Detecting iOS 4 iPhone Touch Screen Gesture Motions (Xcode 4)

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An Example iOS 4 iPhone Touch, Multitouch and Tap Application (Xcode 4)Identifying iPhone Gestures using iOS 4 Gesture Recognizers (Xcode 4)


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The last area of iOS touch screen event handling that we will look at in this book involves the detection of gestures involving movement. As covered in a previous chapter, a gesture refers the activity that takes place in the time between a finger touching the screen and the finger then being lifted from the screen. In the chapter entitled An Example iOS 4 iPhone Touch, Multitouch and Tap Applications we dealt with touches that did not involve any movement across the screen surface. We will now create an example that tracks the coordinates of a finger as it moves across the screen.

Note that the assumption is made throughout this chapter that the reader has already reviewed the Overview of iOS 4 iPhone Multitouch, Taps and Gestures chapter of this book.




The Example iOS 4 iPhone Gesture Application

This example application will detect when a single touch is made on the screen of the iPhone and then report the coordinates of that finger as it is moved across the screen surface.

Creating the Example Project

Start the Xcode environment and select the option to create a new project using the View-based application template option for the iPhone and name the project touchMotion.


Creating Outlets

The application will display the X and Y coordinates of the touch and update these values in real-time as the finger moves across the screen. When the finger is lifted from the screen, the start and end coordinates of the gesture will then be displayed. This will require the creation of two outlets that will later be connected to labels within the user interface of the application. In addition, we need to add an instance variable to our view controller in which to store the start position of the touch. To create these outlets and the instance variable, select the touchMotionViewController.h file and modify it as follows:

#import <UIKit/UIKit.h>

@interface touchMotionViewController : UIViewController {
        UILabel *xCoord;
        UILabel *yCoord;
        CGPoint startPoint;
}
@property (retain, nonatomic) IBOutlet UILabel *xCoord;
@property (retain, nonatomic) IBOutlet UILabel *yCoord;
@property CGPoint startPoint;
@end

Next, edit the touchMotionViewController.m file and add the @synthesize directive:

#import "touchMotionViewController.h"

@implementation touchMotionViewController
@synthesize xCoord, yCoord, startPoint;
.
.
.
@end

Designing the Application User Interface

Select the touchMotionViewController.xib file to load the NIB file into Interface Builder. Within Interface Builder panel, create a user interface such that it resembles the layout in the following figure:

The layout of the iOS 4 iPhone touch motion example


Be sure to stretch the labels so that they both extend to cover most of the width of the view. Once the label objects have been added to the view, establish a connection between the xCoord outlet and the top label by Crtl-clicking on the File’s Owner and dragging the blue line to the top label. Release the pointer and select xCoord from the resulting menu. Repeat this step for the yCoord.

Implementing the touchesBegan Method

When the user first touches the screen the location coordinates need to be saved in the startPoint instance variable and the coordinates reported to the user. This can be achieved by implementing the touchesBegan method in the touchMotionViewController.m file as follows:

- (void) touchesBegan:(NSSet *)touches 
withEvent:(UIEvent *)event {
     UITouch *theTouch = [touches anyObject];
     startPoint = [theTouch locationInView:self.view];
     CGFloat x = startPoint.x;
     CGFloat y = startPoint.y;
     xCoord.text = [NSString stringWithFormat:@"x = %f", x];
     yCoord.text = [NSString stringWithFormat:@"y = %f", y];
}

Implementing the touchesMoved Method

When the user’s finger moves across the screen the touchesMoved event will be called repeatedly until the motion stops. By implementing the touchesMoved method in our view controller, therefore, we can detect the motion and display the revised coordinates to the user:

- (void) touchesMoved:(NSSet *)touches 
withEvent:(UIEvent *)event {
   UITouch *theTouch = [touches anyObject];
   CGPoint touchLocation = 
     [theTouch locationInView:self.view];
   CGFloat x = touchLocation.x;
   CGFloat y = touchLocation.y;
   xCoord.text = [NSString stringWithFormat:@"x = %f", x];
   yCoord.text = [NSString stringWithFormat:@"y = %f", y];
}

Implementing the touchesEnded Method

When the user’s finger lifts from the screen the touchesEnded method of the first responder is called. The final task, therefore, is to implement this method in our view controller such that it displays the start and end points of the gesture:

- (void) touchesEnded:(NSSet *)touches 
withEvent:(UIEvent *)event {
    UITouch *theTouch = [touches anyObject];
    CGPoint endPoint = [theTouch locationInView:self.view];
    xCoord.text = [NSString stringWithFormat:
      @"start = %f, %f", startPoint.x, startPoint.y];
    yCoord.text = [NSString stringWithFormat:
      @"end = %f, %f", endPoint.x, endPoint.y];
}

Building and Running Gesture Example

Build and run the application using the Run button located in the toolbar of the main Xcode project window. When the application starts (either in the iOS Simulator or on a physical device) touch the screen and drag to a new location before lifting your finger from the screen (or mouse button in the case of the iOS Simulator). During the motion the current coordinates will update in real time. Once the gesture is complete the start and end locations of the movement will be displayed.

Summary

Simply by implementing the standard touch event methods the motion of a gesture can easily be tracked by an iPhone application. Much of an iPhone user’s interaction with applications, however, involves some very specific gesture types such as swipes and pinches. To write code to correlate finger movement on the screen with a specific gesture type would be extremely complex. Fortunately, iOS 4 makes this task easy with the introduction of gesture recognizers. In the next chapter, entitled Identifying iPhone Gestures using iOS 4 Gesture Recognizers, we will look at this concept in more detail.

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iOS 10 App Development Essentials Print and eBook (ePub/PDF/Kindle) edition contains over 100 chapters. Learn more...

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PreviousTable of ContentsNext
An Example iOS 4 iPhone Touch, Multitouch and Tap Application (Xcode 4)Identifying iPhone Gestures using iOS 4 Gesture Recognizers (Xcode 4)