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Things I learned implementing my first InAppPurchase

Yesterday I sent my first app off to Apple containing an InAppPurchase (IAP). It’s a free app that gives the user an option to pay a dollar for a premium features. That’s what they call Freemium these days. Free to try, premium to get some more.

The possibility for Freemium was only introduced in October 2009 when Apple finally gave in to developer’s wish to be able to do away with those dreaded Lite versions which have a very low conversion rate (about 1%) anyway. Until that time IAPs where only available for paid apps.

After developing on the premium content for about two weeks I hit my first roadblock. You apparently cannot configure an app id for use in a provisioning profile it it is already configured on another developer’s account.

If you are looking for a full walkthrough, this is not the article to provide that. Troy Brant has the most complete IAP Walkthrough on his blog. Instead this is a summary of my mental notes.

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Uploading UIImages to TwitPic

One day of the week I hope to do something that’s not really business oriented, I call it my “Friday-Project”. This title comes from my time in IT where you would generally avoid do any major changes to the systems you are maintaining on Fridays because nobody likes to work on fixing these on the following weekend. So Friday is a great day to just experiment, prototype or dream. At Google they are supposedly giving their employees 20% of their working time for such projects, so I am dedicating as much time to playful exploration as well.

Today I want to explore and hopefully finish some code to upload an image to TwitPic. So the first step is to have a look at their API documentation which is not very pretty, but at least it’s complete. There we see that we want to implement the uploadAndPost function.

METHOD: http://twitpic.com/api/uploadAndPost

Use this method to upload an image to TwitPic and to send it as a status update to Twitter.

Fields to post in post data should be formatted as multipart/form-data:

- media (required) – Binary image data
– username (required) – Twitter username
– password (required) – Twitter password
– message (optional) – Message to post to twitter. The URL of the image is automatically added.

Sample response:

<?xml version=”1.0″ encoding=”UTF-8″?>
<rsp status=”ok”>
<statusid>1111</statusid>
<userid>11111</userid>
<mediaid>abc123</mediaid>
<mediaurl>http://twitpic.com/abc123</mediaurl>
</rsp>

For this we need to construct an HTTP POST request to the URL http://twitpic.com/api/uploadAndPost and construct a multipart body with binary image data, Twitter username, password and an optional message. The response will be XML giving us the URL we can use to link to the picture.

Having the parameters intermixed makes it a bit harder especially if you have never constructed a multipart body before. The easiest method of image uploading is if you can specify parameters in the URL, but the designer of this API thought it smarter to have it in the body. But don’t worry, we’ll get this figured out as well.

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App Information String Tokens

For my current project DTAboutViewController I need to be able to specify tokens in my strings that would be replaced by information about the app at runtime. Of specific interest are CFBundleDisplayName and CFBundleVersion which are both in info.plist. Usually you could hard-code these into your app strings or maybe do a global #define, but for my component I wanted to have the most flexibility with the least amount of work in subsequent project that would use that.

So I put together a category extension for NSString which also shows off how to use NSScanner to find tokens and replace them with values from your info dictionary.

NSString+Helpers.h

#import 
 
@interface NSString (Helpers)
 
- (NSString *) stringBySubstitutingInfoTokens;
 
@end

NSString+Helpers.m

#import "NSString+Helpers.h"
 
@implementation NSString (Helpers)
 
- (NSString *) stringBySubstitutingInfoTokens
{
	NSMutableString *tmpString = [NSMutableString stringWithString:self];
	NSScanner *scanner = [NSScanner scannerWithString:self];
 
	NSDictionary *infoDict = [[NSBundle mainBundle] infoDictionary];
 
 
	while (![scanner isAtEnd])
	{
		if ([scanner scanString:@"$" intoString:nil])
		{
			NSString *tokenName;
 
			if ([scanner scanCharactersFromSet:[NSCharacterSet alphanumericCharacterSet] intoString:&amp;tokenName])
			{
				id value = [infoDict objectForKey:tokenName];
 
				if (value &amp;&amp; [value isKindOfClass:[NSString class]])
				{
					[tmpString replaceOccurrencesOfString:[@"$" stringByAppendingString:tokenName] withString:value options:NSLiteralSearch range:NSMakeRange(0, [tmpString length])];
				}
			}
		}
 
		[scanner scanUpToString:@"$" intoString:nil];
	}
 
	return [NSString stringWithString:tmpString];
}
@end

The code works by skipping all characters until it encounters a $, which will be our token identifier. From this position the name of the token is until the first non-alphanumberic character, like a space or period. Then it tries to get a value from the bundle’s info dictionary. If it succeeds the token is replaced in a temporary string, if not it is left unchanged.

Note how I am passing nil into scan functions if I am only interested in skipping characters. If I am interested in the scanned result, then I am passing the address of a pointer to an NSString, that’s the reason for the &tokenName. The scan method creates an autoreleased copy of the scanned string and puts the pointer to this string into the parameter the address of which you have passed. That’s unusual in objective-C and seen more often in pure C code, so be aware of that.

Now with this code I gain a very useful way of displaying copyright information as the section footer of my settings table.

- (NSString *)tableView:(UITableView *)tableView titleForFooterInSection:(NSInteger)section
{
	return [@"© 2010 Drobnik.com. $CFBundleDisplayName $CFBundleVersion" stringBySubstitutingInfoTokens];
}

This will look like this, the app’s bundle display name is “About” and the version string is “1.0”.

With this method it’s even thinkable to put your own information into info.plist. Like for example the ID of your app which you would use for a direct link to the review page. Some people complain that Cocoa does not have regular expression support, but actually if you know how to use NSScanner properly you can do even more with it than trying to piece together working regexps.

Making a "Follow us on Twitter" button

I am working on the ultimate “About Page” component at the moment. And of course this won’t be complete without a button to follow the developer on Twitter. Tapbots is one company that has the best role model for modeling this. They have this little button here at the bottom of their about page:

There is something amazing that happened when I tapped on this “Follow us on Twitter” button: it opened up the tapbots user right in Tweetie 2! I was astonished at first, but then it dawned on me that on the iPhone you can have your app respond to a custom URL scheme and obviously some Twitter clients support going to the user’s profile page. I have only Tweetie 2 installed and incidently this supports just that.

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Understanding UIFont

Have you ever really TRULY looked at the documentation of UIFont?

I had to, because I was looking for some metrics information that I could use to custom draw UILabels. And if you want to be independent of what font is set, then you have to get certain metrics, but the Apple SDK documentation of UIFont leaves a bit to desire.

When I asked which characters would be good representatives of a font, I was sent a link to this blog “celebrating the beauty of the ampersand”. Fine, an ampersand will be there as well…

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Drawing Rounded Rectangles

Once you get deeper into coding iPhone apps you find that CoreGraphics starts to become a real friend. A friend who lacks in certain areas, because you will still have to piece together some shapes with the shape drawing functions that CG provides.

For sake of reusability you want to put the creation of distinct shapes into their own respective methods. You could make those into C functions like their CG brethren, but for our purposes objC-methods suffice.

This method creates a CGPath for a rounded rect inside the given rectangle with the given radius. We are alternating adding a straight line segment and then a corner by means of AddArcToPoint.

- (CGPathRef) newPathForRoundedRect:(CGRect)rect radius:(CGFloat)radius
{
	CGMutablePathRef retPath = CGPathCreateMutable();
 
	CGRect innerRect = CGRectInset(rect, radius, radius);
 
	CGFloat inside_right = innerRect.origin.x + innerRect.size.width;
	CGFloat outside_right = rect.origin.x + rect.size.width;
	CGFloat inside_bottom = innerRect.origin.y + innerRect.size.height;
	CGFloat outside_bottom = rect.origin.y + rect.size.height;
 
	CGFloat inside_top = innerRect.origin.y;
	CGFloat outside_top = rect.origin.y;
	CGFloat outside_left = rect.origin.x;
 
	CGPathMoveToPoint(retPath, NULL, innerRect.origin.x, outside_top);
 
	CGPathAddLineToPoint(retPath, NULL, inside_right, outside_top);
	CGPathAddArcToPoint(retPath, NULL, outside_right, outside_top, outside_right, inside_top, radius);
	CGPathAddLineToPoint(retPath, NULL, outside_right, inside_bottom);
	CGPathAddArcToPoint(retPath, NULL,  outside_right, outside_bottom, inside_right, outside_bottom, radius);
 
	CGPathAddLineToPoint(retPath, NULL, innerRect.origin.x, outside_bottom);
	CGPathAddArcToPoint(retPath, NULL,  outside_left, outside_bottom, outside_left, inside_bottom, radius);
	CGPathAddLineToPoint(retPath, NULL, outside_left, inside_top);
	CGPathAddArcToPoint(retPath, NULL,  outside_left, outside_top, innerRect.origin.x, outside_top, radius);
 
	CGPathCloseSubpath(retPath);
 
	return retPath;
}

The method has to be called new-something so that Build&Analyze does not tell you about a memory leak. Having the method name begin with new tells the static analyzer that this method is supposed to return something that the caller has to take care of releasing.

Now, if we want to use this method, then we can do so in any view’s drawRect:

- (void) drawRect:(CGRect)rect
{
	CGContextRef ctx = UIGraphicsGetCurrentContext();
 
	CGRect frame = self.bounds;
 
	CGPathRef roundedRectPath = [self newPathForRoundedRect:frame radius:5];
 
	[[UIColor blueColor] set];
 
	CGContextAddPath(ctx, roundedRectPath);
	CGContextFillPath(ctx);
 
	CGPathRelease(roundedRectPath);
}

Having the rounded rectangle shape as a path object allows us to reuse it several times. You could for example draw a gradient inside after having used the shape for clipping, then draw a border and also add a shadow. All from the same shape. At the end we just release it to clean up.

UILabels with Neon-Effect

For a customer project I needed to have a Neon glow on some text. Being the geek that I am I would not want to settle for simple creating the effect in Photoshop since I wanted to be able to smoothly scale the text. Custom drawing comes to mind, but where?

One might be tempted to first consider create a new UIView for this effect, but then you’d have to also add all those properties that UILabel has on top of what it inherits from UIView. Second idea was to create a UILabel category, but my experiments have shown if I override a standard method like drawRect in my category then this overrides it for all UILabels.

(Background Photo “Seattle by Night” by Alan Bauer)

So the final – and successful – decision was to subclass UILabel: DTGlowingLabel was born.

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Array Self-Sorting

The current Dr. Touch Part that I am working on for my store is about quickly clustering POIs if they would be too close together on a MKMapView. For this purpose I need to calculate distances between all annotation pins and add to this the distances to all newly found clusters. Currently this involves resorting the whole list of distances multiple times. So I put my thinking hat on and built this category extension for NSMutableArray to add numbers of objects in the correct place according to the specified sort order.

To cut down on search time for the insertion point I am using a “divide an conquer” approach. Split the search range in half and check if the value at this index is bigger or smaller than the one I am trying to insert. Continue to divide until the length of my search range is 0. At that point I have found the appropriate place for the insertion.

This approach means that for 1000 values/objects in the array only 10 comparisons are necessary. This is most likely way less than if you are constantly creating sorted copies of the array with one of the three standard sorting methods.

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Easier Version Checking

Sometimes you may need to modify your app’s behavior depending on which OS version it’s running on. As of XCode 3.2 you can choose a different SDK to compile against than you choose for the deployment target. This allows to make an app that runs as low as 2.0, but uses all the bug fixes in the SDKs. And it forces the compiler to make dynamic links to 3.x frameworks. So you can check at run time if such a framework is available and if not present the feature that depends on it.

When working on DTAugmentedRealityController I needed to limit it’s use on devices with a magnetometer and also which run a version of iPhoneOS that’s greater than or equal to 3.1 because this is the version when Apple introduced the capability of overlaying your own view on top of an UIImagePickerController. So I needed an elegant method, I would not settle for comparing strings. That’s why I came up with this extension to UIDevice.

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Adding Last Build Date & Time

If you need or want the date and/or time the last time your app was built then there are two handy macros you can use. Consider the following example:

char *date = __DATE__;  // e.g. 'Dec 15 2009'
char *time = __TIME__;  // e.g. '15:25:56'
 
NSLog(@"Build date: %s", date);
NSLog(@"Build time: %s", time);

I tested it with GCC 4.0, GCC 4.2 and even the new LLVM GCC 4.2. It worked fine with all three compilers.

Those are precompiler macros which work by getting the precompiler to replace them with the current date and time when the file in which they are located in gets built. Bear in mind that generally building is incremental and therefore these will only get updated if there really IS a new build happening. That’s a drawback if you forget it, but you can or should always do a build – clean before you build a release or distribution version to make certain all got updated.

Since those macros date back to C times they get replaced with C-style strings. That’s a pointer to a char array with a binary zero at the end to terminate the string. To convert them to obj-C NSStrings is simple by means of the %s formatter or by using one of the more complicated initializers of NSString.

char *date = __DATE__;
char *time = __TIME__;
 
NSString *myDate = [NSString stringWithCString:date encoding:NSASCIIStringEncoding];
NSString *myTime = [NSString stringWithFormat:@"%s", time];

Finally if you are really so much “pro” that the build time of your app matters, then you will probably also ask if there is a way to force building of certain files to forego the incremental building. Sure you can. All you need to do is to add an extra script to your target to set the last modified time of the file using these macros to the future.

touch -t 2012310000 "${PROJECT_DIR}/Classes/CalendarAppDelegate.m"

Touching Script

It’s not enough to simply set the modified time once because then the next modification sets it back to the current date. Thus the need for this simple unobtrusive script, in this example I am setting the modified time for CalendarAppDelegate.m to Dec 31st 2020 which is sufficiently far away so that this blog article will work for the next 11 years. :-)