How do you know when some process if blocking a UI thread?

Question

Android Engineers · Accepted Answer

## Answer

### How to Detect UI Thread Blocking

**1. Strict Mode (Development)**

```kotlin
StrictMode.setThreadPolicy(
    StrictMode.ThreadPolicy.Builder()
        .detectDiskReads()
        .detectDiskWrites()
        .detectNetwork()
        .penaltyLog()
        .build()
)
```

**2. ANR Detection**
- If the UI thread is blocked for **5+ seconds**, Android shows an ANR (Application Not Responding) dialog
- ANR traces are logged to `/data/anr/traces.txt`

**3. Using Choreographer**

```kotlin
Choreographer.getInstance().postFrameCallback { frameTimeNanos ->
    val jank = (System.nanoTime() - frameTimeNanos) / 1_000_000
    if (jank > 16) Log.w("Jank", "Frame took $jank ms")
}
```

**4. Profiler Tools**
- **Android Studio Profiler**: Shows CPU usage and thread activity
- **Systrace**: Detailed trace of system and app threads
- **Perfetto**: Modern tracing tool for performance analysis

**Key Signs of UI Thread Blocking:**
- Dropped frames (jank)
- Unresponsive UI
- ANR dialogs
- StrictMode violations in Logcat

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Kotlin

Machine Coding

Android Basics

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Advanced Topics

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Common Question

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Answer

How to Detect UI Thread Blocking

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