drawRect not getting called for layer backed drawing with NSView

Question

I am trying to migrate my Opengl application to Metal, and I am using CAMetalLayer for the Metal implementation. I have set the following properties on my inherited NSView class object:

self.layerContentsRedrawPolicy = NSViewLayerContentsRedrawDuringViewResize;
[self setWantsLayer:YES];

I have checked other answers on SO, like NSView subclass - drawRect: not called

So, I tried assigning the delegate as suggested in answers above, something like this

[self.layer setDelegate:self];

and I get the following compilation error:

Cannot initialize a parameter of type 'id<CALayerDelegate> _Nullable' with an lvalue of type 'MyNSView *'

Moreover, I also see the documentation for setting up layer objects at https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/CoreAnimation_guide/SettingUpLayerObjects/SettingUpLayerObjects.html, which states this:

For layer-backed views with custom content, you should continue to override the view’s methods to do your drawing. A layer-backed view automatically makes itself the delegate of its layer and implements the needed delegate methods, and you should not change that configuration. Instead, you should implement your view’s drawRect: method to draw your content.

So, according to this, I don't even have to set the delegate in my case and I am already implementing the drawRect function in MyNSView. Still it doesn't get called.

Is there anything more that needs to be done?

Thanks!

Edit: So, I fixed the compilation error by making my layer follow the protocol CALayerDelegate, but still I don't get any calls to drawLayer or drawRect.

makeBackingLayer

- (CALayer*)makeBackingLayer
{
    id <MTLDevice> device = MTLCreateSystemDefaultDevice();
    CAMetalLayer *backingLayer   = [CAMetalLayer layer];
    backingLayer.opaque          = YES;
    backingLayer.device          = device;
    backingLayer.pixelFormat     = MTLPixelFormatBGRA8Unorm;
    backingLayer.framebufferOnly = YES;

//    [backingLayer setDelegate:self]; //Tried setting the delegate 
                                       //here as well

    return backingLayer;
}

init function

self.layerContentsRedrawPolicy = NSViewLayerContentsRedrawDuringViewResize;
[self setWantsLayer:YES];
[self.layer setDelegate:self];

Draw functions

- (void)displayLayer:(CALayer *)layer
{
    [self drawRect:self.bounds];
}

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx {
    [self drawRect:self.bounds];
}

-(void) drawRect: (NSRect) dirtyRect
{
    [super drawRect:dirtyRect];
}

I have implemented all the draw functions to check if any of them get hit, but I don't receive a call on any of these functions.

For drawing content to the NSView, I draw to offscreen resources and the get the drawable from CALayer of NSView and call presentDrawable before committing on the commandBuffer.

Why aren't you using a subclass of MTKView? How did you arrange to use CAMetalLayer for the layer? Did you override -makeBackingLayer? If so, show that method and any place else you try to configure the layer. — Ken Thomases
@KenThomases It is a limitation to use NSView only for using Metal Rendering. And Yes, I am using makeBackingLayer to generate the layer. I'll update the question with relevant code snippets. — user007
What other methods of NSView do you override? What other properties do you set on it? For example, do you override -wantsUpdateLayer to return true? — Ken Thomases
@KenThomases I have tried overriding -wantsUpdateLayer to return true, and then override UpdateLayer. But, apparently that doesn't get called. I also tried changing the layerContentsRedrawPolicy to NSViewLayerContentsRedrawOnSetNeedsDisplay and calling [nsView setNeedsDisplay:YES], after I call commit on the command buffer, but that doesn't seem to trigger drawing either. — user007

user007 user007 · Accepted Answer · 2018-11-28T15:34:45

Since there wasn't much here that I could debug to find where the bug was introduced, so I started playing with the order in which the instructions were executed. And as it turns out, the order in which you call :

[self setWantsLayer:YES];
self.layerContentsRedrawPolicy = NSViewLayerContentsRedrawDuringViewResize;

was causing the issue. On swapping these two statements, I started receiving call to my displayLayer function. I still couldn't figure out why drawRect doesn't get called for layer backed NSView.

Posting this just in case this swapping fixes the issue for someone else as well.

drawRect not getting called for layer backed drawing with NSView

1 Answers