Digging thorough the development server documentation, I found two options that are making my life much easier:

–use_sqlite

Use sqlite for the local datastore.

–datastore_path=[path to datastore file]

Keep the datastore in the file specified.

By doing these two things, you can now see where GAE is keeping your datastore and you can make intermediate backups and restore them at will. You can also view your datastore using the command line sqlite client. Just add these two parameters to the Extra Flags box in your app’s info in the App Engine Launcher (or to the command line) and you’re good to go. Enjoy!

	[[theApp.managedObjectContext undoManager] disableUndoRegistration];
	detailItem.fieldImageFile = relativePath;
	[[theApp.managedObjectContext undoManager] enableUndoRegistration];

Seemed very clear, but it didn’t work! So, after much poking around on the Internet, I found a posting that alluded to the fact that changes to undo don’t occur until the run loop executes. So, to get the desired effect, I ended up with the following code:

	[[NSRunLoop currentRunLoop] runMode: NSDefaultRunLoopMode beforeDate:[NSDate date]];
	[[theApp.managedObjectContext undoManager] disableUndoRegistration];
	detailItem.fieldImageFile = relativePath;
	[[NSRunLoop currentRunLoop] runMode: NSDefaultRunLoopMode beforeDate:[NSDate date]];
	[[theApp.managedObjectContext undoManager] enableUndoRegistration];

Yet another head-scratching API decision from Apple.

What I wanted to do seemed simple: have a map view with a bunch of moving objects on it. Each object pulls its current position from an XML document on the web. I already had my XML-wrapper object written when I wanted to display it on the map, so I simply implemented the MKAnnotation protocol. All of my objects appeared, and all seemed to be right with the world.

Then, I started updating my objects and found that the annotations didn’t move. At all. I found a bunch of people on the web trying to solve the same problem, but I wasn’t very happy with the solutions I found. Most of them involved removing the annotations and adding them back into the map view. But, thankfully, I found one article that mentioned KVO on the iPhone. I was pretty familiar with KVO from my Flex projects, but didn’t even realize it was supported in Objective C.

To make a long story short, if you don’t use the willChangeValueForKey and didChangeValueForKey before and after updating your coordinate property on your annotation object, the MKMapView will not be aware that it has moved. After bracketing my XML update code with those two calls, all of a sudden my annotations started moving around.

-(void)xmlProperty:(NSString *)newXml
{
	[self willChangeValueForKey:@"coordinate"];
	[xml release];
	xml = [newXml retain];
	[self didChangeValueForKey:@"coordinate"];
}

I was able to extend the NSManagedObject class and add a new xmlString property. By using the entity property to do some simple introspection, I was able to write a serialization function in about 50 lines of code. Enjoy!

NSManagedObject+XMLSync.h

//
//  Created by Scott Means on 1/5/11.
//  Released into the public domain without warranty.
#import <CoreData/CoreData.h>
#import <Foundation/Foundation.h>

@interface NSManagedObject (XMLSync)

@property (nonatomic, readonly) NSString *xmlString;

@end

NSManagedObject+XMLSync.m

//
//  Created by Scott Means on 1/5/11.
//  Released into the public domain without warranty.

#import "NSManagedObject+XMLSync.h"

@implementation NSManagedObject (XMLSync)

- (NSString *)xmlString
{
	NSEntityDescription *ed = self.entity;
	NSURL *uri = self.objectID.URIRepresentation;
	NSMutableString *x = [NSMutableString stringWithFormat:@"<%@ id=\"/%@%@\"",
                ed.name, uri.host, uri.path];

	for (NSString *a in ed.attributesByName.allKeys) {
		id value = [self valueForKey:a];

		if (value) {
			if ([value isKindOfClass:[NSString class]]) {
				[x appendFormat:@" %@=\"%@\"", a, value];
			} else {
				if (![value respondsToSelector:@selector(stringValue)]) {
					NSLog(@"no stringValue");
				}
				[x appendFormat:@" %@=\"%@\"", a, [value stringValue]];
			}
		}
	}

	bool hasChildren = NO;

	for (NSString *r in ed.relationshipsByName) {
		if (!hasChildren) {
			[x appendString:@"/>"];
			hasChildren = YES;
		}

		NSRelationshipDescription *rd = [ed.relationshipsByName objectForKey:r];

		if (rd.isToMany) {
			hasChildren = YES;
			[x appendFormat:@"<%@>", r];

			for (NSManagedObject *c in [self valueForKey:r]) {
				[x appendString:c.xmlString];
			}

			[x appendFormat:@"", r];
		}
	}

	if (hasChildren) {
		[x appendFormat:@"", ed.name];
	}

	return x;
}

@end

Next project: figure out how to do better code formatting in WordPress!

So, wanting to do my bit, I created a new website called Native Code to try to make this easier. My goal is to create a database of common low-level coding idioms (micro-patterns, if you like) and then have them expressed in as many programming languages as possible. And not just expressed, but expressed well. So, please feel free to add your own contributions, create your own idioms, and otherwise capture your coding know-how for the ages. Enjoy!

As usual, there isn’t much on the web about this, but I eventually tracked down a couple of useful articles to get me started:

• keremk’s iphone-static-library-project-template
• Building static libraries with the iPhone SDK

The first article shows you how to add your own custom project type template to XCode so you can easily create shared iPhone libraries. The second one shows you how to reference your new library from your application. With a little trial-and-error I now have a nice shared lib of iPhone classes. Pretty soon, who knows, maybe Apple will allow dynamic linking and we can move iPhone development into the 80s!

<Error>: CGBitmapContextCreate: unsupported parameter combination:
8 integer bits/component; 16 bits/pixel; 1-component colorspace;
kCGImageAlphaPremultipliedLast; 1472 bytes/row.

So the combination of parameters was incorrect. But which one was at fault? Thankfully, I found a helpful post on the web that led me to this article:

CGBitmapContextCreate Supported Color Spaces

Very handy. Wish that this table were actually included in the documentation for the CGBitmapContextCreate method. Imagine how useful that would be! So, for your convenience, here is a function to create a grayscale copy of a UIImage:

UIImage *createGrayCopy(UIImage *source)
{
	int width = source.size.width;
	int height = source.size.height;

	CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();

	CGContextRef context = CGBitmapContextCreate (nil,
						 width,
						 height,
						 8,      // bits per component
						 0,
						 colorSpace,
						 kCGImageAlphaNone);

	CGColorSpaceRelease(colorSpace);

	if (context == NULL) {
		return nil;
	}

	CGContextDrawImage(context,
		CGRectMake(0, 0, width, height), source.CGImage);

	UIImage *grayImage = [UIImage imageWithCGImage:CGBitmapContextCreateImage(context)];
	CGContextRelease(context);

	return grayImage;
}

Never one to pass up a challenge, I told him to come on down. I was careful to set his expectations as low as possible, because one day really isn’t enough time to do a meaningful application. All I could promise was that at the end we would have something to submit to the iTunes App Store, and he was OK with that.

So at approximately 3:00 PM on Monday (Memorial Day), JD pulled up to my house in Columbia, SC. The trip took him 5 hours, instead of the 3 that he had originally thought. After a brief meet & greet (plus various social networking connections, which I guess are the modern equivalent of calling cards), we sat down to do a quick mockup of his site.

We used a new tool that I really, really like: Balsamique Markups. It’s a very nice WYSIWYG tool for laying out web pages, applications, and iPhone apps. For OCD perfectionists (which many developers are), I find the cartoony graphics strangely freeing. It really lets me lay out the gist of something without getting sucked into the minutiae of a full design.

We agreed on a basic application that would:

• display his blog in a browser control
• let the user forward a link to a friend
• take the user to a newsletter signup
• take the user to feedburner

The one rule of iPhone development is that the things you think will be simple will be hard, and the things you think will be hard will be easy. We had a browser window up and running with his blog in it within 30 minutes. But producing a decent-looking home button image took almost an hour.

Possibly the most interesting part of the project was having someone I’d never met come and crash at my house while developing an application. My mother (who lives with me) was a little leery of the idea at first. “What if he’s an axe murderer?”

After JD and I had been working for a while and I mentioned this to him, he assured her that he was not an axe murderer, which I’m not sure had the desired effect. I put it to her like this “It’s like we’re running a bed & breakfast, with iPhone development on the side.” Hmmmm….

In any case, working with JD was a real pleasure, and we had some interesting conversations over cigars at the end of the day. Now, let’s all keep our fingers crossed that Apple will be kind and merciful and approve the app!