Is there a way to reset the scale of the viewport dynamically to 1.0

Question

Im working on a a mobile online-store And got stuck while implementing the product zoom function

After clicking an Image "user-scalable" is allowed and maximum-scale is set to 10.0 When the user zooms in on the product with a pinch gesture, everything works fine. But after closing the zoomed Image the scale is not reset to 1.0.

Is there a way to reset the scale value of the viewport dynamically. The "initial-scale" seems not to work, neither does reseting the "minimum-scale" and "maximum-scale" to 1.0

The problems occurs on iPhone / iPad

There seems to be a solution, but i don't know to which element i should apply the on this post: How to reset viewport scaling without full page refresh?

"You need to use -webkit-transform: scale(1.1); webkit transition."

But I don't know to which element the style is applied.

Here is some code to illustrate the Problem.

In the meta Tag for the viewport looks like this:

<meta name="viewport" content="user-scalable=no, width=device-width, height=device-height, minimum-scale=1.0, maximum-scale=1.0" />

the rest of the page Looks like this:

<div id="page">
    <img src="images/smallProductImage.jpg">
</div>

<div id="zoom">
    <div class="jsZoomImageContainer"></div>
</div>

and this is the javascript::

zoom:{
    img: null,
    initialScreen:null,

    load:function(src){             

        //load the image and show it when loaded

        showLoadingAnimation();
        this.img = new Image();
        this.img.src = src;

        jQuery(this.img).load(function(){
            zoom.show();
        });
    },

    show:function(){

        var screenWidth, screenHeight, imageWidth, imageHeight, scale, ctx;             

        hideLoadingAnimation();
        jQuery("#page").hide();         
        jQuery("#zoom").show();

        jQuery(".jsZoomImageContainer").empty();
        this.initialScreen =[jQuery(window).width(),  jQuery(window).height()]
        jQuery(".jsZoomImageContainer").append(this.img);               


        imageWidth = jQuery(this.img).width();
        imageHeight = jQuery(this.img).height();

        scale = this.initialScreen[0] / imageWidth ;

        jQuery(this.img).width(imageWidth * scale)
        jQuery(this.img).height(imageHeight * scale)


        jQuery(".jsZoomImageContainer").click(function(){
             zoom.hide();
        });

        jQuery('meta[name="viewport"]',"head").attr("content","user-scalable=yes, initial-scale:1.0, minimum-scale=1.0, maximum-scale=10.0")                 

    },

    hide:function(){                        
        jQuery(".jsZoomImageContainer").empty();                        
        jQuery('meta[name="viewport"]',"head").attr("content","user-scalable=no, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0") 

        jQuery("#zoom").hide();
        jQuery("#page").show();

        this.img = null;
        this.initialScreen = null;

    }
}

jQuery("#page img").click(function(){
    zoom.load("images/bigProductImage.jpg");
});

Answer 1

According to ppk, this technique for viewport manipulation works on all modern browsers except for Firefox:

<meta id="testViewport" name="viewport" content="width = 380">
<script>
if (screen.width > 740) {
    var mvp = document.getElementById('testViewport');
    mvp.setAttribute('content','width=740');
}
</script>

Seems like the key is setting an id attribute in the meta tag so you can select it easily with JS and replace the value of the content attribute.