CGContextFillRects: No matching function for call - objective-c

Question

I'm trying to optimize the performance in one of my components. The component needs to draw some (10 to 200) rectangles in it's drawRect method, which is triggered about 20 times per second.
Everything works when I use the CGContextFillRect method on each CGRect separately. I want to test if grouping the drawing into one single call with CGContextFillRects on an array of CGRects would increase performance.
The method CGContextFillRects gives me a compiler error No matching function for call to 'CGContextFillRects'.
This code is inside a .mm file. Should I import something before the CGContextFillRects method can be used?
This is what i'm trying to do:
- (void) drawRect:(CGRect)rect{
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSaveGState(context);
CGContextSetFillColorWithColor(context, self.fillColor.CGColor);
//check if some objects are present
if (self.leftDrawBuffer && self.rightDrawBuffer){
UInt32 xPosForRect = self.leftPadding;
NSMutableArray *rectsToFill = [[NSMutableArray alloc] init];
for (int drawBufferLRIndex = 0; drawBufferLRIndex < 2; drawBufferLRIndex++){
Float32 *drawBuffer_ptr = self.leftDrawBuffer;
if (drawBufferLRIndex > 0){
drawBuffer_ptr = self.rightDrawBuffer;
}
for (int i=0; i< kAmountOfBarsPerChannel; i=i+1){
Float32 amp = drawBuffer_ptr[i];
Float32 blockNumber = 1.0f;
UInt32 yPosForRect = self.bounds.size.height - self.heightPerBlock;
while (blockNumber <= self.blocksPerLine && blockNumber / self.blocksPerLine < amp){
CGRect rect= CGRectMake(xPosForRect, yPosForRect, self.widthPerBlock, self.heightPerBlock);
[rectsToFill addObject:[NSValue valueWithCGRect:rect]];
//Using the method below works and gives me the expected result
//CGContextFillRect(context, rect);
blockNumber++;
yPosForRect -= self.heightPerBlock + self.vPaddingPerBlock;
}
xPosForRect += self.widthPerBlock + self.hPaddingPerBlock;
}
}
//This is the added code where i try to use CGContextFillRects
//1 -> transform to a c array of CGRects
const CGRect *cRects[rectsToFill.count];
for (int i = 0; i < rectsToFill.count; ++i) {
CGRect rect = [[rectsToFill objectAtIndex:i] CGRectValue];
cRects[i] = &rect;
}
size_t size = rectsToFill.count;
//2 -> trigger the method to fill all rects at once
//this method gives me the compiler error 'No matching function for call to 'CGContextFillRects''
CGContextFillRects(context, cRects, size);
}
CGContextRestoreGState(context);
}

Answer 1

The problem is how you convert the rects to a C array. You make pointers to the rects that are temporarily stored on the stack. There are two problems with this. First, the rects are gone with each loop iteration, so you can't do that. Second, You should pass a pointer to an array of CGRects, not an array of pointers to CGRect.
This will likely solve it:
CGRect cRects[rectsToFill.count]; // Replace your lines from this
for (int i = 0; i < rectsToFill.count; ++i) {
CGRect rect = [[rectsToFill objectAtIndex:i] CGRectValue];
cRects[i] = rect;
}
size_t size = rectsToFill.count;
CGContextFillRects(context, cRects, size); // To this
Please note the re-declaration of the cRects array and the change in the assignment.