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pagecurl/src/main/kotlin/eu/wewox/pagecurl/config/CurlConfig.kt

@@ -10,24 +10,53 @@ import androidx.compose.ui.unit.IntSize
 import androidx.compose.ui.unit.dp
 import eu.wewox.pagecurl.ExperimentalPageCurlApi
 
+/**
+ * The configuration for PageCurl.
+ *
+ * @property curl Configures how page curl looks like.
+ * @property interaction Configures interactions with a page curl. Such as if drag or tap to go on next or previous
+ * page is allowed, etc.
+ */
 @ExperimentalPageCurlApi
 public data class PageCurlConfig(
     val curl: CurlConfig = CurlConfig(),
     val interaction: InteractionConfig = InteractionConfig(),
 )
 
+/**
+ * Configures how page curl looks like.
+ *
+ * @property backPage Configures how back-page looks like (color, content alpha).
+ * @property shadow Configures how page shadow looks like (color, alpha, radius).
+ */
 @ExperimentalPageCurlApi
 public data class CurlConfig(
     val backPage: BackPageConfig = BackPageConfig(),
     val shadow: ShadowConfig = ShadowConfig(),
 )
 
+/**
+ * Configures how back-page of the page curl looks like.
+ *
+ * @property color Color of the back-page. In majority of use-cases it should be set to the content background color.
+ * @property contentAlpha The alpha which defines how content is "seen through" the back-page. From 0 (nothing is
+ * visible) to 1 (everything is visible).
+ */
 @ExperimentalPageCurlApi
 public data class BackPageConfig(
     val color: Color = Color.White,
     val contentAlpha: Float = 0.1f,
 )
 
+/**
+ * Configures how page shadow looks like.
+ *
+ * @property color The color of the shadow. In majority of use-cases it should be set to the inverted color to the
+ * content background color. Should be a solid color, see [alpha] to adjust opacity.
+ * @property alpha The alpha of the [color].
+ * @property radius Defines how big the shadow is.
+ * @property offset Defines how shadow is shifted from the page. A little shift may add more realism.
+ */
 @ExperimentalPageCurlApi
 public data class ShadowConfig(
     val color: Color = Color.Black,
@@ -36,16 +65,39 @@ public data class ShadowConfig(
     val offset: DpOffset = DpOffset((-5).dp, 0.dp),
 )
 
+/**
+ * Configures interactions with a page curl.
+ *
+ * @property drag Configures drag interactions.
+ * @property tap Configures tap interactions.
+ */
 @ExperimentalPageCurlApi
 public data class InteractionConfig(
     val drag: Drag = Drag(),
     val tap: Tap = Tap(),
 ) {
+
+    /**
+     * Configures drag interactions.
+     *
+     * @property forward Configures forward drag interaction.
+     * @property backward Configures backward drag interaction.
+     */
     @ExperimentalPageCurlApi
     public data class Drag(
         val forward: Interaction = Interaction(true, rightHalf(), leftHalf()),
         val backward: Interaction = Interaction(true, forward.end, forward.start),
     ) {
+
+        /**
+         * The drag interaction setting.
+         *
+         * @property enabled True if this interaction is enabled or not.
+         * @property start Defines a rectangle where interaction should start. The rectangle coordinates are relative
+         * (from 0 to 1) and then scaled to the PageCurl bounds.
+         * @property end Defines a rectangle where interaction should end. The rectangle coordinates are relative
+         * (from 0 to 1) and then scaled to the PageCurl bounds.
+         */
         @ExperimentalPageCurlApi
         public data class Interaction(
             val enabled: Boolean,
@@ -54,18 +106,41 @@ public data class InteractionConfig(
         )
     }
 
+    /**
+     * Configures tap interactions.
+     *
+     * @property forward Configures forward tap interaction.
+     * @property backward Configures backward tap interaction.
+     * @property custom The custom tap interaction. Could be provided to implement custom taps in the PageCurl, e.g. to
+     * capture taps in the center, etc.
+     */
     @ExperimentalPageCurlApi
     public data class Tap(
         val forward: Interaction = Interaction(true, rightHalf()),
         val backward: Interaction = Interaction(true, leftHalf()),
         val custom: CustomInteraction = CustomInteraction(false)
     ) {
+
+        /**
+         * The tap interaction setting.
+         *
+         * @property enabled True if this interaction is enabled or not.
+         * @property target Defines a rectangle where interaction captured. The rectangle coordinates are relative
+         * (from 0 to 1) and then scaled to the PageCurl bounds.
+         */
         @ExperimentalPageCurlApi
         public data class Interaction(
             val enabled: Boolean,
             val target: Rect = Rect.Zero,
         )
 
+        /**
+         * The custom tap interaction setting.
+         *
+         * @property enabled True if this interaction is enabled or not.
+         * @property onTap The lambda to invoke to check if tap is handled by custom tap or not. Receives the density
+         * scope, the PageCurl size and tap position. Returns true if tap is handled and false otherwise.
+         */
         @ExperimentalPageCurlApi
         public data class CustomInteraction(
             val enabled: Boolean,
@@ -74,6 +149,15 @@ public data class InteractionConfig(
     }
 }
 
+/**
+ * The utility function to create a new copy of the [InteractionConfig] with different enabled states of each
+ * interactions.
+ *
+ * @param dragForwardEnabled True to enable forward drag interaction.
+ * @param dragBackwardEnabled True to enable backward drag interaction.
+ * @param tapForwardEnabled True to enable forward tap interaction.
+ * @param tapBackwardEnabled True to enable backward tap interaction.
+ */
 @ExperimentalPageCurlApi
 public fun InteractionConfig.copy(
     dragForwardEnabled: Boolean = drag.forward.enabled,
@@ -91,6 +175,12 @@ public fun InteractionConfig.copy(
     )
 )
 
+/**
+ * The left half of the PageCurl.
+ */
 private fun leftHalf(): Rect = Rect(0.0f, 0.0f, 0.5f, 1.0f)
 
+/**
+ * The right half of the PageCurl.
+ */
 private fun rightHalf(): Rect = Rect(0.5f, 0.0f, 1.0f, 1.0f)

pagecurl/src/main/kotlin/eu/wewox/pagecurl/page/CurlDraw.kt

@@ -5,6 +5,7 @@ import android.graphics.Canvas
 import android.os.Build
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.CacheDrawScope
+import androidx.compose.ui.draw.DrawResult
 import androidx.compose.ui.draw.drawWithCache
 import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.geometry.toRect
@@ -33,19 +34,20 @@ internal fun Modifier.drawCurl(
     posA: Offset,
     posB: Offset,
 ): Modifier = drawWithCache {
-    fun drawOnlyContent() =
+    // Fast-check if curl is in left most position (gesture is fully completed)
-        onDrawWithContent {
+    // In such case do not bother and draw nothing
-            drawContent()
-        }
-
     if (posA == size.toRect().topLeft && posB == size.toRect().bottomLeft) {
-        return@drawWithCache onDrawWithContent { }
+        return@drawWithCache drawNothing()
     }
 
+    // Fast-check if curl is in right most position (gesture is not yet started)
+    // In such case do not bother and draw the full content
     if (posA == size.toRect().topRight && posB == size.toRect().bottomRight) {
         return@drawWithCache drawOnlyContent()
     }
 
+    // Find the intersection of the curl line ([posA, posB]) and top and bottom sides, so that we may clip and mirror
+    // content correctly
     val topIntersection = lineLineIntersection(
         Offset(0f, 0f), Offset(size.width, 0f),
         posA, posB
@@ -54,14 +56,20 @@ internal fun Modifier.drawCurl(
         Offset(0f, size.height), Offset(size.width, size.height),
         posA, posB
     )
+
+    // Should not really happen, but in case there is not intersection (curl line is horizontal), just draw the full
+    // content instead
     if (topIntersection == null || bottomIntersection == null) {
         return@drawWithCache drawOnlyContent()
     }
 
+    // Limit x coordinates of both intersections to be at least 0, so that page do not look like teared from the book
     val topCurlOffset = Offset(max(0f, topIntersection.x), topIntersection.y)
     val bottomCurlOffset = Offset(max(0f, bottomIntersection.x), bottomIntersection.y)
 
+    // That is the easy part, prepare a lambda to draw the content clipped by the curl line
     val drawClippedContent = prepareClippedContent(topCurlOffset, bottomCurlOffset)
+    // That is the tricky part, prepare a lambda to draw the back-page with the shadow
     val drawCurl = prepareCurl(config, topCurlOffset, bottomCurlOffset)
 
     onDrawWithContent {
@@ -70,16 +78,34 @@ internal fun Modifier.drawCurl(
     }
 }
 
+/**
+ * The simple method to draw the whole unmodified content.
+ */
+private fun CacheDrawScope.drawOnlyContent(): DrawResult =
+    onDrawWithContent {
+        drawContent()
+    }
+
+/**
+ * The simple method to draw nothing.
+ */
+private fun CacheDrawScope.drawNothing(): DrawResult =
+    onDrawWithContent {
+        /* Empty */
+    }
+
 @ExperimentalPageCurlApi
 private fun CacheDrawScope.prepareClippedContent(
     topCurlOffset: Offset,
     bottomCurlOffset: Offset,
 ): ContentDrawScope.() -> Unit {
+    // Make a quadrilateral from the left side to the intersection points
     val path = Path()
     path.lineTo(topCurlOffset.x, topCurlOffset.y)
     path.lineTo(bottomCurlOffset.x, bottomCurlOffset.y)
     path.lineTo(0f, size.height)
     return result@{
+        // Draw a content clipped by the constructed path
         clipPath(path) {
             this@result.drawContent()
         }
@@ -92,8 +118,13 @@ private fun CacheDrawScope.prepareCurl(
     topCurlOffset: Offset,
     bottomCurlOffset: Offset,
 ): ContentDrawScope.() -> Unit {
+    // Build a quadrilateral of the part of the page which should be mirrored as the back-page
+    // In all cases polygon should have 4 points, even when back-page is only a small "corner" (with 3 points) due to
+    // the shadow rendering, otherwise it will create a visual artifact when switching between 3 and 4 points polygon
     val polygon = Polygon(
         sequence {
+            // Find the intersection of the curl line and right side
+            // If intersection is found adds to the polygon points list
             suspend fun SequenceScope<Offset>.yieldEndSideInterception() {
                 val offset = lineLineIntersection(
                     topCurlOffset, bottomCurlOffset,
@@ -102,12 +133,18 @@ private fun CacheDrawScope.prepareCurl(
                 yield(offset)
                 yield(offset)
             }
+
+            // In case top intersection lays in the bounds of the page curl, take 2 points from the top side, otherwise
+            // take the interception with a right side
             if (topCurlOffset.x < size.width) {
                 yield(topCurlOffset)
                 yield(Offset(size.width, topCurlOffset.y))
             } else {
                 yieldEndSideInterception()
             }
+
+            // In case bottom intersection lays in the bounds of the page curl, take 2 points from the bottom side,
+            // otherwise take the interception with a right side
             if (bottomCurlOffset.x < size.width) {
                 yield(Offset(size.width, size.height))
                 yield(bottomCurlOffset)
@@ -117,17 +154,25 @@ private fun CacheDrawScope.prepareCurl(
         }.toList()
     )
 
+    // Calculate the angle in radians between X axis and the curl line, this is used to rotate mirrored content to the
+    // right position of the curled back-page
     val lineVector = topCurlOffset - bottomCurlOffset
-    val angle = Math.PI.toFloat() + atan2(-lineVector.y, lineVector.x) * 2
+    val angle = Math.PI.toFloat() - atan2(lineVector.y, lineVector.x) * 2
+
+    // Prepare a lambda to draw the shadow of the back-page
     val drawShadow = prepareShadow(config, polygon, angle)
 
     return result@{
         withTransform({
+            // Mirror in X axis the drawing as back-page should be mirrored
             scale(-1f, 1f, pivot = bottomCurlOffset)
+            // Rotate the drawing according to the curl line
             rotateRad(angle, pivot = bottomCurlOffset)
         }) {
+            // Draw shadow first
             this@result.drawShadow()
 
+            // And finally draw the back-page with an overlay with alpha
             clipPath(polygon.toPath()) {
                 this@result.drawContent()
 
@@ -146,15 +191,19 @@ private fun CacheDrawScope.prepareShadow(
 ): ContentDrawScope.() -> Unit {
     val shadow = config.shadow
 
+    // Quick exit if no shadow is requested
     if (shadow.alpha == 0f || shadow.radius == 0.dp) {
         return { /* No shadow is requested */ }
     }
 
+    // Prepare shadow parameters
     val radius = shadow.radius.toPx()
     val shadowColor = shadow.color.copy(alpha = shadow.alpha).toArgb()
     val transparent = shadow.color.copy(alpha = 0f).toArgb()
     val shadowOffset = Offset(-shadow.offset.x.toPx(), shadow.offset.y.toPx())
         .rotate(2 * Math.PI.toFloat() - angle)
+
+    // Prepare shadow paint with a shadow layer
     val paint = Paint().apply {
         val frameworkPaint = asFrameworkPaint()
         frameworkPaint.color = transparent
@@ -166,6 +215,8 @@ private fun CacheDrawScope.prepareShadow(
         )
     }
 
+    // Hardware acceleration supports setShadowLayer() only on API 28 and above, thus to support previous API versions
+    // draw a shadow to the bitmap instead
     return if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
         prepareShadowApi28(radius, paint, polygon)
     } else {
@@ -193,6 +244,7 @@ private fun CacheDrawScope.prepareShadowImage(
     paint: Paint,
     polygon: Polygon,
 ): ContentDrawScope.() -> Unit {
+    // Increase the size a little bit so that shadow is not clipped
     val bitmap = Bitmap.createBitmap(
         (size.width + radius * 4).toInt(),
         (size.height + radius * 4).toInt(),
@@ -201,6 +253,7 @@ private fun CacheDrawScope.prepareShadowImage(
     Canvas(bitmap).apply {
         drawPath(
             polygon
+                // As bitmap size is increased we should translate the polygon so that shadow remains in center
                 .translate(Offset(2 * radius, 2 * radius))
                 .offset(radius).toPath()
                 .asAndroidPath(),
@@ -210,6 +263,7 @@ private fun CacheDrawScope.prepareShadowImage(
 
     return {
         drawIntoCanvas {
+            // As bitmap size is increased we should shift the drawing so that shadow remains in center
             it.nativeCanvas.drawBitmap(bitmap, -2 * radius, -2 * radius, null)
         }
     }

pagecurl/src/main/kotlin/eu/wewox/pagecurl/page/CurlGesture.kt

@@ -77,6 +77,7 @@ internal fun Modifier.curlGesture(
         return@pointerInput
     }
 
+    // Use velocity tracker to support flings
     val velocityTracker = VelocityTracker()
     val startRect by lazy { direction.start.multiply(size) }
     val endRect by lazy { direction.end.multiply(size) }
@@ -87,6 +88,7 @@ internal fun Modifier.curlGesture(
                 return@awaitPointerEventScope
             }
 
+            // Change X position to be always on the right side for more convenient gesture tracking
             val dragStart = down.position.copy(x = size.width.toFloat())
 
             onStart()

pagecurl/src/main/kotlin/eu/wewox/pagecurl/page/PageCurl.kt

@@ -10,6 +10,14 @@ import androidx.compose.ui.Modifier
 import eu.wewox.pagecurl.ExperimentalPageCurlApi
 import eu.wewox.pagecurl.config.PageCurlConfig
 
+/**
+ * Shows the pages which may be rotated by drag or tap gestures.
+ *
+ * @param state The state of the PageCurl. Use this to programmatically change the current page or observe changes.
+ * @param modifier The modifier for this composable.
+ * @param config The configuration for PageCurl. Configures how page curl looks like and interacts.
+ * @param content The content lambda to provide the page composable. Receives the page number.
+ */
 @ExperimentalPageCurlApi
 @Composable
 public fun PageCurl(

pagecurl/src/main/kotlin/eu/wewox/pagecurl/page/PageCurlState.kt

@@ -22,6 +22,13 @@ import kotlinx.coroutines.coroutineScope
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 
+/**
+ * Remembers the [PageCurlState].
+ *
+ * @param max The max number of pages.
+ * @param initialCurrent The initial current page.
+ * @return The remembered [PageCurlState].
+ */
 @ExperimentalPageCurlApi
 @Composable
 public fun rememberPageCurlState(
@@ -46,14 +53,27 @@ public fun rememberPageCurlState(
         )
     }
 
+/**
+ * The state of the PageCurl.
+ *
+ * @property max The max number of pages.
+ * @param initialCurrent The initial current page.
+ */
 @ExperimentalPageCurlApi
 public class PageCurlState(
     public val max: Int,
     initialCurrent: Int = 0,
 ) {
+    /**
+     * The observable current page.
+     */
     public var current: Int by mutableStateOf(initialCurrent)
         internal set
 
+    /**
+     * The observable progress as page is rotated.
+     * When going forward it changes from 0 to 1, when going backward it is going from 0 to -1.
+     */
     public val progress: Float get() = internalState?.progress ?: 0f
 
     internal var internalState: InternalState? by mutableStateOf(null)
@@ -75,11 +95,21 @@ public class PageCurlState(
         internalState = InternalState(constraints, left, right, forward, backward)
     }
 
+    /**
+     * Instantly snaps the state to the given page.
+     *
+     * @param value The page to snap to.
+     */
     public suspend fun snapTo(value: Int) {
         current = value.coerceIn(0, max - 1)
         internalState?.reset()
     }
 
+    /**
+     * Go forward with an animation.
+     *
+     * @param block The animation block to animate a change.
+     */
     public suspend fun next(block: suspend Animatable<Edge, AnimationVector4D>.(Size) -> Unit = DefaultNext) {
         internalState?.animateTo(
             target = { current + 1 },
@@ -87,6 +117,11 @@ public class PageCurlState(
         )
     }
 
+    /**
+     * Go backward with an animation.
+     *
+     * @param block The animation block to animate a change.
+     */
     public suspend fun prev(block: suspend Animatable<Edge, AnimationVector4D>.(Size) -> Unit = DefaultPrev) {
         internalState?.animateTo(
             target = { current - 1 },
@@ -146,6 +181,9 @@ public class PageCurlState(
     }
 }
 
+/**
+ * The wrapper to represent a line with 2 points: [top] and [bottom].
+ */
 public data class Edge(val top: Offset, val bottom: Offset) {
 
     internal val centerX: Float = (top.x + bottom.x) * 0.5f