Android: How to get the current X offset of Recycl

问题:

I'm using a Scrollview for an infinite "Time Picker Carousel" and found out, that it is not the best approach (last question)

Now, I found the Recycler View but I am unable to get the current scroll offset in X direction of the recyclerView? (Let's say each item is 100px width, and the second item is only visible to 50%, so the scroll-offset is 150px)

• all items have the same width, but therer is some gap before the first, after the last item
• recyclerView.getScrollX() returns 0 (docs say: initial scroll value)
• the LayoutManager has findFirstVisibleItemPosition, but I cannot calculate the X offset with that

UPDATE

I just found a way to keep tracking the X-Position, while updating the value with the onScrolled callback, but I would prefer getting the actual value instead of tracking it all the time!

private int overallXScroll = 0;
//...
mRecyclerView.setOnScrollListener(new RecyclerView.OnScrollListener() {
        @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);

            overallXScroll = overallXScroll + dx;

            Log.i("check","overallXScroll->" + overallXScroll);

        }
    });

回答1:

RecyclerView already have method to get horizontal and vertical scroll offset

mRecyclerView.computeHorizontalScrollOffset()
mRecyclerView.computeVerticalScrollOffset()

This will work for RecyclerViews containing cells of the same height (for vertical scroll offset) and the same width (for horizontal scroll offset)

回答2:

Solution 1: setOnScrollListener

Save your X-Position as class variable and update each change within the onScrollListener. Ensure you don't reset overallXScroll (f.e. onScreenRotationChange)

 private int overallXScroll = 0;
 //...
 mRecyclerView.setOnScrollListener(new RecyclerView.OnScrollListener() {
    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        super.onScrolled(recyclerView, dx, dy);

        overallXScroll = overallXScroll + dx;
        Log.i("check","overall X  = " + overallXScroll);

    }
 });

Solution 2: Calculate current position.

In my case, I have a horizontal List which is filled with time values (0:00, 0:30, 1:00, 1:30 ... 23:00, 23:30). I'm calculating the time from the time-item, which is in the middle of the screen (calculation point). That's why I need the exact X-Scroll Position of my RecycleView

• Each time item has the same width of 60dp (fyi: 60dp = 30min, 2dp = 1min)

• First item (Header item) has an extra padding, to set 0min to the center

  private int mScreenWidth = 0;
  private int mHeaderItemWidth = 0;
  private int mCellWidth = 0;
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
  
    //init recycle views
    //...
    LinearLayoutManager mLLM = (LinearLayoutManager) getLayoutManager();
    DisplayMetrics displaymetrics = new DisplayMetrics();
    this.getWindowManager().getDefaultDisplay().getMetrics(displaymetrics);
    this.mScreenWidth = displaymetrics.widthPixels;
  
    //calculate value on current device
    mCellWidth = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 60, getResources()
          .getDisplayMetrics());
  
    //get offset of list to the right (gap to the left of the screen from the left side of first item)
    final int mOffset = (this.mScreenWidth / 2) - (mCellWidth / 2);
  
    //HeaderItem width (blue rectangle in graphic)
    mHeaderItemWidth = mOffset + mCellWidth;
  
    mRecyclerView.setOnScrollListener(new RecyclerView.OnScrollListener() {
  
      @Override
      public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
          super.onScrolled(recyclerView, dx, dy);
  
          //get first visible item
          View firstVisibleItem = mLLM.findViewByPosition(mLLM.findFirstVisibleItemPosition());
  
          int leftScrollXCalculated = 0;
          if (firstItemPosition == 0){
                 //if first item, get width of headerview (getLeft() < 0, that's why I Use Math.abs())
              leftScrollXCalculated = Math.abs(firstVisibleItem.getLeft());
          }
          else{
  
                 //X-Position = Gap to the right + Number of cells * width - cell offset of current first visible item
                 //(mHeaderItemWidth includes already width of one cell, that's why I have to subtract it again)
              leftScrollXCalculated = (mHeaderItemWidth - mCellWidth) + firstItemPosition  * mCellWidth + firstVisibleItem.getLeft();
          }
  
          Log.i("asdf","calculated X to left = " + leftScrollXCalculated);
  
      }
  });
  

  }

回答3:

Thanks to @umar-qureshi for the right lead! It appears that you can determine the scroll percentage with Offset, Extent, and Range such that

percentage = 100 * offset / (range - extent)

For example (to be put in an OnScrollListener):

int offset = recyclerView.computeVerticalScrollOffset();
int extent = recyclerView.computeVerticalScrollExtent();
int range = recyclerView.computeVerticalScrollRange();

int percentage = (int)(100.0 * offset / (float)(range - extent));

Log.i("RecyclerView, "scroll percentage: "+ percentage + "%");

回答4:

public class CustomRecyclerView extends RecyclerView {

    public CustomRecyclerView(Context context) {
        super(context);
    }

    public CustomRecyclerView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public CustomRecyclerView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    public int getHorizontalOffset() {
        return super.computeHorizontalScrollOffset();
    }
}

Than where you need to get the offset:

recyclerView.getHorizontalOffset()

回答5:

If you need to know the current page and its offset in comparison to the width of RecyclerView you may wanna try something like this:

recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
    @Override
    public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
        super.onScrollStateChanged(recyclerView, newState);
    }

    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        super.onScrolled(recyclerView, dx, dy);

        final int scrollOffset = recyclerView.computeHorizontalScrollOffset();
        final int width = recyclerView.getWidth();
        final int currentPage = scrollOffset / width;
        final float pageOffset = (float) (scrollOffset % width) / width;

        // the following lines just the example of how you can use it
        if (currentPage == 0) {
            leftIndicator.setAlpha(pageOffset);
        }

        if (adapter.getItemCount() <= 1) {
            rightIndicator.setAlpha(pageOffset);
        } else if (currentPage == adapter.getItemCount() - 2) {
            rightIndicator.setAlpha(1f - pageOffset);
        }
    }
});

If currentPage has index 0 then leftIndicator (arrow) will be transparent, and so will be and rightIndicator if there's only one page (in the example, the adapter always has at least one item to show so there's no check for the empty value).

This way you will basically have almost the same functionality as if you have been using the callback pageScrolled from ViewPager.OnPageChangeListener.

回答6:

by override RecyclerView onScrolled(int dx,int dy) , you can get the scroll offset. but when you add or remove the item, the value may be wrong.

public class TempRecyclerView extends RecyclerView {
   private int offsetX = 0;
   private int offsetY = 0;
   public TempRecyclerView(Context context) {
       super(context);
   }

   public TempRecyclerView(Context context, @Nullable AttributeSet attrs) {
      super(context, attrs);
   }

   public TempRecyclerView(Context context, @Nullable AttributeSet attrs,int defStyle){
      super(context, attrs, defStyle);
   }
/**
 * Called when the scroll position of this RecyclerView changes. Subclasses 
   should usethis method to respond to scrolling within the adapter's data 
   set instead of an explicit listener.
 * <p>This method will always be invoked before listeners. If a subclass 
   needs to perform any additional upkeep or bookkeeping after scrolling but 
   before listeners run, this is a good place to do so.</p>
 */
   @Override
   public void onScrolled(int dx, int dy) {
  //  super.onScrolled(dx, dy);
      offsetX+=dx;
      offsetY+=dy;
     }
 }