Programmatically zooming a UIWebView without gestures - Center Point not calculated correctly

Question

The Problem

I have a UIWebView inside my iPad application which I need to zoom programmatically, but without the use of gestures/taps. The app. has a "+" and a "-" button for zooming in-and-out in user-defined increments (it is an app. for the visually impaired).

This +/- zoom-button functionality previously worked 100% fine when my app. used a UIImageView inside of a UIScrollView (instead of the WebView). The ImageView was the actual view which was zoomed, and the CenterPoint was calculated for the ScrollView's zoomToRect method.

I now have a WebView, which I know contains a ScrollView as a subview. I tried adapting the code which previously worked with the ImageView/ScrollView combo to instead zoom the WebView's ScrollView, but it is no longer calculating the CenterPoint correctly for zoomToRect:.

What Happens:

The WebView zooms correctly in-terms of the zoom-level, but the center point is always wrong. For some reason, the screen always zooms-in on the top-left every time.

Another odd problem, is that after the very first time you zoom-in, you cannot scroll in the WebView past the visible portion of the screen. If you try to, it shows a bit of the content past the bounds of the currently visible area, but it instantly snaps-back.

What I have tried:

I am trying to zoom the UIWebView's ScrollView.

I create a pointer to the ScrollView, and set "self" as its delegate. I then setup various variables, such as scrSize (the size of the view to zoom) and ZoomHandler (explained below):

- (void)viewDidLoad {
  // ... Various UIWebView setup ...    
  [self LoadURL];

  // Zooming & scrollview setup
  zoomHandler = [[ZoomHandler alloc] initWithZoomLevel: ZOOM_STEP];
  scrSize = CGPointMake(self.WebView.frame.size.width, self.WebView.frame.size.height);

  scrollView = [WebView.subviews objectAtIndex:0];
  [scrollView setTag:ZOOM_VIEW_TAG];
  [scrollView setMinimumZoomScale:MINIMUM_SCALE];
  [scrollView setZoomScale:1];
  [scrollView setMaximumZoomScale:10];
  scrollView.bounces = FALSE;
  scrollView.bouncesZoom = FALSE;
  scrollView.clipsToBounds = NO;
  [scrollView setDelegate:self];

  [super viewDidLoad];
}

To override the WebView's default zooming limitations, I inject this Javascript into the loaded webpage in the webViewDidFinishLoad: method:

function increaseMaxZoomFactor() {
  var element = document.createElement('meta');
  element.name = "viewport";
  element.content = "maximum-scale=10 minimum-scale=1 initial-scale=1 user-scalable=yes width=device-width height=device-height;"
  var head = document.getElementsByTagName('head')[0];
  head.appendChild(element);
}

CenterPoint Code:

This code is used to calculate the CenterPoint to pass into zoomToRect:. This worked 100% fine when I was zooming an ImageView inside of a ScrollView.

-(IBAction)zoomOut {
    float newScale = [scrollView zoomScale] / ZOOM_STEP;
    if( [scrollView zoomScale] > MINIMUM_SCALE) {
        [self handleZoomWith:newScale andZoomType: FALSE];
    }
}

-(IBAction)zoomIn {
    float newScale = [scrollView zoomScale] * ZOOM_STEP;
    if( [scrollView zoomScale] < MAXIMUM_SCALE){
        [self handleZoomWith:newScale andZoomType: TRUE];
    }
}

-(void)handleZoomWith: (float) newScale andZoomType:(BOOL) isZoomIn {
    CGPoint newOrigin = [zoomHandler getNewOriginFromViewLocation: [scrollView contentOffset] 
                                                        viewSize: scrSize andZoomType: isZoomIn];
    CGRect zoomRect = [self zoomRectForScale:newScale withCenter:newOrigin];
    [scrollView zoomToRect:zoomRect animated:YES];
}

- (CGRect)zoomRectForScale:(float)scale withCenter:(CGPoint)center {
    CGRect zoomRect;

    //  At a zoom scale of 1.0, it would be the size of the scrollView's bounds.
    //  As the zoom scale decreases, so more content is visible, the size of the rect grows.
    zoomRect.size.height = [WebView frame].size.height / scale;
    zoomRect.size.width  = [WebView frame].size.width  / scale;

    // Choose an origin so as to get the right center.
    zoomRect.origin.x = center.x / scale;
    zoomRect.origin.y = center.y / scale;

    return zoomRect;
}

/**
 Determine the origin [THIS IS INSIDE ZOOMHANDLER]
 */
-(CGPoint) getNewOriginFromViewLocation: (CGPoint) oldOrigin viewSize: (CGPoint) viewSize andZoomType:(BOOL) isZoomIn {

    // Calculate original center (add the half of the width/height of the screen)
    float oldCenterX = oldOrigin.x + (viewSize.x / 2);
    float oldCenterY = oldOrigin.y + (viewSize.y / 2);

    // Xalculate the new center
    CGPoint newCenter;
    if(isZoomIn) {
        newCenter = CGPointMake(oldCenterX * zoomLevel, oldCenterY * zoomLevel);
    } else {
        newCenter = CGPointMake(oldCenterX / zoomLevel, oldCenterY / zoomLevel);
    }

    // Calculate the new origin (deduct the half of the width/height of the screen)
    float newOriginX = newCenter.x - (viewSize.x / 2);
    float newOriginY = newCenter.y - (viewSize.y / 2);

    return CGPointMake(newOriginX, newOriginY);
}

Does anyone have any idea why the CenterPoint is not being calculated correctly? Any help would be GREATLY appreciated; I have been stuck on this for a week now.

Thanks, Alex

Answer 1

Have you tried implementing the zoom using JavaScript rather than the UIWebView's scrollview ?

I believe you can zoom the webview by calling:

[webView stringByEvaluatingJavaScriptFromString:@"document.body.style.zoom = 5.0;"]; (use your ZOOM_STEP as the value)

You could also calculate the center of the browser window using JS:

posY = getScreenCenterY();
posX = getScreenCenterX();

function getScreenCenterY() {
    var y = 0;
    y = getScrollOffset()+(getInnerHeight()/2);
    return(y);
}

function getScreenCenterX() {
    return(document.body.clientWidth/2);
}

function getInnerHeight() {
    var y;
    if (self.innerHeight) // all except Explorer
    {
        y = self.innerHeight;
    }
    else if (document.documentElement && document.documentElement.clientHeight)
    // Explorer 6 Strict Mode
    {
        y = document.documentElement.clientHeight;
    }
    else if (document.body) // other Explorers
    {
        y = document.body.clientHeight;
    }
    return(y);
}

function getScrollOffset() {
    var y;
    if (self.pageYOffset) // all except Explorer
    {
        y = self.pageYOffset;
    }
    else if (document.documentElement &&
document.documentElement.scrollTop) // Explorer 6 Strict
    {
        y = document.documentElement.scrollTop;
    }
    else if (document.body) // all other Explorers
    {
        y = document.body.scrollTop;
    }
    return(y);
}

(source: http://sliceofcake.wordpress.com/2007/09/13/use-javascript-to-find-the-center-of-the-browser/)