Interview questions in Mobile Dev, page 29
View identity in SwiftUI and the .id() modifier
Identity links a view to its persistent state and transitions; structural identity comes from position, explicit identity from .id(); changing the id resets state and forces a new view.
What is a Dart Isolate and how does it differ from threads?
Tests Dart's shared-nothing concurrency model. Define an isolate as an independent heap plus event loop, contrast it with shared-memory threads, and explain UI isolate communication via async message ports.
Describe staged rollout on Google Play Console, benefits, and metrics to monitor
Tests release engineering and risk mitigation. Covers: set percentage and countries, manually ramp, halt on bad signals; benefits are blast-radius reduction and real-world validation. Flag: auto-ramping claims or ignoring halted users keep the bad build.
Why react-native-gesture-handler Replaces the Responder System
The JS responder system runs on the JS thread so gestures lag under load; gesture-handler runs recognition natively for smooth, reliable gestures.
Faulting in Core Data
A fault is a placeholder object whose property data is not yet loaded; accessing a property fires the fault and fetches from the store, saving memory.
When would you implement a CustomPainter and how does shouldRepaint work?
Use CustomPainter for charts; check fields in shouldRepaint to skip frames; pass Listenable to bypass build layout.
Explain R8's shrinking, obfuscation, and optimization phases and how to influence optimization
Separate dead code removal, renaming, and bytecode transformation; cite ProGuard keep rules and selective passes.
Fetching REST Data with Loading and Error State
Track loading, data, and error; fetch in an effect, check response.ok, parse JSON, handle catch and finally.
Minimizing Storyboard merge conflicts
Split into many small storyboards or xibs, or move to programmatic or SwiftUI layout; process-wise coordinate ownership and merge often.
Write the ProGuard keep rule to fix reflection after R8
This tests R8 dead-code removal and exact ProGuard syntax for reflective constructors. A strong answer gives -keep class com.example.T { <init>(...); } because R8 cannot trace dynamic class names. Broad -keep rules or omitting constructor are red flags.
What is Flutter tree shaking and how does it reduce app size?
Tests Dart AOT dead code elimination. Covers compile-time dead code removal, smaller binaries, faster startup, and practices like avoiding dart:mirrors and using const constructors. Red flag: confusing it with minification or runtime stripping.
Draggable Snap-Back Card with Gesture Handler and Reanimated
Shared values track offset, a Pan gesture updates them on the UI thread, an animated style applies translation, onEnd springs back to zero.
Decoding a heterogeneous JSON array by a type field
Peek the type field with a partial decode, switch on it, and decode the concrete type, wrapping each in an enum or boxed protocol value.
How does App Bundle handle ABI splits versus multi-APK?
Tests dynamic delivery of native libs: AAB auto-generates per-ABI config splits, but multi-APK requires manual APK builds per ABI. Misconfiguration ships unused .so bloat or crashes on unsupported devices. Red flag: calling AAB a simple multi-APK zip.
How would you implement debounce for a search input using Streams?
This tests Stream transformation and event throttling. A strong answer outlines a resetting timer that only emits after 300ms of silence, referencing debounceTime or DebounceStreamTransformer. Red flag: manual Timer juggling without Stream composition.
Returning a Promise from a Native Module Method
The exported method takes resolve and reject as its last parameters; do the async work off the main thread, then settle exactly once with a result or an error.
Retain cycles and how to break them
A cycle is two objects (or a closure and its owner) holding mutual strong references so neither deallocates; break it with a [weak] or [unowned] capture list.
What are RecyclerView.Adapter and ViewHolder, and why do they improve scrolling?
This tests RecyclerView's data-to-view separation. A strong answer says Adapter binds data while ViewHolder caches findViewById results, avoiding hierarchy scans on scroll. A red flag is calling ViewHolder a memory saver instead of a lookup cache.
What is a Dart Completer and when should you use it?
This tests bridging callback APIs into Dart's Future ecosystem. An answer defines Completer as a manual Future producer, explains completing with value or error from callbacks, preferring Future() when possible. A red flag is using Completer for simple async.
ViewManager and Exposing Native Component Props
The ViewManager creates the native view and maps it to a JS component; you expose props with a prop-setter that receives the value when the prop changes.
We are hiring for this. Every open role lists the topics its interview covers, so you can prepare for the real thing rather than guessing.
See open roles