Model-View-Controller Architecture

Transcription

1 --Controller Architecture

2 --Controller Architecture Controller --Controller (MVC) is an architectural pattern, a standard design in the field of software architecture. Heavily used by Apple in writing ios apps, so understanding it is critical. MVC divides the objects in your program into three camps.

3 : Encapsulates data and basic behaviors Holds an application s data and defines the logic that manipulates that data. Responds to requests for information about its state, and responds to instructions to change state. May notify other objects when its state changes. Can be as simple as a single integer data member, or as complex as a collection of classes interacting with a database.

4 : Presents information to the user Knows how to display, and might allow users to edit, data from the application s model. Multiple views can exist for a single model for different purposes. A view should not be responsible for storing the data it displays. In ios, each screen full of information is typically a view. UI elements such as buttons, sliders, text fields, etc. are also typically view objects.

5 Controller: Ties the to the Acts as an intermediary between the application s view objects and its model objects. Receives user input from view objects and initiates a response by making calls on model objects. When the state of the model changes, the model communicates that change to the controller, which then requests one or more view objects to update themselves accordingly.

6 It s all about managing communication and separating concerns Limiting and structuring communication makes it easier to design programs. Separating concerns allows s and s to be re-used in different applications.

7 Controllers can always talk directly to their Controller usually maintains object references (pointers) to objects. Object references allow Controller code to send messages to the objects.

8 Outlet Controllers can also talk directly to their A Controller usually maintains a special type of object reference (called an outlet) to objects. Outlets allow Controller code to send messages to the objects.

9 Outlet No communication! The and should not speak to each other!

10 Outlet? No communication! Can the speak to its Controller? Yes, to some degree. Communication is blind and structured. There are several common patterns of communication allowed.

11 Outlet Target-Action No communication! 1. Target-Action Pattern An action is a message emitted automatically by a Cocoa UIControl interface object (a object called a control) when the user does something to it, such as tapping the control. The various user behaviors that will cause a control to emit an action message are called events.

12 Outlet Target-Action No communication! 1. Target-Action Pattern Controller needs to define the action message (as a method of one of the Controller classes). Controller needs to inform the control of three things: o What event should trigger the sending of an action message. o What instance should be sent the action message. o What the action message s name should be. When the event occurs, the control sends the action message to the specified instance, which can then take appropriate action.

13 Outlet Target-Action Delegate No communication! 2. Delegate Pattern Sometimes the needs to synchronize with the Controller. The Controller sets itself as the s delegate. The has an object reference that can point to any object that conforms to a particular delegate protocol. The Controller must conform to that protocol (implement the methods that it defines). That allows the to call those methods in the Controller without knowing the Controller object s exact data type.

14 Outlet Target-Action Delegate Data Source No communication! 3. Data Source s do not own the data that they display. A data source is a type of delegate that allows a to acquire data it needs to display. Controllers are almost always the data source, not the itself. Controllers interpret/format information for the.

15 ? Outlet Target-Action Delegate Data Source No communication! Can the talk directly to the Controller? No. The should be UI independent. So how does the inform other objects that its state has changed?

16 Notification & Key-Value Observing Outlet Target-action Delegate Data source No communication! The can use a radio station -like notification mechanism to broadcast changes to interested objects (called observers). Key-value observing is another mechanism that allows observer objects to be notified of changes to specific properties of an object. Controllers (or another ) tune in to interesting stuff. A might tune in, but probably not to a s station.

17 Larger ios applications consist of many MVC triads, all working together.

18 References APPLE INC., Cocoa fundamentals guide. BUCK, E. M. AND YACKTMAN, D. A., Cocoa Design Patterns. Addison Wesley, Upper Saddle River, NJ.