Every component in Ionic has a lifecycle. Ionic creates, renders the component, checks it when its data-bound properties change and destroys it finally. Ionic offers lifecycle hooks that provide a way to tap into these key moments and trigger an action when they occur. 

Ionic 2 & 3 had these lifecycle events : ionViewDidLoad, ionViewWillEnter, 

ionViewDidEnter, ionViewWillLeave, ionViewDidLeave, ionViewWillUnload, 

ionViewCanEnter, ionViewCanLeave. 

• ionViewDidLoad : Fired only when a view is stored in memory, will not fire if view has been already cached. 
• ionViewWillEnter : fired when entering a page, before it becomes active one. this event is triggered every time user enters in the view. 
• ionViewDidEnter : fired when entering a page, after it becomes active page. 
• ionViewWillLeave : fired when user LEAVES a page, before it stops being the active page. 
• ionViewDidLeave : fired when user leaves a page, after it stops being the active page. 
• ionViewWillUnload : fired when a view is going to be completely removed. 
• ionViewCanEnter : this is a nav GUARD. fired before entering a view. Useful when you want to CONTROL access to pages based on access credentials. 
• ionViewCanLeave : this is also a nav guard, fired before leaving a view. 

Allows you to control whether user can exit the view or not Ionic 4 provides the ANGULAR lifecycle hooks in addition to the above listed Ionic lifecycle hooks. All the angular lifecycle hooks are available. 

• ngOnChanges , NGONINIT, ngDoCheck, ngAfterContentInit, ngAfterContentChecked, 
• ngAfterViewInit, ngAfterViewChecked, ngOnDestroy .