HarmonyOS Flutter Performance Optimization: Slide Response Latency

This article analyzes trace diagnostics for slide response latency in Flutter applications under touch interaction scenarios.

Finger Press (Initial Touch)

The finger press event establishes the foundation for all subsequent interactions, capturing:

• Initial touch coordinates
• Targeted UI controls
• Interaction context

Event Flow:

1. mmi_service thread handles multimodal input events
2. Flutter application listens and responds to touch events

Trace Characteristics:

• In mmi_service thread: "service report touchId:[id], type: down" immediately after "package touchEvent"
• In application main thread: "DispatchTouchEvent" with coordinates and type: 0 (down event)

These traces correspond directly across threads:

Finger Slide (Movement)

Slide events follow a controlled delivery mechanism different from initial presses.

Key Differences:

• Not immediate: Governed by vsync-app signal
• Initial position: Matches press coordinates (first move event)
• Type identifier: type: move (numerical value: 2)

Event Sequence:

1. mmi_service thread detects movement
2. VSyncGenerator produces synchronization signal
3. DVSync-app processes the signal
4. Application main thread receives event

Trace Flow:

Slide Threshold (TouchSlop)

The system-defined minimum recognition distance for slide gestures:

• Default value: 18 units
• Customizable: Developers can adjust per control

Calculation Method:

1. Capture touch coordinates from main thread traces
2. Compute coordinate offsets between events
3. Verify if offset exceeds threshold:

   const distance = Math.sqrt(
     Math.abs(x2 - x1)**2 + 
     Math.abs(y2 - y1)**2
   );
   const isSlide = distance > TOUCH_SLOP;

First Frame Rendering

When slide distance exceeds threshold:

1. Flutter triggers UI update operations
2. Actual rendering waits for next VSync signal
3. One-frame delay occurs before visual update

Trace Pattern:

First Frame Completion

Rendering completion identification:

• Development tracing: RSHardwareThread in RS process
• Automated testing: dpu_gfx_primary thread (hardware signal proxy)

Completion Trace:

Measuring Slide Response Latency

Total response duration spans:

```
Start: mmi_service "package touchEvent" (finger press)
End: RSHardwareThread/dpu_gfx_primary (first slide frame rendered)
```

Optimization Focus Areas:

1. Touch event propagation time
2. VSync signal generation latency
3. Frame scheduling efficiency
4. Render service processing time
5. Hardware composition duration

This analysis methodology enables precise identification of performance bottlenecks in Flutter slide interactions on HarmonyOS, providing actionable insights for latency reduction.