When I call setNeedsDisplayInRect on a UIView, and the drawRect: method fires, am I responsible for making sure I'm not rendering stuff that's outside the CGRect I called, or will the framework handle that for me?
Example:
-(void)drawRect:(CGRect)rect
{
//assume this is called through someMethod below
CGContextRef ctx = UIGraphicsGetCurrentContext();
[[UIColor redColor] setFill];
CGContextFillRect(ctx, rect);
[[UIColor blueColor] setFill];
// is this following line a no-op? or should I check to make sure the rect
// I am making is contained within the rect that is passed in?
CGContextFillRect(ctx, CGRectMake(100.0f, 100.0f, 25.0f, 25.0f));
}
-(void)someMethod
{
[self setNeedsDisplayInRect:CGRectMake(50.0f, 50.0f, 25.0f, 25.0f)];
}
To simplify what Barry said: Yes, the framework will handle it for you.
You can safely ignore the rect, anything you draw outside of it will be ignored.
On the other hand, if you draw outside of the rect you are wasting CPU time, so if you can limit your drawing based on the rect, you should.
The framework will clip your drawing. On OS X (AppKit), drawing is clipped to the dirty areas of the NSView (as of 10.3). I'm not sure what the exact clipping algorithm is in UIKit. Of course, you can speed drawing by checking what needs to be drawn and only drawing in the dirty areas of the view, rather than relying on the framework to clip unnecessary drawing.
Related
I have a shadow image that I would like to draw around the outer edge of a grouped UITableView section. This is the image:
I can get the UIBezierPath that represents the rect I want to draw around, but I can't figure out how to repeat the image along the route of the rect. So far it just fills the rect with the image:
UIImage *patternImg = [UIImage imageNamed:#"cellShadow"];
UIColor *fill = [UIColor colorWithPatternImage:patternImg];
[fill setFill];
CGRect aSectRect = [self rectForSection:0];
UIBezierPath *aSectPath = [self createRoundedPath:aSectRect];
[aSectPath fill];
Is this possible? What do I need to do?
Unfortunately, there's no way to have UIBezierPath use an image as a "brush" which is essentially what you'd want. But you can have CoreGraphics draw a shadow for you:
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetShadow(context, CGSizeZero, myShadowRadius);
// Draw your shape here
Now if you draw just one shape it will get a shadow. But if you draw more shapes, each will get its own shadow which might not be what you want. The solution is called a transparency layer, which is not related to CALayers or something but is just some kind of "grouping" in CoreGraphics:
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetShadow(context, CGSizeZero, myShadowRadius);
CGContextBeginTransparencyLayer(context, NULL);
// Draw your shapes here.
CGContextEndTransparencyLayer(context);
Between the CGContextBeginTransparencyLayer and CGContextEndTransparencyLayer calls the shadow is disabled. After the CGContextEndTransparencyLayer call, the shadow is applied to everything that has been drawn between begin and end as if it were just one shape.
Here is my drawRect:
- (void)drawRect:(CGRect)rect
{
CGContextFillEllipseInRect(UIGraphicsGetCurrentContext(), self.bounds);
}
and the instances of this class are added a subviews the next way:
(#define DOTS_SIZE 30)
[self addSubview:[[VertexView alloc] initWithFrame:CGRectMake(anchor.x-DOTS_SIZE/2, anchor.y-DOTS_SIZE/2, DOTS_SIZE, DOTS_SIZE)]];
As far as I understand, I should get and ellipse (circle my case) in the views bounds. But I get it fully filled with rectangle (square).
By the way, I have logged bounds and their size is 30x30, so I should get nice little circles, but I get squares (T_T)
I'll be thankful for any advise!
The problem is that you have to make some setup before drawing the ellipse. For example:
- (void)drawRect:(CGRect)rect
{
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextSetFillColorWithColor(ctx, [UIColor redColor].CGColor); // Or any other color.
CGContextFillEllipseInRect(ctx, self.bounds);
}
And to make background transparent you can check out: Setting A CGContext Transparent Background
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 use below code in viewLoad (i don't want to draw in - (void)drawRect:(CGRect)rect)for add rect on view but doesn't work.why?
CGRect rect=CGRectMake(10,10,150,140);
CGContextRef context = UIGraphicsGetCurrentContext();
CGFloat red[4] = {1.0f, 0.0f, 0.0f, 1.0f};
CGContextSetStrokeColor(context, red);
CGContextBeginPath(context);
CGContextMoveToPoint(context, 10, 10);
CGContextAddRect(context, rect);
CGContextStrokePath(context);
You can only draw when you've got a drawing context. You can either create an image context anywhere you like to create an image, or you can draw inside drawRect: to draw to your view. You are not supposed to draw to your view outside of drawRect:.
It might look like you're getting a context, but in fact you don't as UIGraphicsGetCurrentContext() returns nil outside of drawRect: (or to be more correct: it only returns a context inside drawRect: or if you've created and pushed an image context yourself).
UIGraphicsGetCurrentContext() will return nil when there is no current context. A context is created for you before drawRect: is called; if you want to create a context manually, use UIGraphicsBeginImageContext or UIGraphicsBeginImageContextWithOptions to start one, UIGraphicsGetImageFromCurrentImageContext() to extract the UIImage, and UIGraphicsEndImageContext() when you're done with it to release the context. See the documentation for details.
You could also, of course, create a CGBitmapContextRef and use CoreGraphics calls directly rather than UIKit calls. There are pros and cons to each approach, the major difference being the default transformation matrix.
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...