上一篇文章我們知道了Eclipse彈出菜單的基本用法。其實(shí)Eclipse的彈出菜單可以用來做很多文章，簡單一點(diǎn)的根據(jù)文件類別，我們可以進(jìn)行不同的文件操作，比如Ant的build.xml我們可以用來build，Java文件我們可以用Java Editor打開，這些基于文件類型的操作我們都可以很容易的實(shí)現(xiàn)。但是還有一種情況，如果文件類型一樣，我們想進(jìn)行不同的操作，該怎么實(shí)現(xiàn)呢？實(shí)際上這樣的應(yīng)用很多，比如同樣是Java文件，含有main方法的Java文件有Run和Debug的選項(xiàng)，其它的都沒有。還有現(xiàn)在的框架都是基于XML文件進(jìn)行配置的，如果一個(gè)項(xiàng)目使用了多個(gè)框架，我們?cè)趺锤鶕?jù)不同的XML文件進(jìn)行框架的區(qū)分呢？答案就是enablement的test。

<!ELEMENT test EMPTY>
<!ATTLIST test
property CDATA #REQUIRED
args     CDATA #IMPLIED
value    CDATA #IMPLIED>

This element is used to evaluate the property state of the object in focus. The set of testable properties can be extended using the propery tester extension point. The test expression returns EvaluationResult.NOT_LOADED if teh property tester doing the actual testing isn't loaded yet.

• property - the name of an object's property to test.
• args - additional arguments passed to the property tester. Multiple arguments are seperated by commas. Each individual argument is converted into a Java base type using the same rules as defined for the value attribute of the test expression.
• value - the expected value of the property. Can be omitted if the property is a boolean property. The test expression is supposed to return EvaluationResult.TRUE if the property matches the value and EvaluationResult.FALSE otherwise. The value attribute is converted into a Java base type using the following rules:
  • the string "true" is converted into Boolean.TRUE
  • the string "false" is converted into Boolean.FALSE
  • if the string contains a dot then the interpreter tries to convert the value into a Float object. If this fails the string is treated as a java.lang.String
  • if the string only consists of numbers then the interpreter converts the value in an Integer object.
  • in all other cases the string is treated as a java.lang.String
  • the conversion of the string into a Boolean, Float, or Integer can be suppressed by surrounding the string with single quotes. For example, the attribute value="'true'" is converted into the string "true"

比如我們要讓含有main方法的Java文件它的右鍵彈出菜單包含一個(gè)額外的選項(xiàng)“This is main class”，需要編寫如下的Plugin.xml：

我們需要檢測在當(dāng)前情況下是否需要顯示這個(gè)菜單項(xiàng)，使用擴(kuò)展點(diǎn) org.eclipse.core.expressions.propertyTesters ：
<!ELEMENT propertyTester EMPTY>
<!ATTLIST propertyTester
id         CDATA #REQUIRED
type       CDATA #REQUIRED
namespace  CDATA #REQUIRED
properties CDATA #REQUIRED
class      CDATA #REQUIRED>

id - unique identifier for the property tester
type - the type to be extended by this property tester
namespace - a unique id determining the name space the properties are added to
properties - a comma separated list of properties provided by this property tester
class - the name of the class that implements the testing methods. The class must be public and extend org.eclipse.core.expressions.PropertyTester with a public 0-argument constructor. 

這里只須注意 propertyTester的namespace和properties正好對(duì)應(yīng)test的property。

至于檢測的邏輯我們?cè)赼dvancedpopupmenus.popup.actions.VisablePropertyTester中實(shí)現(xiàn)，這個(gè)類必須繼承自org.eclipse.core.expressions.PropertyTester。

我們只要判斷接受的Java文件中是否含有main方法，如果有，則返回True，沒有則返回False。

如果我們是要接受一個(gè)Web開發(fā)的配置文件，我們可以這樣寫：

注意和上一個(gè)例子不同的地方，objectClass，nameFileter和type(在上一個(gè)例子中，我們也可以使用objectClass="org.eclipse.core.resources.IFile" nameFilter ="*.java" )，相應(yīng)的我們的VisablePropertyTester類也要做一些改動(dòng)：

這樣根據(jù)不同的xml SystemID，我們就能夠知道到底這是哪一種框架的配置文件了。