NestedScrollingParent 实现复杂交互效果

一时失言乱红尘 2022-07-18 00:06 119阅读 0赞

最终效果：

![img][]

## 1. 接口 ##

`NestedScrollingParent` 用来配置子视图（ `NestedScrollingChild` ）嵌套滚动。

`NestedScrollingParent` 简称 `NP`  
`NestedScrollingChild` 简称 `NC`

1.  NC 产生一个 touch 事件，调用 `startNestedScroll`，表示开始分享出去 touch 事件， NP 的 `onStartNestedScroll` 判断是否需要跟 NC 配合，返回 true 表示接受
2.  NC 调用 `dispatchNestedPreScroll` ，通知 NP 将要进行一个 Touch 事件，如移动 5个像素，NP 的 `onNestedPreScroll` 中收到通知，知道 NC 要准备移动5个像素，这时候 NP 正好也需要滑动 2 个像素，然后在 `onNestedPreScroll` 的参数 `consumed[]` 数组对应的方向（下标为0的x轴，下标为1的y轴）赋值 2，表示消费 2 个像素
3.  NC 拿到 `consumed[]` 数组，知道 NP 消费了 2 个像素，剩下了 3 个像素，然后自己根据需要再移动 3 个像素的距离，如果这时候 NC 由于某个原因只移动了 2 个像素，那么剩下的 1 个像素距离会调用 `dispatchNestedScroll` 给 NP，NP 在 `onNestedScroll` 处理剩下的未消费的 1 像素。最后，NC 调用 `stopNestedScroll` ，最后 NP 调用 `onStopNestedScroll`结束。
4.  同 NestedScroll 类似的还有一套 NestedFling 操作，整体流程类似，NC 快速滑动产生一个 fling 事件，处理流程 NC.`dispatchNestedPreFling` \-> NP.`onNestedPreFling` \-> NC.`dispatchNestedFling` \-> NP.`onNestedFling`

为什么 NC 和 NP 能相互配合滚动？ 主要的源码在 [NestedScrollingChildHelper][] 和 [NestedScrollingParentHelper][]

## 效果实现 ##

自定 ViewGroup ，实现 `NestedScrollingParent` 接口，一个 `NestedScrollingParentHelper` 成员变量作为辅助

布局

<?xml version="1.0" encoding="utf-8"?>
    <xyz.hanks.nestedwebview.nestedscroll.NScrollView
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    
        <android.support.v7.widget.RecyclerView
            android:id="@+id/recyclerView0"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="#20f0"
            android:scrollbarSize="3dp"
            android:scrollbars="vertical"/>
    
        <android.support.v4.widget.NestedScrollView
            android:layout_width="match_parent"
            android:layout_height="wrap_content">
    
            <LinearLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:background="#e1e1e1"
                android:orientation="vertical">
    
                <TextView
                    android:layout_width="match_parent"
                    android:layout_height="100dp"
                    android:text="cdsadadlick"/>
    
                <Button
                    android:layout_width="match_parent"
                    android:layout_height="100dp"
                    android:text="asddasda"/>
    
                <TextView
                    android:layout_width="match_parent"
                    android:layout_height="100dp"
                    android:text="asdadarewe"/>
            </LinearLayout>
        </android.support.v4.widget.NestedScrollView>
    
        <android.support.v7.widget.RecyclerView
            android:id="@+id/recyclerView"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="#2f00"
            android:scrollbarStyle="insideInset"
            android:scrollbars="vertical"/>
    </xyz.hanks.nestedwebview.nestedscroll.NScrollView>

基础的 onMeasure ，onLayout

![onLayout][]

/**
     * NestedScrollView
     * Created by hanks on 16/8/22.
     */
    public class NScrollView extends ViewGroup implements NestedScrollingParent {
        
    
        private static final String TAG = "NScrollView";
        private int maxDistance = 0;
        private int totalHeight = 0;
        private NestedScrollingParentHelper mParentHelper;
        private List<View> nestedScrollingChildList = new ArrayList<>();
        private ScrollerCompat scroller;
    
        public NScrollView(Context context) {
            this(context, null);
        }
    
        public NScrollView(Context context, AttributeSet attrs) {
            this(context, attrs, 0);
        }
    
        public NScrollView(Context context, AttributeSet attrs, int defStyleAttr) {
            super(context, attrs, defStyleAttr);
            setOverScrollMode(View.OVER_SCROLL_NEVER);
            mParentHelper = new NestedScrollingParentHelper(this);
            scroller = ScrollerCompat.create(context);
        }
    
        @Override
        protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
            for (int i = 0; i < getChildCount(); i++) {
                View child = getChildAt(i);
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
    
        @Override
        protected void onLayout(boolean changed, int l, int t, int r, int b) {
    
            int parentHeight = getMeasuredHeight();
            int top = t;
            int lastChildHeight = 0;
            for (int i = 0; i < getChildCount(); i++) {
                View child =  getChildAt(i);
                LayoutParams layoutParams = child.getLayoutParams();
                if (layoutParams.height == LayoutParams.MATCH_PARENT) {
                    layoutParams.height = parentHeight;
                } else {
                    int childMeasuredHeight = child.getMeasuredHeight();
                    layoutParams.height = childMeasuredHeight;
                }
                log(child + "," + layoutParams.height);
                child.setLayoutParams(layoutParams);
                child.layout(l, top, r, top + layoutParams.height);
                top += layoutParams.height;
                lastChildHeight = layoutParams.height;
            }
            maxDistance = top - lastChildHeight;
            totalHeight = top;
            //viewGroup.layout(l, t, r, top);
        }
    
        @Override
        protected void onFinishInflate() {
            super.onFinishInflate();
    
            nestedScrollingChildList.clear();
            for (int i = 0; i < getChildCount(); i++) {
                View child =  getChildAt(i);
                nestedScrollingChildList.add(child);
            }
        }
    
        private void consumeEvent(int dx, int dy, int[] consumed) {
            scrollBy(dx, dy);
            consumed[0] = 0;
            consumed[1] = dy;
            log("parent consumed pre : " + consumed[1]);
        }
    
        public int getCurrentWrapline(View target) {
            int line = 0;
            for (View view : nestedScrollingChildList) {
                if (view == target) {
                    return line;
                }
                line += view.getHeight();
            }
            return line;
        }
    
        @Override
        public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes) {
            return (nestedScrollAxes & SCROLL_AXIS_VERTICAL) != 0;
        }
    
        @Override
        public void onNestedScrollAccepted(View child, View target, int axes) {
            mParentHelper.onNestedScrollAccepted(child, target, axes);
            return;
        }
    
        @Override
        public void onNestedPreScroll(View target, int dx, int dy, int[] consumed) {
            logi("======== onStartNestedScroll =======");
            int scrollY = getScrollY();
            int line = getCurrentWrapline(target);
            boolean targetScrollDown = ScrollUtils.canChildScrollDown(target);
            boolean targetScrollUp = ScrollUtils.canChildScrollUp(target);
            log("onNestedPreScroll: target = [" + target.getClass().getName() + "], dx = [" + dx + "], dy = [" + dy + "], consumed = [" + consumed + "]");
            log("scrollY:" + scrollY + ",lien:" + line + ",scrollDown:" + targetScrollDown + "scrollUp:" + targetScrollUp + ",maxDistance" + maxDistance);
    
            if (scrollY == line
                    && ((dy > 0 && targetScrollDown) || (dy < 0 && targetScrollUp))) {
                return;
            }
    
            if (scrollY + dy < 0 || scrollY + dy > maxDistance) {
                return;
            }
            consumeEvent(0, dy, consumed);
        }
    
        @Override
        public void onNestedScroll(View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
            int scrollY = getScrollY();
            if (scrollY + dyUnconsumed < 0 || scrollY + dyUnconsumed > maxDistance) {
                return;
            }
            scrollBy(dxUnconsumed, dyUnconsumed);
            log("onNestedScroll: target = [" + target.getClass().getName() + "], dxConsumed = [" + dxConsumed + "], dyConsumed = [" + dyConsumed + "], dxUnconsumed = [" + dxUnconsumed + "], dyUnconsumed = [" + dyUnconsumed + "]");
        }
    
        @Override
        public boolean onNestedPreFling(View target, float velocityX, float velocityY) {
            int scrollY = getScrollY();
            log("onNestedPreFling: target = [" + target.getClass().getName() + "], velocityX = [" + velocityX + "], velocityY = [" + velocityY + "]" + ",scrollY:" + scrollY);
            int line = getCurrentWrapline(target);
            if (scrollY != line) {
                fling((int) velocityY, scrollY);
                return true;
            }
            return false;
        }
    
        private void fling(int velocityY, int scrollY) {
            if (getChildCount() > 0) {
                int height = getHeight() - getPaddingTop() - getPaddingBottom();
                int bottom = totalHeight;
                scroller.fling(0, scrollY, 0, velocityY, 0, 0, 0,
                        Math.max(0, bottom - height));
                invalidate();
            }
        }
    
        @Override
        public void computeScroll() {
            if (scroller.computeScrollOffset()) {
                int y = scroller.getCurrY();
                scrollTo(0, y);
                invalidate();
            }
        }
    
        @Override
        public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed) {
            log("onNestedFling: target = [" + target.getClass().getName() + "], velocityX = [" + velocityX + "], velocityY = [" + velocityY + "], consumed = [" + consumed + "]");
            return false;
        }
    
        @Override
        public void onStopNestedScroll(View target) {
            logi("======== onStopNestedScroll =======");
            mParentHelper.onStopNestedScroll(target);
        }
    
        public void log(String s) {
            Log.e(TAG, s);
        }
    
        public void logi(String s) {
            Log.i(TAG, s);
        }
    
        public void logw(String s) {
            Log.w(TAG, s);
        }
    }

> 文章来自： [http://hanks.xyz][http_hanks.xyz]

[img]: /images/20220717/636affa8bb2a46748bc528cdd91c0915.png
[NestedScrollingChildHelper]: http://androidxref.com/7.0.0_r1/xref/frameworks/support/v4/java/android/support/v4/view/NestedScrollingChildHelper.java
[NestedScrollingParentHelper]: http://androidxref.com/7.0.0_r1/xref/frameworks/support/v4/java/android/support/v4/view/NestedScrollingParentHelper.java
[onLayout]: /images/20220717/e4cbc1caec974bca86b22a8f9cecf1b6.png
[http_hanks.xyz]: http://hanks.xyz