Why not JetPack Compose with DiffUtil?
Leveraging DiffUtil-like Efficiency in Jetpack Compose: A Deep Dive into List Optimization Techniques and Their Impact on Performance
DiffUtil
is a tool in Android's RecyclerView library that helps update lists efficiently. It compares an old list with a new list to find the differences. Instead of replacing the whole list and redrawing everything, DiffUtil
figures out the smallest changes needed to update the old list to match the new one. This makes animations smoother and improves performance.
How
DiffUtil
Works
Identifying Changes: It compares items in the old and new lists based on whether the items are the same (
areItemsTheSame
) and whether their content is the same (areContentsTheSame
).Finding Minimal Changes:
DiffUtil
uses Myers' difference algorithm to find the smallest number of insertions, deletions, and moves needed to change the old list into the new one.Generating
DiffResult
: After finding the changes, it creates aDiffResult
object that lists the necessary updates.Applying Changes: The
DiffResult
is then used by aRecyclerView.Adapter
to update the list efficiently, with animations for the changes.
Key Concepts
Myers' Algorithm: This algorithm is key to
DiffUtil
. It compares sequences to find the smallest set of changes needed, making it efficient even for large datasets.Performance:
DiffUtil
boosts performance by reducing the number of operations needed to update the UI and provides smooth animations for changes.Ease of Use: It hides the complexity of comparing lists and applying updates, making it easier for developers to handle dynamic lists in a RecyclerView.
Benefits
Performance optimization: It slashes unnecessary updates to the UI, making everything faster!
Smooth user experience: It effortlessly handles animations for changes, making everything look so cool!
Developer-friendly: It makes updating lists a breeze, saving you tons of time and effort!
In summary, DiffUtil
is an essential tool in the RecyclerView ecosystem. It efficiently manages list updates, boosting performance and enhancing the user experience.
Jetpack Compose with DiffUtil?
In Jetpack Compose, we don't use RecyclerView
directly as you would in a traditional Android View-based UI. Instead, Compose provides the LazyColumn
and LazyRow
components, which can efficiently display lists of items. But guess what? We might still want the functionality of DiffUtil
to efficiently update the list when the underlying data changes!
Here is we you can create a utility function in Jetpack Compose that replicates the behavior of DiffUtil
for a LazyColumn
or LazyRow
:
Create a
DiffUtil
-like function
First, create a function that calculates the difference between two lists:
import androidx.compose.runtime.Composable
import androidx.compose.runtime.remember
import androidx.compose.runtime.snapshots.SnapshotStateList
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.withContext
import androidx.compose.runtime.*
import kotlinx.coroutines.launch
// Define a function to calculate the diff between two lists
suspend fun <T> calculateDiff(
oldList: List<T>,
newList: List<T>,
areItemsTheSame: (T, T) -> Boolean,
areContentsTheSame: (T, T) -> Boolean = areItemsTheSame
): List<DiffResult<T>> {
return withContext(Dispatchers.Default) {
val diffResult = mutableListOf<DiffResult<T>>()
val oldSize = oldList.size
val newSize = newList.size
val usedOldItems = BooleanArray(oldSize)
for (newItem in newList) {
var found = false
for (i in oldList.indices) {
if (!usedOldItems[i] && areItemsTheSame(oldList[i], newItem)) {
found = true
usedOldItems[i] = true
if (!areContentsTheSame(oldList[i], newItem)) {
diffResult.add(DiffResult.ItemUpdated(oldList[i], newItem))
}
break
}
}
if (!found) {
diffResult.add(DiffResult.ItemInserted(newItem))
}
}
for (i in oldList.indices) {
if (!usedOldItems[i]) {
diffResult.add(DiffResult.ItemRemoved(oldList[i]))
}
}
diffResult
}
}
// Define a sealed class to represent the diff result
sealed class DiffResult<T> {
data class ItemInserted<T>(val item: T) : DiffResult<T>()
data class ItemRemoved<T>(val item: T) : DiffResult<T>()
data class ItemUpdated<T>(val oldItem: T, val newItem: T) : DiffResult<T>()
}
Use the
DiffUtil
-like function withLazyColumn
Now, let's create a Composable
that uses this awesome utility function to efficiently update a LazyColumn
!
@Composable
fun <T> DiffableLazyColumn(
items: List<T>,
key: (T) -> Any,
content: @Composable (T) -> Unit,
areItemsTheSame: (T, T) -> Boolean,
areContentsTheSame: (T, T) -> Boolean = areItemsTheSame
) {
val oldItems = remember { mutableStateListOf<T>() }
val diffResult = remember { mutableStateOf<List<DiffResult<T>>>(emptyList()) }
val coroutineScope = rememberCoroutineScope()
LaunchedEffect(items) {
coroutineScope.launch {
diffResult.value = calculateDiff(
oldItems,
items,
areItemsTheSame,
areContentsTheSame
)
oldItems.clear()
oldItems.addAll(items)
}
}
LazyColumn {
itemsIndexed(diffResult.value) { index, diff ->
when (diff) {
is DiffResult.ItemInserted -> content(diff.item)
is DiffResult.ItemRemoved -> {}
is DiffResult.ItemUpdated -> content(diff.newItem)
}
}
}
}
Example Usage
Here is an example of how to utilize the DiffableLazyColumn
in our Compose UI:
@Composable
fun MyScreen() {
var items by remember { mutableStateOf(listOf("Item 1", "Item 2", "Item 3")) }
DiffableLazyColumn(
items = items,
key = { it },
areItemsTheSame = { oldItem, newItem -> oldItem == newItem },
areContentsTheSame = { oldItem, newItem -> oldItem == newItem }
) { item ->
Text(text = item)
}
Button(onClick = {
// Update the list with new items or changes
items = listOf("Item 1", "Item 2", "Item 4")
}) {
Text("Update List")
}
}
Explanation
Diff Calculation: The
calculateDiff
function figures out the difference between the old and new list, just likeDiffUtil
does. How cool is that?!State Management: The
DiffableLazyColumn
keeps track of the old list usingremember
and updates it whenever the new list changes. It's like magic!Rendering: The
LazyColumn
is used to render the items based on the diff results. Super efficient!
This approach gives you the awesome efficiency of DiffUtil
in a Jetpack Compose context, making sure that only the necessary items are re-rendered when the list changes. How amazing is that?!
SUCCESS!!!
BUT!!!
What about Space and Time Complexity?
Time Complexity
The time complexity of the provided calculateDiff
function can be broken down as follows:
Outer Loop (
newList
): This loop goes through each item in thenewList
, so it runs inO(n)
wheren
is the size ofnewList
.Inner Loop (
oldList
): For each item in thenewList
, the function checks each item in theoldList
to find a match. This runs inO(m)
wherem
is the size ofoldList
.
Wait, what?! Since the inner loop runs for each item in the newList
, the overall time complexity is O(n * m)
.
Space Complexity
Space for the Result List: The space needed to store the diff results will be proportional to the number of changes, so in the worst case, it could be
O(n + m)
.Space for the Used Old Items: The function also uses an auxiliary array
usedOldItems
to track which items in theoldList
have been matched, which has a space complexity ofO(m)
.
Can you believe it? Thus, the space complexity is O(n + m)
.
Is It Good to Use with Compose?
While this DiffUtil
-like function is conceptually similar to DiffUtil
in RecyclerView, it may not be the most optimal approach in Compose for the following reasons:
Compose Efficiency: Jetpack Compose is designed to be efficient in re-composition. The Compose framework inherently minimizes unnecessary recompositions, especially when combined with stable, immutable state and key-based optimizations (like the
key
parameter inLazyColumn
).Manual Diffing Overhead: The provided approach introduces manual diffing and adds an extra layer of complexity that might not always be necessary. In many cases, the built-in behavior of
LazyColumn
with proper key handling can be sufficient and more performant, especially for small to medium-sized lists.Composability: Compose encourages thinking in terms of composability and state. The library approach provided might be seen as a more imperative or traditional approach, which may not align well with Compose’s declarative nature.
Recommendations
For Small/Medium Lists: Utilize Compose’s native list handling with
LazyColumn
, ensuring proper use of keys for list items. This approach should be efficient for most scenarios.For Large Lists with Complex Diffing Needs: When dealing with very large lists and aiming to minimize recompositions, consider a more advanced diffing algorithm or structure our data to reduce updates. The current
O(n * m)
approach may become a performance issue with large datasets.Use Stable Keys: Ensure each item in our
LazyColumn
has a stable key by using thekey
parameter to avoid unnecessary recompositions. This is typically sufficient for handling most dynamic list scenarios in Compose.
In summary, while the provided approach is functional, it is generally advisable to rely on Compose's built-in optimizations. Custom diffing logic should only be employed if there is a specific requirement that Compose's native capabilities cannot efficiently address.
That's it for today. Happy Coding...