Method to resize CALayer frame on window resize? - objective-c

Question

I draw a series of images to various CALayer sublayers, then add those sublayers to a superlayer:
- (void)renderImagesFromArray:(NSArray *)array {
CALayer *superLayer = [CALayer layer];
for (id object in array) {
CALayer* subLayer = [CALayer layer];
// Disregard...
NSURL *path = [NSURL fileURLWithPathComponents:#[NSHomeDirectory(), #"Desktop", object]];
NSImage *image = [[NSImage alloc] initWithContentsOfURL:path];
[self positionImage:image layer:subLayer];
subLayer.contents = image;
subLayer.hidden = YES;
[superLayer addSublayer:subLayer];
}
[self.view setLayer:superLayer];
[self.view setWantsLayer:YES];
// Show top layer
CALayer *top = superLayer.sublayers[0];
top.hidden = NO;
}
I then call [self positionImage: layer:] to stretch the CALayer to it's maximum bounds (essentially using the algorithm for the CSS cover property), and position it in the center of the window:
- (void)positionImage:(NSImage *)image layer:(CALayer *)layer{
float imageWidth = image.size.width;
float imageHeight = image.size.height;
float frameWidth = self.view.frame.size.width;
float frameHeight = self.view.frame.size.height;
float aspectRatioFrame = frameWidth/frameHeight;
float aspectRatioImage = imageWidth/imageHeight;
float computedImageWidth;
float computedImageHeight;
float verticalSpace;
float horizontalSpace;
if (aspectRatioImage <= aspectRatioFrame){
computedImageWidth = frameHeight * aspectRatioImage;
computedImageHeight = frameHeight;
verticalSpace = 0;
horizontalSpace = (frameWidth - computedImageWidth)/2;
} else {
computedImageWidth = frameWidth;
computedImageHeight = frameWidth / aspectRatioImage;
horizontalSpace = 0;
verticalSpace = (frameHeight - computedImageHeight)/2;
}
[CATransaction flush];
[CATransaction begin];
CATransaction.disableActions = YES;
layer.frame = CGRectMake(horizontalSpace, verticalSpace, computedImageWidth, computedImageHeight);
[CATransaction commit];
}
This all works fine, except when the window gets resized. I solved this (in a very ugly way) by subclassing NSView, then implementing the only method that was actually called when the window resized, viewWillDraw::
- (void)viewWillDraw{
[super viewWillDraw];
[self redraw];
}
- (void)redraw{
AppDelegate *appDelegate = (AppDelegate *)[[NSApplication sharedApplication] delegate];
CALayer *superLayer = self.layer;
NSArray *sublayers = superLayer.sublayers;
NSImage *image;
CALayer *current;
for (CALayer *view in sublayers){
if (!view.isHidden){
current = view;
image = view.contents;
}
}
[appDelegate positionImage:image layer:current];
}
So... what's the right way to do this? viewWillDraw: get's called too many times which means I have to do unnecessary and redundant calculations, and I can't use viewWillStartLiveResize: because I need to constantly keep the image in its correct position. What am I overlooking?

Answer 1

Peter Hosey was right; my original method was clunky, and I shouldn't have been overriding setNeedsDisplayInRect:. I first made sure that I was using an auto layout in my app, then implemented the following:
subLayer.layoutManager = [CAConstraintLayoutManager layoutManager];
subLayer.autoresizingMask = kCALayerHeightSizable | kCALayerWidthSizable;
subLayer.contentsGravity = kCAGravityResizeAspect;
Basically, I set the sublayer's autoResizingMask to stretch both horizontally and vertically, and then set contentsGravity to preserve the aspect ratio.
That last variable I found by chance, but it's worth noting that you can only use a few contentsGravity constants if, like in my case, you're setting an NSImage as the layer's contents:
That method creates an image that is suited for use as the contents of a layer and that is supports all of the layer’s gravity modes. By contrast, the NSImage class supports only the kCAGravityResize, kCAGravityResizeAspect, and kCAGravityResizeAspectFill modes.
Always fun when a complicated solution can be simplified to 3 lines of code.