Preliminaries in Objective-C

Generator Code 3/9 in Objective-C Preliminaries

As you did in 6, please download and extract images and code for this chapter. If you need refreshing on how to do this, review Figures 6 2 thru 6 4. As usual, start off with a spotlessly clean desktop. Then, open a browser and navigate to http://rorylewis.com/xCode/009_DragRotateAndScale.zip and download its contents to your desktop. Then, extract the files onto your desktop. There will be four text files, one image file, and a folder containing the final working code for DragRotateAndScale (in the event you encounter trouble coding it yourself). The image file is the image I used in the example of my puppy Shaka. The text files consist of sections of boilerplate code from which I will ask you to copy and paste various pieces. You are welcome to use these files in any of your future programs that involve touches. You ll find Translate.rtf, HelperFunctions.rtf, TranslateRotateScale.rtf, and ViewController.rtf. Once you have extracted all the files, remember to delete the 009_DragRotateAndScale.zip and 009_DragRotateAndScale folders. Also, file the DragRotateAndScale Xcode to a safe place, for if you leave it on your desktop it will be overwritten and conflict with your exercise code. Monkeys will start writing Shakespeare, and the world will collapse and disappear all because you did not file it away. After you follow these directions, you will have five files on your desktop.

To start our DragRotateAndScale application, we need to make a new project in Xcode as we ve always done. Open Xcode and select the View-based Application template item (as shown in Figure 7 1).

Make sure the Product menu is set to iPhone before continuing. Name the project DragRotateAndScale and click the Save button, as shown in Figure 7 2.

Next, choose and prepare an image file as your main object for this exercise. Choose whatever is appealing, and save it on your desktop. By the way, we have chosen a smaller image than earlier exercises have used (100 100 pixels), to allow for manipulating and sizing. Copy the handy image file into the project s folder, as shown in Figure 7 3. Click the usual boxes and radio buttons to ensure proper management of this image file down the road.

Figure 7 3. Copy your desired image into the Xcode project. The image used in the example is 100 100 pixels.

We are going to add a new file to our project: a UIImageView subclass called TransformView. This class will intercept touch events and change their transforms accordingly. We are making TransformView a UIImageView subclass so that we can assign an image to our instance and then see it rendered on the screen. Technically, a UIView subclass can do what we want, but, for this example, we want to focus on the transforms and not worry about custom drawing code for a UIView subclass. Creating a custom view isn t strictly necessary for this application, but we will do it anyway to flex our subclassing muscles. The DragRotateAndScaleViewController could technically do everything we need to make this application work, without the need for a custom UIView subclass, but subclassing makes the code simpler and more robust. Thus, we select a new Objective-C class in the New File window, as shown in Figure 7 4.

In the header file, TransformView.h, make sure the superclass that is, the statement after the colon (:) is specified as UIImageView. We do this so we ll be able to assign an image quickly and easily. Your header file should look like the one in Figure 7 5.

For the implementation file, TransformView.m, we want to handle the creation of our view a little differently than normal. UIImageViews do not handle touch input by default, and they therefore reject any touch input they receive. This would not serve us! We want our view to handle not only touch input, but multiple touches. To do this, we override the method (id) initWithImage:(UIImage*)image. Inside of our override, we will insert the lines [self setUserInteractionEnabled:YES] and [self setMultipleTouchEnabled:YES]. Of course, we end each line with a semicolon. The first line will allow our view to respond to touch events. The second will allow TransformView to receive multiple touch events, which we ll need if we re going to enable users to pinch and spread their fingers to scale and rotate. Your code should look like Figure 7 6.