Save canvas then restore, why is that?

Question

I often see the following code

canvas.save().
canvas translate or rotate
some drawing
canvas.restore

I don't understand why we save and then restore. What's the point of undoing what we just did! I am sure I am missing something here Thanks

Answer 1

I understand this question is a bit dated, but for anyone still looking for an answer I can ELI5;

Imagine the canvas is a piece of paper, and you're tasked with drawing a picture of a robot right side up, at the bottom, and another robot upside down, slightly moved to the right, and about 40% smaller at the top. Something like this;

How would you start? What's easier to do first?

You would probably draw the bigger robot at the bottom first since it's right-side up and it's a lot easier to draw in the direction that feels more natural. So you've got the first one done, now how do you approach the second upside down robot?

• You could attempt to draw it as is, but that would be a bit difficult since you're upside down.

or

• You could rotate your paper 180°, move your starting point a bit, and start drawing at a smaller scale, and after you're all done you'd just rotate the paper back.

This is what canvas.save() and canvas.restore() do, they allow you to modify your canvas in any way that makes it easier for you to draw what you need. You don't need to use these methods, but they sure do simplify a lot of the process. The above would look something like

drawRobot()
canvas.save()
canvas.rotate(180)
canvas.translate(100, 0)
canvas.scale(40,40)
drawRobot()
canvas.restore()

If we look at the restore() documentation it says

is used to remove all modifications to the matrix/clip state since the last save call

and to see what those modifications are we take a look at save() it says

translate, scale, rotate, skew, concat or clipRect, clipPath

Well look at that, we did in fact use translate rotate and scale but we also did call drawRobot() so wouldn't calling restore erase our drawing? No, because it doesn't affect the drawing, only the modifications. So when we call restore it will return our canvas to the state that it was in before we started the second drawing.