public abstract class ClosureSignatureHint extends Object
A closure signature hint class is always used in conjunction with the ClosureParams annotation. It is called at compile time (or may be used by IDEs) to infer the types of the parameters of a Closure.
A closure hint class is responsible for generating the list of arguments that a closure accepts. Since closures may accept several signatures, getClosureSignatures(org.codehaus.groovy.ast.MethodNode, org.codehaus.groovy.control.SourceUnit, org.codehaus.groovy.control.CompilationUnit, String[], org.codehaus.groovy.ast.ASTNode) should return a list.
Whenever the type checker encounters a method call that targets a method accepting a closure, it will search for the ClosureParams annotation on the Closure argument. If it is found, then it creates an instance of the hint class and calls the getClosureSignatures(org.codehaus.groovy.ast.MethodNode, org.codehaus.groovy.control.SourceUnit, org.codehaus.groovy.control.CompilationUnit, String[], org.codehaus.groovy.ast.ASTNode) method, which will in turn return the list of signatures.
Note that the signature concept here is used only to describe the parameter types, not the result type, which is found in the generic type argument of the Closure class.
Several predefined hints can be found, which should cover most of the use cases.
| Type Params | Return Type | Name and description |
|---|---|---|
|
public abstract List<ClassNode> |
getClosureSignatures(MethodNode node, SourceUnit sourceUnit, CompilationUnit compilationUnit, String[] options, ASTNode usage) |
|
public static ClassNode |
pickGenericType(ClassNode type, int gtIndex)A helper method which will extract the n-th generic type from a class node. |
|
public static ClassNode |
pickGenericType(MethodNode node, int parameterIndex, int gtIndex)A helper method which will extract the n-th generic type from the n-th parameter of a method node. |
Subclasses should implement this method, which returns the list of accepted closure signatures.
The compiler will call this method each time, in a source file, a method call using a closure literal is encountered and that the target method has the corresponding Closure parameter annotated with ClosureParams. So imagine the following code needs to be compiled:
- groovy.transform.TypeChecked:
- void doSomething() {
println ['a','b'].collect { it.toUpperCase() }
}
The collect method accepts a closure, but normally, the type checker doesn't have enough type information in the sole DefaultGroovyMethods.collect method signature to infer the type of it. With the annotation, it will now try to find an annotation on the closure parameter. If it finds it, then an instance of the hint class is created and the type checker calls it with the following arguments:
Now, the hint instance can return the list of expected parameters. Here, it would have to say that the collect method accepts a closure for which the only argument is of the type of the first generic type of the first argument.
With that type information, the type checker can now infer that the type of it is String, because the first argument (here the receiver of the collect method) is a List<String>
Subclasses are therefore expected to return the signatures according to the available context, which is only the target method and the potential options.
node - the method node for which a Closure parameter was annotated with
ClosureParamssourceUnit - the source unit of the file being compiledcompilationUnit - the compilation unit of the file being compiledoptions - the options, corresponding to the ClosureParams.options found on the annotationusage - the AST node, in the compiled file, which triggered a call to this method. Normally only used for logging/error handlingA helper method which will extract the n-th generic type from a class node.
type - the class node from which to pick a generic typegtIndex - the index of the generic type to extractA helper method which will extract the n-th generic type from the n-th parameter of a method node.
node - the method node from which the generic type should be pickedparameterIndex - the index of the parameter in the method parameter listgtIndex - the index of the generic type to extract