xref: /core/core/src/main/java/androidx/core/view/NestedScrollingParent.java

/*
 * Copyright 2018 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.core.view;

import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;

import androidx.core.view.ViewCompat.ScrollAxis;

import org.jspecify.annotations.NonNull;

/**
 * This interface should be implemented by {@link android.view.ViewGroup ViewGroup} subclasses
 * that wish to support scrolling operations delegated by a nested child view.
 *
 * <p>Classes implementing this interface should create a final instance of a
 * {@link NestedScrollingParentHelper} as a field and delegate any View or ViewGroup methods
 * to the <code>NestedScrollingParentHelper</code> methods of the same signature.</p>
 *
 * <p>Views invoking nested scrolling functionality should always do so from the relevant
 * {@link ViewCompat}, {@link ViewGroupCompat} or {@link ViewParentCompat} compatibility
 * shim static methods. This ensures interoperability with nested scrolling views on Android
 * 5.0 Lollipop and newer.</p>
 */
public interface NestedScrollingParent {
    /**
     * React to a descendant view initiating a nestable scroll operation, claiming the
     * nested scroll operation if appropriate.
     *
     * <p>This method will be called in response to a descendant view invoking
     * {@link ViewCompat#startNestedScroll(View, int)}. Each parent up the view hierarchy will be
     * given an opportunity to respond and claim the nested scrolling operation by returning
     * <code>true</code>.</p>
     *
     * <p>This method may be overridden by ViewParent implementations to indicate when the view
     * is willing to support a nested scrolling operation that is about to begin. If it returns
     * true, this ViewParent will become the target view's nested scrolling parent for the duration
     * of the scroll operation in progress. When the nested scroll is finished this ViewParent
     * will receive a call to {@link #onStopNestedScroll(View)}.
     * </p>
     *
     * @param child Direct child of this ViewParent containing target
     * @param target View that initiated the nested scroll
     * @param axes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL},
     *                         {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both
     * @return true if this ViewParent accepts the nested scroll operation
     */
    boolean onStartNestedScroll(@NonNull View child, @NonNull View target, @ScrollAxis int axes);

    /**
     * React to the successful claiming of a nested scroll operation.
     *
     * <p>This method will be called after
     * {@link #onStartNestedScroll(View, View, int) onStartNestedScroll} returns true. It offers
     * an opportunity for the view and its superclasses to perform initial configuration
     * for the nested scroll. Implementations of this method should always call their superclass's
     * implementation of this method if one is present.</p>
     *
     * @param child Direct child of this ViewParent containing target
     * @param target View that initiated the nested scroll
     * @param axes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL},
     *                         {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both
     * @see #onStartNestedScroll(View, View, int)
     * @see #onStopNestedScroll(View)
     */
    void onNestedScrollAccepted(@NonNull View child, @NonNull View target, @ScrollAxis int axes);

    /**
     * React to a nested scroll operation ending.
     *
     * <p>Perform cleanup after a nested scrolling operation.
     * This method will be called when a nested scroll stops, for example when a nested touch
     * scroll ends with a {@link MotionEvent#ACTION_UP} or {@link MotionEvent#ACTION_CANCEL} event.
     * Implementations of this method should always call their superclass's implementation of this
     * method if one is present.</p>
     *
     * @param target View that initiated the nested scroll
     */
    void onStopNestedScroll(@NonNull View target);

    /**
     * React to a nested scroll in progress.
     *
     * <p>This method will be called when the ViewParent's current nested scrolling child view
     * dispatches a nested scroll event. To receive calls to this method the ViewParent must have
     * previously returned <code>true</code> for a call to
     * {@link #onStartNestedScroll(View, View, int)}.</p>
     *
     * <p>Both the consumed and unconsumed portions of the scroll distance are reported to the
     * ViewParent. An implementation may choose to use the consumed portion to match or chase scroll
     * position of multiple child elements, for example. The unconsumed portion may be used to
     * allow continuous dragging of multiple scrolling or draggable elements, such as scrolling
     * a list within a vertical drawer where the drawer begins dragging once the edge of inner
     * scrolling content is reached.</p>
     *
     * @param target The descendent view controlling the nested scroll
     * @param dxConsumed Horizontal scroll distance in pixels already consumed by target
     * @param dyConsumed Vertical scroll distance in pixels already consumed by target
     * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by target
     * @param dyUnconsumed Vertical scroll distance in pixels not consumed by target
     */
    void onNestedScroll(@NonNull View target, int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed);

    /**
     * React to a nested scroll in progress before the target view consumes a portion of the scroll.
     *
     * <p>When working with nested scrolling often the parent view may want an opportunity
     * to consume the scroll before the nested scrolling child does. An example of this is a
     * drawer that contains a scrollable list. The user will want to be able to scroll the list
     * fully into view before the list itself begins scrolling.</p>
     *
     * <p><code>onNestedPreScroll</code> is called when a nested scrolling child invokes
     * {@link View#dispatchNestedPreScroll(int, int, int[], int[])}. The implementation should
     * report how any pixels of the scroll reported by dx, dy were consumed in the
     * <code>consumed</code> array. Index 0 corresponds to dx and index 1 corresponds to dy.
     * This parameter will never be null. Initial values for consumed[0] and consumed[1]
     * will always be 0.</p>
     *
     * @param target View that initiated the nested scroll
     * @param dx Horizontal scroll distance in pixels
     * @param dy Vertical scroll distance in pixels
     * @param consumed Output. The horizontal and vertical scroll distance consumed by this parent
     */
    void onNestedPreScroll(@NonNull View target, int dx, int dy, int @NonNull [] consumed);

    /**
     * Request a fling from a nested scroll.
     *
     * <p>This method signifies that a nested scrolling child has detected suitable conditions
     * for a fling. Generally this means that a touch scroll has ended with a
     * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds
     * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity}
     * along a scrollable axis.</p>
     *
     * <p>If a nested scrolling child view would normally fling but it is at the edge of
     * its own content, it can use this method to delegate the fling to its nested scrolling
     * parent instead. The parent may optionally consume the fling or observe a child fling.</p>
     *
     * @param target View that initiated the nested scroll
     * @param velocityX Horizontal velocity in pixels per second
     * @param velocityY Vertical velocity in pixels per second
     * @param consumed true if the child consumed the fling, false otherwise
     * @return true if this parent consumed or otherwise reacted to the fling
     */
    boolean onNestedFling(@NonNull View target, float velocityX, float velocityY, boolean consumed);

    /**
     * React to a nested fling before the target view consumes it.
     *
     * <p>This method siginfies that a nested scrolling child has detected a fling with the given
     * velocity along each axis. Generally this means that a touch scroll has ended with a
     * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds
     * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity}
     * along a scrollable axis.</p>
     *
     * <p>If a nested scrolling parent is consuming motion as part of a
     * {@link #onNestedPreScroll(View, int, int, int[]) pre-scroll}, it may be appropriate for
     * it to also consume the pre-fling to complete that same motion. By returning
     * <code>true</code> from this method, the parent indicates that the child should not
     * fling its own internal content as well.</p>
     *
     * @param target View that initiated the nested scroll
     * @param velocityX Horizontal velocity in pixels per second
     * @param velocityY Vertical velocity in pixels per second
     * @return true if this parent consumed the fling ahead of the target view
     */
    boolean onNestedPreFling(@NonNull View target, float velocityX, float velocityY);

    /**
     * Return the current axes of nested scrolling for this NestedScrollingParent.
     *
     * <p>A NestedScrollingParent returning something other than {@link ViewCompat#SCROLL_AXIS_NONE}
     * is currently acting as a nested scrolling parent for one or more descendant views in
     * the hierarchy.</p>
     *
     * @return Flags indicating the current axes of nested scrolling
     * @see ViewCompat#SCROLL_AXIS_HORIZONTAL
     * @see ViewCompat#SCROLL_AXIS_VERTICAL
     * @see ViewCompat#SCROLL_AXIS_NONE
     */
    @ScrollAxis
    int getNestedScrollAxes();
}