Monday, May 31, 2010

Accessorizer 2.0 is out!

acc_saw_clock_icon_512_version14.pngAccessorizer by Kevin Callahan is one my absolute favorite utilities, as I've mentioned before. Well, now, there's a new reason to love it. Kevin just released 2.0 over the weekend. And there's great news for those of you who have bought previous versions: it's a free upgrade.

If you do any serious amount of Objective-C programming, Accessorizer should definitely be in your toolbox.

Friday, May 28, 2010

Core Data and Threading

I've had a number of people ask me lately about the best way to handle using Core Data in a multi-threaded application. I began to put together a tutorial and sample code based on Apple's documentation, which advises to pass NSManagedObjectIDs between threads to get around the fact that threads can't share instances of NSManagedObjectContext or NSManagedObject. I've used this approach in several projects and it works.

While googling around doing research, however, I came across a post by Marcus Zarra on the Mac Developer Network.

Marcus knows Core Data really well, having written a book on the topic, so I figured I'd take a look at what Marcus had to say on the matter. I'm glad I did. Marcus illustrates a really great approach in his post that obviates the need to pass ManagedObjectIDs around between the thread in many situations. You still need to do that if you need to pass a specific object between threads, but if you're doing something like loading or creating managed objects in the background to be displayed in a table, this is so much easier and better.

I won't spoil the surprise, but it's a handy tool for your Core Data toolkit. If you have any thought about using Core Data in a multi-threaded application, you should definitely check it out.

Wednesday, May 26, 2010

The Martian Invasion: Announcing MartianCraft

About four months ago, I joined together with my friend and co-author, Dave Mark and with Rob Rhyne, developer of the awesome Briefs prototyping tool, to form a new mobile software development company specializing in the iPhone and Android platforms. This software development firm is what I've somewhat cryptically been referring to "Super Secret Project A"¹, but its real name can now be revealed as: MartianCraft.

Our original plan had been to do a public announcement long before now, but something unusual (yet good) happened. Before we ever made our announcement, before we ran our first advertisement, before the ink was even dry on our incorporation papers, we got busy. Really, really busy. Our little covert, unannounced software development firm kept us too busy for the first few months to take on new clients. Even finding time to do all the various things that a new business needs to have done was quite a challenge.
Screen shot 2010-05-25 at 7.37.56 PM.png
Having now finished two relatively major projects and a number of smaller ones, we've now got a chance to catch our breath and get all our ducks in a row. We've even got business cards (well, they're coming soon, at least) and some T-shirts to wear at WWDC. We may even have a few extra of those to give out.

We also have something else: the bandwidth to take on a new client or two. If you're looking to have an iPhone, iPad, or Android application developed, or if you've got an existing application in need of a little help drop us a line! We're also available for onsite training, code and architecture reviews, weddings, bar mitzvahs, and bachelor parties! Well, maybe not the last few.

1 Super Secret Project B will be revealed shortly after WWDC, so stay tuned!

Monday, May 24, 2010

Downloading Images for a Table without Threads

It is common practice in many networking libraries in many languages to spawn threads to handle asynchronous communications in the background while the rest of the program continues to function. Although this approach does work on the iPhone, it's really not the right way to do it. I've recently run across two different blog postings that show how to download images for display in an iPhone table view, and they both did synchronous network communications using threads.

Here's my Public Service Announcement for today: Don't. Please… just, please don't, okay? Don't spawn threads for asynchronous communications unless you would have used threads in the same situation if there was no network code..
Note: A few people have suggested that this post is misleading because threads are used behind the scenes. Yes, it is true that the iPhone OS will spawn a small number of non-main threads as the result of using certain networking APIs. You still should be using the asynchronous APIs for your network communications and should not be spawning threads so you can make synchronous networking calls without blocking.
I don't like to knock other people's tutorials. In programming, there are multiple ways of doing nearly everything, and often more than one approach has merit in certain situations, so I'm always hesitant to say that the approach someone else uses is "wrong" if it works. But, this is one of those cases where there is a clear and compelling reason not to do it one particular way. However, because the "right" way is not necessarily intuitive, especially for people whose networking experience has all been thread-based, I thought it worth a short tutorial to show how to download asynchronously for display in a table without spawning any threads.

But first, why shouldn't we use threads this way? Well, simply put, threads have overhead. It's not a huge overhead, but it's non-zero and, in fact, it's non-trivial given the processing power and physical memory available on iPhone OS devices. The overhead of threading absoulutely makes sense in some situations; asynchronous networking is not one of those situations. Apple provides asynchronous APIs specifically for this purpose and they're designed to give the best performance in light of available system resources.

But, don't take my word for it. Here is what Apple says on the matter, in a guide written to help Windows programmers make the transition to programming for the Mac (emphasis mine):
One of the complexities of socket use involves the use of separate threads to handle the exchange of data. In a server environment, where hundreds of sockets are open simultaneously, the code that checks all these threads for activity may cause a significant performance hit. In addition, with threads, data synchronization is often an issue, and locks are often required to guarantee that only one thread has access to a given global variable at a time.


Event-driven code is more complex than code using blocking threads, but it delivers the highest network performance. When you use a run loop to handle multiple sockets in the same thread, you avoid the data synchronization problems associated with a multiple-thread solution.
When Apple says something as specific as this, they usually have a darn good reason for it. This is Apple's polite way of saying "don't use threads for networking, you idiot". They're a lot more polite than I am, obviously.

As you probably know, your application's main thread (the one that gets created automatically when your application is launched) has a loop (contained in UIApplication) that's always running. It's called your application's run loop and it keeps running as long as your application is executing. Every time through this loop, UIApplication does a whole bunch of stuff. It checks for inputs from the hardware and mach messages from the OS, for example, and it looks to see if there are any NSTimers running whose targets need to be called. Among the tasks it does is to check for incoming network data. There are more things the run loop handles, but that last one is the important for today's discussion.

Conceptually speaking, CFNetwork is not the easiest framework to grasp, but it is extremely efficient. It can handle hundreds, maybe even thousands of simultaneous downloads without causing the main thread to seize up. It's considerably more efficient than using synchronous network calls on a thread and not really any harder once you know how to do it.

Let's Try Downloading on the Main Thread

I've written a small iPhone app that downloads images from Deviant Art. You just enter somebody's Deviant Art username into a text field and the application will grab the the images in that person's gallery, download them asynchronously, and display each one in the table once it's finished downloading:

Screen shot 2010-05-24 at 4.28.42 AM.png

Notice that there's an activity indicator showing that some images are in the process of being downloaded. This is all happening on the main thread. I spawn no explicit threads in this application, and yet the UI stays responsive. You can also tap a row to view the image larger, but that's not really important to the process being illustrated.
Note:Although most DeviantArt galleries are SFW, there are many that aren't, so just be aware of that. Edit: Turns out that warning was unnecessary - since we haven't authenticated, DeviartArt filters out all the NSFW images automatically.

Creating the Download Object

In your own applications, you may approach things somewhat differently, but for this sample application, I'm going to put the code to download the image in its own data model class called DeviantDownload. I'm going to override the accessor method for the image so that the first time that accessor is called, it will kick off the download. If I call the image accessor method and get nil, the calling code can make itself the download's delegate to get notified when the download finishes.

In this application, downloads don't get triggered until the first time an image scrolls onto the screen. That may not be what you want in your application, but it will keep things nice and simple for this example. Here's the declaration of our download class:

#import <Foundation/Foundation.h>

#define DeviantDownloadErrorDomain @"Deviant Download Error Domain"
{ DeviantDownloadErrorNoConnection = 1000,

@class DeviantDownload;
@protocol DeviantDownloadDelegate
- (void)downloadDidFinishDownloading:(DeviantDownload *)download;
- (void)download:(DeviantDownload *)download didFailWithError:(NSError *)error;

@interface DeviantDownload : NSObject
{ NSString

*urlString; UIImage

*image; id <NSObject, DeviantDownloadDelegate> delegate;

@private NSMutableData

*receivedData; BOOL


@property (nonatomic, retain) NSString *urlString;
@property (nonatomic, retain) UIImage *image;
@property (nonatomic, readonly) NSString *filename;
@property (nonatomic, assign) id <NSObject, DeviantDownloadDelegate> delegate;

Pretty straightforward, right? I define an error domain and an error code for the one error situation our application generates (the rest come from NSURLConnection). After that, I declare a protocol that defines the methods our delegate has to implement. There are only two: one that gets called when the download finishes successfully, and another that gets called if it fails. In the sample application, our error checking is minimal, but you will do real error checking in your apps, right? RIGHT?! Yes, you will, because that's the way you roll. In a real app, you might also add a third delegate method that to inform the delegate what percentage of the download is complete, but I'll leave that one as an exercise for the student.

Next, I declare the DeviantDownload object, which only has four instances variables, one of which is private. urlString is the url of the image to download, image is the downloaded image, and delegate is the object that wants to get notified about the status of the download. Finally, receivedData is just a buffer to hold the downloaded data until I have it all and can create the image from it.

Okay, let's look at the implementation of our download object:

#import "DeviantDownload.h"

@interface DeviantDownload()
@property (nonatomic, retain) NSMutableData *receivedData;

@implementation DeviantDownload
@synthesize urlString;
@synthesize image;
@synthesize delegate;
@synthesize receivedData;
#pragma mark -

- (UIImage *)image
if (image == nil && !downloading)
if (urlString != nil && [urlString length] > 0)
NSURLRequest *req = [[NSURLRequest alloc] initWithURL:[NSURL URLWithString:self.urlString]];
NSURLConnection *con = [[NSURLConnection alloc]
[con scheduleInRunLoop:[NSRunLoop currentRunLoop]
[con start];

if (con)
NSMutableData *data = [[NSMutableData alloc] init];
[data release];

NSError *error = [NSError errorWithDomain:DeviantDownloadErrorDomain
if ([self.delegate respondsToSelector:@selector(download:didFailWithError:)])
[delegate download:self didFailWithError:error];

[req release];

downloading = YES;


return image;

- (NSString *)filename
return [urlString lastPathComponent];

- (void)dealloc
[urlString release];
[image release];
delegate = nil;
[receivedData release];
[super dealloc];

#pragma mark -
#pragma mark NSURLConnection Callbacks
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
[receivedData setLength:0];

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
[receivedData appendData:data];

- (void)connection:(NSURLConnection *)connection
didFailWithError:(NSError *)error
[connection release];
if ([delegate respondsToSelector:@selector(download:didFailWithError:)])
[delegate download:self didFailWithError:error];

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
self.image = [UIImage imageWithData:receivedData];
if ([delegate respondsToSelector:@selector(downloadDidFinishDownloading:)])
[delegate downloadDidFinishDownloading:self];

[connection release];
self.receivedData = nil;

#pragma mark -
#pragma mark Comparison
- (NSComparisonResult)compare:(id)theOther
DeviantDownload *other = (DeviantDownload *)theOther;
return [self.filename compare:other.filename];


To make memory management more readable, I first declare a property for receivedData in a class extension. This creates a property that is accessible in my class but that isn't advertised to other classes. After that, I synthesize the four properties (three public declared in the header and one private declared in extension)

The first method I've written is an accessor for image. Remember, @synthesize creates an accessor and mutator if one doesn't already exist in your code, so by providing an accessor but not a mutator, @synthesize will create a mutator for me, but will use this method as its accessor. I don't want to download more than once, so I use the instance variable downloading to keep track of whether I've already kicked off a download. If image is nil and I haven't previously kicked off a download, I kick one off asynchronously.

At the end of the method, I return image, even though it will still be nil until the download is complete. That's okay, that will be the calling code's indication that they should become the download's delegate.

The next method, filename just returns the last part of urlString, which is the filename. I don't want to display the whole URL in the table.

The dealloc method is pretty standard stuff, so we can skip it.

The next four methods are the asynchronous callbacks for NSURLConnection. The first, connection:didReceiveResponse: is called when a connection is made. I set the length of receivedData to zero here because if there's a redirect, this will get called multiple times and I want to flush out the payload from the earlier redirected connections. That extra data, when it exists, would prevent our image from being created.

connection:didReceiveData: gets called periodically during the download, whenever there's data sitting in the buffer for us. All I do here is tack the newly received data onto the end of what's already been received.

In connection:didFailWithError:, I release the connection and notify our delegate. Because this is a data model object, I don't show an alert or anything. Dealing with the error isn't this object's responsibility. Once it reports it, its job is done; it'll let the controller figure out what to do with the error. This keeps our object firmly in the "model" component of the MVC paradigm and maximizes the chances I'll be able to reuse this object at some point.

Finally, in connectionDidFinishLoading:, I create an image from the received data, assign it to the image property and notify our delegate.

The last method, compare: is implemented to let us determine if two DeviantDownload objects represent the same file. I'll use this to look at two downloads and see if they are downloading the same file, which we don't want to do. Deviant Art uses a server farm to serve images, so the same image can have multiple URLs, but for a particular user, the filename must be unique, so I compare that.

Using the Download Object

In the table view controller, I pull down a list of the URLs for the given user. I'm not going to go into detail of the logic that does this, but in short, I asynchronously pull down JSON data for the user's gallery in chunks of 24 images, creating new DeviantDownload objects for each one and adding them to the NSMutableArray that drives our table. In our tableView:cellForRowAtIndexPath:, here's what I do:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 

DeviantDisplayCell *cell = (DeviantDisplayCell *)[tableView dequeueReusableCellWithIdentifier:[DeviantDisplayCell reuseIdentifier]];
if (cell == nil)
[[NSBundle mainBundle] loadNibNamed:@"DeviantDisplayCell" owner:self options:nil];
cell = loadCell;
self.loadCell = nil;

DeviantDownload *download = [downloads objectAtIndex:[indexPath row]];
cell.cellLabel.text = download.filename;
UIImage *cellImage = download.image;
if (cellImage == nil)
[cell.cellSpinner startAnimating];
download.delegate = self;

[cell.cellSpinner stopAnimating];

cell.cellImageView.image = cellImage;
return cell;

The first part of this method is standard table view cell stuff, but then I grab the DeviantDownload object for the row being displayed. If the image accessor method returns nil, I show the cell's activity indicator (aka "spinning wait-a s-cond doohickey") and assign the controller as the delegate of the download. If the object doesn't return nil, I make sure the activity indicator isn't animating and stick the download's image into the cell's image view. The first time this gets called for any particular row, image will return nil, which will trigger the download to start.

I also implement the DeviantDownload delegate method that gets called when the download is complete:

- (void)downloadDidFinishDownloading:(DeviantDownload *)download
NSUInteger index = [downloads indexOfObject:download];
NSUInteger indices[] = {0, index};
NSIndexPath *path = [[NSIndexPath alloc] initWithIndexes:indices length:2];
[self.tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:path] withRowAnimation:UITableViewRowAnimationNone];
[path release];
download.delegate = nil;

All I do here is figure out the index of the object that I've been notified has finished download and I build an index path based on it. I then tell the table to reload that one cell's row. Doing that will trigger tableView:cellForRowAtIndexPath: to get called again for the row that has completed and this time that method will get a valid image and will stop the activity indicator and show the downloaded image instead.

Ta da! That's it. It's really not any harder than using threads or NSOperationQueue once you get a feel for using the run-loop based approach.

If you want to try it out, you can download the full project right here. As always, the code is free for use without any restrictions or limitations, I'd just ask that you don't republish it unchanged and claim credit for it.

Saturday, May 22, 2010

Pre-WWDC Pilgrimage Sign-Up

If you think you're interested in traveling from Downtown San Francisco to One Infinite Loop, Cupertino on Saturday Sunday, June 6th to go to Apple's Company Store, or if you live in the area and just want to join us for lunch, please fill out this questionnaire. This does not obligate you to attend, it just will help us make sure that everybody who wants to go down, has a chance.

If there aren't enough drivers to carpool, we may look at what it would cost to charter a bus or van. We will notify everyone before we do so with the cost so people can decide if it's worth it. Hopefully, we'll be able to get everyone down without the hassle though.

Please do spread the word on your blogs and over Twitter so we can include everyone who wants to be included.

Friday, May 21, 2010

Gradient Buttons Yet Again

I added two more button styles to the iPhone Gradient Buttons project and made stroke color and stroke weight configurable options. The two new styles look exactly like the old Black and White style, only the highlight state is a blue gradient. These simulate the style used in alert sheets in several Apple applications.

Screen shot 2010-05-21 at 4.29.17 PM.png

Some of the Core Graphics code was machine-generated and is in kind of rough shape, but it works. I'm thinking about refactoring the different styles methods into something shorter, perhaps a plist, or just C arrays with the different numeric values.

For some strange reason, when I try to use arrayWithObjects: instead of creating a mutable array and manually adding the colors one at a time, I get a NULL gradient back. I have no idea why, but it's working this way at least.

Anyone who wants to contribute, let me know, I'd be happy to add committers.

The Illusion of Open

Today, on Twitter, I've been having some back and forth with John Wilker, one of the founders of the 360|iDev Conferences about Android and the concept of "openness". The discussion really helped to clarify some of my thoughts on the matter (thanks, John!).

Now, I've been somewhat harsh on Android at times, but the things I'm harsh about are details and personal programming platform preferences. It's actually a pretty good platform with a huge amount of potential. It now appears to have reached the critical mass needed to really propel it forward, and I do have high hopes that it will keep moving forward, getting better, and pressuring Apple to do even more amazing things than they would have otherwise done.

Yesterday, Google IO ended, and it was clear from the tone of the conference that Google is planning to put up some fierce competition to Apple on several fronts, and that's good. A lot of Google's pitch was focused on this idea of "openness" - that Google's stuff is inherently more "open" (except, of course, the stuff they make money from, but that's a whole separate topic) and therefore better for the user. Tim Bray, Google's Android Evangelist, went off on a rather enthusiastic but somewhat silly Twitter rant a few days ago about openness and the "curated experience" of the iPhone. It's clear that Google sees "openness" as a competitive advantage over Apple and has made it their battle cry in the mobile space.

But, not too long ago, Google announced that it was ending direct sales of their phone, the Nexus One.

Here's the reality of the Android situation now: if you buy an Android phone, it will most likely be locked down by your carrier, possibly also with some features disabled. Or, to use Tim Bray's term, the reality is that most Android phones that get bought are a "curated experience".

In some places, some carriers will sell unlocked phones, but for a great many people, if you want an open Android phone, you will be required to buy one from a carrier and jailbreak it, which is likely a violation of your subscriber agreement. If you don't jailbreak it, you may not get future Android updates. If you buy an Android phone and don't jailbreak it, you might spend the entire life of your phone using the Android version that shipped on it. Your vendor could even charge you a ridiculous monthly fee for the upgrade, something that at least Verizon has considered doing. Even if your carrier does provide updates for free and regularly, there will be a delay as the vendor and provider add all their customizations and restrictions on top of the official Android release.

For the vast majority of people who will buy Android phones, "open" is an illusion because now that Google has abandoned their direct sales model, Android firmly puts the final decision making power for the overall experience of the phone back into the hands of the traditional carrier/vendor relationship that ruled the space before the iPhone came out. Apple, unlike other phone vendors, is capable of going toe-to-toe with the carriers and is willing to do so to fight for a better user experience. That's why we don't have AT&T branding all over our iPhones. That's why we don't have the mandatory 15-second spiel before voicemail that Verizon users have to suffer through. Apple is at least an equal partner with the carriers who sell their phones. Most of the other phone vendors, to put it bluntly, are the carriers' bitches.

Does Android have some nice features that the iPhone doesn't? Absolutely. Is Android improving? No doubt about it and on a regular basis to boot. But, by putting the real power back in the hand of the carriers and their vendor partners, the user experience is never going to be as important in the decision making process as it is for the iPhone. Even if the Android team manages to make the overall experience better than the iPhone (which I consider unlikely, but possible), the carriers will almost certainly screw it up with their ham-handed customizations and restrictions.

If you're going to have a curated experience, isn't it better to at least have one where the curator is making their decisions primarily around the quality of your experience?

Unless Google resumes direct sales or puts licensing limitations on the carriers to prevent them from locking down Android phones, "open" will be just another empty marketing slogan. And I suspect that's what it will be. Google doesn't really care about the user experience, they just want to keep making money on their proprietary, non-open advertising in the mobile space the way they have on the web, and the more Android phones that are out there, the more phones that will be getting Google Ads. Hell, Google even discussed the possibility of unblockable ads at Google IO!

Right. Nothing screams "open" like unblockable advertisements served using proprietary algorithms based on personal data that's been collected about what you do online.