I am animating an object along a path on the iPhone. The code works great using CAKeyframeAnimation. For debugging purposes, I would like to draw the line on the screen. I can't seem to do that using the current CGContextRef. Everywhere I look it says you need to subclass the UIView then override the drawRect method to draw on the screen. Is there anyway around this? If not, how do I pass data to the drawRect method so it knows what do draw?
EDIT:
Ok. I ended up subclassing UIView and implementing the drawing in the drawRect method. I can get it to draw to the screen by creating another method in the subclassed view (called drawPath) that sets an instance variable then calls setNeedsDisplay. That in turn fires the drawRect method which uses the instance variable to draw to the screen. Is this the best practice? What happens if I want to draw 20+ paths. I shouldn't have to create properties for all of these.
In your drawRect method of UIView put some code like this
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBStrokeColor(context, 1.0, 0,0 , 0.0);
CGContextSetLineWidth(context, 3.f);
CGContextBeginPath(context);
CGContextMoveToPoint(context,x1,y1);
CGContextAddLineToPoint(context, x2 , y2);
CGContextStrokePath(context);
If you want to use Core Graphics drawing use CALayer. If you do not want to subclass it, delegate drawing to the view.
- (id)initWithFrame:(CGRect)frame
{
self = [super initWithFrame:frame];
if (self)
{
CALayer* myLayer = [CALayer new];
myLayer.delegate = self;
self.layer = myLayer;
}
}
- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx
{
// draw your path here
}
Do not forget to call [self.layer setNeedsDisplay]; when you need to redraw it.
Related
The task is, to draw paths at runtime on custom maps which im using in a Scrollview, and then i will have to draw paths at runtime whenever the location coordinates (lat, long) updates. The problem what im trying to solve here is that i have made a class 'graphics' which is a subclass of UIView, in which i code the drawing in the 'drawrect:' method. So when im adding the graphics as subview of the scrollview over image, the line draws, but i need to keep drawing the line as though it were paths. I need to draw the lines at runtime, need to keep updating the points(x,y) of 'CGContextStrokeLineSegments' method. The code:
ViewController:
- (void)loadView {
[[UIApplication sharedApplication] setStatusBarHidden:YES withAnimation:UIStatusBarAnimationNone];
CGRect fullScreenRect=[[UIScreen mainScreen] applicationFrame];
scrollView=[[UIScrollView alloc] initWithFrame:fullScreenRect];
graph = [[graphics alloc] initWithFrame:fullScreenRect];
scrollView.contentSize=CGSizeMake(320,480);
UIImageView *tempImageView2 = [[UIImageView alloc] initWithImage:[UIImage imageNamed:#"fortuneCenter.png"]];
self.view=scrollView;
[scrollView addSubview:tempImageView2];
scrollView.userInteractionEnabled = YES;
scrollView.bounces = NO;
[scrollView addSubview:graph];
}
Graphics.m:
- (id)initWithFrame:(CGRect)frame
{
self = [super initWithFrame:frame];
if (self) {
// Initialization code
self.backgroundColor = [UIColor clearColor];
}
return self;
}
- (void)drawRect:(CGRect)rect
{
CGContextRef context = UIGraphicsGetCurrentContext();
CGPoint point [2] = { CGPointMake(160, 100), CGPointMake(160,300)};
CGContextSetRGBStrokeColor(context, 255, 0, 255, 1);
CGContextStrokeLineSegments(context, point, 2);
}
So how can i draw the lines at runtime. Im just simulating right now, so im not using the realtime data (coordinates). Just want to simulate by using dummy data (coordinates of x,y). Lets say have a button, whenever i press it it updates the coordinates so path extends.
The easiest way would be to add an instance variable representing the points to the UIView subclass.
Then, every time the path changes, update the ivar appropriately and call -setNeedsDisplay or setNeedsDisplayInRect on the custom UIView (or even on its superview). The runtime will then redraw the new path.
You just need to make CGPoint point[] dynamically resizable, from the looks of it.
You can use malloc, a std::vector, or even NSMutableData to store the points you add. Then you pass that array to CGContextStrokeLineSegments.
If 2 points is all you will need, move CGPoint point[2] to an ivar so you may store the positions, then (as Rich noted) invalidate rects appropriately when these values (or the array) are changed.
This subject comes up every now and then, so I created a longer blog post on the general concepts involved with one potential solution, creating and using your own graphics context, here: http://www.musingpaw.com/2012/04/drawing-in-ios-apps.html
Hi i am making a sample app in which i wanto create a square for which i used the following code
- (void)viewDidLoad {
[super viewDidLoad];
[self drawRect:CGRectMake(0, 0, 300, 200)];
[[self view] setNeedsDisplay];
}
- (void) drawRect:(CGRect)rect
{
NSLog(#"drawRect");
CGFloat centerx = rect.size.width/2;
CGFloat centery = rect.size.height/2;
CGFloat half = 100/2;
CGRect theRect = CGRectMake(-half, -half, 100, 100);
// Grab the drawing context
CGContextRef context = UIGraphicsGetCurrentContext();
// like Processing pushMatrix
CGContextSaveGState(context);
CGContextTranslateCTM(context, centerx, centery);
// Uncomment to see the rotated square
//CGContextRotateCTM(context, rotation);
// Set red stroke
CGContextSetRGBStrokeColor(context, 1.0, 0.0, 0.0, 1.0);
{
CGContextSetRGBFillColor(context, 0.0, 1.0, 0.0, 1.0);
}
// Draw a rect with a red stroke
CGContextFillRect(context, theRect);
CGContextStrokeRect(context, theRect);
// like Processing popMatrix
CGContextRestoreGState(context);
[[self view] setNeedsDisplay];
}
But nothing is drawn on screen , dont know wheres the issue is .When i debug it the CGContextRef context was always 0x0 , i dont know why its 0x0 always am i missing something in my code.
It looks like you're trying to draw in a subclass of UIViewController. You need to subclass UIView to override the drawRect: method, which is then called automatically with a valid graphics context in place. You almost never call this method yourself.
To quote the Apple docs:
"To draw to the screen in an iOS application, you set up a UIView object and implement its drawRect: method to perform drawing. The view’s drawRect: method is called when the view is visible onscreen and its contents need updating. Before calling your custom drawRect: method, the view object automatically configures its drawing environment so that your code can start drawing immediately. As part of this configuration, the UIView object creates a graphics context (a CGContextRef opaque type) for the current drawing environment. You obtain this graphics context in your drawRect: method by calling the UIKit function UIGraphicsGetCurrentContext."
So essentailly, your code is on track, you just need to get it in the right place. It needs to be in the View object.
I'v got this piece of code :
CGColorSpaceRef rgb = CGColorSpaceCreateDeviceRGB();
CGFloat colors[] = {
1.0f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f, 0.0f, 1.0f,
};
CGGradientRef gradientRef = CGGradientCreateWithColorComponents(rgb, colors, NULL, sizeof(colors) / (sizeof(colors[0]) * 4));
CGColorSpaceRelease(rgb);
CGContextRef context = UIGraphicsGetCurrentContext();
CGRect rect = theCell.backgroundView.bounds;
CGPoint start = CGPointMake(rect.origin.x, 0);
CGPoint end = CGPointMake(rect.origin.x, rect.size.height/2);
CGContextDrawLinearGradient(context, gradientRef, start, end, kCGGradientDrawsBeforeStartLocation | kCGGradientDrawsAfterEndLocation);
And I wonder how I can make it draw into a designated view and a clipping rect passed in parameters to my function. Tip: I don't mind about drawRect, I'm not subclassing anything.
Tip 2: I don't want to insert any layers that I won't be able to remove later.
Tip 3: This piece of code does not draw anything that my eyes could see..... :-( Missing a graphic port ?
Tip 4: I'd like to erase the draw simply changing the background color, and it's done...
CGContextRef context = UIGraphicsGetCurrentContext(); will get the graphics context for the current view, so if you call this in the drawRect: method of a UIView it will draw.
I don't understand what you mean by:
I don't mind about drawRect, I'm not
subclassing anything
but if you want to do custom drawing you must either override the drawRect: method or use layers. To use layers you would call CGContextRef context = CGLayerGetContext(theLayer); instead of CGContextRef context = UIGraphicsGetCurrentContext();.
Ok, so I looked at the documentation and it says that you can get a CGContextRef by calling UIGraphicsGetCurrentContext() from the drawRectMethod. Here's what it says: In an iOS application, you set up a UIView object to draw to and implement the drawRect: method to perform drawing. Before calling your custom drawRect: method, the view object automatically configures its drawing environment so that your code can start drawing immediately. As part of this configuration, the UIView object creates a graphics context (a CGContextRef opaque type) for the current drawing environment. You obtain this graphics context by calling the UIKit function UIGraphicsGetCurrentContext. You save and restore graphics contexts using the functions UIGraphicsPushContext and UIGraphicsPopContext.
You can create custom graphics context objects in situations where you want to draw somewhere other than your view. For example, you may want to capture a series of drawing commands and use them to create an image or a PDF file. To create the context, you use the CGBitmapContextCreate or CGPDFContextCreate function. After you have the context, you can pass it to the drawing functions needed to create your content.
When creating custom contexts, the coordinate system for those contexts is different from the native coordinate system used by iOS. Instead of the origin being in the upper-left corner of the drawing surface, it is in the lower-left corner and the axes point up and to the right. The coordinates you specify in your drawing commands must take this into consideration or the resulting image or PDF file may appear wrong when rendered. See “Creating a Bitmap Graphics Context” and “Creating a PDF Graphics Context” for details on using CGBitmapContextCreate and CGPDFContextCreate.
Might I recommend you look into the CAGradientLayer, and add it as a sublayer of your view? Lots simpler, and it will be hardware accelerated which matters for table cells.
Example stolen partly from here:
http://tumbljack.com/post/188089679/gpu-accelerated-awesomeness-with-cagradientlayer
#import <QuartzCore/QuartzCore.h> // Also import this framework
......
CAGradientLayer grad = [CAGradientLayer layer];
UIColor *colorOne = [UIColor colorWithHRed:1.0f Green:1.0f Blue:1.0f alpha:1.0f];
UIColor *colorTwo = [UIColor colorWithHRed:0.0f Green:0.0f Blue:0.0f alpha:1.0f];
NSArray *colors = [NSArray arrayWithObjects:(id)colorOne.CGColor, colorTwo.CGColor, nil];
grad.colors = colors;
CGRect rect = theCell.backgroundView.bounds;
rect.size.height = rect.size.height / 2;
grad.frame = rect;
[self.layer addsublayer:grad]
You may have to play with the colors a bit, not sure if you had the gradient tilted or not...
I'm relatively new to Objective-C + Quartz and am running into what is probably a very simple issue. I have a custom UIView sub class which I am using to draw simple rectangles via Quartz. However I am trying to hook up an NSTimer so that it draws a new object every second, the below code will draw the first rectangle but will never draw again. The function is being called (the NSLog is run) but nothing is draw.
Code:
- (void)drawRect:(CGRect)rect {
context = UIGraphicsGetCurrentContext();
[self step:self];
[NSTimer scheduledTimerWithTimeInterval:(NSTimeInterval)(2) target:self selector:#selector(step:) userInfo:nil repeats:TRUE];
}
- (void) step:(id) sender {
static double trans = 0.5f;
CGContextSetRGBFillColor(context, 1, 0, 0, trans);
CGContextAddRect(context, CGRectMake(10, 10, 10, 10));
CGContextFillPath(context);
NSLog(#"Trans: %f", trans);
trans += 0.01;
}
context is in my interface file as:
CGContextRef context;
Any help will be greatly appreciated!
Your example won't work. The reason is that drawRect: is called for you when the view requires drawing, and it can't draw on its own.
Instead, try using your timer from outside of drawRect: (viewDidLoad comes to mind), and each time add an object to draw to a list, and call [view setNeedsDisplay] to request it to be redrawn. There are other techniques if you need stricter control, but it's a good idea to master the basics of application flow first.
I'm trying to implement some very simple line drawing animation for my iPhone app. I would like to draw a rectangle on my view after a delay. I'm using performSelector to run my drawing method.
-(void) drawRowAndColumn: (id) rowAndColumn
{
int rc = [rowAndColumn intValue];
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context, 2.0);
CGContextSetStrokeColorWithColor(context, currentColor.CGColor);
CGRect rect = CGRectMake(rc * 100, 100, 100, 100);
CGContextAddRect(context, rect);
CGContextDrawPath(context, kCGPathFillStroke);
}
Which is then invoked via:
int col = 10;
[self performSelector:#selector(drawRowAndColumn:)
withObject:[NSNumber numberWithInt:col]
afterDelay:0.2];
But it seems that when the drawRowAndColumn: message is actually sent, it no longer has access to a valid CGContextRef, as I get errors such as:
<Error>: CGContextAddRect: invalid context
If I replace the performSelector with a direct call to drawRowAndColumn, it works fine. So my first idea was to also pass in the CGContextRef via the performSelector, but I can't seem to figure out how to pass multiple arguments at the same time (that's another good question.)
What's wrong with above code?
You can't just draw at any time like that. You need to implement the drawRect: method of UIView and put your drawing code in there.
To get drawRect: to fire you need to let Cocoa know that the view needs to be drawn. For that you can call setNeedsDisplay, or setNeedsDisplayInRect:.
Directly translating your attempt this way you'd call setNeedsDisplay using performSelector:withObject:afterDelay - but that's probably not a good way to do animation.
It depends what you're really trying to do - but you could consider, for example, putting your drawing code in drawRect as I suggested, but start the view hidden. You could then call setHidden:NO using performSelector to make it appear after a delay - or you could smoothly animate it in by starting not hidden, but with an alpha of 0, then change alpha to 1 within a UIView animation block (there's lots about this in the docs).