## Filters

If some sections of a document (e.g. a list item in a XHTML help topic) should only be displayed to the user if certain conditions are met, you can specify an enablement expression declaring the criteria that must be met in order to display the element.

Filters are specified by adding an `<enablement>` element as a child of the element that should conditionally be filtered. The syntax used is core expressions, which is the same syntax used to filter menu contributions, etc from the UI.

### System tests

Expressions check criteria by performing tests. One type of test is a system test, which tests a system property against an expected value. Some common system properties to test by are listed below:

Property | Meaning | Possible values
`osgi.os` | operating system | `win32, win32, linux, macosx, aix, solaris, hpux, qnx`
`osgi.ws` | windowing system | `win32, motif, gtk, photon, carbon`
`osgi.arch` | processor architecture | `x86, x86_64, ia64, ia64_32, ppc, PA_RISC, sparc`

Here are a few examples of system tests:

    <p>
       This paragraph should only be displayed on Windows.
       <enablement>
          <systemTest property="osgi.os" value="win32"/>
       </enablement>
    </p>

    <p>
       This paragraph should *not* be displayed on GTK.
       <enablement>
          <not>
             <systemTest property="osgi.ws" value="gtk"/>
          </not>
       </enablement>
    </p>

    <p>
       This paragraph should only be displayed on PowerPC Macs.
       <enablement>
          <systemTest property="osgi.os" value="macosx"/>
          <systemTest property="osgi.arch" value="ppc"/>
       </enablement>
    </p>

Note: When several sub-expressions are listed with no boolean operator, they are by default ANDed together. See the complete expressions syntax specification for more details.

### Property tests

In addition to system tests, you can test any property of an available object as long as there is a property tester available for it.

Tests always perform their test on a property of some object, and each object has different properties you can test. User assistance provides two variables that are used to select the object to test on:

- `platform` - The underlying platform you're running on
- `workbench` - The UI workbench

You can perform a test on one of these by using the `<with>` element, as shown below:

    <enablement>
       <with variable="platform">
          <test property="x" value="y"/>
       </with>
    </enablement>

Each property has a namespace, which is a prefix such as `org.eclipse.ui` that is used to minimize the chances of duplicate properties being defined by two components. The table below shows some common properties you can test by:

Property | Object (variable) | Meaning
`org.eclipse.core.runtime.product` | `platform` | tests if the expected value matches the currently active product's unique id
`org.eclipse.core.runtime.isBundleInstalled` | `platform` | tests if the bundle with the symbolic name (e.g. `org.eclipse.help`) specified as a single argument is installed on the platform
`org.eclipse.ui.isActivityEnabled` | `workbench` | tests if the activity with the id specified as a single argument exists and is currently enabled in the workbench
`org.eclipse.ui.isCategoryEnabled` | `workbench` | tests if the category of activities with the id specified as a single argument exists and is currently enabled in the workbench. A category is enabled if all its activities are enabled

Here are a few examples using these properties:

    <p>
       This paragraph should only be displayed when running the Eclipse SDK.
       <enablement>
          <with variable="platform">
             <test property="org.eclipse.core.runtime.product" value="org.eclipse.sdk.ide"/>
          </with>
       </enablement>
    </p>

    <p>
       This paragraph should only be displayed when either the com.myactivity.a or
       com.myactivity.b activity (or both) is enabled in the workbench.
       <enablement>
          <with variable="workbench">
             <or>
                <test property="org.eclipse.ui.isActivityEnabled" args="com.myactivity.a"/>
                <test property="org.eclipse.ui.isActivityEnabled" args="com.myactivity.b"/>
             <or>
          </with>
       </enablement>
    </p>

    <p>
       This paragraph should only be displayed when the com.mycategory category
       is enabled and the com.mybundle bundle is not installed.
       <enablement>
          <with variable="workbench">
             <test property="org.eclipse.ui.isCategoryEnabled" args="com.mycategory"/>
          </with>
          <with variable="platform">
             <not>
                <test property="org.eclipse.core.runtime.isBundleInstalled" args="com.mybundle"/>
             </not>
          </with>
       </enablement>
    </p>

Note: Make sure you use the `value` and `args` attributes for the appropriate tests. In general, string properties like product will use `value` where boolean tests (`isSomethingTrue`) will use `args`. See the complete expressions syntax specification for more details on how to write expressions.

### Defining your own test

The core expressions framework allows you define your own test that can test on any arbitrary property of any object that is accessible through the variables defined for the context you're in. You do this by defining a property tester.

Note: The variables `platform` and `workbench` resolve to the `org.eclipse.core.runtime.Platform Class` object (we use the class object for "static" tests) and the singleton `org.eclipse.ui.IWorkbench` instance, respectively. Your property tester must declare one of these two classes for its type.

### Filter attributes/elements (deprecated)

Prior to the 3.3, filters were specified using `filter` attributes or elements. The use of these filters is now deprecated, and you should use expressions (described above) instead.

The table below contains a complete list of all the filter properties and their possible values for use with filter elements and attributes.

(deprecated)

Property | Meaning | Possible Values
`os` | operating system | `win32, win32, linux, macosx, aix, solaris, hpux, qnx`
`ws` | windowing system | `win32, motif, gtk, photon, carbon`
`arch` | processor architecture | `x86, x86_64, ia64, ia64_32, ppc, PA_RISC, sparc`
`product` | eclipse product identifier | Any product identifier (e.g., for SDK, `org.eclipse.sdk.ide`)
`plugin` | plug-in presence | Any plug-in identifier (e.g. `org.eclipse.help`)
`category` | category of activities | Any activity category identifier (e.g. for Team category, `org.eclipse.categories.teamCategory`)
`activity` | activity (capability) | Any activity identifier (e.g. for CVS Support activity, `org.eclipse.team.cvs`)

If the name does not match any pre-defined property, the help system will use the JVM's system property of that name. For example, you can pass in any user-defined property at launch, such as `-Dlocation=paris,france` and filter by that property.

There are two ways to specify filters on an element; using attributes, or elements.

### Filter Attribute (deprecated)

The first form is to add a `filter` attribute to the element. The general form is:

       <element filter="[name][operator][value]">
          Some text.
       </element>

The `name` is the name of the property by which to filter, for example, `os` for operating system. The `operator` is either `=` to denote a match (exact match, case sensitive), or `!=` to denote does not match. The `value` is what the property should (or shouldn't) match. For example, for `os`, one of the possible values is `win32` (Windows). A complete list of filter properties and their values is available in a table below.

The example below shows how to display a paragraph of text in an XHTML document when running on Linux only.

       <p filter="os=linux">
          This message will only appear when viewed on Linux.
       </p>

In this second example, the link will only appear when plugin `com.my.plugin` is not installed:

       <a href="..." filter="plugin!=com.my.plugin">
          Click here to download plugin com.my.plugin.
       </a>

### Filter Element (deprecated)

The second form is to use a `filter` element as a child of the element you wish to filter. This form is slightly longer than the attribute form, but it is more powerful because you can specify any number of filters on an element. The general form is:

       <element attribute="value">
          <filter name="[name]" value="[modifier][value]"/>
       </element>

The `name` and `value` here are the same as with the attribute. However, since they are separated, we need another way to specify whether or not it should match. By default, if you do not provide a `modifier`, match is assumed. If it should not match, set the modifier to "`!` "

Here is the first example shown above in the second form:

       <p>
          <filter name="os" value="linux"/>
          This message will only appear when viewed on Linux.
       </p>

And the second example:

       <a href="...">
          <filter name="plugin" value="!com.my.plugin"/>
          Click here to download plugin com.my.plugin.
       </a>

### Infocenter

Filtering support is turned off when running help in infocenter mode, causing all content, including filtered content, to be visible. If you intend to host your documentation in both workbench and infocenter modes, you should use filters in a way that makes sense even if filtering is turned off.

### Where can I use filters?

Filtering can be used in any XML-based user assistance document, such as help XHTML topics, help table of contents, welcome pages, cheat sheets, etc. You cannot use filtering in HTML documents.

In any case, you must not place filters on any element where removing that element would result in invalid XML. For example, you should not place a filter on the `html` element in XHTML, because without that element it is no longer valid XHTML.