平台視圖 (platform views) 讓你可以在 Flutter 應用程式中嵌入原生視圖, 因此你可以從 Dart 對原生視圖進行轉換、裁剪與透明度等操作。

這使你能夠,例如,直接在 Flutter 應用程式中 使用來自 Android 與 iOS SDK 的原生 Google Maps。

iOS 僅使用 Hybrid composition(混合組成), 這代表原生的 UIView 會被加入到視圖階層中。

要在 iOS 上建立平台視圖 (platform view), 請依照以下指示操作:

Dart 端

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在 Dart 端,建立一個 Widget, 並加入 build 實作, 如以下步驟所示。

在 Dart 元件 (Widget) 檔案中,請進行類似於 native_view_example.dart 的修改:

  1. 加入以下 import:

    dart
    import 'package:flutter/foundation.dart';
    import 'package:flutter/services.dart';
  2. 實作一個 build() 方法:

    dart
    Widget build(BuildContext context) {
      // This is used in the platform side to register the view.
      const String viewType = '<platform-view-type>';
      // Pass parameters to the platform side.
      final Map<String, dynamic> creationParams = <String, dynamic>{};
    
      return UiKitView(
        viewType: viewType,
        layoutDirection: TextDirection.ltr,
        creationParams: creationParams,
        creationParamsCodec: const StandardMessageCodec(),
      );
    }

如需更多資訊,請參閱 API 文件: UIKitView

在平台端

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在平台端,可以使用 Swift 或 Objective-C:

實作 factory 與 platform view。 FLNativeViewFactory 會建立 platform view, 而 platform view 則會提供 UIView 的參考。 例如,FLNativeView.swift

swift
import Flutter
import UIKit

class FLNativeViewFactory: NSObject, FlutterPlatformViewFactory {
    private var messenger: FlutterBinaryMessenger

    init(messenger: FlutterBinaryMessenger) {
        self.messenger = messenger
        super.init()
    }

    func create(
        withFrame frame: CGRect,
        viewIdentifier viewId: Int64,
        arguments args: Any?
    ) -> FlutterPlatformView {
        return FLNativeView(
            frame: frame,
            viewIdentifier: viewId,
            arguments: args,
            binaryMessenger: messenger)
    }

    /// Implementing this method is only necessary when the `arguments` in `createWithFrame` is not `nil`.
    public func createArgsCodec() -> FlutterMessageCodec &#x26; NSObjectProtocol {
          return FlutterStandardMessageCodec.sharedInstance()
    }
}

class FLNativeView: NSObject, FlutterPlatformView {
    private var _view: UIView

    init(
        frame: CGRect,
        viewIdentifier viewId: Int64,
        arguments args: Any?,
        binaryMessenger messenger: FlutterBinaryMessenger?
    ) {
        _view = UIView()
        super.init()
        // iOS views can be created here
        createNativeView(view: _view)
    }

    func view() -> UIView {
        return _view
    }

    func createNativeView(view _view: UIView){
        _view.backgroundColor = UIColor.blue
        let nativeLabel = UILabel()
        nativeLabel.text = "Native text from iOS"
        nativeLabel.textColor = UIColor.white
        nativeLabel.textAlignment = .center
        nativeLabel.frame = CGRect(x: 0, y: 0, width: 180, height: 48.0)
        _view.addSubview(nativeLabel)
    }
}

最後,註冊 platform view(平台視圖)。 這可以在應用程式(app)或外掛(plugin)中完成。

若要在應用程式中註冊, 請修改 App 的 AppDelegate.swift

swift
import Flutter
import UIKit

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
    override func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]?
    ) -> Bool {
        GeneratedPluginRegistrant.register(with: self)

        guard let pluginRegistrar = self.registrar(forPlugin: "plugin-name") else { return false }

        let factory = FLNativeViewFactory(messenger: pluginRegistrar.messenger())
        pluginRegistrar.register(
            factory,
            withId: "<platform-view-type>")
        return super.application(application, didFinishLaunchingWithOptions: launchOptions)
    }
}

若要註冊 plugin, 請修改該 plugin 的主要檔案 (例如:FLPlugin.swift):

swift
import Flutter
import UIKit

class FLPlugin: NSObject, FlutterPlugin {
    public static func register(with registrar: FlutterPluginRegistrar) {
        let factory = FLNativeViewFactory(messenger: registrar.messenger())
        registrar.register(factory, withId: "<platform-view-type>")
    }
}

在 Objective-C 中,請加入 factory 和 platform view 的標頭檔。 例如,如 FLNativeView.h 所示:

objc
#import <flutter flutter.h="">

@interface FLNativeViewFactory : NSObject <flutterplatformviewfactory>
- (instancetype)initWithMessenger:(NSObject<flutterbinarymessenger>*)messenger;
@end

@interface FLNativeView : NSObject <flutterplatformview>

- (instancetype)initWithFrame:(CGRect)frame
               viewIdentifier:(int64_t)viewId
                    arguments:(id _Nullable)args
              binaryMessenger:(NSObject<flutterbinarymessenger>*)messenger;

- (UIView*)view;
@end

實作 factory 與 platform view。
FLNativeViewFactory 負責建立 platform view,
而 platform view 則會提供對 UIView 的參考。舉例來說,FLNativeView.m

objc
#import "FLNativeView.h"

@implementation FLNativeViewFactory {
  NSObject<flutterbinarymessenger>* _messenger;
}

- (instancetype)initWithMessenger:(NSObject<flutterbinarymessenger>*)messenger {
  self = [super init];
  if (self) {
    _messenger = messenger;
  }
  return self;
}

- (NSObject<flutterplatformview>*)createWithFrame:(CGRect)frame
                                   viewIdentifier:(int64_t)viewId
                                        arguments:(id _Nullable)args {
  return [[FLNativeView alloc] initWithFrame:frame
                              viewIdentifier:viewId
                                   arguments:args
                             binaryMessenger:_messenger];
}

/// Implementing this method is only necessary when the `arguments` in `createWithFrame` is not `nil`.
- (NSObject<fluttermessagecodec>*)createArgsCodec {
    return [FlutterStandardMessageCodec sharedInstance];
}

@end

@implementation FLNativeView {
   UIView *_view;
}

- (instancetype)initWithFrame:(CGRect)frame
               viewIdentifier:(int64_t)viewId
                    arguments:(id _Nullable)args
              binaryMessenger:(NSObject<flutterbinarymessenger>*)messenger {
  if (self = [super init]) {
    _view = [[UIView alloc] init];
  }
  return self;
}

- (UIView*)view {
  return _view;
}

@end

最後,註冊 platform view(平台視圖)。 這可以在應用程式(app)或外掛(plugin)中完成。

若要在應用程式中註冊, 請修改 App 的 AppDelegate.m

objc
#import "AppDelegate.h"
#import "FLNativeView.h"
#import "GeneratedPluginRegistrant.h"

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application
    didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  [GeneratedPluginRegistrant registerWithRegistry:self];

   NSObject<flutterpluginregistrar>* registrar =
      [self registrarForPlugin:@"plugin-name"];

  FLNativeViewFactory* factory =
      [[FLNativeViewFactory alloc] initWithMessenger:registrar.messenger];

  [[self registrarForPlugin:@"<plugin-name>"] registerViewFactory:factory
                                                          withId:@"<platform-view-type>"];
  return [super application:application didFinishLaunchingWithOptions:launchOptions];
}

@end

若要註冊 plugin, 請修改主要的 plugin 檔案 (例如:FLPlugin.m):

objc
#import <flutter flutter.h="">
#import "FLNativeView.h"

@interface FLPlugin : NSObject<flutterplugin>
@end

@implementation FLPlugin

+ (void)registerWithRegistrar:(NSObject<flutterpluginregistrar>*)registrar {
  FLNativeViewFactory* factory =
      [[FLNativeViewFactory alloc] initWithMessenger:registrar.messenger];
  [registrar registerViewFactory:factory withId:@"<platform-view-type>"];
}

@end

如需更多資訊,請參閱以下的 API 文件:

綜合應用

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當你在 Dart 中實作 build() 方法時, 可以使用 defaultTargetPlatform 來偵測平台,並決定要使用哪個元件(Widget):

dart
Widget build(BuildContext context) {
  // This is used in the platform side to register the view.
  const String viewType = '<platform-view-type>';
  // Pass parameters to the platform side.
  final Map<String, dynamic> creationParams = <String, dynamic>{};

  switch (defaultTargetPlatform) {
    case TargetPlatform.android:
    // return widget on Android.
    case TargetPlatform.iOS:
    // return widget on iOS.
    case TargetPlatform.macOS:
    // return widget on macOS.
    default:
      throw UnsupportedError('Unsupported platform view');
  }
}

效能

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在 Flutter 中使用平台視圖(Platform Views)會帶來效能上的取捨。

針對較複雜的情境,可以採用一些技巧來減輕效能問題。

例如,你可以在 Dart 執行動畫時,使用佔位紋理(placeholder texture)。 換句話說,如果在渲染平台視圖時動畫變慢,可以考慮對原生視圖進行截圖,然後將其作為紋理來渲染。

組合限制

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在組合 iOS 平台視圖(Platform Views)時有一些限制。