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App startup flow

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App Startup Flow Deep Dive

Overview

App startup is the path from launcher tap to first interactive frame. Interviewers care about startup stages and optimization strategy.

Startup Stages

  • Cold start: process not in memory.
  • Warm start: process exists, activity recreated.
  • Hot start: activity already in memory and resumed quickly.

Cold Start Sequence

  1. Launcher sends intent.
  2. ActivityTaskManager/ActivityManager resolves target.
  3. Process created (via Zygote fork).
  4. Application.onCreate() runs.
  5. Activity instance created.
  6. onCreate() -> onStart() -> onResume().
  7. First frame rendered.

Optimization Principles

  • Keep Application.onCreate() minimal.
  • Defer non-critical SDK initialization.
  • Use lazy loading for feature modules and heavy objects.
  • Move disk/network work off main thread.
  • Track startup metrics in CI/release builds.

Measuring Startup

  • Android Studio profiler/traces.
  • reportFullyDrawn() for meaningful readiness signal.
  • Baseline Profiles to improve runtime performance.

Interview Traps

  • "First frame" is not equal to "fully usable UI".
  • Splash screen should not mask heavy startup anti-patterns.

Key Takeaways

  • Startup quality is architecture + measurement + discipline.
  • Optimize cold start first for user-perceived performance.