support external initialized flutter engine

ios/Classes/Boost/FLBFlutterApplicationInterface.h

@@ -35,6 +35,7 @@ NS_ASSUME_NONNULL_BEGIN
 - (id<FLBFlutterProvider>)flutterProvider;
 
 - (void)startFlutterWithPlatform:(id<FLBPlatform>)platform
+                      withEngine:(FlutterEngine* _Nullable)engine
                          onStart:(void (^)(FlutterEngine *engine))callback;
 
 - (FlutterViewController *)flutterViewController;

ios/Classes/Boost/FlutterBoostPlugin.h

@@ -26,6 +26,7 @@
 #import "FLBPlatform.h"
 #import "FLBTypes.h"
 
+NS_ASSUME_NONNULL_BEGIN
 @interface FlutterBoostPlugin : NSObject<FlutterPlugin>
 #pragma mark - Initializer
 + (instancetype)sharedInstance;
@@ -38,6 +39,16 @@
  */
 - (void)startFlutterWithPlatform:(id<FLBPlatform>)platform
                          onStart:(void (^)(FlutterEngine *engine))callback;
+/**
+ * 初始化FlutterBoost混合栈环境。应在程序使用混合栈之前调用。如在AppDelegate中
+ *
+ * @param platform 平台层实现FLBPlatform的对象
+ * @param engine   外部实例化engine后传入
+ * @param callback 启动之后回调
+ */
+- (void)startFlutterWithPlatform:(id<FLBPlatform>)platform
+                          engine:(FlutterEngine* _Nullable)engine
+                         onStart:(void (^)(FlutterEngine *engine))callback;
 
 #pragma mark - Some properties.
 - (BOOL)isRunning;
@@ -109,3 +120,4 @@
 onPageFinished:(void (^)(NSDictionary *))resultCallback
   completion:(void (^)(BOOL))completion;
 @end
+NS_ASSUME_NONNULL_END

ios/Classes/Boost/FlutterBoostPlugin.m

@@ -113,6 +113,13 @@
 
 - (void)startFlutterWithPlatform:(id<FLBPlatform>)platform
                          onStart:(void (^)(FlutterEngine *engine))callback;
+{
+    [self startFlutterWithPlatform:platform engine:nil onStart:callback];
+}
+
+- (void)startFlutterWithPlatform:(id<FLBPlatform>)platform
+                         engine:(FlutterEngine* _Nullable)engine
+                         onStart:(void (^)(FlutterEngine *engine))callback;
 {
     static dispatch_once_t onceToken;
     __weak __typeof__(self) weakSelf = self;
@@ -121,6 +128,7 @@
         self.factory = FLBFactory.new;
         self.application = [self->_factory createApplication:platform];
         [self.application startFlutterWithPlatform:platform
+                                     withEngine:engine
                                        onStart:callback];
     });
 }

ios/Classes/Engine/FLBFlutterApplication.m

@@ -50,12 +50,13 @@
 }
 
 - (void)startFlutterWithPlatform:(id<FLBPlatform>)platform
+                      withEngine:(FlutterEngine* _Nullable)engine
                          onStart:(void (^)(FlutterEngine *engine))callback
 {
     static dispatch_once_t onceToken;
     dispatch_once(&onceToken, ^{
         self.platform = platform;
-        self.viewProvider = [[FLBFlutterEngine alloc] initWithPlatform:platform];
+        self.viewProvider = [[FLBFlutterEngine alloc] initWithPlatform:platform engine:engine];
         self.isRunning = YES;
         if(callback) callback(self.viewProvider.engine);
     });

ios/Classes/Engine/FLBFlutterEngine.h

@@ -28,6 +28,6 @@
 
 NS_ASSUME_NONNULL_BEGIN
 @interface FLBFlutterEngine : NSObject<FLBFlutterProvider>
-- (instancetype)initWithPlatform:(id<FLBPlatform>_Nullable)platform;
+- (instancetype)initWithPlatform:(id<FLBPlatform> _Nullable)platform engine:(FlutterEngine* _Nullable)engine;
 @end
 NS_ASSUME_NONNULL_END

ios/Classes/Engine/FLBFlutterEngine.m

@@ -35,13 +35,17 @@
 
 @implementation FLBFlutterEngine
     
-- (instancetype)initWithPlatform:(id<FLBPlatform> _Nullable)platform
+- (instancetype)initWithPlatform:(id<FLBPlatform> _Nullable)platform engine:(FlutterEngine * _Nullable)engine
 {
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Warc-performSelector-leaks"
     
     if (self = [super init]) {
-        _engine = [[FlutterEngine alloc] initWithName:@"io.flutter" project:nil];
+        if(!engine){
+            _engine = [[FlutterEngine alloc] initWithName:@"io.flutter" project:nil];
+        }else{
+            _engine = engine;
+        }
         if(platform &&
            [platform respondsToSelector: @selector(entryForDart)] &&
            platform.entryForDart){
@@ -69,7 +73,7 @@
 
 - (instancetype)init
 {
-    return [self initWithPlatform:nil];
+    return [self initWithPlatform:nil engine:nil];
 }
 
 - (void)pause