Posts from the “Objective-C” Category

Debouncing Method Dispatch in Objective-C

I’ve been doing a lot of Node.js programming recently, and Javascript / Coffeescript programming style is starting to boil over into the way I think about Objective-C. One of my favorite practices in Javascript is the concept of deferring, throttling, or debouncing method calls. There are tons of uses for this. For example, let’s say your app is updating a model object, and you want to persist that model object when the updates are complete. Unfortunately, the model is touched in several bits of code one after another, and your model gets saved to disk twice. Or three times. Or four times. All in one pass through the run loop.

It’s pretty easy to defer or delay the execution of an objective-c method using “performSelectorAfterDelay” or any number of methods. Debouncing—running a method just once in the next pass through the run loop after it’s been called multiple times—is a bit trickier. However, in the case described above it’s perfect. Touch the model all you want, call “save” a dozen times, and in the next pass through the run loop, it get’s saved just once.

Here’s how I implemented a new debounce method “performSelectorOnMainThreadOnce”:

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@implementation NSObject (AssociationsAndDispatch)
 
- (void)associateValue:(id)value withKey:(void *)key
{
	objc_setAssociatedObject(self, key, value, OBJC_ASSOCIATION_RETAIN);
}
 
- (void)weaklyAssociateValue:(id)value withKey:(void *)key
{
	objc_setAssociatedObject(self, key, value, OBJC_ASSOCIATION_ASSIGN);
}
 
- (id)associatedValueForKey:(void *)key
{
	return objc_getAssociatedObject(self, key);
}
 
- (void)performSelectorOnMainThreadOnce:(SEL)selector
{
    [self associateValue:[NSNumber numberWithBool: YES] withKey: (void*)selector];
 
    dispatch_async(dispatch_get_main_queue(), ^{
        if ([self associatedValueForKey: (void*)selector]) {
            [self performSelector: selector];
            [self associateValue:nil withKey:(void*)selector];
        }
    });
}
 
@end

A more advanced version of this method would allow you to say “perform this selector just once in the next 100 msec” rather than performing it in the next iteration through the run loop. Anybody want to take a stab at that?

stringWithFormat: is slow. Really slow.

I’m working on a project that makes extensive use of NSDictionaries. Buried deep in the model layer, there are dozens of calls to stringWithFormat: used to create dictionary keys. Here’s a quick example:

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- (CGRect)rect:(NSString*)name inDict:(NSDictionary*)dict
{
    float x = [[dict objectForKey:[NSString stringWithFormat: @"%@@0", name]] floatValue];
    float y = [[dict objectForKey:[NSString stringWithFormat: @"%@@1", name]] floatValue];
    float w = [[dict objectForKey:[NSString stringWithFormat: @"%@@2", name]] floatValue];
    float h = [[dict objectForKey:[NSString stringWithFormat: @"%@@3", name]] floatValue];
    return CGRectMake(x, y, w,h);
}

In this example, I’m using stringWithFormat: in a simple way. To read four CGRect values for the rect ‘frame’ from the dictionary, it creates the keys [email protected], [email protected], [email protected], and [email protected] Because of the way my app works, I call stringWithFormat: to create strings like this a LOT. In complex situations, to the tune of 20,000x a second.

I was using Instruments to identify bottlenecks in my code and quickly discovered that stringWithFormat: was responsible for more than 40% of the time spent in the run loop. In an attempt to optimize, I switched to sprintf instead of stringWithFormat. The result was incredible. The code below is nearly 10x faster, and made key creation a negligible task:

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- (NSString*)keyForValueAtIndex:(int)index inPropertySet:(NSString*)name
{
        // the following code just creates %@@%d  - but it's faster than stringWithFormat: by a factor of 10x.
        char cString[25];
        sprintf (cString, "@%d", ii);
        NSString* s = [[[NSString alloc] initWithUTF8String:cString] autorelease];
        return [name stringByAppendingString: s];
}

It’s worth mentioning that I’ve refactored the app even more—to completely avoid saving structs this way (since NSValue is uh… an obvious solution), but I felt like it was worth posting this anyway, since you might not be able to refactor in the way I did.

iPhone Development Tutorial

EXIF Orientation Flags and the iPhone Camera

Layers just came out this past Monday, and it has this great feature that allows you to add a layer to a drawing from your iPhone’s photo library. Simple enough – right? Apple provides the UIImagePicker API, we call a couple functions and get an image back. For most purposes, that would work great! Write some code, test, commit, done. The problem is, the picker interface allows the user to adjust the scale/positioning of the image, and the cropped image is always returned at 320x320px (or less)…

Mac OS X Stack Overflow Status Item!

I’ve become a huge fan of Stack Overflow over the last few weeks. The community there is helpful and fast and there are quite a few questions about Cocoa and Objective-C! It’s gotten to the point where I visit SO whenever my code is compiling – so I thought it was time to take matters into my own hands and make things easier to follow.

I’ve made a Stack Overflow status item for Mac OS X (also a “menu bar item” or a “system icon”) that shows your reputation and lists questions on the front page containing your interesting tags:

SO Status Item in Action

It’s pretty primitive at this point – you can click a question to view it, or click the tiny arrow to go to your user page. It updates every 90 seconds using the RSS and ATOM feeds from the site, so the most active questions are always available at a glance. It’s compatible with Mac OS X 10.5 and 10.6 – so download it and give it a shot! I know there are a lot of things that could be added – so leave a comment and let me know what you think.

Download the Stack Overflow Status Item for Mac OS X (0.5 MB)

PackBits algorithm in Objective-C

The PackBits algorithm is one of the TIFF data compression methods, and it’s also used for pixel data in Photoshop PSD and TGA files. It was originally developed for MacPaint, and although it’s still widely used, there isn’t a whole lot of information online about it. I spent some time this weekend writing a category to extend the NSData class and support the packBits algorithm, and I think I’ve finally got it.

PackBits is really dirt simple. If you have a string of hex values like 00 00 00 FF FF FF FF, it replaces series of  3 or more identical bytes with a count, and then the repeated byte. The input above would be 03 00 04 FF. A prefix is also attached to series of different bytes so the decoder knows how many bytes to pass through before looking for another header byte.

The full documentation of PackBits can be found on Wikipedia. My implementation is a modified version of the one available here: http://michael.dipperstein.com/rle/index.html (source link at very bottom of page). That implementation is a modification of the official PackBits algorithm that allows slightly larger runs of data to be encoded in a single header + byte pair. I was interested in the original implementation only (since data written from any customized encoder could not be opened by a standard decoder) so I changed it back to the standard algorithm.

The primary function provided in the PackBitsAdditions category below is:

[objc] – (NSData*)packedBitsForRange:(NSRange)range skip:(int)skip[/objc]

This function returns the packBits representation of the data in “range”, advancing “skip” bytes with each read. For instance, to read and encode every byte in “range”, you would provide skip = 1. To read and encode only every 4th byte, you would pass skip = 4. This may seem like an odd implementation, but it’s very handy when you need to encode the channels of an RGBA image separately (as in PSD format).

The other function in the category is packedBitsDescription. It describes the packed bits and the process that would be followed to decode them. This function could be easily extended to actually decode the data.

If you find this code useful, please leave a comment! I debugged this for quite a while, and I’d be happy to help if you run into any issues with my implementation!

NSDataPackBitsAdditions.h

[objc]

@interface NSData (PackBitsAdditions)

- (NSString*)packedBitsDescription;
- (NSData*)packedBitsForRange:(NSRange)range skip:(int)skip;

@end
[/objc]

NSDataPackBitsAdditions.m

[objc]
@implementation NSData (PackBitsAdditions)

- (NSString*)packedBitsDescription
{
NSMutableString * description = [NSMutableString string];
char * row = (char*)[self bytes];
int pbOffset = 0;
int pbResultBytes = 0;

while (pbOffset < [self length]){
int headerByte = (int)row[pbOffset];
if (headerByte < 0){
int repeatTimes = 1-headerByte;
UInt8 repeatByte = (UInt8)row[pbOffset+1];
[description appendFormat: @"Printing %u %d times. ", repeatByte, repeatTimes];

pbResultBytes += repeatTimes;
pbOffset += 2;
} else if (headerByte >= 0){
[description appendFormat: @"Printing %d literal bytes. ", headerByte + 1];
pbResultBytes += headerByte + 1;
pbOffset += 2 + headerByte;
}
}

[description appendFormat: @"Total: %d bytes decoded.", pbResultBytes];
return description;
}

- (NSData*)packedBitsForRange:(NSRange)range skip:(int)skip
{
char * bytesIn = [self bytes];
int bytesLength = range.location + range.length;
int bytesOffset = range.location;
NSMutableData * dataOut = [NSMutableData data];

BOOL currIsEOF = NO;
unsigned char currChar; /* current character */
unsigned char charBuf[MAX_READ]; /* buffer of already read characters */
int count; /* number of characters in a run */

/* prime the read loop */
currChar = bytesIn[bytesOffset];
bytesOffset = bytesOffset + skip;
count = 0;

/* read input until there’s nothing left */
while (!currIsEOF)
{
charBuf[count] = (unsigned char)currChar;
count++;

if (count >= MIN_RUN){
int i;

/* check for run charBuf[count - 1] .. charBuf[count - MIN_RUN]*/
for (i = 2; i <= MIN_RUN; i++){
if (currChar != charBuf[count - i]){
/* no run */
i = 0;
break;
}
}

if (i != 0){
/* we have a run write out buffer before run*/
int nextChar;

if (count > MIN_RUN){
/* block size – 1 followed by contents */
UInt8 a = count – MIN_RUN – 1;
[dataOut appendBytes:&a length:sizeof(UInt8)];
[dataOut appendBytes:&charBuf length:sizeof(unsigned char) * (count - MIN_RUN)];
}

/* determine run length (MIN_RUN so far) */
count = MIN_RUN;
while (true){
if (bytesOffset < bytesLength){
nextChar = bytesIn[bytesOffset];
bytesOffset += skip;
} else {
currIsEOF = YES;
nextChar = EOF;
}
if (nextChar != currChar) break;

count++;
if (count == MAX_RUN){
/* run is at max length */
break;
}
}

/* write out encoded run length and run symbol */
UInt8 a = ((int)(1 – (int)(count)));
[dataOut appendBytes:&a length:sizeof(UInt8)];
[dataOut appendBytes:&currChar length:sizeof(UInt8)];

if ((!currIsEOF) && (count != MAX_RUN)){
/* make run breaker start of next buffer */
charBuf[0] = nextChar;
count = 1;
} else {
/* file or max run ends in a run */
count = 0;
}
}
}

if (count == MAX_READ){
int i;

/* write out buffer */
UInt8 a = MAX_COPY – 1;
[dataOut appendBytes:&a length:sizeof(UInt8)];
[dataOut appendBytes:&charBuf[0] length:sizeof(unsigned char) * MAX_COPY];

/* start a new buffer */
count = MAX_READ – MAX_COPY;

/* copy excess to front of buffer */
for (i = 0; i < count; i++)
charBuf[i] = charBuf[MAX_COPY + i];
}

if (bytesOffset < bytesLength)
currChar = bytesIn[bytesOffset];
else
currIsEOF = YES;
bytesOffset += skip;
}

/* write out last buffer */
if (0 != count){
if (count <= MAX_COPY){
/* write out entire copy buffer */
UInt8 a = count – 1;
[dataOut appendBytes:&a length:sizeof(UInt8)];
[dataOut appendBytes:&charBuf length:sizeof(unsigned char) * count];
}
else
{
/* we read more than the maximum for a single copy buffer */
UInt8 a = MAX_COPY – 1;
[dataOut appendBytes:&a length:sizeof(UInt8)];
[dataOut appendBytes:&charBuf length:sizeof(unsigned char) * MAX_COPY];

/* write out remainder */
count -= MAX_COPY;
a = count – 1;
[dataOut appendBytes:&a length:sizeof(UInt8)];
[dataOut appendBytes:&charBuf[MAX_COPY] length:sizeof(unsigned char) * count];
}
}

return dataOut;
}

@end
[/objc]

Custom compiler flags in XCode

So I’ve been messing with compiler flags in XCode for the last hour or so, and it turns out I was totally misunderstanding things. If you get info on a project or target in XCode, there’s a “User-Defined” section at the bottom that allows you to create your own flags for use at compile time. However, these flags are not actually passed into GCC! (Mistake 1!) I assumed that adding a key-value pair to the list would pass the flag straight through to GCC, but it appears that they are for use in build scripts, etc…

To pass a custom flag through to GCC so that you can use it in #ifdef and #if macros at compile time, you have to add a User-Defined Setting called “OTHER_CFLAGS”. For my project, I set the value to “-DIS_PHOTO_CHAT=1″. At compile time, the exact text is passed as an argument to GCC. You can set different values in different targets – and I was able to conditionally include some code in a header file using #if (IS_PHOTO_CHAT==1)…

It seems like you can use XCode’s built-in flags like ${TARGET_NAME} and ${PRODUCT_NAME} to insert variables into the value of OTHER_CFLAGS, but if your target name has spaces, I think you’re at a loss. I tried to set -DTARGET_NAME=${TARGET_NAME} for about an hour, but the target name had a space and I can’t get GCC to accept the value (tried quotes… no luck…) I’m no command-line-compiler-whiz, so I’m sure there’s a trick, but Google hasn’t turned anything up.

I’m still surprised OTHER_CFLAGS wasn’t preset to “” in the target build settings. There’s an empty field for “Other Code Signing Flags” (which seems less useful!) Oh well… Guess XCode is in permanent beta anyway?

Creating a UIImage from a CGLayer

CGLayers are great for drawing – especially when things need to be drawn over and over again. Converting a CGLayer to a UIImage is another story, though. NetSketch uses CGLayers for the drawing canvas, but converts them to UIImages when you go to upload your drawing or email it to a friend. The code below shows how it’s done. The CGLayer is drawn into a bitmap CGContext of the same size, and then a CGImage is created around the CGContext. The CGImage can be turned into a UIImage, and you’re done!

Be sure to leave a comment if you find this function useful!

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UIImage* UIImageFromLayer(CGLayerRef layer)
{
    // Create the bitmap context
    CGContextRef    bitmapContext = NULL;
    void *          bitmapData;
    int             bitmapByteCount;
    int             bitmapBytesPerRow;
    CGSize          size = CGLayerGetSize(layer);
 
 
    // Declare the number of bytes per row. Each pixel in the bitmap in this
    // example is represented by 4 bytes; 8 bits each of red, green, blue, and
    // alpha.
    bitmapBytesPerRow   = (size.width * 4);
    bitmapByteCount     = (bitmapBytesPerRow * size.height);
 
 
    // Allocate memory for image data. This is the destination in memory
    // where any drawing to the bitmap context will be rendered.
    bitmapData = malloc( bitmapByteCount );
 
    if (bitmapData == NULL)
    {
        return nil;
    }
 
    // Create the bitmap context. We want pre-multiplied ARGB, 8-bits
    // per component. Regardless of what the source image format is
    // (CMYK, Grayscale, and so on) it will be converted over to the format
    // specified here by CGBitmapContextCreate.
    bitmapContext = CGBitmapContextCreate (bitmapData, size.width, size.height,8,bitmapBytesPerRow,
                                        CGColorSpaceCreateDeviceRGB(),kCGImageAlphaNoneSkipFirst);
 
 
    if (bitmapContext == NULL)
        // error creating context
        return nil;
 
    CGContextScaleCTM(bitmapContext, 1, -1);
    CGContextTranslateCTM(bitmapContext, 0, -size.height);
 
    // Draw the image to the bitmap context. Once we draw, the memory
    // allocated for the context for rendering will then contain the
    // raw image data in the specified color space.
    CGContextDrawLayerAtPoint(bitmapContext, CGPointZero, layer);
    CGImageRef   img = CGBitmapContextCreateImage(bitmapContext);
    UIImage*     ui_img = [UIImage imageWithCGImage: img];
 
    CGImageRelease(img);
    CGContextRelease(bitmapContext);
    free(bitmapData);
 
    return ui_img;
}

Safari-style iPhone Checkbox

I was finishing up work on an iPhone app earlier today and needed to add a check box to the interface. Interface Builder provides a “checkbox-ish” control that slides between “On” and “Off” states but it didn’t fit in well with the rest of the app. I needed a smaller one like the one that Mobile Safari provides for web forms.

It was pretty easy to create using a UIButton, but I spent a few minutes putting together images for the different button states. Click the link below to grab ‘em – and leave a comment if you find them useful!

iPhone Safari-Style Checkbox.zip