textview - Android transparent text

Question

I need to display a TextView over a gradient background. The TextView itself should have a plain white background, and the text should be transparent.

However, setting a transparent color (#00000000) to the text doesn't work: it only shows a white rectangle, the background doesn't show up where the text is (the text takes the same color as the TextView background).

How can I display a transparent text with a background color on my TextView?

Answer 1

Update, Jan 30, 2016

I made a small library and written a blog post out of this answer, so you don't need to copy and paste code and I do the maintenance for you. :)

Use the view in xml as:

<it.gilvegliach.android.transparenttexttextview.TransparentTextTextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@drawable/view_bg"
    android:text="Hello World" />

Gradle dependency:

 compile 'it.gilvegliach.android:transparent-text-textview:1.0.3'

---

Original Answer

This is how you can achieve that effect:

1. you render the text over a transparent background on a bitmap
2. you use that bitmap to clip the text shape out of the solid white background

Here is a simple subclass of TextView that does that.

final public class SeeThroughTextView extends TextView
{
    Bitmap mMaskBitmap;
    Canvas mMaskCanvas;
    Paint mPaint;

    Drawable mBackground;
    Bitmap mBackgroundBitmap;
    Canvas mBackgroundCanvas;
    boolean mSetBoundsOnSizeAvailable = false;

    public SeeThroughTextView(Context context)
    {
        super(context);

        mPaint = new Paint();
        mPaint.setXfermode(new PorterDuffXfermode(Mode.DST_OUT));
        super.setTextColor(Color.BLACK);
        super.setBackgroundDrawable(new ColorDrawable(Color.TRANSPARENT));
    }

    @Override
    @Deprecated
    public void setBackgroundDrawable(Drawable bg)
    {
        mBackground = bg;
        int w = bg.getIntrinsicWidth();
        int h = bg.getIntrinsicHeight();

        // Drawable has no dimensions, retrieve View's dimensions
        if (w == -1 || h == -1)
        {
            w = getWidth();
            h = getHeight();
        }

        // Layout has not run
        if (w == 0 || h == 0)
        {
            mSetBoundsOnSizeAvailable = true;
            return;
        }

        mBackground.setBounds(0, 0, w, h);
        invalidate();
    }

    @Override
    public void setBackgroundColor(int color)
    {
        setBackgroundDrawable(new ColorDrawable(color));
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh)
    {
        super.onSizeChanged(w, h, oldw, oldh);
        mBackgroundBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mBackgroundCanvas = new Canvas(mBackgroundBitmap);
        mMaskBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mMaskCanvas = new Canvas(mMaskBitmap);

        if (mSetBoundsOnSizeAvailable)
        {
            mBackground.setBounds(0, 0, w, h);
            mSetBoundsOnSizeAvailable = false;
        }
    }

    @Override
    protected void onDraw(Canvas canvas)
    {
        // Draw background
        mBackground.draw(mBackgroundCanvas);

        // Draw mask
        mMaskCanvas.drawColor(Color.BLACK, PorterDuff.Mode.CLEAR);
        super.onDraw(mMaskCanvas);

        mBackgroundCanvas.drawBitmap(mMaskBitmap, 0.f, 0.f, mPaint);
        canvas.drawBitmap(mBackgroundBitmap, 0.f, 0.f, null);
    }
}

Example screenshot: indigo pattern for activity background, pink solid fill for TextView background.

This works both for solid color backgrounds and general drawables. Anyway, this is only a BASIC implementation, some feature such as tiling are not supported.