Problem with CATransaction
Problem with CATransaction
- Subject: Problem with CATransaction
- From: Gabriel Zachmann <email@hidden>
- Date: Sat, 06 Oct 2012 00:15:39 +0200
Dear all,
I have a problem with CA with my screensaver when it runs under Mountain Lion.
My screensaver displays images, a new one every minute. I have used Core Animation to achieve a blending effect
when the image gets replaced by a new one.
Everything else seems to work fine, but the blending effect that I used to get is gone.
I have already googled and it seems to me that others have experienced the same problem, but I am unsure as to how to go about to fix the problem.
Here is my code in a bit more detail.
I define a number of layers in my class:
@interface ArtSaverView : ScreenSaverView <NSTextViewDelegate>
{
[...]
CALayer * mainLayer_;
CALayer * currentLayer_;
CALayer * textLayer_; // holds the image name / error message at the bottom
}
Then I set up some initial layers:
- (id) initWithFrame: (NSRect) frameRect isPreview: (BOOL) preview
{
[...]
// create some layers
// make the view layer-hosting and become the delegate for the root layer
mainLayer_ = [CALayer layer];
mainLayer_.zPosition = 0.0;
mainLayer_.delegate = self;
[mainLayer_ setNeedsDisplay];
// causes the layer content to be drawn in -drawRect:
[self setLayer: mainLayer_];
self.wantsLayer = YES;
[self setNeedsDisplay: YES];
// insert a dummy layer as a placeholder for the image layer later
currentLayer_ = [CALayer layer];
[mainLayer_ addSublayer: currentLayer_];
textLayer_ = [CALayer layer];
textLayer_.zPosition = 0.0;
textLayer_.bounds = NSRectToCGRect( drawRect_ ); // Warning: if bounds==nil or the empty rectangle, then the delegate's drawLayer: won't get called!
TextLayerDelegate * textlayer_delegate = [[TextLayerDelegate alloc] initWith: self];
CFRetain( textlayer_delegate ); // otherwise, it seems, the textlayer_delegate gets collected away by the GC ..
textLayer_.delegate = textlayer_delegate;
textLayer_.position = CGPointMake( 0,0 );
textLayer_.anchorPoint = CGPointMake( 0, 0 );
textLayer_.opacity = 1.0;
[textLayer_ setNeedsDisplay];
[mainLayer_ addSublayer: textLayer_ ];
[...]
}
The idea is that, during runtime, the 'currentLayer' gets replaced by a different layer from time to time,
and letting Core Animation do the fade-out/fade-in effect of the two layers, resp.
I am doing that in the screen savers "workproc":
- (void) animateOneFrame
{
[...]
// create new layer that centeres the new image
CALayer * newlayer;
newlayer = [self makeImageLayer: imageRef withOrientation: img_orientation];
newlayer.contentsGravity = kCAGravityResizeAspectFill;
// swap the old and the new layer
[CATransaction begin];
// causes cross-dissolve animation
[CATransaction setAnimationDuration: fading_duration];
[mainLayer_ replaceSublayer: currentLayer_ with: newlayer];
currentLayer_ = newlayer;
[CATransaction commit];
}
A new layer, containing an image, is created with this sub-routine:
- (CALayer *) makeImageLayer: (CGImageRef) img withOrientation: (int) orientation
{
// create new layer containing the image
CALayer * imgLayer = [CALayer layer];
imgLayer.contents = (id) img;
imgLayer.contentsGravity = kCAGravityResizeAspect; // kCAGravityCenter;
imgLayer.delegate = nil;
imgLayer.opacity = 1.0;
imgLayer.position = CGPointMake( drawRect_.size.width/2.0, drawRect_.size.height/2.0 );
imgLayer.anchorPoint = CGPointMake( 0.5, 0.5 );
imgLayer.zPosition = -1.0; // makes the image appear behind the text
imgLayer.bounds = NSRectToCGRect( drawRect_ );
CGImageRelease( img );
return imgLayer;
}
Unfortunately, I am not very deep into Core Animations.
So, my question is: is there an easy way to make the blending effect reappear, without rewriting too much of my code?
In my current code, the blending is done by just a few lines of code, and I would be happy if the new solution
could be as concise ;-)
Thanks a lot in advance.
Best regards,
Gabriel.
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