setNeedsDisplay no refresh the screen

Question

Iim displaying a huge amount of images each time I call drawRect with setNeedsDisplay. The problem is that cause lags cause each time I call dracRect, I redraw every images (thousands).

Is there a way to not redraw all the images. I mean, keep all images and draw the one I want to draw?

thanks,

What kind of view are you using to display them? If it is a table view for example you could just refresh cell by cell. — talnicolas, Apr 12 '13 at 14:14

Sergey Kalinichenko · Answer 1 · 2013-04-12T14:21:16.047

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One way to avoid redrawing everything is to call setNeedsDisplayInRect: instead of setNeedsDisplay:, and passing only a rectangle containing the image that needs to be redrawn. Of course this means that your drawRect can no longer ignore the rectangle passed in: go through the images, and check if an image overlaps with the rectangle being redrawn. If there is no overlap, skip the image and save some CPU time on a redraw.

Note that this is a very "manual" way of maintaining a view. Consider using some of the components that iOS provides for you, such as UICollectionView, which lets you display lots of stuff on the screen with very little code.

edited Apr 12 '13 at 14:21

answered Apr 12 '13 at 14:16

Sergey Kalinichenko

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@DavidGoncalves Could you be more specific about the result that you get vs. the result that you want then? – Sergey Kalinichenko Apr 12 '13 at 14:20
Yeah, I did It, but the result is not the result I want. I want this result : http://imageshack.us/photo/my-images/826/afterf.png/. my cards have transparency. but I have this result : http://imageshack.us/photo/my-images/40/beforevy.png/. Why? – David Goncalves Apr 12 '13 at 14:24
@DavidGoncalves There is little similarity between the two pictures: it looks like everything has been wiped out before drawing the second one. You need to check that the call of `setNeedsDisplayInRect:` passes the correct rectangle (i.e. one containing the card that has moved) and that your `drawRect:` checks the incoming rectangle before drawing the required images. – Sergey Kalinichenko Apr 12 '13 at 14:29
Yes setNeedsDisplay: passes the correct Rect. The screen you see around the cards in the second screenshot is normally transparent. In the first screen, the transparency is ok. The card that I draw is good, the transparency, to, but the cards drawn before, all have no more transparency. I don't know why. – David Goncalves Apr 12 '13 at 14:37

score 0 · Answer 2 · answered Apr 12 '13 at 14:14

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You will have to split your content into multiple views or layers (CALayer). Then only redraw the view or layer whose content actually needs to change.

answered Apr 12 '13 at 14:14

Mike Weller

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score 0 · Answer 3 · answered Apr 12 '13 at 14:17

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Split the view into different views (or layers), that will enable the system to cache their contents and redraw faster.

I don't know your use case but with hundreds of images, I would consider using OpenGL.

answered Apr 12 '13 at 14:17

Sulthan

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setNeedsDisplay no refresh the screen

3 Answers3