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Title
k

KotlinLeaner

05/02/2023, 4:50 PM
Hi guys, I have a list which is coming from the server. I am learning drawing on
canvas
. I don't know height of
canvas
. So I learnt basic how to measure text using
rememberTextMeasurer
with
TextLayoutResult
for single item and provide height on
canvas
. If I mock 5 item in the list, then how can we measure all text height at a time and provide in
canvas
? I am trying to put my
rememberTextMeasurer
and
TextLayoutResult
inside
movieList
of
forEach
it complains about.
@OptIn(ExperimentalTextApi::class)
@Composable
fun TimeLineComposable() {
    Column(modifier = Modifier.fillMaxSize()) {
        val movieList = getMovieList()
        val heading = AnnotatedString("Welcome to heading")
        val titleTextMeasurer = rememberTextMeasurer()
        val titleTextLayoutResult = remember {
            titleTextMeasurer.measure(heading, TextStyle(fontSize = 16.sp, color = Color.White))
        }
        val canvasHeight = with(LocalDensity.current) {
            titleTextLayoutResult.size.height.toDp()
        }
        Canvas(
            modifier = Modifier
                .fillMaxWidth()
                .background(Color.Black)
                .height(canvasHeight)
        ) {
            drawText(
                textLayoutResult = titleTextLayoutResult,
                topLeft = Offset(
                    x = center.x - titleTextLayoutResult.size.width / 2,
                    y = center.y - titleTextLayoutResult.size.height / 2,
                )
            )
        }
    }
}
Error
@OptIn(ExperimentalTextApi::class)
@Composable
fun TimeLineComposable() {
    Column(modifier = Modifier.fillMaxSize()) {
        val movieList = getMovieList()
        val heading = AnnotatedString("Welcome to heading")
        Canvas(
            modifier = Modifier
                .fillMaxWidth()
                .background(Color.Black)
                .height(canvasHeight)
        ) {
            movieList.forEach {
                val titleTextMeasurer = rememberTextMeasurer()
                val titleTextLayoutResult = remember {
                    titleTextMeasurer.measure(heading, TextStyle(fontSize = 16.sp, color = Color.White))
                }
                val canvasHeight = with(LocalDensity.current) {
                    titleTextLayoutResult.size.height.toDp()
                }
                drawText(
                    textLayoutResult = titleTextLayoutResult,
                    topLeft = Offset(
                        x = center.x - titleTextLayoutResult.size.width / 2,
                        y = center.y - titleTextLayoutResult.size.height / 2,
                    )
                )
            }
        }
    }
}
z

Zach Klippenstein (he/him) [MOD]

05/02/2023, 5:02 PM
remember the text measurer in your composable, then call
measure
on it as many times as you need from in your canvas
note that measuring is expensive, you probably want to measure from the layout phase or the draw cache (using
drawWithCache
)
cc @Halil Ozercan in case i’m super wrong
k

KotlinLeaner

05/02/2023, 5:05 PM
Yes put the
rememberTextMeasurer
outside of canvas block
Now I want the canvas height, how can I find the height ?
z

Zach Klippenstein (he/him) [MOD]

05/02/2023, 5:07 PM
Look at the receiver type of the Canvas lambda, check its properties
k

KotlinLeaner

05/02/2023, 5:11 PM
Sorry I mean how can I provide the height on canvas?
@OptIn(ExperimentalTextApi::class)
@Composable
fun TimeLineComposable() {
    Column(modifier = Modifier.fillMaxSize()) {
        val movieList = getMovieList()
        val titleTextMeasurer = rememberTextMeasurer()
        var canvasHeight by remember { mutableStateOf(0.dp) }
        Canvas(
            modifier = Modifier
                .fillMaxWidth()
                .background(Color.Black)
                .height(canvasHeight)
        ) {
            movieList.forEach {
                val titleTextLayoutResult = titleTextMeasurer.measure(
                    AnnotatedString(it.showDay),
                    TextStyle(fontSize = 16.sp, color = Color.White)
                )
                canvasHeight = titleTextLayoutResult.size.height.toDp()
                drawText(
                    textLayoutResult = titleTextLayoutResult,
                    topLeft = Offset(
                        x = center.x - titleTextLayoutResult.size.width / 2,
                        y = center.y - titleTextLayoutResult.size.height / 2,
                    )
                )
            }
        }
    }
}
This is not working, because height didn't get it by
canvasHeight
Canvas(
            modifier = Modifier
                .fillMaxWidth()
                .background(Color.Black)
                .height(canvasHeight)
        ) {
z

Zach Klippenstein (he/him) [MOD]

05/02/2023, 5:13 PM
How do you want to measure the canvas?
k

KotlinLeaner

05/02/2023, 5:15 PM
I want to measure based on list item size. That's why I used
val titleTextLayoutResult = titleTextMeasurer.measure(
                    AnnotatedString(it.showDay),
                    TextStyle(fontSize = 16.sp, color = Color.White)
                )
                canvasHeight += titleTextLayoutResult.size.height.toDp()
z

Zach Klippenstein (he/him) [MOD]

05/02/2023, 5:20 PM
ok, so then you need to use a layout modifier, make the measure calls in the measure pass, and calculate your layout from there. E.g.
Modifier.layout { measurable, constraints ->
  // 1. Measure all your texts using textMeasurer.measure(…)
  // 2. Calculate height, width based on constraints and text measurement results
  //    e.g. val height = textBounds.sumOf { it.height }.coerceAtMost(constraints.maxHeight)
  // 3. Determine child constraints using that height and whatever you want for width.
  //    e.g. val childConstraints = constraints.copy(minHeight = height, maxHeight = height)
  val placeable = measurable.measure(childConstraints)
  layout(placeable.width, placeable.height) {
    placeable.place(0, 0)
  }
}
but at that point you’re just reimplementing
Column
with a bunch of child `BasicText`s, so why not just do that?
k

KotlinLeaner

05/02/2023, 5:22 PM
Actually, I want to build more complex Ui then these. So I am exploring the use of all these.
I want to build timeline view in compose. This is my github code. I made it in canvas but there is a problem on there. My multiple view get overlap each other..
Is there any example how to use layout modifier in canvas ?
h

Halil Ozercan

05/02/2023, 5:42 PM

https://www.youtube.com/watch?v=xcfEQO0k_gU

You can refer to this video on how to implement a custom layout with a similar functionality https://github.com/android/compose-samples/tree/main/JetLagged This is the repo that contains the project from video.
k

KotlinLeaner

05/02/2023, 5:44 PM
Sure, thanks a lot.. i'll take a look now
a

Arjan van Wieringen

05/02/2023, 5:50 PM
Doesn’t canvas drawscope contain height and width info?
k

KotlinLeaner

05/02/2023, 5:51 PM
It will not give you an accurate height to draw textview.
a

Arjan van Wieringen

05/02/2023, 6:31 PM
Why not accurate? It is literally the size of the canvas in pixels
k

KotlinLeaner

05/02/2023, 6:34 PM
Because you don't know how much in pixel it will draw.
I am drawing on the basics of text height
a

Arjan van Wieringen

05/02/2023, 6:35 PM
You want to know the size of the text height, not the canvas
k

KotlinLeaner

05/02/2023, 6:36 PM
Yes
a

Arjan van Wieringen

05/02/2023, 6:36 PM
But in your first post you write: "I don't know height of
canvas
."
k

KotlinLeaner

05/02/2023, 6:38 PM
After that, I was correct in the next statement..