Basic iPhone Animation using Core Animation (Xcode 4)
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The majority of the visual effects used throughout the iOS 4 user interface on the iPhone are performed using Core Animation. Core Animation provides a simple mechanism for implementing basic animation within an iPhone application. If you need a user interface element to gently fade in or out of view, slide smoothly across the screen or gracefully resize or rotate before the user’s eyes these effects can be achieved using Core Animation in just a few lines of code.
In this chapter we will provide an overview of the basics of Core Animation and work through a simple example. While much can be achieved with Core Animation, however, it should be noted that if you plan to develop a graphics intensive 3D style application then it is more likely that OpenGL ES will need to be used, a subject area to which numerous books are dedicated.
UIView Core Animation Blocks
The concept of Core Animation involves the implementation of so-called animation blocks. Animation blocks are used to mark the beginning and end of a sequence of changes to the appearance of a UIView and its corresponding subviews. Once the end of the block is reached the animation is committed and the changes are performed over a specified duration. For the sake of example, consider a UIView object that contains a UIButton connected to an outlet named theButton. The application requires that the button gradually fade from view over a period of 3 seconds. This can be achieved by making the button transparent through the use of the alpha property:
theButton.alpha = 0;
Simply setting the alpha property to 0, however, causes the button to immediately become transparent. In order to make it fade out of sight gradually we need to place this line of code in an animation block. The start of an animation block is represented by a call to the beginAnimations class method of the UIView class:
[UIView beginAnimations:nil context:nil];
The end of the animation block triggers the animation sequence through a call to the commitAnimations method:
[UIView commitAnimations];
A variety of properties may also be defined within the animation block. For example, the duration of the animation (in our hypothetical example this needs to be 3 seconds) can be declared by a call to the setAnimationDuration class method:
[UIView setAnimationDuration:3];
Bringing this all together gives us a code sequence to gradually fade out a button object over a period of 3 seconds:
[UIView beginAnimations:nil context:nil]; [UIView setAnimationDuration:3]; theButton.alpha = 0; [UIView commitAnimations];
Understanding Animation Curves
In addition to specifying the duration of an animation sequence, the linearity of the animation timeline may also be defined by calling the UIView setAnimationCurve class method. This setting controls whether the animation is performed at a constant speed, whether it starts out slow and speeds up and so on. There are currently four possible animation curve settings:
- UIViewAnimationCurveLinear – The animation is performed at constant speed for the specified duration.
- UIViewAnimationCurveEaseOut – The animation starts out fast and slows as the end of the sequence approaches
- UIViewAnimationCurveEaseIn – The animation sequence starts out slow and speeds up as the end approaches.
- UIViewAnimationCurveEaseInOut – The animation starts slow, speeds up and then slows down again.
Receiving Notification of Animation Completion
Once an animation sequence has been committed and is underway it may be necessary to receive notification when the animation is completed so that the application code can, for example, trigger another animation sequence. The UIView setAnimationDidStopSelector class method allows a method to be specified that will be called when the animation sequence is completed. For example, the following code fragment declares that the method named animationFinished is to be called at the end of the animation sequence:
[UIView setAnimationDidStopSelector:
@selector(animationFinished:finished:context:)];
The animationFinished method would subsequently be declared as follows:
-(void)animationFinished:(NSString *)animationID
finished:(NSNumber *)finished
context:(void *)context
{
// Code to be executed on completion of animation sequence
}
Performing Affine Transformations
Transformations allow changes to be made to the coordinate system of a screen area. This essentially allows the programmer to rotate, resize and translate a UIView object. A call is made to one of a number transformation functions and the result assigned to the transform property of the UIView object.
For example, to change the scale of a UIView object named myView by a factor of 2 in both height and width:
myView.transform = CGAffineTransformMakeScale(2, 2);
Similarly, the UIView object may be rotated using the CGAffineTransformMakeRotation which takes as an argument the angle (in radians) by which the view is to be rotated. The following code, for example, rotates a view by 90 degrees:
myView.transform = CGAffineTransformMakeRotation( 90 * M_PI / 180);
The key point to keep in mind with transformations is that they become animated effects when performed within an animation block. The transformations evolve over the duration of the animation and follow the specified animation curve in terms of timing.
Combining Transformations
Two transformations may be combined to create a single transformation effect via a call to the CGAffineTransformConcat function. This function takes as arguments the two transformation objects that are to be combined. The result may then be assigned to the transform property of the UIView object to be transformed. The following code fragment, for example, both scales and rotates a UIView object named myView:
CGAffineTransform scaleTrans =
CGAffineTransformMakeScale(2, 2);
CGAffineTransform rotateTrans =
CGAffineTransformMakeRotation(angle * M_PI / 180);
myView.transform = CGAffineTransformConcat(scaleTrans, rotateTrans);
Affine transformations offer an extremely powerful and flexible mechanism for creating animations and it is just not possible to do justice to these capabilities in a single chapter. In order to learn more about affine transformations a good starting place is the Transforms chapter of Apple’s Quartz 2D Programming Guide.
Creating the Animation Example Application
The remainder of this chapter is dedicated to the creation of an iPhone application that demonstrates the use of Core Animation. The end result is a simple application on which a blue square appears. When the user touches a location on the screen the box moves to that location. Through the use of affine transformations, the box will rotate 180 degrees as it moves to the new location whilst also changing in size.
Begin by launching Xcode and creating a new View-based iOS application named animate.
Implementing the Interface File
For the purposes of this application we will need a UIView to represent the blue square and variables to contain the rotation angle and scale factor by which the square will be transformed. These need to be declared in the animateViewController.h file as follows:
#import <UIKit/UIKit.h>
@interface animateViewController : UIViewController {
UIView *boxView;
float scaleFactor;
float angle;
}
@property (nonatomic, retain) UIView *boxView;
@end
Drawing in the UIView
Having declared the UIView reference we now need to initialize an instance object and draw a blue square located at a specific location on the screen. We also need to initialize our scaleFactor and angle variables and add boxView as a subview of the application’s main view object. These tasks only need to be performed once when the application first starts up so a good location is the loadView method of the animateViewController.m file. Note that we also need to synthesize access to boxView:
#import "animateViewController.h"
@implementation animateViewController
@synthesize boxView;
.
.
// Implement loadView to create a view hierarchy programmatically, without using a nib.
- (void)loadView {
[super loadView];
scaleFactor = 2;
angle = 180;
CGRect frameRect = CGRectMake(10, 10, 45, 45);
boxView = [[UIView alloc] initWithFrame:frameRect];
boxView.backgroundColor = [UIColor blueColor];
[self.view addSubview:boxView];
}
.
.
@end
Detecting Screen Touches and Performing the Animation
When the user touches the screen the blue box needs to move from its current location to the location of the touch. During this motion, the box will rotate 180 degrees and change in size. The detection of screen touches was covered in detail in An Overview of iOS 4 iPhone Multitouch, Taps and Gestures. For the purposes of this example we want to initiate the animation at the point that the user’s finger is lifted from the screen so need to implement the touchesEnded method in the animateViewController.m file:
-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
UITouch *touch = [touches anyObject];
CGPoint location = [touch locationInView:self.view];
[UIView beginAnimations:nil context:nil];
[UIView setAnimationDelegate:self];
[UIView setAnimationDuration:2];
[UIView setAnimationCurve:UIViewAnimationCurveEaseInOut];
CGAffineTransform scaleTrans =
CGAffineTransformMakeScale(scaleFactor, scaleFactor);
CGAffineTransform rotateTrans =
CGAffineTransformMakeRotation(angle * M_PI / 180);
boxView.transform = CGAffineTransformConcat(scaleTrans, rotateTrans);
angle = (angle == 180 ? 360 : 180);
scaleFactor = (scaleFactor == 2 ? 1 : 2);
boxView.center = location;
[UIView commitAnimations];
}
Before compiling and running the application we need to take some time to describe the actions performed in the above method. First, the method gets the UITouch object from the touches argument and the locationInView method of this object is called to identify the location on the screen where the touch took place:
UITouch *touch = [touches anyObject]; CGPoint location = [touch locationInView:self.view];
The animation block is then started and the current class declared as the delegate. The duration of the animation is set to 2 seconds and curve set to ease in/ease out:
[UIView beginAnimations:nil context:nil]; [UIView setAnimationDelegate:self]; [UIView setAnimationDuration:2]; [UIView setAnimationCurve:UIViewAnimationCurveEaseInOut];
Two transformations are then generated for the view, one to scale the size of the view and one to rotate it 180 degrees. These transformations are then combined into a single transformation and applied to the UIView object:
CGAffineTransform scaleTrans = CGAffineTransformMakeScale(scaleFactor, scaleFactor); CGAffineTransform rotateTrans = CGAffineTransformMakeRotation(angle * M_PI / 180); boxView.transform = CGAffineTransformConcat(scaleTrans, rotateTrans);
Ternary operators are then used to switch the scale and rotation angle variables ready for the next touch. In other words, after rotating 180 degrees on the first touch the view will need to be rotated to 360 degrees on the next animation. Similarly, once the box has been scaled by a factor of 2 it needs to scale back to its original size on the next animation:
angle = (angle == 180 ? 360 : 180); scaleFactor = (scaleFactor == 2 ? 1 : 2);
Finally, the location of the view is moved to the point on the screen where the touch occurred before the animation is committed:
boxView.center = location; [UIView commitAnimations];
Once the touchesEnded method has been implemented it is time to try out the application.
Building and Running the Animation Application
Once the all the code changes have been made and saved, click on the Run button in the Xcode toolbar. Once the application has compiled it will load into the iOS Simulator (refer to [Testing iOS 4 Apps on the iPhone – Developer Certificates and Provisioning Profile for steps on how to run the application on an iPhone device).
When the application loads the blue square should appear near the top left hand corner of the screen. Click (or touch if running on a device) the screen and watch the box glide and rotate to the new location, the size of the box changing as it moves: <google>IOSBOX</google>
Summary
Core Animation provides an easy to implement interface to animation within iOS 4 iPhone applications. From the simplest of tasks such as gracefully fading out a user interface element to basic animation and transformations, Core Animation provides a variety of techniques for enhancing user interfaces. This chapter covered the basics of Core Animation before working step-by-step through an example to demonstrate the implementation of motion, rotation and scaling animation.
Learn SwiftUI and take your iOS Development to the Next Level
|
| Previous | Table of Contents | Next |
| An iOS 4 iPhone Graphics Drawing Tutorial using Quartz 2D (Xcode 4) | Integrating iAds into an iOS 4 iPhone App (Xcode 4) |
