Just an Illusion!

Source: webflow.com

Few years back I wrote an article on creating parallax using ScrollView from XML view here. In this article we’ll do the same but using Jetpack Compose.

End Result

Creating parallax effect is fairly simple in compose. We track the scroll state of our scroller and changes the speed of translation of the views we want the parallax on. So let’s begin.

Firstly, we’ll create parallax using a Column. Column provides vertical arrangement for view hierarchy but aren’t scrollable. To make them scroll, we simply provide it a scrolling state as follows

val scrollState = rememberScrollState()
Column(
    modifier = Modifier
        .fillMaxWidth()
        .verticalScroll(scrollState),
)

Because we’re scrolling vertically so verticalScroll. We remember the scrolling state and hence whichever view will be reading this value will recompose itself accordingly.

In the demo video, it is shown that we’re parallaxing both the tiger image and the header bar. So let’s do it one by one.

Image Parallax

We’ve a column in which we’ve an image and a scrollable long text.

@Composable
fun ImageParallaxScroll() {
    val scrollState = rememberScrollState()
    Column(
        modifier = Modifier
            .fillMaxWidth()
            .verticalScroll(scrollState),
    ) {
        Box(
            modifier = Modifier
                .fillMaxWidth()
                .height(500.dp)
                .background(Color.White),
            contentAlignment = Alignment.Center
        ) {
            Image(
                painterResource(id = R.drawable.ic_tiger),
                contentDescription = "tiger parallax",
                contentScale = ContentScale.Crop,
                modifier = Modifier.fillMaxSize()
            )
        }
        Text(
            text = stringResource(R.string.dummy_text),
            modifier = Modifier.background(
                Color.White
            ),
            style = TextStyle(
                fontSize = 24.sp
            )
        )
    }
}

The above code will give us image and text but both scrollable at same speed.

Now to make the image move slower than the text, we simply modify the Y translation of the image as follows

Box(
    modifier = Modifier
        .fillMaxWidth()
        .height(500.dp)
        .background(Color.White)
        .graphicsLayer {
           translationY = 0.5f * scrollState.value
        },
    contentAlignment = Alignment.Center
)

Using graphicsLayer has the advantage of redrawing only the affected part of the view than recomposing the whole composable. Here we’ve translated Y movement by half of actual scroll.

Bamn! Tiger is parallaxed. But wait, let’s also fades it along by updating its alpha value too as follows

Box(
    modifier = Modifier
        .fillMaxWidth()
        .height(500.dp)
        .background(Color.White)
        .graphicsLayer {
            alpha = 1f - ((scrollState.value.toFloat() / scrollState.maxValue) * 1.5f)
            translationY = 0.5f * scrollState.value
        },
    contentAlignment = Alignment.Center
)

In case of alpha, we calculate the scroll ratio and because the ratio could be very small initially (depending on the scroll height), we magnify it 1.5 times for each value and by trial and error and 1.5 works well for me. You can try with different values. But it gives us the desired result.

Header Bar Parallax

Now in the demo video, we also noticed that the visibility of the header bar changes with scroll movement. So let’s do that too.

@Composable
fun HeaderBarParallaxScroll() {
    val scrollState = rememberScrollState()
    Box {
        Column(
            modifier = Modifier
                .fillMaxWidth()
                .verticalScroll(scrollState),
        ) {
            ...
        }

        Box(
            modifier = Modifier
                .fillMaxWidth()
                .height(60.dp)
                .background(Color.Yellow),
            contentAlignment = Alignment.CenterStart
        ) {
            Text(
                text = "Header bar",
                modifier = Modifier.padding(horizontal = 16.dp),
                style = TextStyle(
                    fontSize = 24.sp,
                    fontWeight = FontWeight.W900,
                    color = Color.Black
                )
            )
        }
    }
}

We use the same composable but we add a new header bar box over the column as shown above. But initially the header bar will be completely visible. We want that is should be hidden and appears as with scroll up movement.

Box(
    modifier = Modifier
        .alpha(min(1f, (scrollState.value.toFloat() / scrollState.maxValue) * 5f))
        .fillMaxWidth()
        .height(60.dp)
        .background(Color.Yellow),
    contentAlignment = Alignment.CenterStart
)

This article was originally published on proandroiddev.com on June 12, 2022