Declarations.md

##Declarations

A declaration introduces a new name or construct into your program.

• Top-Level Code: the top-level code in a Swift source file consists of zero or more statements, declarations, and expressions. By default, variables, constants, and other named declarations that are declared at the top-level of a source file are accessible to code in every source file that is part of the same module.

• Rethrowing Functions and Methods

A function or method can be declared with the rethrows keyword to indicate that it throws an error only if one of its function parameters throws an error. These functions and methods are known as rethrowing functions and rethrowing methods.

• Enumeration declaration

An enueration declaration introduces a named enumeation type into your program. Enumearation delcarations can't contain deinitializer or protocol declarations. 不能继承;没有默认的initializer; 所有的initializer必须明确声明. enumeration是值类型.

• Protocol Declaration不能包含其他的declarations.

• Declaration Modifiers: Declaration modifiers are keywords or context-sensitive keywords that modify the behavior or meaning of a declaration.

  • dynamic: Apply this modifier to any member of a class that can be represented by Objective-C. When you mark a member declaration with the dynamic modifier, access to that member is always dynamically dispatched using the Objective-C runtime.

  • final: Apply this modifier to a class or to a property, method, or subscript member of a class. It's applied to a class to indicate that the class can't be subclassed. It's applied to a property, method, or subscript of a class to indicate that a class member can't be overridden in any subclass.

  • lazy: Apply this modifier to a stored varialbe property fo a class or structure to indicate that the property's initial value is calculaed and stored at most once, when the property is first accessed.

  • optional: Apply this modifier to a protocol's property, method, or subscript members to indicate that a conforming type isn't retuqired to implement those members. You can apply the optional modifier only to protocols that are marked with the objc attribute. As a result, only class types can adopt and conform to a protocol that contains optional member requirements.

  • required: Apply this modifier to a designated or convenience initializer of a class to indicate that every subclass must implement that initializer. The subclass's implementation of that initializer must also be marked with the required modifier.

  • weak: The weak modifier is applied to a variable or a stored variable property to indicate that the variable or property has a weak reference to the object stored as its value. The type of the variable or property must be an optional class type. Use the weak modifier to avoid strong reference cycles.