Movies UI - Lists

Refactor time!

Our app bakes the list support in with the Movie UI. We can make it much more reusable!

We're going to have two types of lists in the application:

Top-level lists of all movies, actors and ratings
Nested lists, such as actors starring in a movie (on the movie's display screen)

Generic List Data

To create consistent list support, we need to separate the LazyColumn and Scaffold from the MovieListUi. We want to keep all selection management, but make it more generic.

But there's a problem. If we make a generic List composable, something like

fun <T> ListScaffold(
  items: List<T>,
  ...
) {
  ...
}

we have several spots that access the item's id. When the item type was explicitly MovieDto, we knew it had an id, but if the item type is a generic parameter T, we can no longer make that assumption.

To fix this, we create a HasId interface in the repository module. (You could create it in the data module, but we don't have a need for it that low.)

interface HasId {
    val id: String
}

The ids defined by our entities are the actual unique IDs in the database. These are simple to define:

show in full file repository/src/main/java/com/androidbyexample/movies/repository/MovieDto.kt

// ...

data class MovieDto(
//  val id: String,
    override val id: String,
    val title: String,
    val description: String,
    val ratingId: String,
//)
): HasId

internal fun MovieEntity.toDto() =
    // ...

show in full file repository/src/main/java/com/androidbyexample/movies/repository/ActorDto.kt

// ...

data class ActorDto(
//  val id: String,
    override val id: String,
    val name: String,
//)
): HasId

internal fun ActorEntity.toDto() =
    // ...

show in full file repository/src/main/java/com/androidbyexample/movies/repository/RatingDto.kt

// ...

data class RatingDto(
//  val id: String,
    override val id: String,
    val name: String,
    val description: String,
//)
): HasId

internal fun RatingEntity.toDto() =
    // ...

But what happens when (later) we display a cast list using RoleWithActorDto? We need unique ids for each of these as well.

If we have

data class RoleWithActorDto(
    val actor: ActorDto,
    val character: String,
    val orderInCredits: Int,
)

We can generate a unique id by appending the orderInCredits to the actor's id. We do this by defining a derived property for the id. A derived property is one whose value is generated from other properties rather than using a backing field to store data.

data class RoleWithActorDto(
    val actor: ActorDto,
    val character: String,
    val orderInCredits: Int,
): HasId {
    override val id: String
        get() = "${actor.id}:$character"
}

But this will raise another issue further down the line... When we display the cast list, what happens when the user clicks on one of those RoleWithActorDtos? The way we've written our list will use the id of the item as the "target" we want to visit when the user clicks. This won't work with this derived id property.

So we really need a separate property to track the targetId. Often this will have the same value as the id, so we can define a default implementation in the HasId interface.

show in full file repository/src/main/java/com/androidbyexample/movies/repository/HasId.kt

package com.androidbyexample.movies.repository

interface HasId {
    val id: String
    val targetId: String
        get() = id
}

Any implementations that need to can override it with their specific implementation

show in full file repository/src/main/java/com/androidbyexample/movies/repository/MovieDto.kt

// ...
data class RoleWithActorDto(
    val actor: ActorDto,
    val character: String,
    val orderInCredits: Int,
//)
): HasId {
    override val id: String
        get() = "${actor.id}:$character"
    override val targetId: String
        get() = actor.id
}

internal fun RoleWithActor.toDto() =
    // ...

show in full file repository/src/main/java/com/androidbyexample/movies/repository/ActorDto.kt

// ...
data class RoleWithMovieDto(
    val movie: MovieDto,
    val character: String,
    val orderInCredits: Int,
//)
): HasId {
    override val id: String
        get() = "${movie.id}:$orderInCredits"
    override val targetId: String
        get() = movie.id
}

internal fun RoleWithMovie.toDto() =
    // ...