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xAcceleration, self.currentPoint.y + yAcceleration);

That ends our calculations, so all that’s left is to update lastUpdateTime with the current time.

lastUpdateTime = [[NSDate alloc] init];

Before you build the app, add the Core Motion framework using the technique mentioned earlier. Once it’s added, go ahead and build and run the app.

NOTE: Unfortunately, Ball just will not do much on the simulator. If you want to experience Ball in all its gravity-obeying grooviness, you’ll need to join the for-pay iOS Developer Program and

install it on your own device.

If all went well, the application will launch, and you should be able to control the movement of the ball by tilting the phone. When the ball gets to an edge of the screen, it should stop. Tip the phone back the other way, and it should start rolling in the other direction. Whee!

Rolling On

Well, we’ve certainly had some fun in this chapter with physics and the amazing iOS accelerometer and gyro. We created a great April Fools’ prank, and you got to see the basics of using the accelerometer as a control device. The possibilities for applications using the accelerometer and gyro are nearly as endless as the universe. So now that you have the basics down, go create something cool and surprise us!

When you feel up to it, we’re going to get into using another bit of iOS hardware: the built-in camera.

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Chapter 20

The Camera and Photo

Library

By now, it should come as no surprise to you that the iPhone, iPad, and iPod touch have a built-in camera and a nifty application called Photos to help you manage all those awesome pictures and videos you’ve taken. What you may not know is that your programs can use the built-in camera to take pictures. Your applications can also allow the user to select from among the media already stored on the device. We’ll look at both of these abilities in this chapter.

Using the Image Picker and UIImagePickerController

Because of the way iOS applications are sandboxed, applications ordinarily can’t get to photographs or other data that live outside their own sandboxes. Fortunately, both the camera and the media library are made available to your application by way of an image picker.

As the name implies, an image picker is a mechanism that lets you select an image from a specified source. When this class first appeared in iOS, it was used only for images. Nowadays, you can use it to capture video as well.

Typically, an image picker will use a list of images and/or videos as its source (see the left side of Figure 20–1). You can, however, specify that the picker use the camera as its source (see the right side of Figure 20–1).

D.Mark et al., Beginning iOS 5 Development

©Dave Mark, Jack Nutting, Jeff LaMarche 2011

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674	CHAPTER 20: The Camera and Photo Library

Figure 20–1. An image picker in action. Users are presented with a list of images (left), and then once an image is selected, they can move and scale the image (right).

The image picker interface is implemented by way of a modal controller class called UIImagePickerController. You create an instance of this class, specify a delegate (as if you didn’t see that coming), specify its image source and whether you want the user to pick an image or a video, and then launch it modally. The image picker will take control of the device to let the user select a picture or video from the existing media library, or to take a new picture or video with the camera. Once the user makes a selection, you can give the user an opportunity to do some basic editing, such as scaling or cropping an image or trimming away a bit of a video clip. All that behavior is implemented by the UIImagePickerController, so you really don’t need to do much heavy lifting here.

Assuming the user doesn’t press cancel, the image or video the user takes or selects from the library will be delivered to your delegate. Regardless of whether the user selects a media file or cancels, your delegate has the responsibility to dismiss the UIImagePickerController so that the user can return to your application.

Creating a UIImagePickerController is extremely straightforward. You just allocate and initialize an instance the way you would with most classes. There is one catch, however. Not every device that runs iOS has a camera. Older iPod touches were the first examples of this, and the first-generation iPad is the latest, but more such devices may roll off Apple’s assembly lines in the future. Before you create an instance of UIImagePickerController, you need to check to see whether the device your program is

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currently running on supports the image source you want to use. For example, before letting the user take a picture with the camera, you should make sure the program is running on a device that has a camera. You can check that by using a class method on

UIImagePickerController, like this:

if ([UIImagePickerController isSourceTypeAvailable: UIImagePickerControllerSourceTypeCamera]) {

In this example, we’re passing UIImagePickerControllerSourceTypeCamera to indicate that we want to let the user take a picture or shoot a video using the built-in camera. The method isSourceTypeAvailable: returns YES if the specified source is currently available. You can specify two other values in addition to

UIImagePickerControllerSourceTypeCamera:

UIImagePickerControllerSourceTypePhotoLibrary specifies that the user should pick an image or video from the existing media library. That image will be returned to your delegate.

UIImagePickerControllerSourceTypeSavedPhotosAlbum specifies that the user will select the image from the library of existing photographs, but that the selection will be limited to the most recent camera roll.

This option will run on a device without a camera, but does not do anything useful.

After making sure that the device your program is running on supports the image source you want to use, launching the image picker is relatively easy:

UIImagePickerController *picker = [[UIImagePickerController alloc] init]; picker.delegate = self;

picker.sourceType = UIImagePickerControllerSourceTypeCamera; [self presentModalViewController:picker animated:YES];

After we’ve created and configured the UIImagePickerController, we use a method that our class inherited from UIView called presentModalViewController:animated: to present the image picker to the user.

TIP: The presentModalViewController:animated: method is not limited to just presenting image pickers. You can present any view controller to the user, modally, by calling

this method on the view controller for a currently visible view.

Implementing the Image Picker Controller Delegate

The object that you want to be notified when the user has finished using the image picker interface needs to conform to the UIImagePickerControllerDelegate protocol. This protocol defines two methods: imagePickerController:didFinishPickingMediaWithInfo: and imagePickerControllerDidCancel:.

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The imagePickerController:didFinishPickingMediaWithInfo: method is called when the user has successfully taken a photo or video, or selected an item from the media library. The first argument is a pointer to the UIImagePickerController that you created earlier. The second argument is an NSDictionary instance that will contain the chosen photo or the URL of the chosen video, as well as optional editing information if you enabled editing and the user actually did some editing. That dictionary will also contain the original, unedited image stored under the key UIImagePickerControllerOriginalImage. Here’s an example of a delegate method that retrieves the original image:

-(void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {

UIImage *selectedImage = [info objectForKey:UIImagePickerControllerEditedImage]; UIImage *originalImage = [info objectForKey:UIImagePickerControllerOriginalImage];

// do something with selectedImage and originalImage

[picker dismissModalViewControllerAnimated:YES];

}

The editingInfo dictionary will also tell you which portion of the entire image was chosen during editing by way of an NSValue object stored under the key

UIImagePickerControllerCropRect. You can convert this string into a CGRect like so:

NSValue *cropValue = [editingInfo objectForKey:UIImagePickerControllerCropRect]; CGRect cropRect = [cropValue CGRectValue];

After this conversion, cropRect will specify the portion of the original image that was selected during the editing process. If you do not need this information, you can just ignore it.

CAUTION: If the image returned to your delegate comes from the camera, that image will not be

stored in the photo library. It is your application’s responsibility to save the image, if necessary.

The other delegate method, imagePickerControllerDidCancel:, is called if the user decides to cancel the process without capturing or selecting any media. When the image picker calls this delegate method, it’s just notifying you that the user is finished with the picker and didn’t choose anything.

Both of the methods in the UIImagePickerControllerDelegate protocol are marked as optional, but they really aren’t, and here is why: modal views like the image picker must be told to dismiss themselves. As a result, even if you don’t need to take any application-specific actions when the user cancels an image picker, you still need to dismiss the picker. At a bare minimum, your imagePickerControllerDidCancel: method will need to look like this in order for your program to function correctly:

-(void)imagePickerControllerDidCancel:(UIImagePickerController *)picker {

[picker dismissModalViewControllerAnimated:YES];

}

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