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Browser detection is old hat for web developers, who have long known the need for special handling of the differences between supposedly equal implementations of the HTML standard We can achieve this in two different ways: at run time something you
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4 Working with Browsers
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have probably already done on your own or at compile time, which is a special characteristic of GWT Let s analyze first the classic way and then move on GWT provides JSNI (JavaScript Native Interface) to mix Java and JavaScript code, and we ll see more of it in 8, Mixing in JavaScript However, let s get ahead of ourselves, because using a native (ie, JavaScript) method is the easiest way to get at the user agent For example, the following code (the /*-{ and }-*/ delimiters are part of the JSNI magic ; we ll explain it later) does just that in an economical way Note that producing the result in lowercase helps writing further tests
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public static native String getUserAgent() /*-{ return navigatoruserAgenttoLowerCase(); }-*/;
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Given this code, you may write code such as
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if (getUserAgent()contains("gecko"))
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and act accordingly This test will be done, however, at run time, and your generated application will have to include code for all possible agents you want to consider (Do you really want to include Firefox-optimum code for IE users And what about Opera, Safari, or Chrome users; do they also need that baggage ) So, although this method works, and is easy to understand, you probably want to move on to more advanced ways and do things in GWT s own way7
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The Deferred Binding Way
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The other way to recognize the browser type is far subtler and uses GWT s deferred binding replacement technique In server-side Java, you have dynamic binding that lets you, at runtime, select the appropriate subclass and create an instance of it However, GWT doesn t support reflection but provides deferred binding instead: Think dynamic class loading at compile time When your source code is compiled, the GWT compiler produces a different version of your code for each specific configuration it finds; this is how separate versions of your application are created for Firefox, IE, and so on, and for each language (We see more on internationalization in 12, Internationalization and Localization ) How could you use this Let s work out a simple example, by writing a greeter that will work differently for IE and for other browsers First, let s set up a general class, which we can use everywhere we need it (This allows hiding the special GWTcreate() idiom from the rest of the application)
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7 By the way, if you want to detect the operating system in use, you could use JSNI to get the value of navigatoruserAgent and then analyze its contents to determine the operating system There are plenty of such routines online; google for javascript detection os OR operating system and you ll get plenty of appropriate hits
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public class HelloBrowser { HelloBrowserStdImpl helloImpl = GWTcreate(HelloBrowserStdImplclass);
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public void salute() { helloImplsayHello(); } }
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Note that the class doesn t do anything on its own; it delegates the work to the
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helloImpl object, which was created by GWT We have different classes for IE and for
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public class HelloBrowserStdImpl { public void sayHello() { Windowalert("You don't have IE"); } }
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And for IE:
public class HelloBrowserIEImpl extends HelloBrowserStdImpl { @Override public void sayHello() { Windowalert("If you are seeing this, you have IE"); } }
If we run the code as is, it will always show a standard hello message; we need to tell GWT about both implementations of the class and when to use it In the XML file for your project, add, before the final </module> line:
<replace-with class="comfkerekimvptestclientHelloBrowserIEImpl"> <when-type-is class="comfkerekimvptestclientHelloBrowserStdImpl"/> <any> <when-property-is name="useragent" value="ie6"/> <when-property-is name="useragent" value="ie8"/> </any> </replace-with>
Note the <any></any> construct, which lets you test for any of several agents at the same time You can also use <all></all> (which requires that all conditions must be satisfied) or <none></none> (which requires that no conditions are satisfied) as alternatives Of course, if you just want to check a single condition (say, for ie6) you could have written more simply:
<replace-with class="comfkerekimvptestclientHelloBrowserIEImpl"> <when-type-is class="comfkerekimvptestclientHelloBrowserStdImpl"/> <when-property-is name="useragent" value="ie6"/> </replace-with>