A Tasty Pixel » Cocoa

Easy inclusion of OpenSSL into iOS projects

Oddly, iOS doesn’t provide any OpenSSL implementation at all — If you want to do anything with crypto (like checking signatures, checksumming, etc.), you have to build in the library yourself.

I came across a great XCode project wrapper for OpenSSL yesterday, by Stephen Lombardo. This is an XCode project file that contains a target to build OpenSSL from source, and works with both Mac and iOS projects. I made some modifications to it, in order to make it work by just dropping in the OpenSSL source tarball, without having to dirty up your source tree with the extracted OpenSSL distribution.

Here’s how to use it:

Download the OpenSSL source.
Put the downloaded OpenSSL source tar.gz into the same folder
as openssl.xcodeproj (I put it in Library/openssl within my project tree).
Drag the openssl.xcodeproj file into your main project tree in XCode.
Right-click on your project target, and add openssl.xcodeproj under “Direct
Dependencies” on the General tab.
On the Build tab for your project’s target, find the “Header Search Paths”
option, and add the path:
> $(SRCROOT)/Library/openssl/build/openssl.build/openssl/include

(Assuming you’ve put openssl.xcodeproj at the path Library/openssl — adjust as necessary).
Expand your target’s “Link Binary With Libraries” build stage, and drag
libcrypto.a from the openssl.xcodeproj group.

Then, you can just import and use as normal (#import, etc).

Download it here

Avoid the clobber: Nest your network activity indicator updates

On the iPhone, when you are doing anything that uses the network, you’re supposed to let the user know something’s going on, via -[UIApplication setNetworkActivityIndicatorVisible:]. This takes a boolean.

That’s all well and good, but if you have more than one object in your app that may do things with the network simultaneously, you’re going to clobber yourself.

A nice and easy solution: Maintain an activity counter and create a category on UIApplication to maintain it, and show or hide the indicator as appropriate. Then, whenever you start doing something with the network:

[[UIApplication sharedApplication] showNetworkActivityIndicator];

…And when you’re done:

[[UIApplication sharedApplication] hideNetworkActivityIndicator];

Here’s a category that’ll do it:

Getting Data out of the iPhone while Debugging

Here’s a utility I whipped up quickly to save out a file from a hex string, as from [NSData description] — kinda a reverse hexdump.

While doing some debugging, I realised I needed to visualise an intermediate UIImage from the iPhone’s camera. Not being able to use the simulator, and thus be able to write to a file easily, this was my solution: po UIImageJPEGRepresentation(photo, 0.8) to print out the data as a hex string, then copied it to the clipboard, saved it as a text file, and used an NSScanner to scan in each int, fix the endianness and write it out as a file.

Insane? Maybe, but it did the trick.

Easy Delayed Messaging using NSProxy and NSInvocation

Sometimes it’s necessary to perform an action some time in the future, whether it’s disabling a button for a certain time interval after it’s pressed, performing an animation after a short wait, or triggering a reload of some data.

NSTimer is great for that purpose, as well as repeatedly performing actions, but it’s most convenient utility method, scheduledTimerWithTimeInterval:target:selector:userInfo:repeats: only takes one ‘id‘ argument. Using the NSInvocation equivalent, scheduledTimerWithTimeInterval:invocation:repeats: requires creating and setting up the NSInvocation itself, which is always verbose and a pain.

NSProxy is a wonderful little construct that lets us interact with it like we were talking to the original object. I first learned how it works from a post by Shaun Wexler which shows an easier way to send a message on the main thread, by using the NSInvocation given via the NSProxy’s forwardInvocation: method. The same technique can be used to easily create a configured NSTimer.

So, instead of some awful thing like this:

NSInvocation *i = [NSInvocation invocationWithMethodSignature:[mapView methodSignatureForSelector:@selector(selectAnnotation:animated:)]];
[i setTarget:mapView];
MKAnnotation *annotation = view.annotation;
[i setArgument:&amp;annotation atIndex:3];
BOOL flag=YES;
[i setArgument:&amp;flag atIndex:4];
[NSTimer scheduledTimerWithTimeInterval:0.5 invocation:i repeats:NO];

We can do this:

[(MKMapView*)[TPTimerProxy scheduledTimerProxyWithTarget:mapView timeInterval:0.5 repeats:NO] selectAnnotation:view.annotation animated:YES];

Here’s the juice:

iPhone debugging tip: Breaking on exceptions and reading their content

Just a quick one: This may be obvious to many devs, but it’s worth noting. One common and useful debugging technique is breaking on exceptions, so that you can see exactly where in your app’s flow a breakpoint occurs.

This can be done by adding -[NSException raise] and objc_exception_throw to your breakpoints list.

Once an exception happens, you can then check out the exception itself to see what went wrong. The approach varies between platforms. If you’re in the simulator (or any Mac OS X app running on Intel), the exception will be stored in the $eax register. Take a look by typing:

po $eax

If you’re on the iPhone, it’ll be $r0, so:

po $r0

A trick for capturing all touch input for the duration of a touch

If you’ve ever tried to implement an interactive control that makes use of gestures within a UITableView, or tried to implement a view that you can drag around, like pins on a MKMapView, you’ll know that you’re either generally thwarted by the scroll view (and the table view will just scroll, instead of your control getting the vertical drag gesture), or as soon as the touch moves outside the bounds of your view, you’ll get no more touchesMoved events, making for a very frustrating dragging interaction.

Some platforms give you a way to capture all pointer input for a time, but this doesn’t appear to be available out of the box on the iPhone — at least, I haven’t found it.

Here’s a way to make it work:

Subclass UIWindow and replace sendEvent: with your own method
Provide a way for objects to be registered with your UIWindow subclass to gain ‘touch priority’ – either add a property that taken a UIView/UIResponder, or add a mutable array to be able to register multiple views.
When you receive a touch began event, check to see if the touch was within the bounds of any ‘priority’ views – if they are, the view subsequently gets all events until the touch ended event.

iPhone/Mac animation for custom classes: Property animation for more than just CALayer

I recently wrote a custom view — a 3D vintage-looking pull lever — that provided a continuous property to control the state. I wanted to animated this smoothly, a-la CABasicAnimation, but couldn’t find a built-in way to do so.

So, I wrote a class that provides similar functionality to CABasicAnimation, but works on any object. I thought I’d share it.

Features:

From/to value settings (currently only supports NSNumber and scalar numeric types, but easily extendable)
Duration, delay settings
Timing functions: Linear, ease out, ease in, and ease in/ease out
Animation chaining (specify another configured animation for chainedAnimation, and it’ll be fired once the first animation completes)
Delegate notification of animation completion
Uses a single timer for smooth lock-step animation
Uses CADisplayLink if available, to update in sync with screen updates

Use it like this:

- (void)startMyAnimation {
  TPPropertyAnimation *animation = [TPPropertyAnimation propertyAnimationWithKeyPath:@"state"];
  animation.toValue = [NSNumber numberWithFloat:1.0]; // fromValue is taken from current value if not specified
  animation.duration = 1.0;
  animation.timing = TPPropertyAnimationTimingEaseIn;
  [animation beginWithTarget:self];
}

Make sure you also include the QuartzCore framework, used to access CADisplayLink, if it’s available.

It’s BSD-licensed.

Grab it here:
TPPropertyAnimation.zip

Links for May 30th through August 8th

Links for May 30th through August 8th:

cufón – fonts for the people A very impressive framework that embeds any font into a website, via javascript and the canvas element. Great cross-browser support.
mikeash.com: Method Replacement for Fun and Profit Method replacement and method swizzling in Objective-C.
Core Data Tutorial: How To Use NSFetchedResultsController | Ray Wenderlich
TwitThis – Use Multiple Twitter Clients on your iPhone Application The class TwitterClientManager loads a list list of supported Twitter clients is loaded from a plist file, which can be extended to support more clients in the future;
Each Twitter client is represented by an instance of the TwitterClient class;
The user can choose his preferred Twitter client at any time, and launch the application by a simple touch; the TwitterClientManager class stores the selected value in the user settings.