I just spent the past two days wrestling with trying to get a working implementation of tap to zoom working correctly in a UIScrollView.

I’m writing an iPhone application that generates a report view, and I really need to allow users to tap to zoom in, tap to zoom out, and still be able to use pinch zooming and panning.

A long search of the Apple demo code, and internet articles did not turn anything definitive up, besides the fact that almost everyone else seems to be having similar problems, including but not limited to: fuzzy renderings when expanding, confused and incorrectly zooming UIScrollViews after manual tap zooming, and much else besides.

I ran into all these problems, and eventually I concluded that the layers underneath a UIView are used to cache zoom renderings, and also carry other state, that it just is not possible to get at through the published api. There *is* a private api, but it is just that, private, and I’d like to get this app on the App Store, so I can’t use it.

Ok, so the approach I created is simply to recreate the scroll view and content view whenever I need to. It turns out that this works well enough for me, both in the simulator, and on the device.

The following is my TapZoomDemo I created to show this solution:

TapZoomAppDelegate.h

//
//  TapZoomDemoAppDelegate.h
//  TapZoomDemo
//
//  Created by Jonathan Watmough on 12/10/08.
//  Copyright __MyCompanyName__ 2008. All rights reserved.
//

#import 

@interface TapZoomDemoAppDelegate : NSObject 
{
    UIWindow *window;
}

// properties
@property (nonatomic, retain) IBOutlet UIWindow *window;

// methods
- (void)createScrollView:(CGRect)scrollFrame contentOffset:(CGPoint)contentOffset contentFrame:(CGRect)contentFrame
           scalingFactor:(float)scalingFactor;

@end

TapZoomAppDelegate.m

//
//  TapZoomDemoAppDelegate.m
//  TapZoomDemo
//
//  Created by Jonathan Watmough on 12/10/08.
//  Copyright __MyCompanyName__ 2008. All rights reserved.
//

#import "TapZoomDemoAppDelegate.h"
#import "MyContentView.h"

@implementation TapZoomDemoAppDelegate

@synthesize window;

// content min and max widths (used to calculate min/max scale factors)
// hardcoded to same as and 2x width of scroll view
static const float kMinWidth = 320;
static const float kMaxWidth = 640;

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

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

    // Override point for customization after application launch

    // put up a backing checked view
    UIView * checks = [[UIView alloc] initWithFrame:[[UIScreen mainScreen] applicationFrame]];
    checks.backgroundColor = [UIColor colorWithPatternImage:[UIImage imageNamed:@"4x4check.png"]];
    [window addSubview:checks];

    // use app frame for scroll view
    CGRect scrollFrame = [[UIScreen mainScreen] applicationFrame];

    // display our view initially at max size (2x), offset to be centered
    CGRect contentFrame = CGRectMake(0, 0, scrollFrame.size.width*2, scrollFrame.size.height*2);
    CGPoint contentOffset = CGPointMake((contentFrame.size.width-scrollFrame.size.width)/2,
                                        (contentFrame.size.height-scrollFrame.size.height)/2);

    // create scroll view and content view
    [self createScrollView:scrollFrame contentOffset:contentOffset contentFrame:contentFrame scalingFactor:0.0];

    // register for double-tap on content view
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doubleTapZoom:)
                                                 name:@"DoubleTapZoom" object:nil];
    // bring up app window
    [window makeKeyAndVisible];
}

// helper method for creating a scroll view and content view
- (void)createScrollView:(CGRect)scrollFrame contentOffset:(CGPoint)contentOffset contentFrame:(CGRect)contentFrame
           scalingFactor:(float)scalingFactor
{
    // create content frame at required size, with optional scaling
    MyContentView * content = [[MyContentView alloc] initWithFrame:contentFrame];
    if( scalingFactor!=0 )
        [content setTransform:CGAffineTransformMakeScale(scalingFactor, scalingFactor)];

    // create a new scroll view and add to window
    UIScrollView * scrollView = [[UIScrollView alloc] initWithFrame:scrollFrame];
    [window addSubview:scrollView];

    // reset min and max scaling
    [scrollView setMinimumZoomScale:kMinWidth/contentFrame.size.width];
    [scrollView setMaximumZoomScale:kMaxWidth/contentFrame.size.width];

    // constrain the content offset
    float offsetX = contentOffset.x;
    if( offsetX<0 )
        offsetX = 0;
    if( offsetX>contentFrame.size.width-scrollFrame.size.width )
        offsetX = contentFrame.size.width-scrollFrame.size.width;
    float offsetY = contentOffset.y;
    if( offsetY<0 )
        offsetY = 0;
    if( offsetY>contentFrame.size.height-scrollFrame.size.height )
        offsetY = contentFrame.size.height-scrollFrame.size.height;

    // setup the scroll view, size is passed, (possible bad) offset is passed
    // we'll go ahead and use the passed offset, but animate back to constrained
    [scrollView addSubview:content];
    [scrollView setContentSize:contentFrame.size];
    [scrollView setContentOffset:contentOffset];
    [scrollView setDelegate:self];

    // be nice and animate back to constrained offset, with optional scaling back to 1.0 (tap zoom)
    [UIView beginAnimations:@"" context:nil];
    [scrollView setContentOffset:CGPointMake(offsetX,offsetY)];
    if( scalingFactor!=0.0 )
        [content setTransform:CGAffineTransformIdentity];
    [UIView commitAnimations];

    // rather than cluttering up autorelease pool
    [content release];
    [scrollView release];
}

// scroll delegate methods

- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
    // return the first subview of the scroll view
    return [scrollView.subviews objectAtIndex:0];
}

- (void)scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(float)scale
{
    // get the view
    UIView * content = [scrollView.subviews objectAtIndex:0];

    // get size and offset of (dynamic scaled) content view
    CGPoint contentOffset = [scrollView contentOffset];
    CGSize contentSize = [scrollView contentSize];

    // dump the old views
    [content removeFromSuperview];
    [scrollView removeFromSuperview];

    // scale the size of the frame to scaled size
    CGRect contentFrame = CGRectMake(0, 0, contentSize.width, contentSize.height);

    // create a new scroll view
    CGRect scrollFrame = [[UIScreen mainScreen] applicationFrame];

    // create our new content view and containing scroll view
    [self createScrollView:scrollFrame contentOffset:contentOffset contentFrame:contentFrame scalingFactor:0.0];
}

// Double Tap Zoom
- (void)doubleTapZoom:(NSNotification *)notification
{
    // scroll view passed as object, get content subview
    UIScrollView * scrollView = [notification object];
    UIView * content = [[scrollView subviews] objectAtIndex:0];

    // get dictionary holding event, and event
    NSDictionary * dict = [notification userInfo];
    UIEvent * event = [dict objectForKey:@"event"];

    // get touch from set of touches for view
    UITouch * touch = [[event touchesForView:content] anyObject];

    // need scroll frame, content frame and content offset
    CGRect scrollFrame = scrollView.frame;
    CGRect contentFrame;
    CGPoint contentOffset;

    // zoom in if not at max size, other zoom out to fit to page
    float scalingFactor = 0.0;
    float currentWidth = [content frame].size.width;
    if( currentWidth < kMaxWidth )
    {
        contentFrame = CGRectMake(0, 0, scrollFrame.size.width*2, scrollFrame.size.height*2);
        scalingFactor = kMaxWidth / currentWidth;
        CGPoint touchPoint = [touch locationInView:content];
        contentOffset = CGPointMake(touchPoint.x*scalingFactor/2, touchPoint.y*scalingFactor/2);
    }
    else
    {
        scalingFactor = kMinWidth / currentWidth;
        contentFrame = CGRectMake(0, 0, scrollFrame.size.width, scrollFrame.size.height);
        contentOffset = CGPointMake(0,0);
    }

    // dump the old views
    [content removeFromSuperview];
    [scrollView removeFromSuperview];

    // create scroll view and content view
    [self createScrollView:scrollFrame contentOffset:contentOffset contentFrame:contentFrame scalingFactor:1.0/scalingFactor];
}

@end

MyContentView.m

//
//  MyContentView.m
//  TapZoomDemo
//
//  Created by Jonathan Watmough on 12/10/08.
//  Copyright 2008 __MyCompanyName__. All rights reserved.
//

#import "MyContentView.h"

@implementation MyContentView

- (id)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        // Initialization code
        [self setBackgroundColor:[UIColor whiteColor]];
    }
    return self;
}

- (void)drawRect:(CGRect)rect
{
    // Drawing code - This will be called with different bounds at the end of each
    // tap/pinch zoom operation.
    // For illustration, I'm scaling so that a '1' glyph fills the area.

    // context
    CGContextRef context = UIGraphicsGetCurrentContext();

    // Fill view with a 20-point '1'
    UIFont * f = [UIFont systemFontOfSize:20];
    [[UIColor darkGrayColor] set];

    // get bounds
    CGRect b = [self bounds];

    // draw a string
    NSString * text = @"1";
    CGSize sz = [text sizeWithFont:f];

    // scale to bounds / text size
    CGContextScaleCTM(context, b.size.width/sz.width, b.size.height/sz.height);

    // draw
    [text drawAtPoint:CGPointMake(0,0) withFont:f];
}

// look for double taps
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    // get any touch
    UITouch * t = [touches anyObject];
    if( [t tapCount]>1 )
    {
        NSDictionary *dict = [NSDictionary dictionaryWithObject:event forKey:@"event"];
        [[NSNotificationCenter defaultCenter] postNotificationName:@"DoubleTapZoom"
                                                            object:[self superview]
                                                          userInfo:dict];
    }
}

- (void)dealloc {
    [super dealloc];
}

@end