Incompatible block pointer types sending 'int (^)(__strong id, __strong id)' to parameter of type 'NSComparator'

Question

Getting below error in GPUImage Library when moved from xcode 5.0 to 5.1. After searching on google i found that i need to send int like this [NSNumber numberWithInt:number] but issue is that i dont have control on the value getting passed to sortedArrayUsingComparator in below given code.

Error Log:

Incompatible block pointer types sending 'int (^)(__strong id, _strong id)' to parameter of type 'NSComparator' (aka 'NSComparisonResult (^)(_strong id, __strong id)')

Error on this line : NSArray *sortedPoints = [points sortedArrayUsingComparator:^(id a, id b) {

- (id) initWithCurveFile:(NSString*)curveFile
{    
    self = [super init];
    if (self != nil)
    {
        NSString *bundleCurvePath = [[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent: curveFile];

        NSFileHandle* file = [NSFileHandle fileHandleForReadingAtPath: bundleCurvePath];

        if (file == nil)
        {
            NSLog(@"Failed to open file");

            return self;
        }

        NSData *databuffer;

        // 2 bytes, Version ( = 1 or = 4)
        databuffer = [file readDataOfLength: 2];
        version = CFSwapInt16BigToHost(*(int*)([databuffer bytes]));

        // 2 bytes, Count of curves in the file.
        [file seekToFileOffset:2];
        databuffer = [file readDataOfLength:2];
        totalCurves = CFSwapInt16BigToHost(*(int*)([databuffer bytes]));

        NSMutableArray *curves = [NSMutableArray new];

        float pointRate = (1.0 / 255);
        // The following is the data for each curve specified by count above
        for (NSInteger x = 0; x<totalCurves; x++)
        {
            // 2 bytes, Count of points in the curve (short integer from 2...19)
            databuffer = [file readDataOfLength:2];            
            short pointCount = CFSwapInt16BigToHost(*(int*)([databuffer bytes]));

            NSMutableArray *points = [NSMutableArray new];
            // point count * 4
            // Curve points. Each curve point is a pair of short integers where 
            // the first number is the output value (vertical coordinate on the 
            // Curves dialog graph) and the second is the input value. All coordinates have range 0 to 255. 
            for (NSInteger y = 0; y<pointCount; y++)
            {
                databuffer = [file readDataOfLength:2];
                short y = CFSwapInt16BigToHost(*(int*)([databuffer bytes]));
                databuffer = [file readDataOfLength:2];
                short x = CFSwapInt16BigToHost(*(int*)([databuffer bytes]));

                [points addObject:[NSValue valueWithCGSize:CGSizeMake(x * pointRate, y * pointRate)]];
            }

            [curves addObject:points];
        }

        [file closeFile];

        rgbCompositeCurvePoints = [curves objectAtIndex:0];
        redCurvePoints = [curves objectAtIndex:1];
        greenCurvePoints = [curves objectAtIndex:2];
        blueCurvePoints = [curves objectAtIndex:3];
    }

    return self;

}


- (NSArray *)getPreparedSplineCurve:(NSArray *)points

{

    if (points && [points count] > 0) 

    {

        // Sort the array.

        NSArray *sortedPoints = [points sortedArrayUsingComparator:^(id a, id b) {

            float x1 = [(NSValue *)a CGPointValue].x;

            float x2 = [(NSValue *)b CGPointValue].x;  

            return x1 > x2;

        }];

        // Convert from (0, 1) to (0, 255).

        NSMutableArray *convertedPoints = [NSMutableArray arrayWithCapacity:[sortedPoints count]];

        for (int i=0; i<[points count]; i++){

            CGPoint point = [[sortedPoints objectAtIndex:i] CGPointValue];

            point.x = point.x * 255;

            point.y = point.y * 255;

            [convertedPoints addObject:[NSValue valueWithCGPoint:point]];

        }


        NSMutableArray *splinePoints = [self splineCurve:convertedPoints];

        // If we have a first point like (0.3, 0) we'll be missing some points at the beginning

        // that should be 0.

        CGPoint firstSplinePoint = [[splinePoints objectAtIndex:0] CGPointValue];

        if (firstSplinePoint.x > 0) {

            for (int i=0; i <=firstSplinePoint.x; i++) {

                CGPoint newCGPoint = CGPointMake(0, 0);

                [splinePoints insertObject:[NSValue valueWithCGPoint:newCGPoint] atIndex:0];

            }

        }


        // Prepare the spline points.

        NSMutableArray *preparedSplinePoints = [NSMutableArray arrayWithCapacity:[splinePoints count]];

        for (int i=0; i<[splinePoints count]; i++) 

        {

            CGPoint newPoint = [[splinePoints objectAtIndex:i] CGPointValue];

            CGPoint origPoint = CGPointMake(newPoint.x, newPoint.x);

            float distance = sqrt(pow((origPoint.x - newPoint.x), 2.0) + pow((origPoint.y - newPoint.y), 2.0));

            if (origPoint.y > newPoint.y) 

            {

                distance = -distance;

            }

            [preparedSplinePoints addObject:[NSNumber numberWithFloat:distance]];

        }

        return preparedSplinePoints;

    }

    return nil;
}

Answer 1

A comparison block must return a NSComparisonResult, which is NSOrderedAscending, NSOrderedSame, or NSOrderedDescending (a.k.a. -1, 0, +1), depending on whether the first element is smaller, equal to or larger than the second element).

Returning x1 > x2 does not fulfill this requirement, and the compiler complains because it "infers" the return type as int from the last statement.

To solve this problem, you can

• use the technique from Is there a standard sign function (signum, sgn) in C/C++? to return -1, 0, or +1, and
• declare the return type of the block explicitly as NSComparisonResult  to make the compiler happy.

You can also simplify the code slightly by declaring the block parameters as NSValue *.

NSArray *sortedPoints = [points sortedArrayUsingComparator:^NSComparisonResult(NSValue *a, NSValue *b) {
    float x1 = [a CGPointValue].x;
    float x2 = [b CGPointValue].x;
    return (x1 > x2) - (x2 > x1); // -1, 0, or +1
}];