Android NestedScrollView悬浮固定顶部

前言

项目中有页面涉及到多个元素组 需要NestedScrollView包裹来上下滑动
接到需求 一些标题在滑动到顶部时需要置顶

我之前做过关于Android Behavior之ViewPager+Fragment+RecyclerView实现吸顶效果

大概就是这种效果 只不过这次是随意的组件 比如TextView或布局组件RelativeLayout等
废话不多说来看下实现效果

动态效果图

关于标题title勘测信息这个View组件
在向下滑动时 滑动到屏幕的顶部 需要悬浮顶部 实现这种效果

代码实现

写一个自定义View继承NestedScrollView 需要实现构造
设置标记Tag STICKY_TAG = "sticky"

使用说明 凡是被NestedScrollView的子组件 只要在xml中标记sticky
就可以实现悬浮

public class StickyScrollView extends NestedScrollView {


    /**
     * 标记应粘贴并具有恒定图形的视图。例如TextViews、ImageViews等
     */
    public static final String STICKY_TAG = "sticky";

    /**
     * 应粘贴并具有非恒定图形的视图的标志。例如按钮、进度条等
     */
    public static final String FLAG_NONCONSTANT = "-nonconstant";

    /**
     * 具有非完全不透明的视图的标志
     */
    public static final String FLAG_HASTRANSPARANCY = "-hastransparancy";

    /**
     * 在粘贴视图下方向外窥视的阴影的默认高度。
     */
    private static final int DEFAULT_SHADOW_HEIGHT = 10; // dp;

    private ArrayList<View> stickyViews;
    private View currentlyStickingView;
    private float stickyViewTopOffset;
    private int stickyViewLeftOffset;
    private boolean redirectTouchesToStickyView;
    private boolean clippingToPadding;
    private boolean clipToPaddingHasBeenSet;

    private int mShadowHeight;
    private Drawable mShadowDrawable;

    private final Runnable invalidateRunnable = new Runnable() {

        @Override
        public void run() {
            if (currentlyStickingView != null) {
                int l = getLeftForViewRelativeOnlyChild(currentlyStickingView);
                int t = getBottomForViewRelativeOnlyChild(currentlyStickingView);
                int r = getRightForViewRelativeOnlyChild(currentlyStickingView);
                int b = (int) (getScrollY() + (currentlyStickingView.getHeight() + stickyViewTopOffset));
                invalidate(l, t, r, b);
            }
            postDelayed(this, 16);
        }
    };

    public StickyScrollView(Context context) {
        this(context, null);
    }

    public StickyScrollView(Context context, AttributeSet attrs) {
        this(context, attrs, android.R.attr.scrollViewStyle);
    }

    public StickyScrollView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        setup();

        TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.StickyScrollView, defStyle, 0);

        final float density = context.getResources().getDisplayMetrics().density;
        int defaultShadowHeightInPix = (int) (DEFAULT_SHADOW_HEIGHT * density + 0.5f);

        mShadowHeight = a.getDimensionPixelSize(
                R.styleable.StickyScrollView_stuckShadowHeight,
                defaultShadowHeightInPix);

        int shadowDrawableRes = a.getResourceId(
                R.styleable.StickyScrollView_stuckShadowDrawable, -1);

        if (shadowDrawableRes != -1) {
            mShadowDrawable = context.getResources().getDrawable(
                    shadowDrawableRes);
        }

        a.recycle();
    }

    /**
     * 设置阴影可绘制的高度（以像素为单位）。
     *
     * @param height
     */
    public void setShadowHeight(int height) {
        mShadowHeight = height;
    }


    public void setup() {
        stickyViews = new ArrayList<View>();
    }

    private int getLeftForViewRelativeOnlyChild(View v) {
        int left = v.getLeft();
        while (v.getParent() != getChildAt(0)) {
            v = (View) v.getParent();
            left += v.getLeft();
        }
        return left;
    }

    private int getTopForViewRelativeOnlyChild(View v) {
        int top = v.getTop();
        while (v.getParent() != getChildAt(0)) {
            v = (View) v.getParent();
            top += v.getTop();
        }
        return top;
    }

    private int getRightForViewRelativeOnlyChild(View v) {
        int right = v.getRight();
        while (v.getParent() != getChildAt(0)) {
            v = (View) v.getParent();
            right += v.getRight();
        }
        return right;
    }

    private int getBottomForViewRelativeOnlyChild(View v) {
        int bottom = v.getBottom();
        while (v.getParent() != getChildAt(0)) {
            v = (View) v.getParent();
            bottom += v.getBottom();
        }
        return bottom;
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
        if (!clipToPaddingHasBeenSet) {
            clippingToPadding = true;
        }
        notifyHierarchyChanged();
    }

    @Override
    public void setClipToPadding(boolean clipToPadding) {
        super.setClipToPadding(clipToPadding);
        clippingToPadding = clipToPadding;
        clipToPaddingHasBeenSet = true;
    }

    @Override
    public void addView(View child) {
        super.addView(child);
        findStickyViews(child);
    }

    @Override
    public void addView(View child, int index) {
        super.addView(child, index);
        findStickyViews(child);
    }

    @Override
    public void addView(View child, int index, android.view.ViewGroup.LayoutParams params) {
        super.addView(child, index, params);
        findStickyViews(child);
    }

    @Override
    public void addView(View child, int width, int height) {
        super.addView(child, width, height);
        findStickyViews(child);
    }

    @Override
    public void addView(View child, android.view.ViewGroup.LayoutParams params) {
        super.addView(child, params);
        findStickyViews(child);
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
        super.dispatchDraw(canvas);
        if (currentlyStickingView != null) {
            canvas.save();
            canvas.translate(getPaddingLeft() + stickyViewLeftOffset, getScrollY() + stickyViewTopOffset + (clippingToPadding ? getPaddingTop() : 0));

            canvas.clipRect(0, (clippingToPadding ? -stickyViewTopOffset : 0),
                    getWidth() - stickyViewLeftOffset,
                    currentlyStickingView.getHeight() + mShadowHeight + 1);

            if (mShadowDrawable != null) {
                int left = 0;
                int right = currentlyStickingView.getWidth();
                int top = currentlyStickingView.getHeight();
                int bottom = currentlyStickingView.getHeight() + mShadowHeight;
                mShadowDrawable.setBounds(left, top, right, bottom);
                mShadowDrawable.draw(canvas);
            }

            canvas.clipRect(0, (clippingToPadding ? -stickyViewTopOffset : 0), getWidth(), currentlyStickingView.getHeight());
            if (getStringTagForView(currentlyStickingView).contains(FLAG_HASTRANSPARANCY)) {
                showView(currentlyStickingView);
                currentlyStickingView.draw(canvas);
                hideView(currentlyStickingView);
            } else {
                currentlyStickingView.draw(canvas);
            }
            canvas.restore();
        }
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            redirectTouchesToStickyView = true;
        }

        if (redirectTouchesToStickyView) {
            redirectTouchesToStickyView = currentlyStickingView != null;
            if (redirectTouchesToStickyView) {
                redirectTouchesToStickyView =
                        ev.getY() <= (currentlyStickingView.getHeight() + stickyViewTopOffset) &&
                                ev.getX() >= getLeftForViewRelativeOnlyChild(currentlyStickingView) &&
                                ev.getX() <= getRightForViewRelativeOnlyChild(currentlyStickingView);
            }
        } else if (currentlyStickingView == null) {
            redirectTouchesToStickyView = false;
        }
        if (redirectTouchesToStickyView) {
            ev.offsetLocation(0, -1 * ((getScrollY() + stickyViewTopOffset) - getTopForViewRelativeOnlyChild(currentlyStickingView)));
        }
        return super.dispatchTouchEvent(ev);
    }

    private boolean hasNotDoneActionDown = true;

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        if (redirectTouchesToStickyView) {
            ev.offsetLocation(0, ((getScrollY() + stickyViewTopOffset) - getTopForViewRelativeOnlyChild(currentlyStickingView)));
        }

        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            hasNotDoneActionDown = false;
        }

        if (hasNotDoneActionDown) {
            MotionEvent down = MotionEvent.obtain(ev);
            down.setAction(MotionEvent.ACTION_DOWN);
            super.onTouchEvent(down);
            hasNotDoneActionDown = false;
        }

        if (ev.getAction() == MotionEvent.ACTION_UP || ev.getAction() == MotionEvent.ACTION_CANCEL) {
            hasNotDoneActionDown = true;
        }

        return super.onTouchEvent(ev);
    }

    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        super.onScrollChanged(l, t, oldl, oldt);
        doTheStickyThing();
    }

    private void doTheStickyThing() {
        View viewThatShouldStick = null;
        View approachingView = null;
        for (View v : stickyViews) {
            int viewTop = getTopForViewRelativeOnlyChild(v) - getScrollY() + (clippingToPadding ? 0 : getPaddingTop());
            if (viewTop <= 0) {
                if (viewThatShouldStick == null || viewTop > (getTopForViewRelativeOnlyChild(viewThatShouldStick) - getScrollY() + (clippingToPadding ? 0 : getPaddingTop()))) {
                    viewThatShouldStick = v;
                }
            } else {
                if (approachingView == null || viewTop < (getTopForViewRelativeOnlyChild(approachingView) - getScrollY() + (clippingToPadding ? 0 : getPaddingTop()))) {
                    approachingView = v;
                }
            }
        }
        if (viewThatShouldStick != null) {
            stickyViewTopOffset = approachingView == null ? 0 : Math.min(0, getTopForViewRelativeOnlyChild(approachingView) - getScrollY() + (clippingToPadding ? 0 : getPaddingTop()) - viewThatShouldStick.getHeight());
            if (viewThatShouldStick != currentlyStickingView) {
                if (currentlyStickingView != null) {
                    stopStickingCurrentlyStickingView();
                }
                // only compute the left offset when we start sticking.
                stickyViewLeftOffset = getLeftForViewRelativeOnlyChild(viewThatShouldStick);
                startStickingView(viewThatShouldStick);
            }
        } else if (currentlyStickingView != null) {
            stopStickingCurrentlyStickingView();
        }
    }

    private void startStickingView(View viewThatShouldStick) {
        currentlyStickingView = viewThatShouldStick;
        if (getStringTagForView(currentlyStickingView).contains(FLAG_HASTRANSPARANCY)) {
            hideView(currentlyStickingView);
        }
        if (((String) currentlyStickingView.getTag()).contains(FLAG_NONCONSTANT)) {
            post(invalidateRunnable);
        }
    }

    private void stopStickingCurrentlyStickingView() {
        if (getStringTagForView(currentlyStickingView).contains(FLAG_HASTRANSPARANCY)) {
            showView(currentlyStickingView);
        }
        currentlyStickingView = null;
        removeCallbacks(invalidateRunnable);
    }

    /**
     * 通知已在“视图”层次结构中的一个或多个视图中添加或删除粘性属性
     */
    public void notifyStickyAttributeChanged() {
        notifyHierarchyChanged();
    }

    private void notifyHierarchyChanged() {
        if (currentlyStickingView != null) {
            stopStickingCurrentlyStickingView();
        }
        stickyViews.clear();
        findStickyViews(getChildAt(0));
        doTheStickyThing();
        invalidate();
    }

    private void findStickyViews(View v) {
        if (v instanceof ViewGroup) {
            ViewGroup vg = (ViewGroup) v;
            for (int i = 0; i < vg.getChildCount(); i++) {
                String tag = getStringTagForView(vg.getChildAt(i));
                if (tag != null && tag.contains(STICKY_TAG)) {
                    stickyViews.add(vg.getChildAt(i));
                } else if (vg.getChildAt(i) instanceof ViewGroup) {
                    findStickyViews(vg.getChildAt(i));
                }
            }
        } else {
            String tag = (String) v.getTag();
            if (tag != null && tag.contains(STICKY_TAG)) {
                stickyViews.add(v);
            }
        }
    }

    private String getStringTagForView(View v) {
        Object tagObject = v.getTag();
        return String.valueOf(tagObject);
    }

    private void hideView(View v) {
        if (Build.VERSION.SDK_INT >= 11) {
            v.setAlpha(0);
        } else {
            AlphaAnimation anim = new AlphaAnimation(1, 0);
            anim.setDuration(0);
            anim.setFillAfter(true);
            v.startAnimation(anim);
        }
    }

    private void showView(View v) {
        if (Build.VERSION.SDK_INT >= 11) {
            v.setAlpha(1);
        } else {
            AlphaAnimation anim = new AlphaAnimation(0, 1);
            anim.setDuration(0);
            anim.setFillAfter(true);
            v.startAnimation(anim);
        }
    }

}

资源文件

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="StickyScrollView">
        <attr name="stuckShadowHeight" format="dimension" />
        <attr name="stuckShadowDrawable" format="reference" />
    </declare-styleable>
</resources>

布局文件使用

重点关注 该子组件标记了 android:tag="sticky" 所以在滑动时会悬浮顶部
具体参考代码

 private void findStickyViews(View v) {
        if (v instanceof ViewGroup) {
            ViewGroup vg = (ViewGroup) v;
            for (int i = 0; i < vg.getChildCount(); i++) {
                String tag = getStringTagForView(vg.getChildAt(i));
                if (tag != null && tag.contains(STICKY_TAG)) {
                    stickyViews.add(vg.getChildAt(i));
                } else if (vg.getChildAt(i) instanceof ViewGroup) {
                    findStickyViews(vg.getChildAt(i));
                }
            }
        } else {
            String tag = (String) v.getTag();
            if (tag != null && tag.contains(STICKY_TAG)) {
                stickyViews.add(v);
            }
        }
    }

重点关注 该子组件标记了 android:tag="sticky" 所以在滑动时会悬浮顶部
具体参考代码

 <RelativeLayout
                android:id="@+id/rl_survey"
                android:layout_width="match_parent"
                android:layout_height="@dimen/dp_40"
                android:layout_margin="@dimen/dp_5"
                android:background="@drawable/shape_rec_3c79f7_radius_10dp"
                android:focusable="true"
                android:focusableInTouchMode="true"
                android:gravity="center_vertical"
                android:orientation="horizontal"
                android:padding="@dimen/dp_5"
                android:tag="sticky"
                app:layout_constraintRight_toRightOf="parent"
                app:layout_constraintTop_toTopOf="parent">

                <TextView
                    android:id="@+id/tv_tip_survey"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:layout_marginStart="@dimen/dp_10"
                    android:text="勘测信息"
                    android:textColor="@color/white"
                    android:textSize="@dimen/sp_15" />

                <ImageView
                    android:id="@+id/iv_arrow_survey"
                    android:layout_width="@dimen/dp_14"
                    android:layout_height="@dimen/dp_14"
                    android:layout_alignParentEnd="true"
                    android:layout_marginEnd="@dimen/dp_10"
                    android:rotation="180"
                    android:src="@mipmap/ic_arrow_down_white" />

            </RelativeLayout>