620	CHAPTER 17: Taps, Touches, and Gestures

Detecting Multiple Taps

In the TouchExplorer application, we printed the tap count to the screen, so you’ve already seen how easy it is to detect multiple taps. It’s not quite as straightforward as it seems, however, because often you will want to take different actions based on the number of taps. If the user triple-taps, you get notified three separate times. You get a single-tap, a double-tap, and finally a triple-tap. If you want to do something on a double-tap but something completely different on a triple-tap, having three separate notifications could cause a problem.

Fortunately, the engineers at Apple anticipated this situation, and they provided a mechanism to let multiple gesture recognizers play nicely together, even when they’re faced with ambiguous inputs that could seemingly trigger any of them. The basic idea is that you place a constraint on a gesture recognizer, telling it to not trigger its associated method unless some other gesture recognizer fails to trigger its own method.

That seems a bit abstract, so let’s make it real. One commonly used gesture recognizer is represented by the UITapGestureRecognizer class. A tap recognizer can be configured to do its thing when a particular number of taps occur. Imagine we have a view for which we want to define distinct actions that occur when the user taps once or double-taps. You might start off with something like the following:

UITapGestureRecognizer *singleTap = [[UITapGestureRecognizer alloc] initWithTarget: self action:@selector(doSingleTap)];

singleTap.numberOfTapsRequired = 1; [self.view addGestureRecognizer:singleTap];

UITapGestureRecognizer *doubleTap = [[UITapGestureRecognizer alloc] initWithTarget: self action:@selector(doDoubleTap)];

doubleTap.numberOfTapsRequired = 2; [self.view addGestureRecognizer:doubleTap];

The problem with this piece of code is that the two recognizers are unaware of each other, and they have no way of knowing that the user’s actions may be better suited to another recognizer. With the preceding code, if the user double-taps the view, the doDoubleTap method will be called, but the doSingleMethod will also be called—twice!— once for each tap.

The way around this is to create a failure requirement. We tell singleTap that it should trigger its action only if doubleTap doesn’t recognize and respond to the user input by adding this single line:

[singleTap requireGestureRecognizerToFail:doubleTap];

This means that when the user taps once, singleTap doesn’t do its work immediately. Instead, singleTap waits until it knows that doubleTap has decided to stop paying attention to the current gesture (that is, the user didn’t tap twice). We’re going to build on this further with our next project.

In Xcode, create a new project with the Single View Application template. Call this new project TapTaps, and use the Device Family popup to choose iPhone.

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This application is going to have four labels: one each that informs us when it has detected a single-tap, double-tap, triple-tap, and quadruple-tap (see Figure 17–4).

Figure 17–4. The TapTaps application detects up to four simultaneous taps.

We need outlets for the four labels, and we also need separate methods for each tap scenario to simulate what you would have in a real application. We’ll also include a method for erasing the text fields. Single-click BIDViewController.h, and make the following changes:

#import <UIKit/UIKit.h>

@interface BIDViewController : UIViewController

@property (weak, nonatomic) IBOutlet UILabel *singleLabel; @property (weak, nonatomic) IBOutlet UILabel *doubleLabel; @property (weak, nonatomic) IBOutlet UILabel *tripleLabel; @property (weak, nonatomic) IBOutlet UILabel *quadrupleLabel;

-(void)tap1;

-(void)tap2;

-(void)tap3;

-(void)tap4;

-(void)eraseMe:(UILabel *)label;

@end

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622	CHAPTER 17: Taps, Touches, and Gestures

Save the file. Then select BIDViewController.xib to edit the GUI. Once you’re there, add four Labels to the view from the library. Make all four labels stretch from blue guideline to blue guideline, set their alignment to centered, and then format them however you see fit. Feel free to make each label a different color, but that is by no means necessary. When you’re finished, control-drag from the File’s Owner icon to each label, and connect each one to singleLabel, doubleLabel, tripleLabel, and quadrupleLabel, respectively. Finally, make sure you double-click each label and press the delete key to get rid of any text.

Save your changes. Then select BIDViewController.m, and add the following code at the top of the file:

#import "BIDViewController.h"

@implementation BIDViewController

@synthesize singleLabel; @synthesize doubleLabel; @synthesize tripleLabel; @synthesize quadrupleLabel;

- (void)tap1 {

singleLabel.text = @"Single Tap Detected"; [self performSelector:@selector(eraseMe:)

withObject:singleLabel afterDelay:1.6f];

}

- (void)tap2 {

doubleLabel.text = @"Double Tap Detected"; [self performSelector:@selector(eraseMe:)

withObject:doubleLabel afterDelay:1.6f];

}

- (void)tap3 {

tripleLabel.text = @"Triple Tap Detected"; [self performSelector:@selector(eraseMe:)

withObject:tripleLabel afterDelay:1.6f];

}

- (void)tap4 {

quadrupleLabel.text = @"Quadruple Tap Detected"; [self performSelector:@selector(eraseMe:)

withObject:quadrupleLabel afterDelay:1.6f];

}

- (void)eraseMe:(UILabel *)label { label.text = @"";

}

.

.

.

Insert the following lines into the existing viewDidUnload method:

- (void)viewDidUnload {

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[super viewDidUnload];

//Release any retained subviews of the main view.

//e.g. self.myOutlet = nil;

self.singleLabel = nil; self.doubleLabel = nil; self.tripleLabel = nil; self.quadrupleLabel = nil;

}

Now, add the following code to viewDidLoad:

- (void)viewDidLoad { [super viewDidLoad];

// Do any additional setup after loading the view, typically from a nib.

UITapGestureRecognizer *singleTap = [[UITapGestureRecognizer alloc] initWithTarget:self

action:@selector(tap1)];

singleTap.numberOfTapsRequired = 1; singleTap.numberOfTouchesRequired = 1; [self.view addGestureRecognizer:singleTap];

UITapGestureRecognizer *doubleTap = [[UITapGestureRecognizer alloc] initWithTarget:self

action:@selector(tap2)];

doubleTap.numberOfTapsRequired = 2; doubleTap.numberOfTouchesRequired = 1; [self.view addGestureRecognizer:doubleTap];

[singleTap requireGestureRecognizerToFail:doubleTap];

UITapGestureRecognizer *tripleTap = [[UITapGestureRecognizer alloc] initWithTarget:self

action:@selector(tap3)];

tripleTap.numberOfTapsRequired = 3; tripleTap.numberOfTouchesRequired = 1; [self.view addGestureRecognizer:tripleTap];

[doubleTap requireGestureRecognizerToFail:tripleTap];

UITapGestureRecognizer *quadrupleTap = [[UITapGestureRecognizer alloc] initWithTarget:self

action:@selector(tap4)];

quadrupleTap.numberOfTapsRequired = 4; quadrupleTap.numberOfTouchesRequired = 1; [self.view addGestureRecognizer:quadrupleTap];

[tripleTap requireGestureRecognizerToFail:quadrupleTap];

}

The four tap methods do nothing more in this application than set one of the four labels and use performSelector:withObject:afterDelay: to erase that same label after 1.6 seconds. The eraseMe: method erases the text from any label that is passed into it.

The interesting part of this is what occurs in the viewDidLoad method. We start off simply enough, by setting up a tap gesture recognizer and attaching it to our view.

UITapGestureRecognizer *singleTap = [[UITapGestureRecognizer alloc] initWithTarget:self

action:@selector(tap1)];

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624	CHAPTER 17: Taps, Touches, and Gestures

singleTap.numberOfTapsRequired = 1; singleTap.numberOfTouchesRequired = 1; [self.view addGestureRecognizer:singleTap];

Note that we set both the number of taps (touches in the same position, one after another) required to trigger the action and touches (number of fingers touching the screen at the same time) to 1. After that, we set up another tap gesture recognizer to handle a double-tap.

UITapGestureRecognizer *doubleTap = [[UITapGestureRecognizer alloc] initWithTarget:self

action:@selector(tap2)];

doubleTap.numberOfTapsRequired = 2; doubleTap.numberOfTouchesRequired = 1; [self.view addGestureRecognizer:doubleTap];

[singleTap requireGestureRecognizerToFail:doubleTap];

This is pretty similar to the previous code, right up until that last line, in which we give singleTap some additional context. We are effectively telling singleTap that it should trigger its action only in case some other gesture recognizer—in this case doubleTap— decides that the current user input isn’t what it’s looking for.

Let’s think about what this means. With those two tap gesture recognizers in place, a single tap in the view will immediately make singleTap think, “Hey, this looks like it’s for me.” At the same time, doubleTap will think, “Hey, this looks like it might be for me, but I’ll need to wait for one more tap.” Because singleTap is set up to wait for doubleTap’s “failure,” it doesn’t send its action method right away; instead, it waits to see what happens with doubleTap.

After that first tap, if another tap occurs immediately, then doubleTap says, “Hey, that’s mine all right,” and fires its action. At that point, singleTap will realize what happened and give up on that gesture. On the other hand, if a particular amount of time goes by (the amount of time that the system considers to be the maximum length of time between taps in a double-tap), doubleTap will give up, and singleTap will see the failure and finally trigger its event.

The rest of the method goes on to define gesture recognizers for three and four taps, and at each point configure one gesture to be dependent on the failure of the next.

UITapGestureRecognizer *tripleTap = [[UITapGestureRecognizer alloc] initWithTarget:self

action:@selector(tap3)];

tripleTap.numberOfTapsRequired = 3; tripleTap.numberOfTouchesRequired = 1; [self.view addGestureRecognizer:tripleTap];

[doubleTap requireGestureRecognizerToFail:tripleTap];

UITapGestureRecognizer *quadrupleTap = [[UITapGestureRecognizer alloc] initWithTarget:self

action:@selector(tap4)];

quadrupleTap.numberOfTapsRequired = 4; quadrupleTap.numberOfTouchesRequired = 1; [self.view addGestureRecognizer:quadrupleTap];

[tripleTap requireGestureRecognizerToFail:quadrupleTap];

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