One Android Architecture Mistake: Skipping the Mapper Layer

One Android Architecture Mistake: Skipping the Mapper Layer

When API DTOs leak directly into UI state, every backend change breaks screens. A small mapper layer prevents this coupling and keeps refactors cheap.

Step 1: Separate transport and UI models

data class LessonDto(val id: String, val title: String?, val level: String)

data class LessonCard(val id: String, val title: String, val badge: String)

Step 2: Centralize transformation rules

fun LessonDto.toCard(): LessonCard = LessonCard(
    id = id,
    title = title ?: "Untitled lesson",
    badge = level.uppercase()
)

Step 3: Test mapper behavior with edge payloads

@Test fun null_title_falls_back() {
    val dto = LessonDto("1", null, "beginner")
    assertEquals("Untitled lesson", dto.toCard().title)
}

Common pitfall

Applying fallback rules in composables. You will duplicate business logic across screens.

What to check

• UI never imports raw DTO types.
• Mapper rules are unit-tested.
• Backend schema changes affect one mapper file first.