/*
* Copyright 2021 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.
*/
package androidx.compose.foundation.relocation
import androidx.compose.foundation.ExperimentalFoundationApi
import androidx.compose.runtime.remember
import androidx.compose.ui.Modifier
import androidx.compose.ui.composed
import androidx.compose.ui.geometry.Rect
import androidx.compose.ui.layout.LayoutCoordinates
import androidx.compose.ui.modifier.ModifierLocalProvider
import androidx.compose.ui.modifier.ProvidableModifierLocal
import androidx.compose.ui.platform.debugInspectorInfo
import kotlinx.coroutines.Job
import kotlinx.coroutines.coroutineScope
import kotlinx.coroutines.job
import kotlinx.coroutines.launch
/**
* A parent that can respond to [bringChildIntoView] requests from its children, and scroll so that
* the item is visible on screen. To apply a responder to an element, pass it to the
* [bringIntoViewResponder] modifier.
*
* When a component calls [BringIntoViewRequester.bringIntoView], the
* [BringIntoView ModifierLocal][ModifierLocalBringIntoViewParent] is read to gain access to the
* [BringIntoViewResponder], which is responsible for, in order:
*
* 1. Calculating a rectangle that its parent responder should bring into view by returning it from
* [calculateRectForParent].
* 2. Performing any scroll or other layout adjustments needed to ensure the requested rectangle is
* brought into view in [bringChildIntoView].
*
* Here is a sample defining a custom [BringIntoViewResponder]:
* @sample androidx.compose.foundation.samples.BringIntoViewResponderSample
*
* Here is a sample where a composable is brought into view:
* @sample androidx.compose.foundation.samples.BringIntoViewSample
*
* Here is a sample where a part of a composable is brought into view:
* @sample androidx.compose.foundation.samples.BringPartOfComposableIntoViewSample
*
* @see BringIntoViewRequester
*/
@ExperimentalFoundationApi
interface BringIntoViewResponder {
/**
* Return the rectangle in this node that should be brought into view by this node's parent,
* in coordinates relative to this node. If this node needs to adjust itself to bring
* [localRect] into view, the returned rectangle should be the destination rectangle that
* [localRect] will eventually occupy once this node's content is adjusted.
*
* @param localRect The rectangle that should be brought into view, relative to this node. This
* will be the same rectangle passed to [bringChildIntoView].
* @return The rectangle in this node that should be brought into view itself, relative to this
* node. If this node needs to scroll to bring [localRect] into view, the returned rectangle
* should be the destination rectangle that [localRect] will eventually occupy, once the
* scrolling animation is finished.
*/
@ExperimentalFoundationApi
fun calculateRectForParent(localRect: Rect): Rect
/**
* Bring this specified rectangle into bounds by making this scrollable parent scroll
* appropriately.
*
* This method should ensure that only one call is being handled at a time. If you use Compose's
* `Animatable` you get this for free, since it will cancel the previous animation when a new
* one is started while preserving velocity.
*
* @param localRect The rectangle that should be brought into view, relative to this node. This
* is the same rectangle that will have been passed to [calculateRectForParent].
*/
@ExperimentalFoundationApi
suspend fun bringChildIntoView(localRect: Rect)
}
/**
* A parent that can respond to [BringIntoViewRequester] requests from its children, and scroll so
* that the item is visible on screen. See [BringIntoViewResponder] for more details about how
* this mechanism works.
*
* @sample androidx.compose.foundation.samples.BringIntoViewResponderSample
* @sample androidx.compose.foundation.samples.BringIntoViewSample
*
* @see BringIntoViewRequester
*/
@ExperimentalFoundationApi
fun Modifier.bringIntoViewResponder(
responder: BringIntoViewResponder
): Modifier = composed(debugInspectorInfo {
name = "bringIntoViewResponder"
properties["responder"] = responder
}) {
val defaultParent = rememberDefaultBringIntoViewParent()
val modifier = remember(defaultParent) {
BringIntoViewResponderModifier(defaultParent)
}
modifier.responder = responder
return@composed modifier
}
/**
* A modifier that holds state and modifier implementations for [bringIntoViewResponder]. It has
* access to the next [BringIntoViewParent] via [BringIntoViewChildModifier] and additionally
* provides itself as the [BringIntoViewParent] for subsequent modifiers. This class is responsible
* for recursively propagating requests up the responder chain.
*/
@OptIn(ExperimentalFoundationApi::class)
private class BringIntoViewResponderModifier(
defaultParent: BringIntoViewParent
) : BringIntoViewChildModifier(defaultParent),
ModifierLocalProvider<BringIntoViewParent?>,
BringIntoViewParent {
lateinit var responder: BringIntoViewResponder
override val key: ProvidableModifierLocal<BringIntoViewParent?>
get() = ModifierLocalBringIntoViewParent
override val value: BringIntoViewParent
get() = this
/**
* Stores the rectangle and coroutine [Job] of the newest request to be handled by
* [bringChildIntoView].
*
* This property is not guarded by a lock since requests should only be made on the main thread.
*
* ## Request queuing
*
* When multiple requests are received concurrently, i.e. a new request comes in before the
* previous one finished, they are put in a queue. The queue is implicit – this property only
* stores a reference to the tail of the queue. The head of the queue is the oldest request that
* hasn't finished executing yet. Each subsequent request in the queue waits for the previous
* request to finish by [joining][Job.join] on its [Job]. When the oldest request finishes (or
* is cancelled), it is "removed" from the queue by simply returning from [bringChildIntoView].
* This completes its job, which resumes the next request, and makes that the new "head". When
* the last request finishes it "clears" the queue by setting this property to null.
*
* ## Interruption
*
* There is one case not covered above: If a request comes in for a rectangle that is
* not [fully-overlapped][completelyOverlaps] by the previous request, the new request will
* "interrupt" all previous requests and indirectly remove them from the queue. The reason only
* overlapping requests are queued is because if rectangle A contains rectangle B, then when A
* is fully on screen, then B is also fully on screen. By satisfying A first, both A and B can
* be satisfied in a single call to the [BringIntoViewResponder]. If B were to interrupt A, A
* might never be fully brought into view (see b/216790855).
*
* Interruption works by immediately dispatching the new request to the
* [BringIntoViewResponder]. Note that this class does not actually cancel the previously-
* running request's [Job] – the responder is responsible for cancelling any in-progress request
* and immediately start handling the new one. In particular, if the responder is using
* Compose's animation, the animation will handle cancellation itself and preserve velocity.
* When the previously-running request is cancelled, its job will complete and the next request
* in the queue will try to run. However, it will see that another request was already
* dispatched, and return early, completing its job as well. Etc etc until all requests that
* were received before the interrupting one have returned from their [bringChildIntoView]
* calls.
*
* The way a request determines if it's been interrupted is by comparing the previous request
* to the value of [newestDispatchedRequest]. [newestDispatchedRequest] is only set just before
* the [BringIntoViewResponder] is called.
*/
private var newestReceivedRequest: Pair<Rect, Job>? = null
/**
* The last queued request to have been sent to the [BringIntoViewResponder], or null if no
* requests are in progress.
*/
private var newestDispatchedRequest: Pair<Rect, Job>? = null
/**
* Responds to a child's request by first converting [rect] into this node's [LayoutCoordinates]
* and then, concurrently, calling the [responder] and the [parent] to handle the request.
*/
override suspend fun bringChildIntoView(rect: Rect, childCoordinates: LayoutCoordinates) {
coroutineScope {
val layoutCoordinates = layoutCoordinates ?: return@coroutineScope
if (!childCoordinates.isAttached) return@coroutineScope
val localRect = layoutCoordinates.localRectOf(childCoordinates, rect)
// Immediately make this request the tail of the queue, before suspending, so that
// any requests that come in while suspended will join on this one.
// Note that this job is not the caller's job, since coroutineScope introduces a child
// job.
// For more information about how requests are queued, see the kdoc on newestRequest.
val requestJob = coroutineContext.job
val thisRequest = Pair(localRect, requestJob)
val previousRequest = newestReceivedRequest
newestReceivedRequest = thisRequest
try {
// In the simplest case there are no ongoing requests, so just dispatch.
if (previousRequest == null ||
// If there's an ongoing request but it won't satisfy this request, then
// interrupt it.
!previousRequest.first.completelyOverlaps(localRect)
) {
dispatchRequest(thisRequest, layoutCoordinates)
return@coroutineScope
}
// The previous request completely overlaps this one, so wait for it to finish since
// it will probably satisfy this request.
// Note that even if the previous request fails or is cancelled, join will return
// normally. It could be cancelled either because a newer job interrupted it, or
// because the caller/sender of that request was cancelled.
previousRequest.second.join()
// If a newer request interrupted this one while we were waiting, then it will have
// already dispatched so we should consider this request cancelled and not dispatch.
if (newestDispatchedRequest === previousRequest) {
// Any requests queued up previously to us have finished, and nothing new came
// in while we were waiting.
dispatchRequest(thisRequest, layoutCoordinates)
}
} finally {
// Only the last job in the queue should clear the dispatched request, since if
// there's another job waiting to start it needs to know what the last dispatched
// request was.
if (newestDispatchedRequest === newestReceivedRequest) {
newestDispatchedRequest = null
}
// Only the last job in the queue should clear the queue.
if (newestReceivedRequest === thisRequest) {
newestReceivedRequest = null
}
}
}
}
/**
* Marks [request] as the [newestDispatchedRequest] and then dispatches it to both the
* [responder] and the [parent].
*/
private suspend fun dispatchRequest(
request: Pair<Rect, Job>,
layoutCoordinates: LayoutCoordinates
) {
newestDispatchedRequest = request
val localRect = request.first
val parentRect = responder.calculateRectForParent(localRect)
coroutineScope {
// For the item to be visible, if needs to be in the viewport of all its
// ancestors.
// Note: For now we run both of these concurrently, but in the future we could
// make this configurable. (The child relocation could be executed before the
// parent, or parent before the child).
launch {
// Bring the requested Child into this parent's view.
responder.bringChildIntoView(localRect)
}
// TODO I think this needs to be in launch, since if the parent is cancelled (this
// throws a CE) due to animation interruption, the child should continue animating.
parent.bringChildIntoView(parentRect, layoutCoordinates)
}
// Don't try to null out newestDispatchedRequest here, bringChildIntoView will take care of
// that.
}
}
/**
* Translates [rect], specified in [sourceCoordinates], into this [LayoutCoordinates].
*/
private fun LayoutCoordinates.localRectOf(
sourceCoordinates: LayoutCoordinates,
rect: Rect
): Rect {
// Translate the supplied layout coordinates into the coordinate system of this parent.
val localRect = localBoundingBoxOf(sourceCoordinates, clipBounds = false)
// Translate the rect to this parent's local coordinates.
return rect.translate(localRect.topLeft)
}
/**
* Returns true if [other] is fully contained inside this [Rect], using inclusive bound checks.
*/
private fun Rect.completelyOverlaps(other: Rect): Boolean {
return left <= other.left &&
top <= other.top &&
right >= other.right &&
bottom >= other.bottom
}