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Exclamation point indicating a default value exist for this variable

• weak keyword - the object is not in memory thus prevent reference cycle (system keep deallocating memories to object). The Superview maintains the strong reference, and the subviews maintains the weak

• actions - defines the application response. When event occurs, the action method is executed. Created the same way outlets are created

ctrl + drag can be done from the reverse way, dragging code to UI

Pattern

• target action pattern - one object sends a message to another object when event occurs; action method calling on the target/sender and pass in the target/sender

Swift related

• Enumeration - a dot abbreviation for known type

  
  

• Present - execute on self object, asks view controller to present the custom view controller. completion can create callback

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