Derive Edit Position from Markdown

Sat Feb 8, 2020 by Bill Farmer. Categories: Hacking .

I had a request to switch to the edit view from the markdown view at the double tap position in my Diary app. The app already has a gesture detector so I just had to add an onDoubleTap function.

To get the tap position in the markdown view it is necessary to compensate for the position of the view on the screen, scrolling, and the display density.

        // onDoubleTap
        @Override
        public boolean onDoubleTap(MotionEvent e)
        {
            if (shown)
            {
                int[] l = new int[2];
                markdownView.getLocationOnScreen(l);

                // Get tap position
                float y = e.getY() - l[1];

                int scrollY = markdownView.getScrollY();
                int contentHeight = markdownView.getContentHeight();
                float density = getResources().getDisplayMetrics().density;

                // Get markdown position
                final float p = (y + scrollY) / (contentHeight * density);

The function to get the view location on screen is a bit arcane in that you must provide a two element array for the X and Y co-ordinates. The position as a proportion of the total length of the displayed markdown is calculated from the corrected Y position, the scroll position and the density.

Switch to the edit view and close any open search.

                // Animation
                animateEdit();

                // Close text search
                if (searchItem.isActionViewExpanded())
                    searchItem.collapseActionView();

It is necessary to scroll after a delay, otherwise it gets ignored. To find the position in the edit view it is necessary to get the total height of the text, calculate the Y position as a proportion, and then use a series of functions to get the text line, the offset in the text to set the selection, and the position of the line to scroll to it.


                // Scroll after delay
                textView.postDelayed(() ->
                {
                    int h = textView.getLayout().getHeight();
                    int v = Math.round(h * p);

                    // Get line
                    int line = textView.getLayout().getLineForVertical(v);
                    int offset = textView.getLayout()
                        .getOffsetForHorizontal(line, 0);
                    textView.setSelection(offset);

                    // get text position
                    int position = textView.getLayout().getLineBaseline(line);

                    // Scroll to it
                    int height = scrollView.getHeight();
                    scrollView.smoothScrollTo(0, position - height / 2);
                }, POSITION_DELAY);

                shown = false;
                return true;
            }
            return false;
        }

This method is not completely accurate because of embedded images, text styles, etc, but good enough to get the cursor and scroll position in about the right place.

Bill Farmer

Derive Edit Position from Markdown

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