FlutterActivity

我們知道南誊，原生連接Flutter需要用到繼承FlutterActivity或者FlutterFragment锻煌，以FlutterActivity為例，它的onCreate如下：

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
  switchLaunchThemeForNormalTheme();

  super.onCreate(savedInstanceState);

  delegate = new FlutterActivityAndFragmentDelegate(this);
  delegate.onAttach(this);
  delegate.onRestoreInstanceState(savedInstanceState);

  lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);

  configureWindowForTransparency();

  setContentView(createFlutterView());

  configureStatusBarForFullscreenFlutterExperience();
}

可見(jiàn)，自定義的子類(lèi)的onCreate中必須要先調(diào)用super.onCreate方法完成FlutterActivity的默認(rèn)配置。

之前我們解析了createFlutterView方法焙格，知道了FlutterEngine渲染的UI通過(guò)FlutterView來(lái)添加到Activity中，現(xiàn)在我們來(lái)看前面的FlutterActivityAndFragmentDelegate夷都，和原生通信的邏輯也從它開(kāi)始眷唉。

FlutterActivityAndFragmentDelegate

首先看一下它的onAttach方法：

void onAttach(@NonNull Context context) {
  ensureAlive();

  if (flutterEngine == null) {
    setupFlutterEngine();
  }

  if (host.shouldAttachEngineToActivity()) {
    flutterEngine.getActivityControlSurface().attachToActivity(this, host.getLifecycle());
  }
  
  platformPlugin = host.providePlatformPlugin(host.getActivity(), flutterEngine);

  host.configureFlutterEngine(flutterEngine);
}

首先會(huì)調(diào)用setupFlutterEngine方法保證先創(chuàng)建FlutterEngine，然后調(diào)用host的configureFlutterEngine方法來(lái)對(duì)FlutterEngine進(jìn)行相關(guān)配置囤官，比如可以重寫(xiě)FlutterActivity的子類(lèi)的configureFlutterEngine來(lái)添加自定義的Plugin冬阳。

看一下setupFlutterEngine方法：

void setupFlutterEngine() {

  // First, check if the host wants to use a cached FlutterEngine.
  String cachedEngineId = host.getCachedEngineId();
  if (cachedEngineId != null) {
    flutterEngine = FlutterEngineCache.getInstance().get(cachedEngineId);
    isFlutterEngineFromHost = true;
    if (flutterEngine == null) {
      throw new IllegalStateException(
          "The requested cached FlutterEngine did not exist in the FlutterEngineCache: '"
              + cachedEngineId
              + "'");
    }
    return;
  }

  // Second, defer to subclasses for a custom FlutterEngine.
  flutterEngine = host.provideFlutterEngine(host.getContext());
  if (flutterEngine != null) {
    isFlutterEngineFromHost = true;
    return;
  }

  // Our host did not provide a custom FlutterEngine. Create a FlutterEngine to back our
  // FlutterView.
  flutterEngine =
      new FlutterEngine(
          host.getContext(),
          host.getFlutterShellArgs().toArray(),
          /*automaticallyRegisterPlugins=*/ false,
          /*willProvideRestorationData=*/ host.shouldRestoreAndSaveState());
  isFlutterEngineFromHost = false;
}

在方法的一開(kāi)始，首先會(huì)通過(guò)FlutterEngineCache.getInstance().get(cachedEngineId)方法嘗試從緩存中取FlutterEngine党饮，所以我們可以在子類(lèi)Activity中重寫(xiě)getCachedEngineId方法來(lái)設(shè)置在每一次重新打開(kāi)該Activity的時(shí)候都能使用之前的FlutterEngine摩泪，這會(huì)極大地提高性能。getCachedEngineId方法在FlutterActivity中有默認(rèn)實(shí)現(xiàn)：

@Override
@Nullable
public String getCachedEngineId() {
  return getIntent().getStringExtra(EXTRA_CACHED_ENGINE_ID);
}

可以看到劫谅，你也可以通過(guò)intent的方式傳遞cachedEngineId见坑。

當(dāng)然我們需要事先創(chuàng)建FlutterEngine然后把它put到FlutterEngineCache中，這一步你可以放在Application類(lèi)的初始化中捏检，但不是更好的做法荞驴，我們接著往下看。

如果cache中沒(méi)有拿到之前使用過(guò)的FlutterEngine的話就會(huì)嘗試調(diào)用子類(lèi)Activity重寫(xiě)的provideFlutterEngine方法來(lái)獲取贯城，所以你可以重寫(xiě)這個(gè)方法然后在這里創(chuàng)建FlutterEngine熊楼，然后可以在這里把它放到cache中，這就是比上面放在Application中更好的做法能犯，為什么呢鲫骗？因?yàn)榉旁贏pplication中的話，F(xiàn)lutterEngine并不一定會(huì)被使用踩晶，有可能使用過(guò)程中從沒(méi)有打開(kāi)過(guò)Flutter頁(yè)面执泰，這就會(huì)導(dǎo)致始終持有這個(gè)FlutterEngine對(duì)象，造成資源浪費(fèi)渡蜻；還有一個(gè)好處就是這樣一來(lái)每個(gè)頁(yè)面都有自己的獨(dú)立的FlutterEngine术吝，每個(gè)FlutterEngine的業(yè)務(wù)相對(duì)獨(dú)立而且不需要維護(hù)其他Flutter業(yè)務(wù)的相關(guān)信息计济。當(dāng)然如果是App的所有頁(yè)面都是Flutter頁(yè)面或者原生頁(yè)面占很少一部分的時(shí)候在Application中配置會(huì)更好。

最后一步則是new一個(gè)新的FlutterEngine對(duì)象排苍，context和host相關(guān)聯(lián)沦寂。

使用緩存FlutterEngine的作用一個(gè)是明顯地提高頁(yè)面打開(kāi)速度，再一個(gè)會(huì)保存之前Flutter頁(yè)面的操作信息淘衙。

需要注意的是传藏，如果使用緩存id的話，則需要手動(dòng)設(shè)置切入點(diǎn)彤守，哪怕是main的默認(rèn)入口也要手動(dòng)設(shè)置漩氨。因?yàn)樵贔lutterActivity的onStart方法中調(diào)用了一個(gè)doInitialFlutterViewRun方法：

private void doInitialFlutterViewRun() {
  // Don't attempt to start a FlutterEngine if we're using a cached FlutterEngine.
  if (host.getCachedEngineId() != null) {
    return;
  }

  ... ...

  String appBundlePathOverride = host.getAppBundlePath();
  if (appBundlePathOverride == null || appBundlePathOverride.isEmpty()) {
    appBundlePathOverride = FlutterInjector.instance().flutterLoader().findAppBundlePath();
  }

  // Configure the Dart entrypoint and execute it.
  DartExecutor.DartEntrypoint entrypoint =
      new DartExecutor.DartEntrypoint(
          appBundlePathOverride, host.getDartEntrypointFunctionName());
  flutterEngine.getDartExecutor().executeDartEntrypoint(entrypoint);
}

可見(jiàn)，如果使用緩存id的話則不會(huì)走到下面的默認(rèn)切入點(diǎn)設(shè)置代碼遗增。

FlutterPlugin

我們這里分析的是和原生通信，所以必須要看FlutterPlugin款青，它是通信的橋梁。

前面我們說(shuō)到可以重寫(xiě)FlutterActivity的configureFlutterEngine方法來(lái)配置Plugin：

@Override
    public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
        flutterEngine.getPlugins().add((FlutterPlugin) getPlugin());
        super.configureFlutterEngine(flutterEngine);
    }

getPlugins方法獲取的是FlutterEngineConnectionRegistry，它的add方法中有如下代碼：

if (has(plugin.getClass())) {
  return;
}
plugins.put(plugin.getClass(), plugin);
plugin.onAttachedToEngine(pluginBinding);

可見(jiàn)會(huì)按照plugin的class作為key保存到plugins中勘天，然后就調(diào)用plugin的onAttachedToEngine方法龙助，所以我們通常都會(huì)重寫(xiě)這個(gè)方法，在里面初始化各種Channel康震。

下面看一個(gè)demo：

class MyHomePlugin : FlutterPlugin, EventChannel.StreamHandler, MethodChannel.MethodCallHandler {

    private var batteryEventChannel : EventChannel? = null
    private var methodChannel : MethodChannel? = null
    private var batteryEventSink : EventChannel.EventSink? = null

    override fun onAttachedToEngine(binding : FlutterPlugin.FlutterPluginBinding) {
        batteryEventChannel = EventChannel(binding.binaryMessenger, "battery")
        batteryEventChannel?.setStreamHandler(this)

        methodChannel = MethodChannel(binding.binaryMessenger, "allMethod")
        methodChannel?.setMethodCallHandler(this)
    }

    override fun onDetachedFromEngine(binding : FlutterPlugin.FlutterPluginBinding) {
        batteryEventChannel?.setStreamHandler(null)
        batteryEventChannel = null
        methodChannel?.setMethodCallHandler(null)
        methodChannel = null
    }

    override fun onListen(arguments : Any?, events : EventChannel.EventSink?) {
        batteryEventSink = events
        events?.success(666)
          //events?.error("007","Wrong~",null)
        //events?.endOfStream()
    }

    override fun onCancel(arguments : Any?) {
        batteryEventSink?.endOfStream()
        batteryEventSink = null
    }

    override fun onMethodCall(call : MethodCall, result : MethodChannel.Result) {
        when(call.method){
            "getName"->{
                LogUtil.d(this,"getName() has been called!")
            }
        }
    }
}

對(duì)于EventChannel來(lái)說(shuō)燎含，調(diào)用setStreamHandler方法設(shè)置一個(gè)EventChannel.StreamHandler，重寫(xiě)onListen和onCancel方法后即可接收Flutter發(fā)送的數(shù)據(jù)腿短，onListen回調(diào)中屏箍，使用一個(gè)變量持有events的引用，這樣在需要發(fā)送給Flutter的時(shí)候直接使用batteryEventSink發(fā)送即可橘忱，和MethodChannel不同赴魁，EventSink使用success、error和endOfStream方法發(fā)送數(shù)據(jù)钝诚，分別對(duì)應(yīng)Flutter監(jiān)聽(tīng)端的幾個(gè)回調(diào)颖御，F(xiàn)lutter端的監(jiān)聽(tīng)寫(xiě)法是：

@override
void initState() {
  super.initState();
  eventChannel.receiveBroadcastStream().listen(
    (event) {
      onReceiveBatteryChange(event);
    },
    onError: onReceiveBatteryWrong,
  );
}

這里調(diào)用了listen方法之后就會(huì)回調(diào)到上面原生代碼的onListen方法中，使用方法參數(shù)events調(diào)用相關(guān)方法后凝颇，最終又會(huì)回調(diào)到這里listen方法里設(shè)置的對(duì)應(yīng)回調(diào)函數(shù)潘拱。

對(duì)于MethodChannel來(lái)說(shuō)，以Flutter端發(fā)送拧略、原生接收的方向來(lái)看芦岂，F(xiàn)lutter端的發(fā)送代碼是：

Future result = await methodChannel.invokeMethod("getName");

原生接收需要調(diào)用setMethodCallHandler給MethodChannel設(shè)置MethodChannel.MethodCallHandler，重寫(xiě)其onMethodCall回調(diào)方法處理接收邏輯垫蛆。反過(guò)來(lái)盔腔，原生發(fā)送、Flutter接收的方向也是一樣的，只不過(guò)設(shè)置的內(nèi)容交換一下即可弛随。

總結(jié)

本文分析了FlutterActivity瓢喉、FlutterPlugin、Channel之間的連接關(guān)系舀透，以及FlutterEngine的緩存策略栓票，并知道了如何使用相關(guān)Channel來(lái)使Flutter端和原生交互起來(lái)，至于EventChannel和MethodChannel的原理涉及的代碼比較多愕够，另起一篇來(lái)整理走贪。