How to add red wavy line below text in Android's TextView

Question

I'm trying to add red wavy line below errors in texts, such as:

Unfortunately I can't find a proper *Span class to wrap the error text with.

How should I implement such a feature in Android?

Answer 1

I've solved the problem by implementing a custom Span:

Add error_underline.png to your resources:

<-- tiny 6x3 pixels here

Then use this class to create spans:

static class ErrorSpan extends DynamicDrawableSpan {

    private BitmapDrawable mRedWavy;
    private int mWidth;
    private int mBmpHeight;

    ErrorSpan(Resources resources) {
        super(DynamicDrawableSpan.ALIGN_BASELINE);
        mRedWavy = new BitmapDrawable(resources, BitmapFactory.decodeResource(resources, R.drawable.error_underline));
        mBmpHeight = mRedWavy.getIntrinsicHeight();
        mRedWavy.setTileModeX(TileMode.REPEAT);
    }

    @Override
    public Drawable getDrawable() {
        return mRedWavy;
    }

    @Override
    public int getSize(Paint paint, CharSequence text,
                         int start, int end,
                         Paint.FontMetricsInt fm) {
        mWidth = (int) paint.measureText(text, start, end);
        return mWidth;
    }


    @Override
    public void draw(Canvas canvas, CharSequence text,
                     int start, int end, float x, 
                     int top, int y, int bottom, Paint paint) {

        mRedWavy.setBounds(0, 0, mWidth, mBmpHeight);
        canvas.save();
        canvas.translate(x, bottom-mBmpHeight);
        mRedWavy.draw(canvas);
        canvas.restore();
        canvas.drawText(text.subSequence(start, end).toString(), x, y, paint);
    }
}

Answer 2

Here is another solution without touching resources:

public class WavyUnderlineSpan implements LineBackgroundSpan {

private int color;
private int lineWidth;
private int waveSize;

public WavyUnderlineSpan() {
    this(Color.RED, 1, 3);
}

public WavyUnderlineSpan(int color, int lineWidth, int waveSize) {
    this.color = color;
    this.lineWidth = lineWidth;
    this.waveSize = waveSize;
}

@Override
public void drawBackground(Canvas canvas, Paint paint, int left, int right, int top, int baseline, int bottom,
                           CharSequence text, int start, int end, int lnum) {
    Paint p = new Paint(paint);
    p.setColor(color);
    p.setStrokeWidth(lineWidth);

    int width = (int) paint.measureText(text, start, end);
    int doubleWaveSize = waveSize * 2;
    for (int i = left; i < left + width; i += doubleWaveSize) {
        canvas.drawLine(i, bottom, i + waveSize, bottom - waveSize, p);
        canvas.drawLine(i + waveSize, bottom - waveSize, i + doubleWaveSize, bottom, p);
    }
}

}

Answer 3

@bpronin's code did not work for me - the span was too small on high resolution screen and it spanned the entire text, not only the error spanned.

But following his idea I updated my answer to remove the need for added resource:

public class ErrorSpan extends DynamicDrawableSpan {

    private int width;
    int lineWidth;
    int waveSize;
    int color;


    public ErrorSpan(Resources resources) {
        this(resources, Color.RED, 1, 3);
    }

    public ErrorSpan(Resources resources, int color, int lineWidth, int waveSize) {
        super(DynamicDrawableSpan.ALIGN_BASELINE);
        // Get the screen's density scale
        final float scale = resources.getDisplayMetrics().density;
        // Convert the dps to pixels, based on density scale
        this.lineWidth = (int) (lineWidth * scale + 0.5f);
        this.waveSize = (int) (waveSize * scale + 0.5f);
        this.color = color;
    }

    @Override
    public Drawable getDrawable() {
        return null;
    }

    @Override
    public int getSize(Paint paint, CharSequence text,
                         int start, int end,
                         Paint.FontMetricsInt fm) {
        width = (int) paint.measureText(text, start, end);
        return width;
    }


    @Override
    public void draw(Canvas canvas, CharSequence text,
                     int start, int end, float x, 
                     int top, int y, int bottom, Paint paint) {

        Paint p = new Paint(paint);
        p.setColor(color);
        p.setStrokeWidth(lineWidth);

        int doubleWaveSize = waveSize * 2;
        for (int i = (int)x; i < x + width; i += doubleWaveSize) {
            canvas.drawLine(i, bottom, i + waveSize, bottom - waveSize, p);
            canvas.drawLine(i + waveSize, bottom - waveSize, i + doubleWaveSize, bottom, p);
        }
        canvas.drawText(text.subSequence(start, end).toString(), x, y, paint);
    }
}