面试题：Objective-C的SceneKit中复杂3D场景优化与性能调优

渲染机制优化

减少渲染面：
- 思路：使用SCNGeometry的- (void)removeFacesWithIndices:(NSIndexSet *)faceIndices方法，剔除不可见的面。例如，对于一个复杂的3D模型，如果其背面始终不可见，可以剔除背面的多边形。
- 代码示例：

SCNGeometry *geometry = [SCNGeometry geometryWithSources:geometrySources elements:geometryElements];
NSMutableIndexSet *faceIndicesToRemove = [NSMutableIndexSet indexSet];
// 假设通过某种算法计算出要移除的面的索引
[faceIndicesToRemove addIndex:0];
[faceIndicesToRemove addIndex:1];
[geometry removeFacesWithIndices:faceIndicesToRemove];

层级细节（LOD）：
- 思路：根据相机与模型的距离切换不同精度的模型。可以创建多个不同精度的模型版本，当模型距离相机较远时，加载低精度模型；距离较近时，加载高精度模型。
- 代码示例：

SCNNode *lodNode = [SCNNode node];
SCNNode *lowDetailNode = [SCNNode nodeWithGeometry:lowDetailGeometry];
SCNNode *highDetailNode = [SCNNode nodeWithGeometry:highDetailGeometry];
[lodNode addChildNode:lowDetailNode];
[lodNode addChildNode:highDetailNode];
lowDetailNode.hidden = NO;
highDetailNode.hidden = YES;
// 在相机移动的代理方法中根据距离切换
- (void)renderer:(id<SCNSceneRenderer>)renderer updateAtTime:(NSTimeInterval)time {
    SCNVector3 cameraPosition = self.sceneView.pointOfView.position;
    SCNVector3 nodePosition = lodNode.position;
    float distance = sqrt(pow(cameraPosition.x - nodePosition.x, 2) + pow(cameraPosition.y - nodePosition.y, 2) + pow(cameraPosition.z - nodePosition.z, 2));
    if (distance < 10) {
        lowDetailNode.hidden = YES;
        highDetailNode.hidden = NO;
    } else {
        lowDetailNode.hidden = NO;
        highDetailNode.hidden = YES;
    }
}

批处理：
- 思路：将多个小的模型合并为一个大的模型进行渲染。可以使用SCNGeometryElement来构建合并后的几何体。
- 代码示例：

NSMutableArray<SCNGeometrySource *> *sources = [NSMutableArray array];
NSMutableArray<SCNGeometryElement *> *elements = [NSMutableArray array];
// 假设已有多个SCNGeometry对象
for (SCNGeometry *geometry in geometriesArray) {
    for (SCNGeometrySource *source in geometry.sources) {
        [sources addObject:source];
    }
    for (SCNGeometryElement *element in geometry.elements) {
        [elements addObject:element];
    }
}
SCNGeometry *combinedGeometry = [SCNGeometry geometryWithSources:sources elements:elements];
SCNNode *combinedNode = [SCNNode nodeWithGeometry:combinedGeometry];

内存管理优化

纹理压缩：
- 思路：使用SCNMaterialProperty的- (void)setContents:(id)contents fromData:(NSData *)data options:(NSDictionary<NSString *,id> *)options方法加载压缩后的纹理。例如，将PNG纹理转换为ETC2等压缩格式。
- 代码示例：

NSData *compressedTextureData = [NSData dataWithContentsOfFile:@"compressedTexture.etc2"];
SCNMaterial *material = [SCNMaterial material];
[material.diffuse setContents:compressedTextureData fromData:compressedTextureData options:@{SCNSceneSourceImportFlattenHierarchyKey: @YES}];

资源缓存：
- 思路：使用单例模式创建一个资源缓存类，缓存已经加载的模型和纹理。当需要再次使用相同资源时，直接从缓存中获取。
- 代码示例：

@interface ResourceCache : NSObject
@property (nonatomic, strong) NSMutableDictionary<NSString *, id> *cacheDictionary;
+ (instancetype)sharedCache;
- (id)cachedResourceForKey:(NSString *)key;
- (void)cacheResource:(id)resource forKey:(NSString *)key;
@end

@implementation ResourceCache
+ (instancetype)sharedCache {
    static ResourceCache *sharedCache = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedCache = [[ResourceCache alloc] init];
        sharedCache.cacheDictionary = [NSMutableDictionary dictionary];
    });
    return sharedCache;
}

- (id)cachedResourceForKey:(NSString *)key {
    return self.cacheDictionary[key];
}

- (void)cacheResource:(id)resource forKey:(NSString *)key {
    self.cacheDictionary[key] = resource;
}
@end

使用时：

ResourceCache *cache = [ResourceCache sharedCache];
SCNGeometry *geometry = [cache cachedResourceForKey:@"model1"];
if (!geometry) {
    geometry = [SCNGeometry geometryWithSource:source element:element];
    [cache cacheResource:geometry forKey:@"model1"];
}

资源加载优化

异步加载：
- 思路：使用dispatch_async在后台线程加载模型和纹理。
- 代码示例：

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    SCNScene *scene = [SCNScene sceneWithURL:[NSURL fileURLWithPath:@"model.scn"] options:nil error:nil];
    dispatch_async(dispatch_get_main_queue(), ^{
        self.sceneView.scene = scene;
    });
});

资源预加载：
- 思路：在应用启动或者空闲时间，预先加载可能用到的资源。例如，通过监听UIApplicationDidEnterBackground通知，在应用进入后台时进行预加载。
- 代码示例：

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(preloadResources) name:UIApplicationDidEnterBackgroundNotification object:nil];
- (void)preloadResources {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        SCNScene *scene1 = [SCNScene sceneWithURL:[NSURL fileURLWithPath:@"model1.scn"] options:nil error:nil];
        SCNScene *scene2 = [SCNScene sceneWithURL:[NSURL fileURLWithPath:@"model2.scn"] options:nil error:nil];
        // 缓存预加载的资源
        ResourceCache *cache = [ResourceCache sharedCache];
        [cache cacheResource:scene1 forKey:@"scene1"];
        [cache cacheResource:scene2 forKey:@"scene2"];
    });
}

星途面试题库

面试题：Objective-C的SceneKit中复杂3D场景优化与性能调优

知识考点

面试题答案

渲染机制优化

内存管理优化

资源加载优化