[macOS] Make FlutterEngine support multiple views

shell/platform/darwin/macos/framework/Headers/FlutterEngine.h

@@ -76,10 +76,10 @@ FLUTTER_DARWIN_EXPORT
 - (BOOL)runWithEntrypoint:(nullable NSString*)entrypoint;
 
 /**
- * The default `FlutterViewController` associated with this engine, if any.
+ * The default `FlutterViewController` of this engine, if any.
  *
- * The default view always has ID kFlutterDefaultViewId, and is the view
- * operated by the APIs that do not have a view ID specified.
+ * The default view is the first view added to the engine, always has ID
+ * kFlutterDefaultViewId, and is used by legacy APIs that assume a single view.
  *
  * Setting this field from nil to a non-nil view controller also updates
  * the view controller's engine and ID.
@@ -92,6 +92,11 @@ FLUTTER_DARWIN_EXPORT
  */
 @property(nonatomic, nullable, weak) FlutterViewController* viewController;
 
+/**
+ * The `FlutterViewController` associated with the given view ID, if any.
+ */
+- (nullable FlutterViewController*)viewControllerForId:(uint64_t)viewId;
+
 /**
  * The `FlutterBinaryMessenger` for communicating with this engine.
  */

shell/platform/darwin/macos/framework/Headers/FlutterPluginRegistrarMacOS.h

@@ -36,12 +36,18 @@ FLUTTER_DARWIN_EXPORT
 @property(nonnull, readonly) id<FlutterTextureRegistry> textures;
 
 /**
- * The view displaying Flutter content. May return |nil|, for instance in a headless environment.
+ * The default view displaying Flutter content.
  *
- * WARNING: If/when multiple Flutter views within the same application are supported (#30701), this
- * API will change.
+ * This method may return |nil|, for instance in a headless environment.
+ *
+ * The default view is a special view operated by single-view APIs.
+ */
+- (nullable NSView*)view;
+
+/**
+ * The `NSView` associated with the given view ID, if any.
  */
-@property(nullable, readonly) NSView* view;
+- (nullable NSView*)viewForId:(uint64_t)viewId;
 
 /**
  * Registers |delegate| to receive handleMethodCall:result: callbacks for the given |channel|.

shell/platform/darwin/macos/framework/Headers/FlutterViewController.h

@@ -89,7 +89,9 @@ FLUTTER_DARWIN_EXPORT
     NS_DESIGNATED_INITIALIZER;
 - (nonnull instancetype)initWithCoder:(nonnull NSCoder*)nibNameOrNil NS_DESIGNATED_INITIALIZER;
 /**
- * Initializes this FlutterViewController with the specified `FlutterEngine`.
+ * Initializes this FlutterViewController with an existing `FlutterEngine`.
+ *
+ * The initialized view controller will add itself to the engine as part of this process.
  *
  * This initializer is suitable for both the first Flutter view controller and
  * the following ones of the app.

shell/platform/darwin/macos/framework/Source/FlutterCompositor.mm

@@ -19,7 +19,7 @@
   // TODO(dkwingsmt): This class only supports single-view for now. As more
   // classes are gradually converted to multi-view, it should get the view ID
   // from somewhere.
-  FlutterView* view = [view_provider_ getView:kFlutterDefaultViewId];
+  FlutterView* view = [view_provider_ viewForId:kFlutterDefaultViewId];
   if (!view) {
     return false;
   }
@@ -37,7 +37,7 @@
 bool FlutterCompositor::Present(uint64_t view_id,
                                 const FlutterLayer** layers,
                                 size_t layers_count) {
-  FlutterView* view = [view_provider_ getView:view_id];
+  FlutterView* view = [view_provider_ viewForId:view_id];
   if (!view) {
     return false;
   }

shell/platform/darwin/macos/framework/Source/FlutterCompositorTest.mm

@@ -11,6 +11,8 @@
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterViewProvider.h"
 #import "flutter/testing/testing.h"
 
+extern const uint64_t kFlutterDefaultViewId;
+
 @interface FlutterViewMockProvider : NSObject <FlutterViewProvider> {
   FlutterView* _defaultView;
 }
@@ -30,7 +32,7 @@ - (nonnull instancetype)initWithDefaultView:(nonnull FlutterView*)view {
   return self;
 }
 
-- (nullable FlutterView*)getView:(uint64_t)viewId {
+- (nullable FlutterView*)viewForId:(uint64_t)viewId {
   if (viewId == kFlutterDefaultViewId) {
     return _defaultView;
   }