Ios – Can UIPinchGestureRecognizer and UIPanGestureRecognizer Be Merged

concatenationiosmatrix-multiplicationuigesturerecognizer

I am struggling a bit trying to figure out if it is possible to create a single combined gesture recognizer that combines UIPinchGestureRecognizer with UIPanGestureRecognizer.

I am using pan for view translation and pinch for view scaling. I am doing incremental matrix concatenation to derive a resultant final transformation matrix that is applied to the view. This matrix has both scale and translation. Using separate gesture recognizers leads to a jittery movement/scaling. Not what I want. Thus, I want to handle concatenation of scale and translation once within a single gesture. Can someone please shed some light on how to do this?

Best Answer

6/14/14: Updated Sample Code for iOS 7+ with ARC.

The UIGestureRecognizers can work together and you just need to make sure you don't trash the current view's transform matrix. Use the CGAffineTransformScale method and related methods that take a transform as input, rather than creating it from scratch (unless you maintain the current rotation, scale, or translation yourself.

Download Xcode Project

Sample UIPinchGesture project on Github.

Note: iOS 7 behaves weird with UIView's in IB that have Pan/Pinch/Rotate gestures applied. iOS 8 fixes it, but my workaround is to add all views in code like this code example.

Demo Video

Add them to a view and conform to the UIGestureRecognizerDelegate protocol

@interface ViewController () <UIGestureRecognizerDelegate>

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    UIView *blueView = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 150, 150)];
    blueView.backgroundColor = [UIColor blueColor];
    [self.view addSubview:blueView];
    [self addMovementGesturesToView:blueView];

    // UIImageView's and UILabel's don't have userInteractionEnabled by default!
    UIImageView *imageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"BombDodge.png"]]; // Any image in Xcode project
    imageView.center = CGPointMake(100, 250);
    [imageView sizeToFit];
    [self.view addSubview:imageView];
    [self addMovementGesturesToView:imageView];

    // Note: Changing the font size would be crisper than zooming a font!
    UILabel *label = [[UILabel alloc] init];
    label.text = @"Hello Gestures!";
    label.font = [UIFont systemFontOfSize:30];
    label.textColor = [UIColor blackColor];
    [label sizeToFit];
    label.center = CGPointMake(100, 400);
    [self.view addSubview:label];
    [self addMovementGesturesToView:label];
}

- (void)addMovementGesturesToView:(UIView *)view {
    view.userInteractionEnabled = YES;  // Enable user interaction

    UIPanGestureRecognizer *panGesture = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handlePanGesture:)];
    panGesture.delegate = self;
    [view addGestureRecognizer:panGesture];

    UIPinchGestureRecognizer *pinchGesture = [[UIPinchGestureRecognizer alloc] initWithTarget:self action:@selector(handlePinchGesture:)];
    pinchGesture.delegate = self;
    [view addGestureRecognizer:pinchGesture];
}

Implement gesture methods

- (void)handlePanGesture:(UIPanGestureRecognizer *)panGesture {
    CGPoint translation = [panGesture translationInView:panGesture.view.superview];

    if (UIGestureRecognizerStateBegan == panGesture.state ||UIGestureRecognizerStateChanged == panGesture.state) {
        panGesture.view.center = CGPointMake(panGesture.view.center.x + translation.x,
                                             panGesture.view.center.y + translation.y);
        // Reset translation, so we can get translation delta's (i.e. change in translation)
        [panGesture setTranslation:CGPointZero inView:self.view];
    }
    // Don't need any logic for ended/failed/canceled states
}

- (void)handlePinchGesture:(UIPinchGestureRecognizer *)pinchGesture {

    if (UIGestureRecognizerStateBegan == pinchGesture.state ||
        UIGestureRecognizerStateChanged == pinchGesture.state) {

        // Use the x or y scale, they should be the same for typical zooming (non-skewing)
        float currentScale = [[pinchGesture.view.layer valueForKeyPath:@"transform.scale.x"] floatValue];

        // Variables to adjust the max/min values of zoom
        float minScale = 1.0;
        float maxScale = 2.0;
        float zoomSpeed = .5;

        float deltaScale = pinchGesture.scale;

        // You need to translate the zoom to 0 (origin) so that you
        // can multiply a speed factor and then translate back to "zoomSpace" around 1
        deltaScale = ((deltaScale - 1) * zoomSpeed) + 1;

        // Limit to min/max size (i.e maxScale = 2, current scale = 2, 2/2 = 1.0)
        //  A deltaScale is ~0.99 for decreasing or ~1.01 for increasing
        //  A deltaScale of 1.0 will maintain the zoom size
        deltaScale = MIN(deltaScale, maxScale / currentScale);
        deltaScale = MAX(deltaScale, minScale / currentScale);

        CGAffineTransform zoomTransform = CGAffineTransformScale(pinchGesture.view.transform, deltaScale, deltaScale);
        pinchGesture.view.transform = zoomTransform;

        // Reset to 1 for scale delta's
        //  Note: not 0, or we won't see a size: 0 * width = 0
        pinchGesture.scale = 1;
    }
}

- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer {
    return YES; // Works for most use cases of pinch + zoom + pan
}

Resources

Related Solutions

Java – How to concatenate two arrays in Java

I found a one-line solution from the good old Apache Commons Lang library.
ArrayUtils.addAll(T[], T...)

Code:

String[] both = ArrayUtils.addAll(first, second);

Ios – What’s the difference between the atomic and nonatomic attributes

The last two are identical; "atomic" is the default behavior (~~note that it is not actually a keyword; it is specified only by the absence of nonatomic~~ -- atomic was added as a keyword in recent versions of llvm/clang).

Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.

In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".

What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.

Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.

Adding to this:

atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.

Consider:

 @property(atomic, copy) NSString *firstName;
 @property(atomic, copy) NSString *lastName;
 @property(readonly, atomic, copy) NSString *fullName;

In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.

To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.