When it's necessary to execute invalidate() on a View?

Question

My answer to this question was just accepted but I started to wonder when exactly one needs to invalidate() a View and when it is not necessary?

After a bit of think

Answer 1

I had this problem when I wanted to draw a textPaint! My code was

    canvas.drawPaint(textPaintNumber)
    canvas.drawText("MyText", 30F, 63F, textPaintNumber)

I cleared the first lint and the problem was solved

    canvas.drawText("MyText", 30F, 63F, textPaintNumber)