1.Use Foundation to:Create and manage collections such as arrays and dictionaries.Access images and other resources stored in your app.Create and manage strings.Post and observe notifications.Create date and time objects.Automatically discover devices on IP networks.Manipulate URL streams.Perform code asynchronously.2.Use UIKit to:Construct and manage your user interface.Handle touch and motion-based events.Present text and web content.Optimize your app for multitasking.Create custom user-interface elements.3.Use Core Data to:Save and retrieve objects from storage.Support basic undo/redo.Validate property values automatically.Filter, group, and organize data in memory.Manage results in a table view with [NSFetchedResultsController].Support document-based applications  

1.You need to specify a delegate object for the text field because the text field sends a message to its delegate when the user taps the Done button in the keyboard (recall that a delegate is an object that acts on the behalf of another object).

开放的app在放到ios设备前需要在Xcode Organizer window为app生成签名！设备还要安装相关的provisioning profile

1. A weak reference implies that the referring object does not own the referenced object. The lifetime of an object is determined by how many strong references there are to it. An object is not freed as long as there is a strong reference to it.