Jetpack Compose declarative UI fundamentals

package com.example.myapp

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.*
import androidx.compose.material3.*
import androidx.compose.runtime.*
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MyAppTheme {
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    color = MaterialTheme.colorScheme.background
                ) {
                    PostsList()
                }
            }
        }
    }
}

@Composable
fun PostsList() {
    var posts by remember { mutableStateOf(listOf<Post>()) }
    var isLoading by remember { mutableStateOf(false) }

    LaunchedEffect(Unit) {
        isLoading = true
        // Simulate loading
        kotlinx.coroutines.delay(1000)
        posts = getDummyPosts()
        isLoading = false
    }

    Column(modifier = Modifier.fillMaxSize()) {
        TopAppBar(
            title = { Text("Posts") }
        )

        if (isLoading) {
            Box(
                modifier = Modifier.fillMaxSize(),
                contentAlignment = androidx.compose.ui.Alignment.Center
            ) {
                CircularProgressIndicator()
            }
        } else {
            LazyColumn(
                modifier = Modifier.fillMaxSize(),
                contentPadding = PaddingValues(16.dp),
                verticalArrangement = Arrangement.spacedBy(12.dp)
            ) {
                items(posts.size) { index ->
                    PostCard(post = posts[index])
                }
            }
        }
    }
}

@Composable
fun PostCard(post: Post) {
    var isLiked by remember { mutableStateOf(false) }

    Card(
        modifier = Modifier.fillMaxWidth(),
        elevation = CardDefaults.cardElevation(defaultElevation = 4.dp)
    ) {
        Column(modifier = Modifier.padding(16.dp)) {
            Text(
                text = post.title,
                style = MaterialTheme.typography.titleLarge
            )

            Spacer(modifier = Modifier.height(8.dp))

            Text(
                text = post.body,
                style = MaterialTheme.typography.bodyMedium,
                maxLines = 3
            )

            Spacer(modifier = Modifier.height(12.dp))

            Row(
                modifier = Modifier.fillMaxWidth(),
                horizontalArrangement = Arrangement.SpaceBetween
            ) {
                IconButton(onClick = { isLiked = !isLiked }) {
                    Icon(
                        imageVector = if (isLiked) Icons.Filled.Favorite else Icons.Outlined.FavoriteBorder,
                        contentDescription = "Like",
                        tint = if (isLiked) Color.Red else Color.Gray
                    )
                }

                Text(
                    text = post.author,
                    style = MaterialTheme.typography.bodySmall,
                    modifier = Modifier.align(androidx.compose.ui.Alignment.CenterVertically)
                )
            }
        }
    }
}

@Preview(showBackground = true)
@Composable
fun PostCardPreview() {
    MyAppTheme {
        PostCard(
            post = Post(
                id = 1,
                title = "Sample Post",
                body = "This is a sample post body",
                author = "John Doe"
            )
        )
    }
}

data class Post(
    val id: Int,
    val title: String,
    val body: String,
    val author: String
)