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11.1.1 Loading a UIImage
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The UIImage class offers seven different ways to create an instance of an image. The four factory methods are probably the easiest to use, and they re the ones we ve listed in table 11.1. You can also use some equivalent init methods if you prefer. The image data can be of several file types, including BMP, CUR, GIF, JPEG, PNG, and TIFF. In this book, we use mostly JPEGs (because they re small) and PNGs (because they look good and are accelerated on the iPhone hardware). You can also create a UIImage from a Quartz 2D object; this is the SDK s fundamental graphics package, which we ll talk about more in chapter 13. There s one suggested restriction when you re creating UIImages: the images shouldn t be larger than 1024 x 1024.
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Table 11.1 Factory methods for creating a UIImage Summary Creates a UIImage based on a file in the main bundle. Creates a UIImage from a Quartz 2D object. This is the same as initWithCGImage:. Creates a UIImage from a complete file path that you specify, as discussed in chapter 16. This is the same as initWithContentsOfFile:. Creates a UIImage from NSData. This is the same as initWithData:.
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Factory method imageNamed: imageWithCGImage:
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imageWithContentsOfFile:
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imageWithData:
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Media: images and the camera
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After you import an image into your program, you can display it. If you re going to stay entirely within the simple methods of the UIKit, you should use the UIImageView class to display the image.
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11.1.2 Drawing a UIImageView
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You ve already used the UIImageView in your programs when displaying pictures. We re now ready to talk about the details of how it works. You can initialize a UIImageView two ways. First, you can use the initWithImage: method, which allows you to pass a UIImage, as follows:
UIImage *myImage1 = [UIImage imageNamed:@"sproul1.jpg"]; UIImageView *myImageView = [[UIImageView alloc] initWithImage:myImage1]; [self.view addSubview:myImageView];
Alternatively, you can use a plain initWithFrame: method and modify the object s properties by hand. Table 11.2 shows a few of the properties and methods you re most likely to use when doing more extensive work with a UIImageView.
Table 11.2 A few properties and methods of note for UIImageView Type Property Property Summary Specifies how often an animation cycles Identifies an NSArray of images to load into the UIImageView Specifies how many times to run an animation cycle Identifies a single image to load into a UIImageView Starts the animation Stops the animation
Method or property animationDuration animationImages
animationRepeatCount image startAnimating stopAnimating
Property Property Method Method
To load a normal image, you can use the image property, but there s usually little reason to use it rather than the initWithImage: method unless you re dynamically changing your image. If you want to create a set of images to animate, it s useful to take advantage of the other UIImageView methods and properties. You can load an array of images into a UIImageView, declare how fast and how often they should animate, and start and stop them as you see fit. A simple example of this is shown in listing 11.1.
Listing 11.1 Using UIImageView to animate images
- (void)viewDidLoad { UIImage *myImage1 = [UIImage imageNamed:@"sproul1.jpg"]; UIImage *myImage2 = [UIImage imageNamed:@"sproul2.jpg"];
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UIImage *myImage3 = [UIImage imageNamed:@"sproul3.jpg"]; UIImage *myImage4 = [UIImage imageNamed:@"sproul4.jpg"]; UIImageView *myImageView = [[UIImageView alloc] initWithFrame:[[UIScreen mainScreen] bounds]]; myImageView.animationImages = [NSArray arrayWithObjects:myImage1, myImage2,myImage3,myImage4,nil]; myImageView.animationDuration = 4; [myImageView startAnimating]; [self.view addSubview:myImageView]; [myImageView release]; [super viewDidLoad]; }
This code first loads the images, then creates a UIView, and finally starts the animation. Taking advantage of UIImageView s animation capability is one of the main reasons you may want to load images by hand rather than do it through Interface Builder.
11.1.3 Modifying an image in the UIKit
You ve seen how to create images and load them into image views programmatically. The next thing to do is to start modifying them. Unfortunately, you have only a limited ability to do so while working with UIImageView. You can make some changes, based on simple manipulations of the view. For example, if you resize your UIImageView, it automatically resizes the picture it contains. Likewise, you can decide where to draw your UIImageView by setting its frame to something other than the whole screen. You can even layer multiple images by using multiple UIImageViews. This starts to get unwieldy quickly, though, and you can t do anything fancier, like transforming images or modifying how they stack through blending or alpha transparency options. To do that sort of work (and to stack graphics, not just views), you need to learn about Core Graphics. UIImage offers some simple ways to access Core Graphics functionality that doesn t require going out to the Core Graphics framework (or learning about contexts or the other complexities that underlie its use). We ll talk about those briefly here; but for the most part, Core Graphics will wait for the next chapter, which concentrates on the entire Quartz 2D graphics engine.
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