IGListKit框架解析（一）

Instagram在2016年年底发布了基于数据驱动的UICollectionView框架IGListKit。使用数据驱动去创造更为快速灵活的列表控件。以下是该框架的特点：

• 数据驱动（数据改变 -> Diff算法 -> update界面）
• 可重复单元和组件的更好体系结构
• 解耦的差异算法
• 可以为数据模型自定义差异算法
• 可扩展的API

IGListKit架构图

3637572-bd1b004bc054287f.png

IGListKit的管理者--IGListAdapter

如上图所示，该框架直接通过IGListAdapter对象去管理UICollectionView并封装了UICollectionViewDelegate、UICollectionViewDataSource的相关实现，同时Adapter和Controller互相持有，用户只需要实现IGListAdapterDataSource就可以实现一个灵活的列表。

IGListAdapter的三个参数：

• IGListAdapterUpdater : 是一个实现了IGListUpdatingDelegate协议的对象，负责处理row和section的刷新。
• Controller : Adapter和Controller互相持有的同时，也负责管理着SectionController。
• WorkingRange : 可以为不可见范围的section准备内容。

IGListAdapter是如何管理UICollectionView的呢？

我们在和IGListAdapter中寻找和UICollectionViewDelegate、UICollectionViewDataSource相关的方法，发现在IGListAdapter+UICollectionView中有相关实现，以下是代码：

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
    return self.sectionMap.objects.count;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    IGListSectionController * sectionController = [self sectionControllerForSection:section];
    IGAssert(sectionController != nil, @"Nil section controller for section %zi for item %@. Check your -diffIdentifier and -isEqual: implementations.",
            section, [self.sectionMap objectForSection:section]);
    const NSInteger numberOfItems = [sectionController numberOfItems];
    IGAssert(numberOfItems >= 0, @"Cannot return negative number of items %zi for section controller %@.", numberOfItems, sectionController);
    return numberOfItems;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    IGListSectionController *sectionController = [self sectionControllerForSection:indexPath.section];

    // flag that a cell is being dequeued in case it tries to access a cell in the process
    _isDequeuingCell = YES;
    UICollectionViewCell *cell = [sectionController cellForItemAtIndex:indexPath.item];
    _isDequeuingCell = NO;

    IGAssert(cell != nil, @"Returned a nil cell at indexPath <%@> from section controller: <%@>", indexPath, sectionController);

    // associate the section controller with the cell so that we know which section controller is using it
    [self mapView:cell toSectionController:sectionController];

    return cell;
}

- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
    IGListSectionController *sectionController = [self sectionControllerForSection:indexPath.section];
    id <IGListSupplementaryViewSource> supplementarySource = [sectionController supplementaryViewSource];
    UICollectionReusableView *view = [supplementarySource viewForSupplementaryElementOfKind:kind atIndex:indexPath.item];
    IGAssert(view != nil, @"Returned a nil supplementary view at indexPath <%@> from section controller: <%@>, supplementary source: <%@>", indexPath, sectionController, supplementarySource);

    // associate the section controller with the cell so that we know which section controller is using it
    [self mapView:view toSectionController:sectionController];

    return view;
}

通过这段代码，我们可以清晰地看到IGListAdapter通过管理着IGListSectionController去实现UICollectionViewDelegate和UICollectionViewDataSource的相关实现，现在，我们去看看几个关键性的方法的实现去探索IGListAdapter的结构。

- (nullable IGListSectionController *)sectionControllerForSection:(NSInteger)section {
    IGAssertMainThread();
    
    return [self.sectionMap sectionControllerForSection:section];
}

- (NSInteger)sectionForSectionController:(IGListSectionController *)sectionController {
    IGAssertMainThread();
    IGParameterAssert(sectionController != nil);

    return [self.sectionMap sectionForSectionController:sectionController];
}

- (IGListSectionController *)sectionControllerForObject:(id)object {
    IGAssertMainThread();
    IGParameterAssert(object != nil);

    return [self.sectionMap sectionControllerForObject:object];
}

- (id)objectForSectionController:(IGListSectionController *)sectionController {
    IGAssertMainThread();
    IGParameterAssert(sectionController != nil);

    const NSInteger section = [self.sectionMap sectionForSectionController:sectionController];
    return [self.sectionMap objectForSection:section];
}

上述几个方法，都是IGListAdapter+UICollectionView中实现UICollectionViewDelegate、UICollectionViewDataSource时经常使用的方法，可以清晰地看到sectionMap是这些方法共同使用的属性，sectionMap是IGListKit中自定义的Map类型（IGListSectionMap），通过IGListSectionMap的代码，来一窥IGListAdapter实现管理者的方式。

@interface IGListSectionMap : NSObject <NSCopying>

- (instancetype)initWithMapTable:(NSMapTable *)mapTable NS_DESIGNATED_INITIALIZER;

/**
 The objects stored in the map.
 */
@property (nonatomic, strong, readonly) NSArray *objects;

/**
 Update the map with objects and the section controller counterparts.

 @param objects The objects in the collection.
 @param sectionControllers The section controllers that map to each object.
 */
- (void)updateWithObjects:(NSArray <id <NSObject>> *)objects sectionControllers:(NSArray <IGListSectionController *> *)sectionControllers;

/**
 Fetch a section controller given a section.

 @param section The section index of the section controller.

 @return A section controller.
 */
- (nullable IGListSectionController *)sectionControllerForSection:(NSInteger)section;

/**
 Fetch the object for a section

 @param section The section index of the object.

 @return The object corresponding to the section.
 */
- (nullable id)objectForSection:(NSInteger)section;

/**
 Fetch a section controller given an object. Can return nil.

 @param object The object that maps to a section controller.

 @return A section controller.
 */
- (nullable id)sectionControllerForObject:(id)object;

/**
 Look up the section index for a section controller.

 @param sectionController The list to look up.

 @return The section index of the given section controller if it exists, NSNotFound otherwise.
 */
- (NSInteger)sectionForSectionController:(id)sectionController;

/**
 Look up the section index for an object.

 @param object The object to look up.

 @return The section index of the given object if it exists, NSNotFound otherwise.
 */
- (NSInteger)sectionForObject:(id)object;

这是IGListSectionMap中主要的几个实现，也是经常用到的几个，我们再看看IGListSectionMap中的变量，很简单。

@interface IGListSectionMap ()

// both of these maps allow fast lookups of objects, list objects, and indexes
@property (nonatomic, strong, readonly, nonnull) NSMapTable<id, IGListSectionController *> *objectToSectionControllerMap;
@property (nonatomic, strong, readonly, nonnull) NSMapTable<IGListSectionController *, NSNumber *> *sectionControllerToSectionMap;

@property (nonatomic, strong, nonnull) NSMutableArray *mObjects;

@end

通过这两个变量的名称以及注释，也可以判断出该结构支持“双向查找”。再看看更新方法，确认一下是不是这么回事。

- (void)updateWithObjects:(NSArray *)objects sectionControllers:(NSArray *)sectionControllers {
    IGParameterAssert(objects.count == sectionControllers.count);

    [self reset];

    self.mObjects = [objects mutableCopy];

    id firstObject = objects.firstObject;
    id lastObject = objects.lastObject;

    [objects enumerateObjectsUsingBlock:^(id object, NSUInteger idx, BOOL *stop) {
        IGListSectionController *sectionController = sectionControllers[idx];

        // set the index of the list for easy reverse lookup
        [self.sectionControllerToSectionMap setObject:@(idx) forKey:sectionController];
        [self.objectToSectionControllerMap setObject:sectionController forKey:object];

        sectionController.isFirstSection = (object == firstObject);
        sectionController.isLastSection = (object == lastObject);
        sectionController.section = (NSInteger)idx;
    }];
}

由此可见，该结构是很灵活的，在IGListSectionMap结构的辅助下，IGListAdapter可以控制UICollectionView下层的IGListSectionController，通过IGListSectionController也通过IGListBindingSectionControllerDataSource去管理着UICollectionViewCell、ViewModel以及Cell的展示。

下一期，具体讲一下IGlistKit中运用的技巧、IGListBindingSectionController以及IGListKit的“灵魂”IGListDiff算法。