Swift-Properties-Property monitor (Didset, Willset)

Property Monitor

1. Property Monitor Function

Property Observer control and response property changes , each time the property is set to the value of the Call Property observer , even if the new value and the old value of the same time 　　 is no exception.

　　Property observers can be added by overloading the property's methods for who can inherit ( including storage properties and computed properties )

　　 Note: You do not need to add a property observer for computed properties that cannot be overloaded , because you can directly monitor and respond to changes in values by setter

2. Property Monitor Method

1. Willset

　　　　Features: called before a new value is set

Description: The Illset observer will Pass in the new attribute value as a fixed parameter, in Willset 　　　　 A name for this parameter can be specified in the Item code , and If not specified, the default name of the parameter is represented by NewValue

2.Didset

　　　　function: called immediately after the new value has been set

Gong　　　　 can: The dIDSet Observer passes the old property values as parameters , with the default name of the parameter : OldValue

3. Attention

1. Willset and Didset observers are not called during property initialization , they are called only when the value of the property is set outside of the initialization

2. If you assign a value to a property in the Didset Observer , this value replaces the value set by the Observer.

4. code example

Class Person:nsobject {//Age attribute var age:int?//name attribute var name:string? {willset{println ("Willset")}didset{println ("Didset")}}}

Swift-Properties-Property monitor (Didset, Willset)