[移动开发]Swift Programming-Note 05-MVVM

Lecture03:

Mdel-View-ViewModel (MVVM)

????????A "coding organizing" architectural design paradigm.

????????Must be adhered to for SwiftUI to work.

????????It is different from MVC (Model View Controller) that UIKit (old-style iOS) uses.

?

Architecture

• MVVM

????????Design paradigm

• Varieties of Types

????????struct

????????class

????????protocol

????????"Don't care" type (aka generics)

????????enum

????????functions

?

• Both struct adn class have ... pretty much exactly the same syntax.?
  • Stored vars (the kind you are used to, ie., stored in memory)
  • computed vars (i.e. those whose value is the result of evaluating some code)
  • constant lets (i.e. vars whose values never change)

  • var body: some View {
    return Text("Hello World")
    }

    functions

  • // 显式输入两个参数operand 和 by
    func multiply(operand: Int, by: Int) -> Int {
        return operand * by
    }
    multiply(operand: 5, by: 6)
    
    //只显式输入一个参数otheroperand，另一个没有提示 默认输入到operand参数
    func multiply(_ operand:Int, by otherOperand: Int) -> Int {
        return operand * otherOperand
    }
    multiply(5, by: 6)

    initializers (i.e. special functions that are called when creating a struct or class)

  • struct RoundedRectangle {
        init( cornerRadius: CGFloat) {
            // initialize this rectangle with that cornerRadius
        }
        init(cornerSize: CGSize) {
            // initialize this rectangle with that cornerSize
        }
    }
    
    
    struct MemoryGame {
        init(numberOfPairsOfCards: Int){
            // create a game with that many pairs of cards
        }
    }

    So what's the difference between struct and class?

Generics

• ????????Sometimes we just don't care

We may want to manipulate data structures that we are "type agnostic" about.

In other words, we don't know what type something is and we don't care.

But Swift is a strongly-typed language, so we don't use variables and such that are "untyped".

So how do we specify the type of soemthing when we don't vare what type it is?

We use a "don't care" type (we calll this feature "genetics")--范型

How Array uses a "don't care" type

Array's declearation looks something like this ...

struct Array<Element> {
    ...
    func append(_ element: Element){ ... }
}
    

The type of the argument to append is Element. A "don't care: type.

Array's implementation of append knows nothing about that argument and it does not care.

Element is not any known struct or class or protocal, it's just a placeholder for a type.

The code for using an Array looks something like this...

var a = Array<Int>()
a.append(5)
a.append(22)

When someone uses Array, that's when Element gets determined(by Array<Int>)

Note that Array has to let the world know the names of all of its "don't care" types in its API.

It does this with the < > notation on its struct declaration Array<Element> above.

That's how users of Array know that they have to say what type Element actually is.

var a = Array<Int>()

It is perfectly legal to have multiple "don't care" types in the above (e.g. <Element, foo>)

• Type Parameter

I willl often refer to these types like element in Array as a "don't care" type

If you ask Array what type its elements are, it will say "I don't care".

But its actual name is Type Parameter.

Other languages most of you may know (e.g. Java) hava a similar feature.

However, Swift combines this with protocols to take it all to? the next level.

We'll talk about that next week!

Functions as Types

• Functions are people* too! ( * er, types)

You can declare a variable (or parameter to a func or whatever) to be of type "function".

The syntax for this includes the types of the arguments and return value.

You can do this anywhere any other type is allowed.

Examples ...

(Int, Int)   -> Bool     // takes two Ints and returns a Bool
(Double)     -> Void     // takes a Double and returns nothing
() -> Array<String>      // takes no arguments and returns an Array of Strings
() -> Void               // takes no arguments and returns nothing (this is a common one)

All off the above a just types. No different than Bool or View or Array<Int>. All are types

var foo: (Double) -> Void //foo's type: "function that takes a Double, returns nothing"
func doSomething(what: () -> Bool) // what's type: "function, takes nothing, returns Bool"

Example ...

var operation: (Double) -> Double
// This is a var called operation.
// It is of type"function that takes a Double and returns a Double".

// Here a simple function that takes a Double and return a Double ...
func square(operand: Double) -> Double {
    return operand * operand
}

operation = square //just assigning a value to the operation var, nothing more
let result1 = operand(4) // result1 would equal 16
// Note that we don't use argument labels (e.g. operand:) when executing function types.
operation = sqrt // sqrt is a built-in function which happens to take and return a Double
let retult2 = operation(4) // result2 would be 2
// We'll soon see an example of using a function type for a parameter to a function in our demo

Closures 闭包

It's so common to pass functions around that we are very often "inlining" them.

We call such an inlined function a "closure" and there's special language support for it.

We'll cover this in the demo and again later in the quarter.

Remember that we are mostly doing "functional programming" in SwiftUI.

As the very name implies, "funcitons as types" is a very important concept in Swift. Very.

Architecture

• MVVM

????????Design paradigm

• Varieties of Types

????????struct

????????class

????????protocol

????????"Don't care" type (aka generics)

????????enum

????????functions

• MVVM and Types in Action

Now that we know about MVVM, let's implement it in our Memorize application

In doing so, we'll see a lot of what we just talked about ...

We're going to use the special init function (in both our Model and our ViewModel)

We're going to use generics in our implementation of our Model

We're going to use a function as a type in our Model

We're going to see a class for the first time(our ViewModel will be a class)

We're going to implement an "Intent" in our MVVM

And finally, we will make our UI"reactive" through our MVVM design

Whew! Let's get started ...