Question

First of all, this problem did not exist in iOS 4.3. I can still run the program in iOS 4.3 and won’t see any problem. Secondly, this is an iPad app.

The problem is that when I hold down an image and drag it, it won’t be dragged. It worked in iOS 4.3 but not iOS 5.

I created new testing project and cleared everything that’s not needed. The project I chose is a Single View Application and did not alter any of the AppDelegate files.

Here is the code.

myUIScrollViewClass.h

#import <UIKit/UIKit.h>


@interface myUIScrollViewClass : UIScrollView
{
}

@end

myUIScrollViewClass.m

#import "myUIScrollViewClass.h"


@implementation myUIScrollViewClass


- (id)init
{
    self = [super init];

    if (self) 
     {
     }

    return self;
}

- (void)didReceiveMemoryWarning
{
    [self didReceiveMemoryWarning];
}


-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
     [self.nextResponder touchesBegan: touches withEvent:event]; 
}

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

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event

{
     [self.nextResponder touchesEnded: touches withEvent:event]; 
}

-(void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event

{
     [self.nextResponder touchesCancelled: touches withEvent:event]; 
}

@end

ViewController.h

#import <UIKit/UIKit.h>
#import "myUIScrollViewClass.h"

@interface ViewController : UIViewController <UIScrollViewDelegate>
{
     myUIScrollViewClass *mainScrollView_;
     UIImageView *aTouchedImage_;
}

@property (retain, nonatomic) myUIScrollViewClass *mainScrollView_;
@property (retain, nonatomic) UIImageView *aTouchedImage_;

@end

ViewController.m

#import "ViewController.h"

@implementation ViewController

@synthesize mainScrollView_;
@synthesize aTouchedImage_;

- (void)didReceiveMemoryWarning
{
     [super didReceiveMemoryWarning];
     // Release any cached data, images, etc that aren't in use.
}

#pragma mark - View lifecycle

- (void)viewDidLoad
{
     mainScrollView_ = [[myUIScrollViewClass alloc] initWithFrame:self.view.frame];
     [self.view addSubview:mainScrollView_];

     mainScrollView_.delegate = self;
     mainScrollView_.contentSize = CGSizeMake(1024, 768);
     mainScrollView_.clipsToBounds = YES;
     mainScrollView_.bounces = NO;
     mainScrollView_.bouncesZoom = NO;
     mainScrollView_.showsVerticalScrollIndicator = NO;
     mainScrollView_.showsHorizontalScrollIndicator = NO;
     mainScrollView_.backgroundColor = [UIColor whiteColor];
     mainScrollView_.userInteractionEnabled = YES;

     aTouchedImage_ = [[UIImageView alloc] initWithImage: [UIImage imageNamed:@"image1.png"]];
     [aTouchedImage_ setFrame:CGRectMake(0, 0, 80, 80)];
     [mainScrollView_ addSubview:aTouchedImage_];
     [aTouchedImage_ setCenter:CGPointMake(512, 334)];


     [super viewDidLoad];
     // Do any additional setup after loading the view, typically from a nib.
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;
}

- (void)viewWillAppear:(BOOL)animated
{     
    [super viewWillAppear:animated];
}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
}

- (void)viewWillDisappear:(BOOL)animated
{
     [super viewWillDisappear:animated];
}

- (void)viewDidDisappear:(BOOL)animated
{
     [super viewDidDisappear:animated];
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
     // Return YES for supported orientations
     return YES;
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)anEvent
{
     mainScrollView_.scrollEnabled = NO;


     for (UITouch *touch in touches) 
     {
          [aTouchedImage_ setCenter:[touch locationInView:self.view]];
     }
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)anEvent
{           
     mainScrollView_.scrollEnabled = NO;


     for (UITouch *touch in touches) 
     {
          [aTouchedImage_ setCenter:[touch locationInView:self.view]];
     }
}

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)anEvent
{
     mainScrollView_.scrollEnabled = YES;


     for (UITouch *touch in touches)
     {        
     }
}

-(void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)anEvent
{
    mainScrollView_.scrollEnabled = YES;

    for (UITouch *touch in touches) 
    {
    }
}

@end

Answer 1

It seems I will answer my own question.

I have requested a support from Apple Developer Technical Support, and they told me to place my code in the UIScrollView rather than passing the touch events to another view. Apple themselves admitting that there is a problem with passing touch events.

Here is an extract of their reply:

“It looks like not all touch events are getting through to your view controller. Your UIScrollView is blocking touch events from getting through, even though is forwards those events to the next responder. The behaviour is very erratic, some touch events are being forwarded to your view controller, but not all of them.”

“Looks like a behaviour change in iOS 5.0.x. My guess it’s because of the view controller containment feature that was introduced.”

The other way to do it is to use UIGestureRecognizer to implement dragging of views. Something like the code below:

- (void)viewDidLoad 
{
    // ... other initialization code here

    UILongPressGestureRecognizer *longPress = [[[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(dragChild:)] autorelease];
    [longPress setMinimumPressDuration:0]; // recognize immediately when a finger comes down
    [draggableView addGestureRecognizer:longPress];
}

- (void)dragChild:(UILongPressGestureRecognizer *)longPress 
{
    [[longPress view] setCenter:[longPress locationInView:[[longPress view] superview]];
}