onScrollChange() is not being called for every pixel scrolled on ScrollView

Question

I want to add new child views after every 50 pixels of scrolling but the statement Log.d("scroll",scrollY+":"+oldScrollY); is missing some values i.e. scrollY = 149 and scrollY = 151 are there in Log but scrollY = 150 is missing. So the code doesn't run for scrollY = 150 and new child views is not added.

Although it works as it should for most of the values of scrollY but for few values of scrollY, the event is not being triggered.

How can i resolve this issue? Or is there any other way to achieve this.

public class MainActivity extends AppCompatActivity implements ScrollViewListener {

    RelativeLayout parent;
    TextView height;
    ObservableScrollView scrollView;
    int position=1;
    View view1,view2;

    @RequiresApi(api = Build.VERSION_CODES.M)
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        parent = (RelativeLayout) findViewById(R.id.parent);
        scrollView = (ObservableScrollView) findViewById(R.id.scroll_view);

        LayoutInflater inflater = LayoutInflater.from(this);
        view1 = inflater.inflate(R.layout.education,null, false);
        view1.setId(R.id.parent+position);

        RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,
                                                                             ViewGroup.LayoutParams.WRAP_CONTENT);
        params.addRule(RelativeLayout.LEFT_OF,R.id.divider);
        params.topMargin = 100;
        params.rightMargin = 3;
        parent.addView(view1, params);
        position++;

        final Animation leftAnimation = new TranslateAnimation(view1.getX()-50,view1.getX(),view1.getY()+100,view1.getY());
        leftAnimation.setDuration(1000);
        view1.setAnimation(leftAnimation);

        scrollView.setOnScrollChangeListener(new View.OnScrollChangeListener() {
            @Override
            public void onScrollChange(View view, int scrollX, int scrollY, int oldScrollX, int oldScrollY) {
                Log.d("scroll",scrollY+":"+oldScrollY);
                if((scrollY-oldScrollY)>0 && scrollY%50 == 0)
                {
                    Log.d("scroll","abcghfg");

                    if(position%2==0)
                    {
                        RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,
                                ViewGroup.LayoutParams.WRAP_CONTENT);
                        params.addRule(RelativeLayout.RIGHT_OF,R.id.divider);
                       // params.topMargin = (position-1)*(view1.getLayoutParams().height)+position*100;
                        params.topMargin = 30;
                        params.leftMargin = 3;
                        params.addRule(RelativeLayout.BELOW,parent.getChildAt(position-1).getId());
                        Log.d("scroll","abc");
                        view2 = LayoutInflater.from(MainActivity.this).inflate(R.layout.education,null,false);
                        view2.setId(R.id.parent+position);

                        Animation rightAimation = new TranslateAnimation(view2.getX()+50,view2.getX(),view2.getY()+100,view2.getY());
                        rightAimation.setDuration(1000);
                        view2.startAnimation(rightAimation);

                        parent.addView(view2, params);
                    }
                    else
                    {
                        RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,
                                ViewGroup.LayoutParams.WRAP_CONTENT);
                        params.addRule(RelativeLayout.LEFT_OF,R.id.divider);
                        //params.topMargin = (position-1)*(view1.getLayoutParams().height)+position*100;
                        params.topMargin = 30;
                        params.rightMargin = 3;
                        params.addRule(RelativeLayout.BELOW,parent.getChildAt(position-1).getId());
                        Log.d("scroll","def");
                        view2 = LayoutInflater.from(MainActivity.this).inflate(R.layout.education,null,false);
                        view2.setId(R.id.parent+position);

                        Animation animation = new TranslateAnimation(view2.getX()-50,view2.getX(),view2.getY()+100,view2.getY());
                        animation.setDuration(1000);
                        view2.startAnimation(animation);

                        parent.addView(view2, params);
                    }
                    position++;
                }
            }
        });
    }
}

Thanks in advance.

score 0 · Accepted Answer · answered Nov 06 '17 at 12:45

0

Android doesn't render every pixel in a scroll, because it would be bad for performance. If you scroll really slow, you will see that your callback will be called for every pixel, but if you scroll fast, it will be called just when the render triggers.

In your case, you need to check if it rendered 50 or more pixels since last time you rendered the last view.

answered Nov 06 '17 at 12:45

jonathanrz

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you're right. But then again it will be problem if scrolling is done from 49 pixels to 201 pixels directly then those views which were supposed to be added at 50, 100, 150 and 200 pixels will be missed. Isn't there any other solution to draw such UI in android which adds new views smoothly with animation upon scrolling(does not depend upon speed of scrolling) just like we see in websites – brok Nov 06 '17 at 16:22
i am facing one more problem. After fast scrolling down when i go back to top then sometimes all the child views(previously added) are removed. – brok Nov 06 '17 at 16:24
There are any reason why you are not using a RecyclerView? – jonathanrz Nov 06 '17 at 16:27
How could i use RecyclerView? I am actually adding these views left and right of a vertical line alternatively one below other – brok Nov 06 '17 at 16:45
You could have a row that have the left and right view and define with one will be visible in the onBindView of the adapter. – jonathanrz Nov 06 '17 at 16:49
main issue is i want to make child views visible upon scrolling with animation and when setting listener on ScrollView same issue will be there no matter if i use RecyclerView or what i am using currently. – brok Nov 06 '17 at 17:28

onScrollChange() is not being called for every pixel scrolled on ScrollView

1 Answers1