在Objective-C的2点之间画线

Question

I want to draw line between 2 points. But UIGraphicsGetCurrentContext() is return null

ViewController.m

self.drawLine.firstPoint = self.btnPointOne.bounds.origin;
self.drawLine.secondPoint = self.btnPointTwo.bounds.origin;
[self.drawLine drawRect:self.drawWatchModeView.bounds];

DrawLine.h

@interface DrawLine : UIView{
    CGContextRef context;
}

@property (assign, nonatomic) CGPoint firstPoint;
@property (assign, nonatomic) CGPoint secondPoint;

DrawLine.m

- (id)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
    }
    return self;
}

- (void)drawRect:(CGRect)rect {
    context = UIGraphicsGetCurrentContext();
    // Drawing code
    CGContextSetStrokeColorWithColor(context, [UIColor blackColor].CGColor);
    //line width
    CGContextSetLineWidth(context, 1.0);
    CGContextMoveToPoint(context, self.firstPoint.x, self.firstPoint.y);
    CGContextAddLineToPoint(context, self.secondPoint.x, self.secondPoint.y);
    // and now draw the Path!
    CGContextStrokePath(context);
}

But it error

CGContextSetStrokeColorWithColor: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextSetLineWidth: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextMoveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddLineToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextSetStrokeColorWithColor: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextSetLineWidth: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextMoveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddLineToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextSetStrokeColorWithColor: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextSetLineWidth: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextMoveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddLineToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextSetStrokeColorWithColor: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextSetLineWidth: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextMoveToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextAddLineToPoint: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.

I try to UIViewControllerBasedStatusBarAppearance set YES it not work for me I try to remove UIViewControllerBasedStatusBarAppearance it not work for me

Answer 1

You can't just call drawRect: whenever you want, so your line:

[self.drawLine drawRect:self.drawWatchModeView.bounds];

needs to change to ask the OS to run a draw cycle. When that happens the drawing context will be setup and everything will be ready. When you run it explicitly that context doesn't exist. So, change that line to:

[self.drawLine setNeedsDisplay];

or

[self.drawLine setNeedsDisplayInRect:self.drawWatchModeView.bounds];