android/support/v4/view/NestedScrollingParent2.java

/*
 * Copyright (C) 2017 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 android.support.v4.view;

import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.view.ViewCompat.NestedScrollType;
import android.support.v4.view.ViewCompat.ScrollAxis;
import android.view.MotionEvent;
import android.view.View;

/**
 * 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 all versions
 * of Android.</p>
 */
public interface NestedScrollingParent2 extends 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, int)}.
     * </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
     * @param type the type of input which cause this scroll event
     * @return true if this ViewParent accepts the nested scroll operation
     */
    boolean onStartNestedScroll(@NonNull View child, @NonNull View target, @ScrollAxis int axes,
            @NestedScrollType int type);

    /**
     * React to the successful claiming of a nested scroll operation.
     *
     * <p>This method will be called after
     * {@link #onStartNestedScroll(View, View, int, 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
     * @param type the type of input which cause this scroll event
     * @see #onStartNestedScroll(View, View, int, int)
     * @see #onStopNestedScroll(View, int)
     */
    void onNestedScrollAccepted(@NonNull View child, @NonNull View target, @ScrollAxis int axes,
            @NestedScrollType int type);

    /**
     * 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
     * @param type the type of input which cause this scroll event
     */
    void onStopNestedScroll(@NonNull View target, @NestedScrollType int type);

    /**
     * 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, 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
     * @param type the type of input which cause this scroll event
     */
    void onNestedScroll(@NonNull View target, int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed, @NestedScrollType int type);

    /**
     * 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
     * @param type the type of input which cause this scroll event
     */
    void onNestedPreScroll(@NonNull View target, int dx, int dy, @Nullable int[] consumed,
            @NestedScrollType int type);

}