How to get percentage of scroll in NestedScrollView any time (smoothly)?

Question

I want to change the alpha of toolbar base on scroll, like below:

At first, the toolbar is transparent and by scrolling to the bottom it will more and more visible and at the end it will be fully opaque (visible).

The structure of my layout is:

<android.support.design.widget.CoordinatorLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">

    <android.support.design.widget.AppBarLayout
    android:id="@+id/app_bar_layout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <android.support.design.widget.CollapsingToolbarLayout
        android:id="@+id/collapsing_toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:contentScrim="?attr/colorPrimary"
        app:layout_scrollFlags="scroll|exitUntilCollapsed">

        <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:layout_collapseMode="parallax">

            ....

        </RelativeLayout>

    </android.support.design.widget.CollapsingToolbarLayout>

    </android.support.design.widget.AppBarLayout>

    <android.support.v4.widget.NestedScrollView
    android:id="@+id/scroll"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:clipToPadding="false"
    app:layout_behavior="@string/appbar_scrolling_view_behavior">

         ....

    </android.support.v4.widget.NestedScrollView>

</android.support.design.widget.CoordinatorLayout>

Content of nested scrollview will be change dynamically from server so I don't know it's height.

I just found 2 ways to detect scrollY:

1. addOnScrollChangedListener:

   scroll.getViewTreeObserver().addOnScrollChangedListener(new ViewTreeObserver.OnScrollChangedListener() {
    @Override
    public void onScrollChanged() {
       // use scroll.getScrollY()     
   }
   });
   

2. setOnScrollChangeListener:

   scroller.setOnScrollChangeListener(new NestedScrollView.OnScrollChangeListener() {
   
   @Override
   public void onScrollChange(NestedScrollView v, int scrollX, int scrollY, int oldScrollX, int oldScrollY) {
   
   }
   });
   

But none of these ways are not smooth. for example if you check the value of scrollY (by using log.d()) you will see something like:

scrollY: 58
scrollY: 117
scrollY: 167
scrollY: 192
scrollY: 195
scrollY: 238
scrollY: 281
scrollY: 338 

There is a large gap between numbers.

My question is: How to get percentage of scroll every moment (smoothly)? Or any other way to change alpha of toolbar base on current position of scroll?

Answer 1

You can add an scrollListener to ScrollView.

The calculation of scrolled percentage of your scrollView:

double percent = ((((float) scrollY) / ((float) (scrollContentHeight - screenHeight + statusBarHeight))));

---

---

• The top edge of the displayed part of your view: scrollY = scrollView.getScrollY()

• The scrollView has one child and scrollContentHeight = scrollView.getChildAt(0).getHeight() is the height of the content of scrollView.

• We need to calculate the max height of top edge in full scrolled state: scrollContentHeight - screenHeight + statusBarHeight

• Finished! percent = scrollY*100 / (scrollContentHeight - screenHeight + statusBarHeight)

• And set the color: view.setBackgroundColor(Color.argb((int) (255.0 * (percent/100)), r, g, b));

---

scrollView.getViewTreeObserver().addOnScrollChangedListener(new ViewTreeObserver.OnScrollChangedListener() {
        @Override
        public void onScrollChanged() {
            int scrollY = scrollView.getScrollY();
            int scrollContentHeight = scrollView.getChildAt(0).getHeight();
            int screenHeight = Utility.getScreenHeight(context);
            int statusBarHeight = Utility.getStatusBarHeight(context);

            int color = getResources().getColor(R.color.colorPrimary);
            int r = (color >> 16) & 0xFF;
            int g = (color >> 8) & 0xFF;
            int b = (color >> 0) & 0xFF;

            double percent = ((((float) scrollY) / ((float) (scrollContentHeight - screenHeight + statusBarHeight))));
            if (percent >= 0 && percent <= 1)
                toolbar.setBackgroundColor(Color.argb((int) (255.0 * percent), r, g, b));
        }
    });

Answer 2

try this

scrollview.getViewTreeObserver().addOnScrollChangedListener(new ViewTreeObserver.OnScrollChangedListener() {
            @Override
            public void onScrollChanged() {
                double scrollViewHeight = scrollview.getChildAt(0).getBottom() - scrollview.getHeight();
                double getScrollY = scrollview.getScrollY();
                double scrollPosition = (getScrollY / scrollViewHeight) * 100d;
                Log.i("scrollview", "scroll Percent Y: " + (int) scrollPosition);
            }
        });

it will show the percentage from 0 to 100%

Answer 3

Try this method:

    nestedScrollView.setOnScrollChangeListener((NestedScrollView.OnScrollChangeListener) (v, scrollX, scrollY, oldScrollX, oldScrollY) -> {
                int verticalScrollableHeight = v.getChildAt(0).getMeasuredHeight() - v.getMeasuredHeight();
                float verticalPercentage = ((float) scrollY) / verticalScrollableHeight;