iphone - What is the difference between -viewWillAppear: and -viewDidAppear:? -

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what difference between -[uiviewcontroller viewwillappear:] , -[uiviewcontroller viewdidappear:]?

in general, do:

1) viewdidload - whenever i'm adding controls view should appear view, right away, put in viewdidload method. method called whenever view loaded memory. example, if view form 3 labels, add labels here; view never exist without forms.

2) viewwillappear: use viewwillappear update data on form. so, example above, use load data domain form. creation of uiviews expensive, , should avoid as possible doing on viewwillappear method, becuase when gets called, means iphone ready show uiview user, , heavy here impact performance in visible manner (like animations being delayed, etc).

3) viewdidappear: finally, use viewdidappear start off new threads things take long time execute, example doing webservice call data form above.the thing because view exists , being displayed user, can show nice "waiting" message user while data.


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