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10
Building Complex UI in Jetpack Compose Written by Tino Balint

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Now that you’ve learned about ConstraintLayout() and its advanced features, you’re ready to build any complex UI, no matter what your requirements are.

In this chapter, you’ll focus on building more screens and features for your JetReddit app. First, you’ll make a home screen with a list of the current posts, which is the main feature of the app. Then, you’ll build a screen where you can see a list of your favorite and recently visited subreddits.

Building the home screen

To understand your task, take a look at the following example from the original Reddit app:

Reddit Home Screen
Reddit Home Screen

Here, you see a home screen with two posts. The screen consists of a header, content and post actions. There are two types of content, a text and an image. Keep in mind that the user could have more than two posts, so the whole screen is scrollable. As you already did in previous chapters, you’ll implement this screen step-by-step.

Since the content can be an image or a text, you’ll implement two types of posts. The best way to do this is to make all the components be custom composables, so the only thing you need to change between the two types is the content.

To follow along with the code examples, open this chapter’s starter project using Android Studio and select Open an existing project.

Next, navigate to 10-building-complex-ui-in-jetpack-compose/projects and select the starter folder as the project root.

Once the project opens, let it build and sync and you’re ready to go!

You might already be familiar with the project hierarchy from the previous chapter, but in case you aren’t, check out this image:

Project Hierarchy
Project Hierarchy

There are several packages here, but you’ll only change the code within screens, to implement new features of the app, and components for custom composables — for example, Post(), which those screens need.

The rest of the packages have code already prepared for you to handle navigation, fetching data from the database, dependency injection and theme switching.

Once you’re familiar with the file organization, build and run the app. You’ll see a screen like this:

Starting Screen
Starting Screen

It’s an empty home screen. It only contains the app drawer from the previous chapter.

You’re ready to go now. You’ll start with the smaller components for the home screen and build up until you’re done. Your first task is to implement the post’s header.

Adding a post header

Each post on the home screen has a header that contains the following information: the subreddit it belongs to, the name of the user who posted it, how old the post is and its title.

@Composable
fun Header(post: PostModel) {
   Row(modifier = Modifier.padding(start = 16.dp)) {
     Image(
         ImageBitmap.imageResource(id = R.drawable.subreddit_placeholder),
         contentDescription = stringResource(id = R.string.subreddits),
         Modifier
             .size(40.dp)
             .clip(CircleShape)
     )
     Spacer(modifier = Modifier.width(8.dp))
     Column(modifier = Modifier.weight(1f)) {
       Text(
           text = stringResource(R.string.subreddit_header, post.subreddit),
           fontWeight = FontWeight.Medium,
           color = MaterialTheme.colors.primaryVariant
       )
       Text(
           text = stringResource(R.string.post_header, post.username, post.postedTime),
           color = Color.Gray
       )
     }
     MoreActionsMenu()
   }

   Title(text = post.title)
}
Header Preview
Piuqed Yseyees

Building the voting action button

The voting action button has two images and a text, which makes it slightly different from other action buttons. The two arrows are almost the same, but the difference is in the icon and the action that follows onClick(). Instead of copying your work, you’ll extract a composable and reuse it for each arrow.

@Composable
fun ArrowButton(onClickAction: () -> Unit, arrowResourceId: Int) {
  IconButton(onClick = onClickAction, modifier = Modifier.size(30.dp)) {
    Icon(
      imageVector = ImageVector.vectorResource(arrowResourceId),
      contentDescription = stringResource(id = R.string.upvote),
      modifier = Modifier.size(20.dp),
      tint = Color.Gray
    )
  }
}
Arrow Button Preview
Iqwiw Xevsir Ymejoaf

@Composable
fun VotingAction(
  text: String,
  onUpVoteAction: () -> Unit,
  onDownVoteAction: () -> Unit
) {
  Row(verticalAlignment = Alignment.CenterVertically) {
    ArrowButton(onUpVoteAction, R.drawable.ic_baseline_arrow_upward_24)
    Text(
      text = text,
      color = Color.Gray,
      fontWeight = FontWeight.Medium,
      fontSize = 12.sp
    )
    ArrowButton(onDownVoteAction, R.drawable.ic_baseline_arrow_downward_24)
  }
}
Voting Action Button Preview
Wepekh Uvdoij Naysij Wlenieh

Building the post

You might wonder how you’ll build Post() without first implementing the content. To find out how — and why — make the following changes to Post():

@Composable
fun Post(post: PostModel, content: @Composable () -> Unit = {}) {
  Card(shape = MaterialTheme.shapes.large) {
    Column(modifier = Modifier.padding(
      top = 8.dp,
      bottom = 8.dp)
    ) {
      Header(post)
      Spacer(modifier = Modifier.height(4.dp))
      content.invoke()
      Spacer(modifier = Modifier.height(8.dp))
      PostActions(post)
    }
  }
}
Post Preview
Quqy Gdumeoy

Adding the content

Look at TextPost() and ImagePost():

@Composable
fun TextPost(post: PostModel) {
  Post(post) {
    TextContent(post.text)
  }
}

@Composable
fun ImagePost(post: PostModel) {
  Post(post) {
    ImageContent(post.image ?: R.drawable.compose_course)
  }
}
@Preview
@Composable
fun ImagePostPreview() {
  Post(DEFAULT_POST) {
    ImageContent(DEFAULT_POST.image ?: R.drawable.compose_course)
  }
}
Image Post Preview
Enoza Mits Dhotaox

Adding multiple posts

To finish the home screen, you need to add the ability to display multiple posts using Post(), which you just made. The posts should vary by type and content.

@Composable
fun HomeScreen(viewModel: MainViewModel) {
  val posts: List<PostModel> by viewModel.allPosts.observeAsState(listOf())

  LazyColumn(modifier = Modifier.background(color = MaterialTheme.colors.secondary)) {
    items(posts) {
      if (it.type == PostType.TEXT) {
        TextPost(it)
      } else {
        ImagePost(it)
      }
      Spacer(modifier = Modifier.height(6.dp))
    }
  }
}
Home Screen
Gule Pjyaal

Building the Subreddits screen

First, take a look at the image below to understand what you’ll build:

Subreddit Screen Example
Xepgoktal Cfkuob Ijihdha

Building the subreddit body

Look at the subreddit body from the example image once more. It consists of a background image, an icon and three texts. Since some elements overlap, you’ll use ConstraintLayout() for flexibility.

@Composable
fun SubredditBody(subredditModel: SubredditModel, modifier: Modifier = Modifier) {
  ConstraintLayout(
    modifier = modifier
      .fillMaxSize()
      .background(color = MaterialTheme.colors.surface)
  ) {
    val (backgroundImage, icon, name, members, description) = createRefs() // 1

    SubredditImage( // 2
      modifier = modifier.constrainAs(backgroundImage) {
        centerHorizontallyTo(parent)
        top.linkTo(parent.top)
      }
    )

    SubredditIcon( // 3
      modifier = modifier.constrainAs(icon) {
        top.linkTo(backgroundImage.bottom)
        bottom.linkTo(backgroundImage.bottom)
        centerHorizontallyTo(parent)
      }.zIndex(1f)
    )

    SubredditName( // 4
      nameStringRes = subredditModel.nameStringRes,
      modifier = modifier.constrainAs(name) {
        top.linkTo(icon.bottom)
        centerHorizontallyTo(parent)
      }
    )

    SubredditMembers( // 5
      membersStringRes = subredditModel.membersStringRes,
      modifier = modifier.constrainAs(members) {
        top.linkTo(name.bottom)
        centerHorizontallyTo(parent)
      }
    )

    SubredditDescription( // 6
      descriptionStringRes = subredditModel.descriptionStringRes,
      modifier = modifier.constrainAs(description) {
        top.linkTo(members.bottom)
        centerHorizontallyTo(parent)
      }
    )
  }
}
Subreddit Body Preview
Puysadbor Risq Tzavuad

Adjusting the elements’ height and shadowing

The elements’ positions are correct, but their height is wrong and there are no visible shadows at the edge.

@Composable
fun Subreddit(subredditModel: SubredditModel, modifier: Modifier = Modifier) {
  Card(
    backgroundColor = MaterialTheme.colors.surface,
    shape = RoundedCornerShape(4.dp),
    modifier = modifier
      .size(120.dp)
      .padding(
        start = 2.dp,
        end = 2.dp,
        top = 4.dp,
        bottom = 4.dp
      )
  ) {
    SubredditBody(subredditModel)
  }
}
Subreddit Preview
Dicfajfir Yjiciaj

Building the community item

The community item is fairly simple; it only has an icon and a text. To build it, change Community() code to:

@Composable
fun Community(text: String, modifier: Modifier = Modifier, onCommunityClicked: () -> Unit = {}) {
  Row(modifier = modifier
    .padding(start = 16.dp, top = 16.dp)
    .fillMaxWidth()
    .clickable { onCommunityClicked.invoke() }
  ) {
    Image(
        bitmap = ImageBitmap.imageResource(id = R.drawable.subreddit_placeholder),
        contentDescription = stringResource(id = R.string.community_icon),
        modifier = modifier
            .size(24.dp)
            .clip(CircleShape)
    )
    Text(
        fontSize = 10.sp,
        color = MaterialTheme.colors.primaryVariant,
        text = text,
        fontWeight = FontWeight.Bold,
        modifier = modifier
            .padding(start = 16.dp)
            .align(Alignment.CenterVertically)
    )
  }
}
Community Preview
Gohvaracg Jjipeav

Adding a community list

Next, you’ll build the list that contains all the main and added communities.

@Composable
fun Communities(modifier: Modifier = Modifier) {
  mainCommunities.forEach {
    Community(text = stringResource(it))
  }

  Spacer(modifier = modifier.height(4.dp))

  BackgroundText(stringResource(R.string.communities))

  communities.forEach {
    Community(text = stringResource(it))
  }
}
Communities Preview
Yezboqewoej Tdasuaz

Finishing the screen

The last part of the puzzle to build SubredditsScreen() is to combine everything you’ve built so far into a list that the user can scroll horizontally and vertically.

@Composable
fun SubredditsScreen(modifier: Modifier = Modifier) {
  Column(modifier = modifier.verticalScroll(rememberScrollState())) {
    Text(
      modifier = modifier.padding(16.dp),
      text = stringResource(R.string.recently_visited_subreddits),
      fontSize = 12.sp,
      style = MaterialTheme.typography.subtitle1
    )

    LazyRow(
      modifier = modifier.padding(end = 16.dp)
    ) {
      items(subreddits) { Subreddit(it) }
    }
    Communities(modifier)
  }
}
Subreddits Screen
Teckuvnazp Swmiey

Key points

  • Build your app by first implementing the most basic components.
  • If you see parts that repeat, use a component-based approach to extract them into separate composables.
  • Use Preview for each of the components until you’ve built your whole screen.
  • Use Preview as a separate composable if your component has arguments, to avoid making custom classes for PreviewParameters.
  • Use emptyContent() to display empty content inside the composable.
  • Use zIndex() if multiple composables overlap and you want to change their order of display in the z-orientation.

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