Scrolling with two fingers with a UIScrollView

Question

I have an app where my main view accepts both touchesBegan and touchesMoved, and therefore takes in single finger touches, and drags. I want to implement a UIScrollView, and I have it working, but it overrides the drags, and therefore my contentView never receives them. I'd like to implement a UIScrollview, where a two finger drag indicates a scroll, and a one finger drag event gets passed to my content view, so it performs normally. Do I need create my own subclass of UIScrollView?

Here's my code from my appDelegate where I implement the UIScrollView.

@implementation MusicGridAppDelegate

@synthesize window;
@synthesize viewController;
@synthesize scrollView;


- (void)applicationDidFinishLaunching:(UIApplication *)application {    

    // Override point for customization after app launch    
    //[application setStatusBarHidden:YES animated:NO];
    //[window addSubview:viewController.view];

    scrollView.contentSize = CGSizeMake(720, 480);
    scrollView.showsHorizontalScrollIndicator = YES;
    scrollView.showsVerticalScrollIndicator = YES;
    scrollView.delegate = self;
    [scrollView addSubview:viewController.view];
    [window makeKeyAndVisible];
}


- (void)dealloc {
    [viewController release];
    [scrollView release];
    [window release];
    [super dealloc];
}

Answer 1

Bad news: iPhone SDK 3.0 and up, don't pass touches to -touchesBegan: and -touchesEnded: **UIScrollview**subclass methods anymore. You can use the touchesShouldBegin and touchesShouldCancelInContentView methods that is not the same.

If you really want to get this touches, have one hack that allow this.

In your subclass of UIScrollView override the hitTest method like this:

- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event {

  UIView *result = nil;
  for (UIView *child in self.subviews)
    if ([child pointInside:point withEvent:event])
      if ((result = [child hitTest:point withEvent:event]) != nil)
        break;

  return result;
}

This will pass to you subclass this touches, however you can't cancel the touches to UIScrollView super class.

Answer 2

In SDK 3.2 the touch handling for UIScrollView is handled using Gesture Recognizers.

If you want to do two-finger panning instead of the default one-finger panning, you can use the following code:

for (UIGestureRecognizer *gestureRecognizer in scrollView.gestureRecognizers) {     
    if ([gestureRecognizer  isKindOfClass:[UIPanGestureRecognizer class]]) {
        UIPanGestureRecognizer *panGR = (UIPanGestureRecognizer *) gestureRecognizer;
        panGR.minimumNumberOfTouches = 2;               
    }
}

Answer 3

For iOS 5+, setting this property has the same effect as the answer by Mike Laurence:

self.scrollView.panGestureRecognizer.minimumNumberOfTouches = 2;

One finger dragging is ignored by panGestureRecognizer and so the one finger drag event gets passed to the content view.

Answer 4

What I do is have my view controller set up the scroll view:

[scrollView setCanCancelContentTouches:NO];
[scrollView setDelaysContentTouches:NO];

And in my child view I have a timer because two-finger touches usually start out as one finger followed quickly by two fingers.:

- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    // Hand tool or two or more touches means a pan or zoom gesture.
    if ((selectedTool == kHandToolIndex) || (event.allTouches.count > 1)) {
        [[self parentScrollView] setCanCancelContentTouches:YES];
        [firstTouchTimer invalidate];
        firstTouchTimer = nil;
        return;
    }

    // Use a timer to delay first touch because two-finger touches usually start with one touch followed by a second touch.
    [[self parentScrollView] setCanCancelContentTouches:NO];
    anchorPoint = [[touches anyObject] locationInView:self];
    firstTouchTimer = [NSTimer scheduledTimerWithTimeInterval:kFirstTouchTimeInterval target:self selector:@selector(firstTouchTimerFired:) userInfo:nil repeats:NO];
    firstTouchTimeStamp = event.timestamp;
}

If a second touchesBegan: event comes in with more than one finger, the scroll view is allowed to cancel touches. So if the user pans using two fingers, this view would get a touchesCanceled: message.

Answer 5

Check out my solution:

#import “JWTwoFingerScrollView.h”

@implementation JWTwoFingerScrollView

- (id)initWithCoder:(NSCoder *)coder {
    self = [super initWithCoder:coder];
    if (self) {
        for (UIGestureRecognizer* r in self.gestureRecognizers) {
            NSLog(@“%@”,[r class]);
            if ([r isKindOfClass:[UIPanGestureRecognizer class]]) {
                [((UIPanGestureRecognizer*)r) setMaximumNumberOfTouches:2];
                [((UIPanGestureRecognizer*)r) setMinimumNumberOfTouches:2];
            }
        }
    }
    return self;
}

-(void)firstTouchTimerFired:(NSTimer*)timer {
    [self setCanCancelContentTouches:NO];
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    [self setCanCancelContentTouches:YES];
    if ([event allTouches].count == 1){
        touchesBeganTimer = [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(firstTouchTimerFired:) userInfo: nil repeats:NO];
        [touchesBeganTimer retain];
        [touchFilter touchesBegan:touches withEvent:event];
    }
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    [touchFilter touchesMoved:touches withEvent:event];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    NSLog(@“ended %i”,[event allTouches].count);
    [touchFilter touchesEnded:touches withEvent:event];
}

-(void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event {
    NSLog(@“canceled %i”,[event allTouches].count);
    [touchFilter touchesCancelled:touches withEvent:event];
}

@end

It does not delays the first touch and does not stop when the user touches with two fingers after using one. Still it allows to cancel a just started one touch event using a timer.

Answer 6

Yes, you'll need to subclass UIScrollView and override its -touchesBegan: and -touchesEnded: methods to pass touches "up". This will probably also involve the subclass having a UIView member variable so that it knows what it's meant to pass the touches up to.