How to draw on a UIView with core graphics? - ios

Question

I can't figure out why I can't draw on the UIView layer. Is that possible?
My example code i am using:
#implementation Menu
UIView *window;
UIWindow *mainWindow;
CGSize viewwidth;
viewwidth=[[UIScreen mainScreen] bounds].size;
// viewwidth.width or viewwidth.height
//My UIView
window = [[UIView alloc]initWithFrame:CGRectMake(0, 0, viewwidth.width, viewwidth.height)];
window.backgroundColor = [UIColor whiteColor];
window.layer.opacity = 1.0f;
[window setNeedsDisplay];
[mainWindow addSubview: window];
My method drawRect for drawing:
- (void)drawRect:(CGRect)rect
{
// Drawing code
CGRect rectangle = CGRectMake(0, 0, 320, 100);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBFillColor(context, 1.0, 1.0, 1.0, 0.0); //this is the transparent color
CGContextSetRGBStrokeColor(context, 0.0, 0.0, 0.0, 0.5);
CGContextFillRect(context, rectangle);
CGContextStrokeRect(context, rectangle); //this will draw the border
}
Am I doing something wrong, the problem is i get a empty white screen?

Answer 1

When you create a project and chose to use Storyboards you will set a ViewController to your Main.storyboard in Interface Builder.
There are differences between iOS Apps that use SceneDelegates and those who do not (iOS before 13.0). But your chosen ViewController will be assigned to be used from within your Main Window/Scene when the App will be loaded. In AppDelegate.m (before iOS 13) or SceneDelegate.m (starting iOS 13) you could directly access or even assign this yourself to the self.window.rootViewController property.
Do to inheritance of UIViewController which come with a basic UIView ready to work with under self.view (in this example inside ViewController.m) you can place more Subviews to that property with [self.view addSubview:yourOtherView].
-(void)drawRect:(CGRect)rect is a method that belongs to UIView and will be called when the view is set to update or layout. But it is important to know that this is not the only method where you can place drawing code. UIView:drawRect can be avoided to lower the Energy impact and workload you app is consuming when you don't need repeatedly redrawing the graphics. Also the workload is heavy influenced to be lower when you are able to lower the CGRect size to draw inside.
One word to value naming.. you will have less confusing code when values are named after the content they belong to. So naming a property "window" which is actually a UIView will stress you when your project becomes more complex. So you created a local value with name mainWindow while window is already existing. When doing so you will have to assign your mainWindow to the apps UIWindow yourself to make it available. And the UIView you named window can confuse you while _window or self.window is already existing as part of the AppDelegate or SceneDelegate. And as mentioned above you have a UIView that belongs to the UIViewController you assigned in Storyboard or in code.
Your drawing code is valid. It may be invisible to you because of transparency you choose to go for.
below some code to compare to for your ViewController.
#import "ViewController.h"
#import "MyGraphicView.h"
#interface ViewController ()
#property (nonatomic) MyGraphicView *graphicView;
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view.
_graphicView = [[MyGraphicView alloc] initWithFrame:self.view.frame];
[self.view addSubview:_graphicView];
}
#end
and a MyGraphicView.h
#import <UIKit/UIKit.h>
NS_ASSUME_NONNULL_BEGIN
#interface MyGraphicView : UIView
#end
NS_ASSUME_NONNULL_END
and corresponding MyGraphicView.h
#import "MyGraphicView.h"
#implementation MyGraphicView
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
// This method is called when a view is first displayed or when an event occurs that invalidates a visible part of the view.
- (void)drawRect:(CGRect)rect {
CGRect rectangle = CGRectMake(0, 0, 320, 100);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBFillColor(context, 1.0, 1.0, 1.0, 0.0);
CGContextSetRGBStrokeColor(context, 0.0, 1.0, 0.0, 0.5);
CGContextFillRect(context, rectangle);
CGContextStrokeRect(context, rectangle);
}
#end