value of optional type "CGContext?" not unwrapped - swift

Question

I try to draw a line in the view, but my code fail to complier due to optional type error. I am new to swift and objective-c and spend many hours to search answer. The issue does not fix so far. So, Can anyone provide some clue to fix this issue?
Code:
import UIKit
class DrawLines: UIView {
// Only override draw() if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
override func draw(_ rect: CGRect) {
//Drawing code
// context
let context = UIGraphicsGetCurrentContext()
CGContextSetLineWidth(context, 3.0)
CGContextSetStrokeColorWithColor(context, UIColor.purpleColor().cgColor)
//create a path
CGContextMoveToPoint(context,0,0)
CGContextAddLineToPoint(context,250,320)
}
}
Error:

Answer 1

UIGraphicsGetCurrentContext() returns optional, to use it in your example you need to call context!.
The best way to use it it wrap it in if-let:
if let context = UIGraphicsGetCurrentContext() {
// Use context here
}
or even better use guard let:
guard let context = UIGraphicsGetCurrentContext() else { return }
// Use context here

Answer 2

In this case the solution is to use ! when getting the context:
let context = UIGraphicsGetCurrentContext()!
The app will crash when there is no current context which means you have done something very wrong.

Answer 3

Just forced unwrap the context, it is 100% safe but solves only one issue.
From the documentation of UIGraphicsGetCurrentContext:
The current graphics context is nil by default. Prior to calling its drawRect: method, view objects push a valid context onto the stack, making it current.
In Swift 3 (assumed from the draw signature) the graphics syntax changed significantly:
class DrawLines: UIView {
override func draw(_ rect: CGRect) {
let context = UIGraphicsGetCurrentContext()!
context.setLineWidth(3.0)
context.setStrokeColor(UIColor.purple.cgColor)
//create a path
// context.beginPath()
context.move(to: CGPoint())
context.addLine(to: CGPoint(x:250, y:320))
// context.strokePath()
}
}
PS: But to draw the line you should uncomment the beginPath() and strokePath() lines.