How to Load a UIImage From a URL

Early on in development of Addicus, we discovered how easy it was to load images from the web. This was a huge boon to us since it meant our artists could modify assets on our server and they would update live in the test builds, without having to make a new build.

This all hinged on the ability to load a UIImage from a URL. And as it happens, this can be accomplished in as little as one line of code:

UIImage * img = [[UIImage alloc] initWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://example.com/image.png"]]];

This eventually led to us writing a resource manager singleton class that handles all of our resource loading and can be toggled between local and web mode.

Eliminating class dependencies on your application delegate

Ever have the need to call a method inside your app delegate from an arbitrary place in your code?  We’ve had to deal with this issue in a few cases working on our new project.  We’re making use of a UIScrollView instance to give us that popular flick scroll movement that is so popular on the iPhone.  We’ve found that alot of people have been interested in doing the same thing and you can read our blog on that particular challenge here.  We have a UIScrollView instance sitting inside our app delegate and there are certain parts of our code where we need to move that UIScrollView among different layers in Cocos 2D.  To do that we need access to the scroll view instance itself.  That results in code like the following…

myAppDelegate *app = [[UIApplication sharedApplication] delegate];
UIScrollView *appOverlayScrollView  = (UIScrollView *)app.view;
[[app window] sendSubviewToBack:appOverlayScrollView];

This creates a messy dependency between an arbitrary class and the app delegate class.  Wouldn’t it be great if we could send a message to the app delegate and tell it to move the scroll view to the front or back maintaining encapsulation?  Turns out you can indeed do this.  The UIApplication class contains a method called sendAction which is normally used to send an action message identified by a selector to a specified target.  This method can easily be used to call selectors in any object so the above code can be converted to…

[[UIApplication sharedApplication]
sendAction:@selector(bringScrollViewToFront)
to:[[UIApplication sharedApplication] delegate]
from:self
forEvent:nil];

The only downside to this approach is that I haven’t found a way to supply arguments along with the selector so you need to be calling a method with no arguments.  If you know of a way around this issue please do let me know.

How to Display an Activity Indicator with UIActivityIndicatorView

We all wish that our apps could run without latency and pauses, but the reality is loading screens are needed from time to time. Today’s advent tip is how to display an activity indicator in your app like the one seen below. It’s great for soothing the savage, impatient user.

It can actually be done with just a few lines of code. First, we create and position the UIActivityIndicatorView by doing the following. It needs to be added to a view, so be sure to replace “myView” with your own view.

UIActivityIndicatorView *activityView = [[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge] autorelease];
[myView addSubview: activityView];
activityView.center = CGPointMake(240,160);

So now the activity indicator is created and positioned but it isn’t visible yet. You can make it visible at any time by running the following one line:

[activityView startAnimating];

Finally, once you’ve finished loading the copious amounts of awesomesauce that fuel your app, you can make the activity indicator go away by running the following solitary line of code:

[activityView stopAnimating];

How to Show an Alert with UIAlertView

Showing an alert message to the user isn’t just a handy interface lick, it’s also an invaluable debugging tool. Thankfully, this functionality is built into the iPhone SDK.

To pop an alert with a title, message and OK button, we use the UIAlertView class like so:

UIAlertView *alert = [[UIAlertView alloc]
			initWithTitle: @"Announcement"
			message: @"It turns out that you are playing Addicus!"
			delegate: nil
			cancelButtonTitle:@"OK"
			otherButtonTitles:nil];
[alert show];
[alert release];

But what if you want to have more than one button on the alert and collect user input from it? Well that is pretty simple too. To do that, you first need to create a class that employs the UIAlertViewDelegate delegate like so:

@interface MyClass : NSObject <UIAlertViewDelegate>

Then, in your class, you need to override the alertView method in order to receive input like so:

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex {
	if (buttonIndex == 0) {
		NSLog(@"user pressed OK");
	}
	else {
		NSLog(@"user pressed Cancel");
	}
}

Finally, we create the UIAlertView and pass in the delegate. Also note that we’ve labeled the Cancel button “Cancel” and added the “OK” button to the other button titles, which differs slightly from the above example.

UIAlertView *alert = [[UIAlertView alloc]
			initWithTitle: @"Announcement"
			message: @"It turns out that you are playing Addicus!"
			delegate: MY_DELEGATE
			cancelButtonTitle:@"Cancel"
			otherButtonTitles:@"OK",nil];
[alert show];
[alert release];

How to Open a URL in Safari

Tis the season to broaden your iPhone dev chops! We have been developing on the iPhone platform for about 6 months now and it turns out that in that time, you tend to learn lots of little tips and tricks. Since we’re overcome by the spirit of giving around this time of year, we are going to be posting 24 of these bite-sized iPhone development tips, 1 every day between now and Christmas day. Consider it an advent calendar of iPhone dev tip goodies.

On With The First Tip!

Say you would like to open a URL in Safari. We used this on the “Get Set” button on the main menu of Addicus. Here is how to do it with just one line of code using the UIApplication class:

[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"http://getsetgames.com/"]];