aggressive truncation logic that discarded all subsequent content, keeping only the first line.\n\n**Core Idea:** Print the full state text before and after each chunk arrives. Compare the difference with the UI display to confirm if it's a data loss or a render block.\n\nInstead of guessing, we let the logs speak. Here is the logic shift:\n\n```\n// Before: Blind concatenation\nonData: (chunk) {\n  currentText = chunk; // Wrong: Overwriting\n  setState(() {});\n}\n\n// After: Strict log comparison\nonData: (chunk) {\n  print('Before: ${currentText.length}');\n  print('Chunk: ${chunk.length}');\n  currentText += chunk; // Correct: Appending\n  print('After: ${currentText.length}');\n  setState(() {});\n}\n```\n\nThese few lines helped us pinpoint the exact moment of data loss in a black-box scenario.\n\n**Minimalist Fix**\n\nOnce the logs identified the problem, we applied the most minimal fix possible: remove the complex splitting logic and return to basic string appending.\n\n```\n// Before: Over-truncation shows only the first line\nfinal lines = currentText.split('\\n');\nsetState(() {\n  displayText = lines.first; // Wrong logic\n});\n\n// After: Atomic append\nsetState(() {\n  displayText = currentText + incomingChunk;\n});\n```\n\n| Decision | Alternative | Rationale |\n|---|---|---|\nChosen: Log comparison / Rejected: Blind guessing |\nStreaming bugs often reproduce under specific chunk sequences. Only by comparing Before/After states can we catch non-linear logic errors. | |\nChosen: Keep existing pipeline / Rejected: Rewrite StreamBuilder |\nTo control risk, we only fixed the core Append logic, avoiding unknown side effects from a framework rewrite. |", "url": "https://wpnews.pro/news/i-fixed-a-flutter-streaming-bug-by-comparing-logs", "canonical_source": "https://dev.to/quarktimes/i-fixed-a-flutter-streaming-bug-by-comparing-logs-40ma", "published_at": "2026-06-15 02:56:34+00:00", "updated_at": "2026-06-15 03:11:20.553282+00:00", "lang": "en", "topics": ["developer-tools", "artificial-intelligence", "ai-products"], "entities": ["Flutter", "tkstock", "SSE", "Server-Sent Events"], "alternates": {"html": "https://wpnews.pro/news/i-fixed-a-flutter-streaming-bug-by-comparing-logs", "markdown": "https://wpnews.pro/news/i-fixed-a-flutter-streaming-bug-by-comparing-logs.md", "text": "https://wpnews.pro/news/i-fixed-a-flutter-streaming-bug-by-comparing-logs.txt", "jsonld": "https://wpnews.pro/news/i-fixed-a-flutter-streaming-bug-by-comparing-logs.jsonld"}}