Hello, SwiftUI

来体验一把声明式语法吧

import SwiftUI

struct ContentView: View {

var body: some View {

VStack {

MapView()

.edgesIgnoringSafeArea(.top)

.frame(height: 300)

CircleImage()

.offset(x: 0, y: -130)

.padding(.bottom, -130)

VStack(alignment: .leading) {

Text("Turtle Rock")

.font(.title)

HStack(alignment: .top) {

Text("Joshua Tree National Park")

.font(.subheadline)

Spacer()

Text("California")

.font(.subheadline)

}

}

.padding()

Spacer()

}

}

}

#if DEBUG

struct ContentView_Previews: PreviewProvider {

static var previews: some View {

ContentView()

}

}

#endif

我们看看先分析一下它是怎么玩的

上面代码中诡异的出现啦几个陌生的东东

var body: some View {

struct ContentView: View {

PreviewProvider

那么能够时时预览那么肯定跟这三个哥们有关系

下面我们先看看这View在swiftUI中是何方神

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)

public protocol View : _View {

/// The type of view representing the body of this view.

///

/// When you create a custom view, Swift infers this type from your

/// implementation of the required `body` property.

associatedtype Body : View

/// Declares the content and behavior of this view.

var body: Self.Body { get }

}

和明显这个所谓的body就是我们实时预览的真正实体

接着我们看PreviewProvider

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)

public protocol PreviewProvider : _PreviewProvider {

/// The type of the previews variable.

associatedtype Previews : View

/// Generates a collection of previews.

///

/// Example:

///

/// struct MyPreviews : PreviewProvider {

/// static var previews: some View {

/// return Group {

/// GreetingView("Hello"),

/// GreetingView("Guten Tag"),

///

///ForEach(otherGreetings.identified(by: \.self_)) {

/// GreetingView($0)

/// }

/// ]

/// .previewDevice("iPhone X")

/// }

/// }

static var previews: Self.Previews { get }

/// Returns which platform to run the provider on.

///

/// When `nil`, Xcode infers the platform based on the file the

/// `PreviewProvider` is defined in. This should only be provided when the

/// file is in targets that support multiple platforms.

static var platform: PreviewPlatform? { get }

}

咋看之下混混然，仔细一看有一个 static var previews: Self.Previews { get } 还是个static变量，从而我们可以臆断这个就是渲染的主要地方和实现

接下来我们看看怎么在开发中使用

在使用之前我们会发现多了个类

SceneDelegate

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
        // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
        // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).

        // Use a UIHostingController as window root view controller
        let window = UIWindow(frame: UIScreen.main.bounds)
        window.rootViewController = UIHostingController(rootView: ContentView())
        self.window = window
        window.makeKeyAndVisible()
    }

细心的朋友会发现多个了UIHostingController

而Appdelegate则

    // MARK: UISceneSession Lifecycle
    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        // Called when a new scene session is being created.
        // Use this method to select a configuration to create the new scene with.
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }

自从新的功能就基本串联起来，后续我研究怎么使用swiftUI