CollapsingAppBarList

CollapsingAppBarList.kt

/* Copyright 2024 Garret Yoder

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License. */

import androidx.compose.animation.AnimatedContent
import androidx.compose.animation.core.animateDpAsState
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.PaddingValues
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.height
import androidx.compose.foundation.layout.offset
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.LazyListScope
import androidx.compose.foundation.lazy.rememberLazyListState
import androidx.compose.runtime.Composable
import androidx.compose.runtime.derivedStateOf
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.input.nestedscroll.NestedScrollConnection
import androidx.compose.ui.input.nestedscroll.NestedScrollSource
import androidx.compose.ui.input.nestedscroll.nestedScroll
import androidx.compose.ui.platform.LocalDensity
import androidx.compose.ui.unit.Dp
import androidx.compose.ui.unit.IntOffset
import androidx.compose.ui.unit.dp
import kotlin.math.roundToInt

@Composable
fun CollapsingAppBarList(modifier: Modifier = Modifier, collapsedHeight: Dp = 64.dp, collapsedContent: @Composable () -> Unit, expandedHeight: Dp = 128.dp, expandedContent: @Composable () -> Unit, content: LazyListScope.() -> Unit) {
    val listState = rememberLazyListState()
    val listIsTop by remember {
        derivedStateOf {
            listState.firstVisibleItemIndex == 0 && listState.firstVisibleItemScrollOffset == 0
        }
    }
    val toolbarHeight: Dp by animateDpAsState(if (listIsTop) expandedHeight else collapsedHeight)
    val toolbarHeightPx = with(LocalDensity.current) { collapsedHeight.roundToPx().toFloat() }
    var toolbarOffsetHeightPx by remember { mutableStateOf(0f) }
    val nestedScrollConnection = remember {
        object : NestedScrollConnection {
            override fun onPreScroll(available: Offset, source: NestedScrollSource): Offset {
                toolbarOffsetHeightPx = (toolbarOffsetHeightPx + available.y).coerceIn(-toolbarHeightPx, 0f)
                return Offset.Zero
            }
        }
    }
    Box(modifier=modifier.nestedScroll(nestedScrollConnection)) {
        LazyColumn (Modifier,
            contentPadding = PaddingValues(top = toolbarHeight),
            state = listState
        ){
            content()
        }
        Box(Modifier
            .fillMaxWidth()
            .height(toolbarHeight)
            .offset { IntOffset(x = 0, y = toolbarOffsetHeightPx.roundToInt()) },
            contentAlignment = Alignment.Center)
        {
            AnimatedContent(listIsTop) {
                when (it) {
                    true -> {
                        expandedContent()
                    }
                    false -> {
                        collapsedContent()
                    }
                }
            }
        }
    }
}