Flutter Android多窗口方案落地实战

前言

通过此篇文章，你将了解到：

Flutter如何在Android上实现多窗口机制；

Flutter与Android的事件机制和冲突解决；

Flutter多窗口存在的隐患和展望。

Flutter在桌面端的多窗口需求，一直是个历史巨坑。随着Flutter的技术在我们windows、android桌面设备落地，我们发现多窗口需求必不可少，突破这个技术壁垒已经刻不容缓。

实现原理

1. 基本原理

对于Android移动设备来说，多窗口的应用大多是用于直播/音视频的悬浮弹窗，让用户离开应用后还能在小窗口中观看内容。实现原理是通过WindowManager创建和管理窗口，包括视图内容、拖拽、事件等操作。

我们都清楚Flutter只是一个可以做业务逻辑的UI框架，在Flutter中想要实现多窗口，也必须依赖Android的窗口管理机制。基于原生的Window，显示Flutter绘制的UI，从而实现跨平台的视图交互和业务逻辑。

2. 具体步骤

Android端基于Window Manager创建Window，管理窗口的生命周期和拖拽逻辑；
使用FlutterEngineGroup来管理Flutter Engine，通过引擎吸附Flutter的UI，加入到原生的FlutterView；
把FlutterView通过addView的方式加入到Window上。

3. 原理图

插件实现

基于上述原理，可以在Android的窗口显示Flutter的UI。但要真正提供给Flutter层使用，还需要再封装一个插件层。

通过单例管理多个窗口由于是多窗口，可能项目中多个地方都会调用到，因此需要使用单例来统一管理所有窗口的生命周期，保证准确创建、及时销毁。

//引擎生命钩子回调，让调用方感知引擎状态
interface EngineCallback {
    fun onCreate(id:String)
    fun onEngineDestroy(id: String)
}
class EngineManager private constructor(context: Context) {
    // 单例对象
    companion object :
        SingletonHolder<EngineManager, Context>(::EngineManager)
    // 窗口类型；如果是单一类型，那么同名窗口将返回上一次的未销毁的实例。
    private val TYPE_SINGLE: String = "single"
    init {
        Log.d("EngineManager", "EngineManager init")
    }
    data class Entry(
        val engine: FlutterEngine,
        val window: AndroidWindow?
    )
    private var myContext: Context = context
    private var engineGroup: FlutterEngineGroup = FlutterEngineGroup(myContext)
    // 每个窗口对应一个引擎，基于引擎ID和名称存储多窗口的信息，以及查找
    private val engineMap = ConcurrentHashMap<String, Entry>() //搜索引擎，用作消息分发
    private val nameIdMap = ConcurrentHashMap<String, String>() //判断是否存在了任务
    private val idNameMap = ConcurrentHashMap<String, String>() //根据任务获取name并清除
    private val engineCallback =
        ConcurrentHashMap<String, EngineCallback>() //通知调用方引擎状态-create 1-attach 2-destroy
    fun showWindow(
        params: HashMap<String, Any>,
        engineStatusCallback: EngineCallback
    ): String? {
        val entry: String?
        if (params.containsKey("entryPoint")) {
            entry = params["entryPoint"] as String
        } else {
            return null
        }
        val name: String?
        if (params.containsKey("name")) {
            name = params["name"] as String
        } else {
            return null
        }
        val type = params["type"]
        if (type == TYPE_SINGLE && nameIdMap[name] != null) {
            return nameIdMap[name]
        }
        val windowUid = UUID.randomUUID().toString()
        if (type == TYPE_SINGLE) {
            nameIdMap[name] = windowUid
            idNameMap[windowUid] = name
            engineCallback[windowUid] = engineStatusCallback
        }
        val dartEntrypoint = DartExecutor.DartEntrypoint(findAppBundlePath(), entry)
        val args = mutableListOf(windowUid)
        var user: List<String>? = null
        if (params.containsKey("params")) {
            user = params["params"] as List<String>
        }
        if (user != null) {
            args.addAll(user)
        }
        // 把调用方传递的参数回调给Flutter
        val option =
            FlutterEngineGroup.Options(myContext).setDartEntrypoint(dartEntrypoint)
                .setDartEntrypointArgs(
                    args
                )
        val engine = engineGroup.createAndRunEngine(option)
        val draggable = params["draggable"] as Boolean? ?: true
        val width = params["width"] as Int? ?:
        val height = params["height"] as Int? ?:
        val config = GravityConfig()
        config.paddingX = params["paddingX"] as Double? ?:.0
        config.paddingY = params["paddingY"] as Double? ?:.0
        config.gravityX = GravityForX.values()[params["gravityX"] as Int? ?:]
        config.gravityY = GravityForY.values()[params["gravityY"] as Int? ?:]
        // 把创建好的引擎传给AndroidWindow，由其去创建窗口
        val androidWindow =
            AndroidWindow(myContext, draggable, width, height, config, engine)
        engineMap[windowUid] = Entry(engine, androidWindow)
        androidWindow.open()
        engine.platformViewsController.attach(
            myContext,
            engine.renderer,
            engine.dartExecutor
        )
        return windowUid
    }
    fun setPosition(id: String?, x: Int, y: Int): Boolean {
        id ?: return false
        val entry = engineMap[id]
        entry ?: return false
        entry.window?.setPosition(x, y)
        return true
    }
    fun setSize(id: String?, width: double, height: double): Boolean {
        // ......
    }
}

通过代码我们可以看到，每个窗口都对应一个engine，通过name和生成的UUID做唯一标识，然后把engine传给AndroidWindow，在那里加入WindowManger，以及Flutter UI的获取。

AndroidWindow的实现；通过context.getSystemService(Service.WINDOW_SERVICE) as WindowManager获取窗口管理器；同时创建FlutterView和LayoutInfalter，通过engine拿到视图吸附到FlutterView，把FlutterView加到Layout中，最后把Layout通过addView加到WindowManager中显示。

class AndroidWindow(
    private val context: Context,
    private val draggable: Boolean,
    private val width: Int,
    private val height: Int,
    private val config: GravityConfig,
    private val engine: FlutterEngine
) {
    private var startX =f
    private var startY =f
    private var initialX =
    private var initialY =
    private var dragging = false
    private lateinit var flutterView: FlutterView
    private var windowManager = context.getSystemService(Service.WINDOW_SERVICE) as WindowManager
    private val inflater =
        context.getSystemService(Service.LAYOUT_INFLATER_SERVICE) as LayoutInflater
    private val metrics = DisplayMetrics()
    @SuppressLint("InflateParams")
    private var rootView = inflater.inflate(R.layout.floating, null, false) as ViewGroup
    private val layoutParams = WindowManager.LayoutParams(
        dippx(context, width.toFloat()),
        dippx(context, height.toFloat()),
        WindowManager.LayoutParams.TYPE_SYSTEM_ALERT, // 系统应用才可使用此类型
        WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE,
        PixelFormat.TRANSLUCENT
    )
    fun open() {
        @Suppress("Deprecation")
        windowManager.defaultDisplay.getMetrics(metrics)
        layoutParams.gravity = Gravity.START or Gravity.TOP
        selectMeasurementMode()
        // 设置位置
        val screenWidth = metrics.widthPixels
        val screenHeight = metrics.heightPixels
        when (config.gravityX) {
            GravityForX.Left -> layoutParams.x = config.paddingX!!.toInt()
            GravityForX.Center -> layoutParams.x =
                ((screenWidth - layoutParams.width) / + config.paddingX!!).toInt()
            GravityForX.Right -> layoutParams.x =
                (screenWidth - layoutParams.width - config.paddingX!!).toInt()
            null -> {}
        }
        when (config.gravityY) {
            GravityForY.Top -> layoutParams.y = config.paddingY!!.toInt()
            GravityForY.Center -> layoutParams.y =
                ((screenHeight - layoutParams.height) / + config.paddingY!!).toInt()
            GravityForY.Bottom -> layoutParams.y =
                (screenHeight - layoutParams.height - config.paddingY!!).toInt()
            null -> {}
        }
        windowManager.addView(rootView, layoutParams)
        flutterView = FlutterView(inflater.context, FlutterSurfaceView(inflater.context, true))
        flutterView.attachToFlutterEngine(engine)
        if (draggable) {
            @Suppress("ClickableViewAccessibility")
            flutterView.setOnTouchListener { _, event ->
                when (event.action) {
                    MotionEvent.ACTION_MOVE -> {
                        if (dragging) {
                            setPosition(
                                initialX + (event.rawX - startX).roundToInt(),
                                initialY + (event.rawY - startY).roundToInt()
                            )
                        }
                    }
                    MotionEvent.ACTION_UP -> {
                        dragEnd()
                    }
                    MotionEvent.ACTION_DOWN -> {
                        startX = event.rawX
                        startY = event.rawY
                        initialX = layoutParams.x
                        initialY = layoutParams.y
                        dragStart()
                        windowManager.updateViewLayout(rootView, layoutParams)
                    }
                }
                false
            }
        }
        @Suppress("ClickableViewAccessibility")
        rootView.setOnTouchListener { _, event ->
            when (event.action) {
                MotionEvent.ACTION_DOWN -> {
                    layoutParams.flags =
                        layoutParams.flags or WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                    windowManager.updateViewLayout(rootView, layoutParams)
                    true
                }
                else -> false
            }
        }
        engine.lifecycleChannel.appIsResumed()
        rootView.findViewById<FrameLayout>(R.id.floating_window)
            .addView(
                flutterView,
                ViewGroup.LayoutParams(
                    ViewGroup.LayoutParams.MATCH_PARENT,
                    ViewGroup.LayoutParams.MATCH_PARENT
                )
            )
        windowManager.updateViewLayout(rootView, layoutParams)
    }
    // .....

插件层封装。插件层就很简单了，创建好MethodCallHandler之后，直接持有单例的EngineManager就可以了。

class FlutterMultiWindowsPlugin : FlutterPlugin, MethodCallHandler {
    companion object {
        private const val TAG = "MultiWindowsPlugin"
    }
    @SuppressLint("LongLogTag")
    override fun onAttachedToEngine(@NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
        Log.i(TAG, "onMessage: onAttachedToEngine")
        Log.i(TAG, "onAttachedToEngine: ${Thread.currentThread().name}")
        MessageHandle.init(flutterPluginBinding.applicationContext)
        MethodChannel(
            flutterPluginBinding.binaryMessenger,
            "flutter_multi_windows.messageChannel",
        ).setMethodCallHandler(this)
    }
    override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
        Log.i(TAG, "onDetachedFromEngine: ${Thread.currentThread().name}")
    }
    override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
        Log.i(TAG, "onMethodCall: thread : ${Thread.currentThread().name}")
        MessageHandle.onMessage(call, result)
    }
}
@SuppressLint("StaticFieldLeak")
internal object MessageHandle {
    private const val TAG = "MessageHandle"
    private var context: Context? = null
    private var manager: EngineManager? = null
    fun init(context: Context) {
        this.context = context
        if (manager != null)
            return
        // 必须单例调用
        manager = EngineManager.getInstance(this.context!!)
    }
    // 处理消息，所有管道通用。需要共享Flutter Activity
    fun onMessage(
        call: MethodCall, result: MethodChannel.Result
    ) {
        val params = call.arguments as Map<*, *>
        when (call.method) {
            "open" -> {
                Log.i(TAG, "onMessage: open")
                val map: HashMap<String, Any> = HashMap()
                map["needShowWindow"] = true
                map["name"] = params["name"] as String
                map["entryPoint"] = params["entryPoint"] as String
                map["width"] = (params["width"] as Double).toInt()
                map["height"] = (params["height"] as Double).toInt()
                map["gravityX"] = params["gravityX"] as Int
                map["gravityY"] = params["gravityY"] as Int
                map["paddingX"] = params["paddingX"] as Double
                map["paddingY"] = params["paddingY"] as Double
                map["draggable"] = params["draggable"] as Boolean
                map["type"] = params["type"] as String
                if (params["params"] != null) {
                    map["params"] = params["params"] as ArrayList<String>
                }
                result.success(manager?.showWindow(map, object : EngineCallback {
                    override fun onEngineDestroy(id: String) {
                    }
                }))
            }
            "close" -> {
                val windowId = params["windowId"] as String
                manager?.dismissWindow(windowId)
            }
            "executeTask" -> {
                Log.i(TAG, "onMessage: executeTask")
                val map: HashMap<String, Any> = HashMap()
                map["name"] = params["name"] as String
                map["entryPoint"] = params["entryPoint"] as String
                map["type"] = params["type"] as String
                result.success(manager?.executeTask(map))
            }
            "finishTask" -> {
                manager?.finishTask(params["taskId"] as String)
            }
            "setPosition" -> {
                val res = manager?.setPosition(
                    params["windowId"] as String,
                    params["x"] as Int,
                    params["y"] as Int
                )
                result.success(res)
            }
            "setAlpha" -> {
                val res = manager?.setAlpha(
                    params["windowId"] as String,
                    (params["alpha"] as Double).toFloat(),
                )
                result.success(res)
            }
            "resize" -> {
                val res = manager?.resetWindowSize(
                    params["windowId"] as String,
                    params["width"] as Int,
                    params["height"] as Int
                )
                result.success(res)
            }
            else -> {
            }
        }
    }
}

同时需要清楚，Engine通过传入的entryPoint，就可以找到Flutter层中的方法入口点，在入口点中runApp即可。

实现过程中的坑

在实现过程中我们遇到的值得分享的坑，就是Flutter GestureDetector和Window滑动事件的冲突。由于悬浮窗是需要可滑动的，因此在原生层需要监听对应的事件；而Flutter的事件,是Android层分发给FlutterView的，两者形成冲突，导致Flutter内部滑动的时候，原生层也会捕获到，最终造成冲突。

如何解决？从需求上来看，悬浮窗是否需要滑动，应该交给调用方决定，也就是由Flutter层来决定是否Android是否要对Flutter的滑动事件进行监听，即flutterView.setOnTouchListener。这里我们使用一种更轻量级的操作，FlutterView的监听默认加上，然后在事件处理中，我们通过变量来做处理；而Flutter通过MethodChannel改变这个变量，加快了通信速度，避免了事件来回监听和销毁。

flutterView.setOnTouchListener { _, event ->
    when (event.action) {
        MotionEvent.ACTION_MOVE -> {
            if (dragging) {
                setPosition(
                    initialX + (event.rawX - startX).roundToInt(),
                    initialY + (event.rawY - startY).roundToInt()
                )
            }
        }
        MotionEvent.ACTION_UP -> {
            dragEnd()
        }
        MotionEvent.ACTION_DOWN -> {
            startX = event.rawX
            startY = event.rawY
            initialX = layoutParams.x
            initialY = layoutParams.y
            dragStart()
            windowManager.updateViewLayout(rootView, layoutParams)
        }
    }
    false
}

dragging则是通过Flutter层去驱动的：FlutterMultiWindowsPlugin().dragStart();

private fun dragStart() {
    dragging = true
}
private fun dragEnd() {
    dragging = false
}

使用方式

目前我们内部已在4个应用落地了这个方案。应用方式有两种：一种是Flutter通过插件调用，也可以直接通过后台Service打开。效果尚佳，目的都是为了让Flutter的UI跨端使用。

另外，Flutter的方法入口点必须声明@pragma('vm:entry-point')。

写在最后

目前来看这种方式可以完美支持Flutter在Android上开启多窗口，且能精准控制。但由于一个engine对应一个窗口，过多engine带来的内存隐患还是不可忽视的。我们希望Flutter官方能尽快的支持engine对应多个入口点，并且共享内存，只不过目前来看还是有点天方夜谭~~

这篇文章，需要有一定原生基础的同学才能看懂。只讲基础原理，代码不全，仅供参考！ 另外多窗口的需求，不知道大家需求量如何，热度可以的话我再出个windows的多窗口实现！