docs/html/guide/webapps/migrating.jd

page.title=Migrating to WebView in Android 4.4
@jd:body

<div id="qv-wrapper">
<div id="qv">

<h2>In this document</h2>
<ol>
  <li><a href="#UserAgent">User Agent Changes</a></li>
  <li><a href="#Threads">Multi-threading and Thread Blocking</a></li>
  <li><a href="#URLs">Custom URL Handling</a></li>
  <li><a href="#Viewport">Viewport Changes</a>
    <ol>
      <li><a href="#TargetDensity">Viewport target-densitydpi no longer supported</a></li>
      <li><a href="#SmallViewport">Viewport zooms in when small</a></li>
      <li><a href="#MultiViewport">Multiple viewport tags not supported</a></li>
      <li><a href="#Zoom">Default zoom is deprecated</a></li>
    </ol>
  </li>
  <li><a href="#Style">Styling Changes</a>
    <ol>
      <li><a href="#BackgroundSize">The background CSS shorthand overrides background-size</a></li>
      <li><a href="#Pixels">Sizes are in CSS pixels instead of screen pixels</a></li>
      <li><a href="#Columns">NARROW_COLUMNS and SINGLE_COLUMN no longer supported</a></li>
    </ol>
  </li>
  <li><a href="#TouchCancel">Handling Touch Events in JavaScript</a></li>
</ol>

</div>
</div>










<p>Android 4.4 (API level 19) introduces a new version of {@link android.webkit.WebView} that is
based on <a href="http://www.chromium.org/Home">Chromium</a>. This change upgrades
{@link android.webkit.WebView} performance and standards support for HTML5, CSS3, and JavaScript
to match the latest web browsers. Any apps using {@link android.webkit.WebView} will inherit these
upgrades when running on Android 4.4 and higher.</p>

<p>This document describes additional changes
to {@link android.webkit.WebView} that you should be aware of if you set your
<a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#target">{@code
targetSdkVersion}</a> to "19" or higher.</p>

<p class="note"><strong>Note:</strong>
If your <a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#target">{@code
targetSdkVersion}</a> is set to "18" or lower, {@link android.webkit.WebView} operates in
"quirks mode" in order to avoid some of the behavior changes described below, as closely
as possible&mdash;while still providing your app the performance and web standards upgrades.
Beware, though, that <a href="#Columns">single and narrow column layouts</a> and <a
href="#Zoom">default zoom levels</a> are
<strong>not supported at all</strong> on Android 4.4, and there may be other behavioral differences
that have not been identified, so be sure to test your app on Android 4.4
or higher even if you keep your <a
href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#target">{@code
targetSdkVersion}</a> set to "18" or lower. </p>

<p>To help you work through any issues you may encounter when migrating your app to
{@link android.webkit.WebView} in Android 4.4, you can enable remote debugging through
Chrome on your desktop by calling
{@link android.webkit.WebView#setWebContentsDebuggingEnabled setWebContentsDebuggingEnabled()}.
This new feature in {@link android.webkit.WebView} allows
you to inspect and analyze your web content, scripts, and network activity while running in
a {@link android.webkit.WebView}. For more information, see <a
href="https://developers.google.com/chrome-developer-tools/docs/remote-debugging">Remote
Debugging on Android</a>.</p>




<h2 id="UserAgent">User Agent Changes</h2>

<p>If you serve content to your {@link android.webkit.WebView} based on the user agent, you should
to be aware of the user agent string has changed slightly and now includes the Chrome version:</p>

<pre class="no-pretty-print">
Mozilla/5.0 (Linux; Android 4.4; Nexus 4 Build/KRT16H) AppleWebKit/537.36
(KHTML, like Gecko) Version/4.0 <strong>Chrome/30.0.0.0</strong> Mobile Safari/537.36
</pre>

<p>If you need to retrieve the user agent but don't need to store it for your app or
do not want to instantiate {@link android.webkit.WebView}, you should use
the static method, {@link android.webkit.WebSettings#getDefaultUserAgent
getDefaultUserAgent()}. However, if you intend to override the user agent string in your
{@link android.webkit.WebView}, you may instead want to use
{@link android.webkit.WebSettings#getUserAgentString getUserAgentString()}.</p>




<h2 id="Threads">Multi-threading and Thread Blocking</h2>

<p>If you call methods on {@link android.webkit.WebView} from any thread other than your app's
UI thread, it can cause unexpected results. For example, if your app uses multiple threads,
you can use the {@link android.app.Activity#runOnUiThread runOnUiThread()} method
to ensure your code executes on the UI thread:</p>

<pre>
runOnUiThread(new Runnable() {
    &#64;Override
    public void run() {
        // Code for WebView goes here
    }
});
</pre>

<p>Also be sure that you <a href="{@docRoot}guide/components/processes-and-threads.html#Threads">
never block the UI thread</a>. A situation in which some apps make this mistake is while waiting for
a JavaScript callback. For example, <strong>do not</strong> use code like this:</p>

<pre>
// This code is BAD and will block the UI thread
webView.loadUrl("javascript:fn()");
while(result == null) {
  Thread.sleep(100);
}
</pre>

<p>You can instead use a new method, {@link android.webkit.WebView#evaluateJavascript
evaluateJavascript()}, to run JavaScript asynchronously.</p>





<h2 id="URLs">Custom URL Handling</h2>

<p>The new {@link android.webkit.WebView} applies additional restrictions when requesting resources
and resolving links that use a custom URL scheme. For example, if you implement callbacks such as
{@link android.webkit.WebViewClient#shouldOverrideUrlLoading shouldOverrideUrlLoading()} or
{@link android.webkit.WebViewClient#shouldInterceptRequest shouldInterceptRequest()}, then
{@link android.webkit.WebView} invokes them only for valid URLs.</p>

<p>If you are using a custom URL scheme or a base URL and
notice that your app is receiving fewer calls to these callbacks or failing
to load resources on Android 4.4, ensure that the requests specify valid URLs that conform to
<a href="http://tools.ietf.org/html/rfc3986#appendix-A">RFC 3986</a>.

<p>For example, the new {@link android.webkit.WebView} may not call your
{@link android.webkit.WebViewClient#shouldOverrideUrlLoading shouldOverrideUrlLoading()} method
for links like this:</p>

<pre>&lt;a href="showProfile">Show Profile&lt;/a></pre>

<p>The result of the user clicking such a link can vary:
<ul>
  <li>If you loaded the page by calling {@link android.webkit.WebView#loadData
loadData()} or {@link android.webkit.WebView#loadDataWithBaseURL
loadDataWithBaseURL()} with an invalid or null base URL, then you will not receive the
{@link android.webkit.WebViewClient#shouldOverrideUrlLoading shouldOverrideUrlLoading()} callback
for this type of link on the page.
  <p class="note"><strong>Note:</strong>
  When you use {@link android.webkit.WebView#loadDataWithBaseURL
loadDataWithBaseURL()} and the base URL is invalid or set null, all links in the content
you are loading must be absolute.</p>
  <li>If you loaded the page by calling {@link android.webkit.WebView#loadUrl
loadUrl()} or provided a valid base URL with {@link android.webkit.WebView#loadDataWithBaseURL
loadDataWithBaseURL()}, then you will receive the
{@link android.webkit.WebViewClient#shouldOverrideUrlLoading shouldOverrideUrlLoading()} callback
for this type of link on the page, but the URL you receive will be absolute, relative
to the current page. For example, the URL you receive will be
<code>"http://www.example.com/showProfile"</code> instead of just <code>"showProfile"</code>.
</ul>


<p>Instead of using a simple string in a link as shown above, you can use a custom scheme such
as the following:</p>

<pre>&lt;a href="example-app:showProfile">Show Profile&lt;/a></pre>

<p>You can then handle this URL in your
{@link android.webkit.WebViewClient#shouldOverrideUrlLoading shouldOverrideUrlLoading()} method
like this:</p>

<pre>
// The URL scheme should be non-hierarchical (no trailing slashes)
private static final String APP_SCHEME = "example-app:";

&#64;Override
public boolean shouldOverrideUrlLoading(WebView view, String url) {
    if (url.startsWith(APP_SCHEME)) {
        urlData = URLDecoder.decode(url.substring(APP_SCHEME.length()), "UTF-8");
        respondToData(urlData);
        return true;
    }
    return false;
}
</pre>

<p>If you can't alter the HTML then you may be able to use
{@link android.webkit.WebView#loadDataWithBaseURL loadDataWithBaseURL()} and set a base URL
consisting of a custom scheme and a valid host, such as
<code>"example-app://&lt;valid_host_name>/"</code>. For example:</p>

<pre>
webView.loadDataWithBaseURL("example-app://example.co.uk/", HTML_DATA,
        null, "UTF-8", null);
</pre>

<p>The valid host name should conform to
<a href="http://tools.ietf.org/html/rfc3986#appendix-A">RFC 3986</a>
and it's important to include the trailing slash at the end, otherwise, any requests from the
loaded page may be dropped.</p>



<h2 id="Viewport">Viewport Changes</h2>


<h3 id="TargetDensity">Viewport target-densitydpi no longer supported</h3>

<p>Previously, {@link android.webkit.WebView} supported a viewport property called
<code>target-densitydpi</code> to help web pages specify their intended screen density. This
property is no longer supported and you should migrate to using standard solutions with
images and CSS as discussed in <a
href="http://developers.google.com/chrome/mobile/docs/webview/pixelperfect">Pixel-Perfect UI in
the WebView</a>.</p>


<h3 id="SmallViewport">Viewport zooms in when small</h3>

<p>Previously, if you set your viewport width to a value less than or equal to "320"
it would be set to "device-width", and if you set the viewport height to a value less than or
equal to the {@link android.webkit.WebView} height, it would be set to "device-height". However,
when running in the new {@link android.webkit.WebView}, the width or height value is adhered and
the {@link android.webkit.WebView} zooms in to fill the screen width.</p>


<h3 id="MultiViewport">Multiple viewport tags not supported</h3>

<p>Previously, if you included multiple viewport tags in a web page, {@link android.webkit.WebView}
would merge the properties from all the tags.
In the new {@link android.webkit.WebView}, only the last viewport is
used and all others are ignored.</p>


<h3 id="Zoom">Default zoom is deprecated</h3>

<p>The methods {@link android.webkit.WebSettings#getDefaultZoom()} and
{@link android.webkit.WebSettings#setDefaultZoom setDefaultZoom()} for getting and setting
the initial zoom level on a page have are no longer supported and you should instead define
the appropriate viewport in the web page.</p>

<p class="caution"><strong>Caution:</strong> These APIs are not supported on Android 4.4 and higher
at all. Even if your <a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#target"
>{@code targetSdkVersion}</a> is set to "18" or lower, these APIs have no effect.</p>

<p>For information about how to define the viewport properties in your HTML, read
<a href="http://developers.google.com/chrome/mobile/docs/webview/pixelperfect" class="external-link"
>Pixel-Perfect UI in the WebView</a>.

<p>If you cannot set the width of the viewport in the HTML, then you should call
{@link android.webkit.WebSettings#setUseWideViewPort setUseWideViewPort()} to ensure the page
is given a larger viewport. For example:</p>

<pre>
WebSettings settings = webView.getSettings();
settings.setUseWideViewPort(true);
settings.setLoadWithOverviewMode(true);
</pre>





<h2 id="Style">Styling Changes</h2>

<h3 id="BackgroundSize">The background CSS shorthand overrides background-size</h3>

<p>Chrome and other browser have behaved this way for a while, but now
{@link android.webkit.WebView} will also override a CSS setting for {@code background-size}
if you also specify the {@code background} style. For example, the size here will be reset
to a default value:</p>

<pre class="no-pretty-print">
.some-class {
  background-size: contain;
  background: url('images/image.png') no-repeat;
}
</pre>

<p>The fix is to simply switch the two properties around.</p>

<pre class="no-pretty-print">
.some-class {
  background: url('images/image.png') no-repeat;
  background-size: contain;
}
</pre>


<h3 id="Pixels">Sizes are in CSS pixels instead of screen pixels</h3>

<p>Previously, size parameters such as <a
href="https://developer.mozilla.org/en-US/docs/Web/API/Window.outerWidth" class="external-link">
<code>window.outerWidth</code></a> and
<a href="https://developer.mozilla.org/en-US/docs/Web/API/Window.outerHeight"
class="external-link"><code>window.outerHeight</code></a> returned a value in actual screen pixels.
In the new {@link android.webkit.WebView}, these return a value based on CSS pixels.</p>

<p>It's generally bad practice to try and calculate the physical size in pixels for
sizing elements or other calculations. However, if you've disabled zooming and the initial-scale
is set to 1.0, you can use <code>window.devicePixelRatio</code>
to get the scale, then multiply the CSS pixel value by that. Instead,
you can also <a href="{@docRoot}guide/webapps/webview.html#BindingJavaScript">create a
JavaScript binding</a> to query the pixel size from the {@link android.webkit.WebView} itself.</p>

<p>For more information, see <a class="external-link"
href="http://www.quirksmode.org/blog/archives/2012/03/windowouterwidt.html">quirksmode.org</a>.</p>



<h3 id="Columns">NARROW_COLUMNS and SINGLE_COLUMN no longer supported</h3>

<p>The {@link android.webkit.WebSettings.LayoutAlgorithm#NARROW_COLUMNS} value for {@link
android.webkit.WebSettings.LayoutAlgorithm} is not be supported in the new {@link
android.webkit.WebView}.</p>

<p class="caution"><strong>Caution:</strong> These APIs are not supported on Android 4.4 and higher
at all. Even if your <a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#target"
>{@code targetSdkVersion}</a> is set to "18" or lower, these APIs have no effect.</p>

<p>You can handle this change in the following ways:</p>

<ul>
  <li>Alter the styles of your application:
    <p>If you have control of the HTML and CSS on the page, you may find that altering the design
    of your content may be the most reliable approach. For example, for screens where you cite
    licenses, you may want wrap text inside of a
    <code>&lt;pre></code> tag, which you could do with the following styles:
    <pre>&lt;pre style="word-wrap: break-word; white-space: pre-wrap;"></pre>
    <p>This may be especially helpful if you have not defined the viewport properties for
    your page.</p>
  </li>
  <li>Use the new {@link android.webkit.WebSettings.LayoutAlgorithm#TEXT_AUTOSIZING} layout
  algorithm:
    <p>If you were using narrow columns as a way to make a broad spectrum of desktop
    sites more readable on mobile devices and you aren't able to change the HTML content, the new
    {@link android.webkit.WebSettings.LayoutAlgorithm#TEXT_AUTOSIZING} algorithm may be a
    suitable alternative to {@link android.webkit.WebSettings.LayoutAlgorithm#NARROW_COLUMNS}.</p>
  </li>
</ul>

<p>Additionally, the {@link android.webkit.WebSettings.LayoutAlgorithm#SINGLE_COLUMN} value&mdash;which
was previously deprecated&mdash;is also not supported in the new {@link
android.webkit.WebView}.</p>




<h2 id="TouchCancel">Handling Touch Events in JavaScript</h2>

<p>If your web page is directly handling touch events in a {@link android.webkit.WebView},
be sure you are also handling the <a
href="https://developer.mozilla.org/en-US/docs/Web/Reference/Events/touchcancel"
class="external-link"><code>touchcancel</code></a>
event. There are a few scenarios where <code>touchcancel</code> will be called, which can
cause problems if not received:</p>

<ul>
  <li>An element is touched (so <code>touchstart</code> and <code>touchmove</code> are called)
  and the page is scrolled, causing a <code>touchcancel</code> to be thrown.</li>
  <li>An element is touched (<code>touchstart</code> is called) but
  <code>event.preventDefault()</code> is not called, resulting earlier enough that
  <code>touchcancel</code> is thrown (so
  {@link android.webkit.WebView} assumes you don't want to consume the touch events).</li>
</ul>