Groovy读书笔记4

Groovy读书笔记4

Closure

Use Cases for Closures
   
    separating iteration logic from what to do on each iteration

    handling resources, 
        open
        do the job   // in a closure, as parameter
        close

Implicit Variable of Closure
    
    it                     default parameter

    this                enclosing class
    owner           enclosing object, class or closure
    delegate       same as owner, but changeable 

closure without parameters
   { -> ... }

closure with one parameter
   { ... }, { x -> ... }

Scope of a closure

   a closure must remember original context frame. it is closed under declaration context.

Return from a closure or method
  
  end return
      the value of  last expression is returned.

  prematurely return with return keyword

Groovy Control Structure

Groovy Truth

   Boolean Test: evaluate an expression to be true or false
   
   any non-void type can be evaluated as Boolean.
        to be true
              non empty, non zero, non null

Typo, forget a =
  
   if(x = 1) is not allowed in groovy.

  if(1 == x) is safe than if(x == 1)

Loop
  an object can be iterable.

assert
  
  如果没有assert， code一样产生够用的错误信息，那么就不需要assert

  assert can clarify the code intents. runnable comment!

Return Value

  method with void do not return a value
  closure always return a value, null if no value

OO in Groovy

dynamic, object oriented and scripting

classes and scripts definition

script’s binding 
    undeclared variable

    transfer variables between Script and Calller

default visibility of fields ==> property definition

default visibility of methods ==> public

Reference a field

    Class and Map has some similiarity.

    obj.field
    obj[’field’]

    override get and set methods

Parameter Type
   
  optional, if not specified, then means Object.

Groovy’s method dispatch
   GroovyObject.invokeMethod(name, params[])

Safe dereferencing with ?. operator

    protected from NPE

    null?.foo()   no foo called, return null

Constructor

    called by 
        1. new ClassName(...)
        2. as with list
        3. implicit coercion with list, implicit construction

    named parameters for default constructor
        available if no constructor is defined

Type aliasing

       import ... as ..

Typing

     Interface

     Design by Contract

     Duck Typing, Dynamic Typing, checked by tests

Multimethods

    method look up take the dynamic type of arguments into account.

    method dispatch at run time(Groovy) or compile time(Java)

GroovyBeans

    obj.property mapped to obj.getProperty() or obj.setProperty(value) method

dot-@ operator

    obj.@field access directly the field

event in groovy bean

   btn.actionPerformed = { ... }

   groovy通过bean introspection确定是否为添加一个Listener，如果是， 生成一个ClosureListener，
   a proxy implementation of the required listener interface，调用后面的closure，添加

GPath

      objs.p1,p2*.p3

Spread Operator *

     *list 打散

mix-in with use

    dynamic 增加method

    use(category) {
          ....
    }

   category中定义static methods， use后， method的第一个argument的类得到一个新的method

Groovy's Meta-Object Protocoll(MOP)

how Groovy performs its magic?
      interception points

All classes programmed in Groovy are constructed by the GroovyClassGenerator, such that it implements GroovyObject interface.

Method Dispatching

     this.invokeMethod()
     getMetaClass().invokeMethod()
     MetaClassRegistry.getMetaClass(this.class).invokeMethod()

Usability

    intercept
    relay
    pretend