dashedBorder modifier for android compose

DashedBorder.kt

import androidx.compose.foundation.BorderStroke

/*
 * Copyright 2020 The Android Open Source Project
 *
 * 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.ui.Modifier
import androidx.compose.ui.composed
import androidx.compose.ui.draw.drawWithCache
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.geometry.Size
import androidx.compose.ui.geometry.isSimple
import androidx.compose.ui.graphics.*
import androidx.compose.ui.graphics.drawscope.Stroke
import androidx.compose.ui.graphics.drawscope.clipRect
import androidx.compose.ui.graphics.drawscope.withTransform
import androidx.compose.ui.platform.debugInspectorInfo
import androidx.compose.ui.unit.Dp

/**
 * Modify element to add border with appearance specified with a [border] and a [shape], pad the
 * content by the [BorderStroke.width] and clip it.
 *
 * @sample androidx.compose.foundation.samples.BorderSample()
 *
 * @param border [BorderStroke] class that specifies border appearance, such as size and color
 * @param shape shape of the border
 */
fun Modifier.dashedBorder(border: BorderStroke, shape: Shape = RectangleShape, on: Dp, off: Dp) =
    dashedBorder(width = border.width, brush = border.brush, shape = shape, on, off)

/**
 * Returns a [Modifier] that adds border with appearance specified with [width], [color] and a
 * [shape], pads the content by the [width] and clips it.
 *
 * @sample androidx.compose.foundation.samples.BorderSampleWithDataClass()
 *
 * @param width width of the border. Use [Dp.Hairline] for a hairline border.
 * @param color color to paint the border with
 * @param shape shape of the border
 * @param on the size of the solid part of the dashes
 * @param off the size of the space between dashes
 */
fun Modifier.dashedBorder(width: Dp, color: Color, shape: Shape = RectangleShape, on: Dp, off: Dp) =
    dashedBorder(width, SolidColor(color), shape, on, off)

/**
 * Returns a [Modifier] that adds border with appearance specified with [width], [brush] and a
 * [shape], pads the content by the [width] and clips it.
 *
 * @sample androidx.compose.foundation.samples.BorderSampleWithBrush()
 *
 * @param width width of the border. Use [Dp.Hairline] for a hairline border.
 * @param brush brush to paint the border with
 * @param shape shape of the border
 */
fun Modifier.dashedBorder(width: Dp, brush: Brush, shape: Shape, on: Dp, off: Dp): Modifier =
    composed(
        factory = {
            this.then(
                Modifier.drawWithCache {
                    val outline: Outline = shape.createOutline(size, layoutDirection, this)
                    val borderSize = if (width == Dp.Hairline) 1f else width.toPx()

                    var insetOutline: Outline? = null // outline used for roundrect/generic shapes
                    var stroke: Stroke? = null // stroke to draw border for all outline types
                    var pathClip: Path? = null // path to clip roundrect/generic shapes
                    var inset = 0f // inset to translate before drawing the inset outline
                    // path to draw generic shapes or roundrects with different corner radii
                    var insetPath: Path? = null
                    if (borderSize > 0 && size.minDimension > 0f) {
                        if (outline is Outline.Rectangle) {
                            stroke = Stroke(
                                borderSize, pathEffect = PathEffect.dashPathEffect(
                                    floatArrayOf(on.toPx(), off.toPx())
                                )
                            )
                        } else {
                            // Multiplier to apply to the border size to get a stroke width that is
                            // large enough to cover the corners while not being too large to overly
                            // square off the internal shape. The resultant shape will be
                            // clipped to the desired shape. Any value lower will show artifacts in
                            // the corners of shapes. A value too large will always square off
                            // the internal shape corners. For example, for a rounded rect border
                            // a large multiplier will always have squared off edges within the
                            // inner section of the stroke, however, having a smaller multiplier
                            // will still keep the rounded effect for the inner section of the
                            // border
                            val strokeWidth = 1.2f * borderSize
                            inset = borderSize - strokeWidth / 2
                            val insetSize = Size(
                                size.width - inset * 2,
                                size.height - inset * 2
                            )
                            insetOutline = shape.createOutline(insetSize, layoutDirection, this)
                            stroke = Stroke(
                                strokeWidth, pathEffect = PathEffect.dashPathEffect(
                                    floatArrayOf(on.toPx(), off.toPx())
                                )
                            )
                            pathClip = if (outline is Outline.Rounded) {
                                Path().apply { addRoundRect(outline.roundRect) }
                            } else if (outline is Outline.Generic) {
                                outline.path
                            } else {
                                // should not get here because we check for Outline.Rectangle
                                // above
                                null
                            }

                            insetPath =
                                if (insetOutline is Outline.Rounded &&
                                    !insetOutline.roundRect.isSimple
                                ) {
                                    // Rounded rect with non equal corner radii needs a path
                                    // to be pre-translated
                                    Path().apply {
                                        addRoundRect(insetOutline.roundRect)
                                        translate(Offset(inset, inset))
                                    }
                                } else if (insetOutline is Outline.Generic) {
                                    // Generic paths must be created and pre-translated
                                    Path().apply {
                                        addPath(insetOutline.path, Offset(inset, inset))
                                    }
                                } else {
                                    // Drawing a round rect with equal corner radii without
                                    // usage of a path
                                    null
                                }
                        }
                    }

                    onDrawWithContent {
                        drawContent()
                        // Only draw the border if a have a valid stroke parameter. If we have
                        // an invalid border size we will just draw the content
                        if (stroke != null) {
                            if (insetOutline != null && pathClip != null) {
                                val isSimpleRoundRect = insetOutline is Outline.Rounded &&
                                        insetOutline.roundRect.isSimple
                                withTransform({
                                    clipPath(pathClip)
                                    // we are drawing the round rect not as a path so we must
                                    // translate ourselves othe
                                    if (isSimpleRoundRect) {
                                        translate(inset, inset)
                                    }
                                }) {
                                    if (isSimpleRoundRect) {
                                        // If we don't have an insetPath then we are drawing
                                        // a simple round rect with the corner radii all identical
                                        val rrect = (insetOutline as Outline.Rounded).roundRect
                                        drawRoundRect(
                                            brush = brush,
                                            topLeft = Offset(rrect.left, rrect.top),
                                            size = Size(rrect.width, rrect.height),
                                            cornerRadius = rrect.topLeftCornerRadius,
                                            style = stroke
                                        )
                                    } else if (insetPath != null) {
                                        drawPath(insetPath, brush, style = stroke)
                                    }
                                }
                                // Clip rect to ensure the stroke does not extend the bounds
                                // of the composable.
                                clipRect {
                                    // Draw a hairline stroke to cover up non-anti-aliased pixels
                                    // generated from the clip
                                    if (isSimpleRoundRect) {
                                        val rrect = (outline as Outline.Rounded).roundRect
                                        drawRoundRect(
                                            brush = brush,
                                            topLeft = Offset(rrect.left, rrect.top),
                                            size = Size(rrect.width, rrect.height),
                                            cornerRadius = rrect.topLeftCornerRadius,
                                            style = Stroke(
                                                Stroke.HairlineWidth,
                                                pathEffect = PathEffect.dashPathEffect(
                                                    floatArrayOf(on.toPx(), off.toPx())
                                                )
                                            )
                                        )
                                    } else {
                                        drawPath(
                                            pathClip, brush = brush, style = Stroke(
                                                Stroke.HairlineWidth,
                                                pathEffect = PathEffect.dashPathEffect(
                                                    floatArrayOf(on.toPx(), off.toPx())
                                                )
                                            )
                                        )
                                    }
                                }
                            } else {
                                // Rectangular border fast path
                                val strokeWidth = stroke.width
                                val halfStrokeWidth = strokeWidth / 2
                                drawRect(
                                    brush = brush,
                                    topLeft = Offset(halfStrokeWidth, halfStrokeWidth),
                                    size = Size(
                                        size.width - strokeWidth,
                                        size.height - strokeWidth
                                    ),
                                    style = stroke
                                )
                            }
                        }
                    }
                }
            )
        },
        inspectorInfo = debugInspectorInfo {
            name = "border"
            properties["width"] = width
            if (brush is SolidColor) {
                properties["color"] = brush.value
                value = brush.value
            } else {
                properties["brush"] = brush
            }
            properties["shape"] = shape
        }
    )

Thanks for sharing this, really helpful way to do re-usable dashed borders.

I did have issues with the hairline rect within the clip block (starting at line 181) being rendered and I had to cut that all out to work properly:

            .dashedBorder(
                width = 2.dp,
                color = colorResource(id = R.color.blue),
                shape = MaterialTheme.shapes.medium, on = 4.dp, off = 4.dp
            )

Great effort, thanks

Had the same problem as @benbeverly so I added a simple alpha = 0f to the drawRoundRect and drawPath functions to not render the weird border. I don't know if the clip still works as intended and only the border doesn't get draw or something, but it seems to work:

                                // of the composable.
                                clipRect {
                                    // Draw a hairline stroke to cover up non-anti-aliased pixels
                                    // generated from the clip
                                    if (isSimpleRoundRect) {
                                        val rrect = (outline as Outline.Rounded).roundRect
                                        drawRoundRect(
                                            brush = brush,
                                            alpha = 0f,
                                            topLeft = Offset(rrect.left, rrect.top),
                                            size = Size(rrect.width, rrect.height),
                                            cornerRadius = rrect.topLeftCornerRadius,
                                            style = Stroke(
                                                Stroke.HairlineWidth,
                                                pathEffect = PathEffect.dashPathEffect(
                                                    floatArrayOf(on.toPx(), off.toPx())
                                                )
                                            )
                                        )
                                    } else {
                                        drawPath(
                                            pathClip, alpha = 0f, brush = brush, style = Stroke(
                                                Stroke.HairlineWidth,
                                                pathEffect = PathEffect.dashPathEffect(
                                                    floatArrayOf(on.toPx(), off.toPx())
                                                )
                                            )
                                        )
                                    }
                                }

RESPECT!

Thanks for this! Works really great, have to remove 'hairline stroke to cover up non-anti-aliased pixels' section like @benbeverly and @DDihanov mentioned though, as the hairline is not in sync with the dotted borders and looks out of place.