Objective-C object into C Callback function

Question

The target: to use patterns images to draw in Quartz 2D. At the present moment:

const CGPatternCallbacks kPatternCallbacks = {0, routine, NULL};

void routine(void *info, CGContextRef contextRef) {

    UIImage *brushTexture = [UIImage imageNamed:@"PatternImage1"];

    CGContextDrawImage(contextRef, CGRectMake(0, 0, 50, 50), brushTexture.CGImage);
}

To use in:

    CGPatternRef strokePattern = CGPatternCreate(NULL,
                                                 CGRectMake(0, 0, 50, 50),
                                                 CGAffineTransformIdentity,
                                                 50, // horizontal spacing
                                                 50, // vertical spacing
                                                 kCGPatternTilingNoDistortion,
                                                 true,
                                                 &kPatternCallbacks);

All this stuff inside implementation class (ViewController.m). And perfect, I can draw a pattern image.

The problem: to change something inside C callback function dynamically, in example:

In "ViewController.h"

@interface ViewController : UIViewController {
    NSString *imageName;
}

@property (nonatomic, strong) NSString *imageName;

And in the implementation class (ViewController.m)

@synthesize imageName;

imageName = @"PatternImage1";

void routine(void *info, CGContextRef contextRef) {

    //UIImage *brushTexture = [UIImage imageNamed:@"PatternImage1"];

    //CGContextDrawImage(contextRef, CGRectMake(0, 0, 50, 50), brushTexture.CGImage);
    CGContextDrawImage(contextRef, CGRectMake(0, 0, 50, 50), imageName.CGImage);
}

And ERROR!!! -> Use of undeclared identifier 'imageName'

And I've already tried...

void routine(void *info, CGContextRef contextRef) {

    //NSString *imageName = CFBridgingRelease(info);     //<- not working

    //NSString *imageName = (__bridge NSString *)(info);  //<- not working

    NSString *imageName = (__bridge NSString *) info;     //<- not working


    //UIImage *brushTexture = [UIImage imageNamed:@"PatternImage1"];

    CGContextDrawImage(contextRef, CGRectMake(0, 0, 50, 50), imageName.CGImage);
}

The routine structure can not change because it is defined by CGPatternCallbacks.

void routine(void *info, CGContextRef contextRef) { }

I searched a lot but have not found the solution. Topics like:

"Pass an Objective-C object to C function"
"Mix Objective-C and C"
"Callback function"
"C Callbacks"

Nothing, nothing and nothing.

Impossible? I do not think so! I will have to abandon... F***!

You are trying to access an instance variable of your class from a C function - that's not possible. Just a suggestion - use a global variable and set it to point to your view controller before invoking CGPatternCreate. That way you'll be able to access the property in the callback (ex. - controller.imageName.CGImage). — Ivan Genchev
– Ivan Genchev, Commented Jun 18, 2014 at 16:03
Thank you. I found it. I used the sentence: -> static NSString *imageName = @"PatternImage1"; <- And now, I can change the pattern image dynamically. — Markus
– Markus, Commented Jun 18, 2014 at 17:44

Ken Thomases · Accepted Answer · 2014-06-18 18:32:56Z

2

The info parameter the gets passed to your callback is whatever value you passed as the info parameter in your call to CGPatternCreate(). In Objective-C code, you would typically pass (__bridge void*)self. Then, in your callback, you'd cast it back to the appropriate type and call a method to do the real work:

void routine(void *info, CGContextRef contextRef) {
    ViewController* self = (__bridge ViewController*)info;
    [self drawPattern:contextRef];
}

- (void) drawPattern:(CGContextRef)contextRef {
    UIImage *brushTexture = [UIImage imageNamed:imageName]; // <- imageName can be an ivar
    CGContextDrawImage(contextRef, CGRectMake(0, 0, 50, 50), brushTexture.CGImage);
}

In your very simple case where you just want to pass an image name string in, you could pass that NSString* as the info pointer, instead. Or even the UIImage* created from the name so you don't have to create it each time. That avoids the need to call a method on the recovered self. However, it is very common to have more complicated needs, even if your needs right now are simple, so it's often worth the trouble to pass in self.

answered Jun 18, 2014 at 18:32

Ken Thomases

90.9k7 gold badges126 silver badges165 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Markus Over a year ago

I appreciate your comments very much. I did not know the syntax to call a method in Objective-C "- (void)" inside Callback Function, and could not find it searching forums. I will use your proposal to avoid creating NSObjects repeatedly in each function call. Because of course, my project makes more operations, such as changing the colour of the pattern.

Markus Over a year ago

Sorry, but I tried the alternative you propose, but does NOT WORK. Now I can not draw using the selected pattern image.

Markus Over a year ago

I made a copy/paste of the code. But it is possible that my application has some peculiarity: perhaps this fact alter the result. My App is an application to draw (with Quartz2D) and I use the method: - (void) touchesMoved.

Markus · Accepted Answer · 2014-06-18 17:42:32Z

Found it !!!

Found iiiiiiiit !!!

static NSString *imageName = @"PatternImage1";

It is visible inside callback function !!!!

Yabba Dabba Doo !!!

Complete source code. Only in "ViewController.m" :

#import "ViewController.h"

@implementation ViewController

static NSString *imageName = @"PatternImage1";

In some part of code:

const CGPatternCallbacks kPatternCallbacks = {0, routine, NULL};

In some part of code:

CGPatternRef strokePattern = CGPatternCreate(NULL, CGRectMake(0,0,50,50),
                                                 CGAffineTransformIdentity,
                                                 50, 50,
                                                 kCGPatternTilingNoDistortion,
                                                 true,
                                                 &kPatternCallbacks);

In some part of code, an "if/else" statement:

if () {
  imageName = @"PatternImage2";

} else if () {
  imageName = @"PatternImage3";

} else {
  imageName = @"PatternImage4";
}

And finally, the callback function:

void routine(void *info, CGContextRef contextRef) {

    UIImage *brushTexture = [UIImage imageNamed:imageName];

    CGContextDrawImage(contextRef, CGRectMake(0, 0, 50, 50), brushTexture.CGImage);
}

Collectives™ on Stack Overflow

Objective-C object into C Callback function

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related