A one-file example of rendering into a UIView with Metal and CADisplayLink

MinimalMetal.m

#import <UIKit/UIKit.h>
#import <QuartzCore/CAMetalLayer.h>
#import <Metal/Metal.h>

@class MetalView;

@protocol MetalViewDelegate
- (void)drawInView:(MetalView *)view;
@end

@interface MetalView : UIView
@property (nonatomic, strong) id<MTLDevice> device;
@property (nonatomic, weak) id<MetalViewDelegate> delegate;
@property (nonatomic, nullable, strong) CADisplayLink *displayLink;
@end

@implementation MetalView

+ (Class)layerClass {
    return [CAMetalLayer class];
}

- (id<MTLDevice>)device {
    return ((CAMetalLayer *)self.layer).device;
}

- (void)setDevice:(id<MTLDevice>)device {
    ((CAMetalLayer *)self.layer).device = device;
}

- (id<CAMetalDrawable>)currentDrawable {
    return [((CAMetalLayer *)self.layer) nextDrawable];
}

- (void)didMoveToWindow {
    if (self.window == nil) {
        [self deactivateDisplayLink];
    } else {
        [self activateDisplayLink];
    }
}

- (void)activateDisplayLink {
    self.displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(displayLinkDidFire:)];
    [self.displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
}

- (void)deactivateDisplayLink {
    [self.displayLink invalidate];
    self.displayLink = nil;
}

- (void)displayLinkDidFire:(CADisplayLink *)displayLink {
    [self drawRect:self.bounds];
}

- (void)drawRect:(CGRect)rect {
    [self.delegate drawInView:self];
}

@end

@interface ViewController : UIViewController <MetalViewDelegate>
@property (nonatomic, strong) UIWindow *window;
@property (nonatomic, strong) id<MTLDevice> device;
@property (nonatomic, strong) id<MTLCommandQueue> commandQueue;
@end

@implementation ViewController

- (instancetype)init {
    if (self = [super init]) {
        _device = MTLCreateSystemDefaultDevice();
        _commandQueue = [_device newCommandQueue];
    }
    return self;
}

- (void)loadView {
    CGRect bounds = UIScreen.mainScreen.bounds;
    MetalView *view = [[MetalView alloc] initWithFrame:bounds];
    view.delegate = self;
    self.view = view;
}

- (void)drawInView:(MetalView *)view {
    id<CAMetalDrawable> drawable = [view currentDrawable];
    if (drawable == nil) {
        return;
    }

    MTLRenderPassDescriptor *passDescriptor = [MTLRenderPassDescriptor new];
    passDescriptor.colorAttachments[0].texture = drawable.texture;
    passDescriptor.colorAttachments[0].clearColor = MTLClearColorMake(0, 0.5, 1, 1);
    passDescriptor.colorAttachments[0].loadAction = MTLLoadActionClear;
    passDescriptor.colorAttachments[0].storeAction = MTLStoreActionStore;

    id<MTLCommandBuffer> commandBuffer = [self.commandQueue commandBuffer];
    id<MTLRenderCommandEncoder> renderCommandEncoder = [commandBuffer renderCommandEncoderWithDescriptor:passDescriptor];
    [renderCommandEncoder endEncoding];
    [commandBuffer presentDrawable:drawable];
    [commandBuffer commit];
}

@end

@interface AppDelegate : UIResponder <UIApplicationDelegate>
@property (nonatomic, strong) UIWindow *window;
@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    ViewController *vc = [[ViewController alloc] init];
    self.window = [[UIWindow alloc] initWithFrame:vc.view.bounds];
    self.window.rootViewController = vc;
    [self.window makeKeyAndVisible];
    return YES;
}

@end

int main(int argc, char * argv[]) {
    return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
}