摘自：優化UITableViewCell高度計算Swift版、優化UITableViewCell高度計算的那些事

本文帶大家詳細探索那篇文章所對應的庫(1.2版)，這個庫就是利用緩存tableviewcell的高度提高滑動的流暢性。

主要是利用Runloop在空閑狀態時，後臺計算tableviewcell的高度並緩存起來。然後在使用的時候就直接從緩存中去，這裏都放在一個數組裏存在內存。

對Runloop以及幾個mode不懂的可以看sunnyxx blog中的視頻 視頻可戳 ， 文章的話可以看看 深入理解RunLoop、 【iOS程序啟動與運轉】- RunLoop個人小結。

其實就是在kCFRunLoopDefaultMode模式下BeforWaitting狀態去執行計算的。

下面來探究源碼。首先在UITableView+FDTemplateLayoutCell 下載源碼，下載1.2版本。

然後你得到的庫就只有兩個文件：

.m文件大概只有500行代碼。

下面看下作者的視線思路：

1. 創建了一個_FDTemplateLayoutCellHeightCache類，就是管理Cache的一個類，裏面有兩個屬性四個方法。

屬性：

• sections 這個變量就是用來存儲緩存的height的一個二維數組。（因為tableview有section和row組成所以必須二維）

• _FDTemplateLayoutCellHeightCacheAbsentValue 這個是一個靜態常量，就是用來標記沒有緩存高度的row 。

方法：

• buildHeightCachesAtIndexPathsIfNeeded:indexPaths
  這個方法傳入indexPaths數組來給sections中還沒有初始化的元素進行初始化
• hasCachedHeightAtIndexPath:indexPath 根據下標索引判斷是否有緩存（其實就是判斷是否等於上面那個靜態常量）
• cacheHeight:height:byIndexPath 根據indexPath給sections賦值。
• cachedHeightAtIndexPath:indexPath 根據indexPath取值

這個類主要是操作和存儲緩存的。這個類的代碼如下：

@interface _FDTemplateLayoutCellHeightCache : NSObject
@property (nonatomic, strong) NSMutableArray *sections;　　
@end

static CGFloat const _FDTemplateLayoutCellHeightCacheAbsentValue = -1;

@implementation _FDTemplateLayoutCellHeightCache

- (void)buildHeightCachesAtIndexPathsIfNeeded:(NSArray *)indexPaths {
    if (indexPaths.count == 0) {
        return;
    }
    
    if (!self.sections) {
        self.sections = @[].mutableCopy;
    }
    
    // Build every section array or row array which is smaller than given index path.
    [indexPaths enumerateObjectsUsingBlock:^(NSIndexPath *indexPath, NSUInteger idx, BOOL *stop) {
        for (NSInteger section = 0; section <= indexPath.section; ++section) {
            if (section >= self.sections.count) {
                self.sections[section] = @[].mutableCopy;
            }
        }
        NSMutableArray *rows = self.sections[indexPath.section];
        for (NSInteger row = 0; row <= indexPath.row; ++row) {
            if (row >= rows.count) {
                rows[row] = @(_FDTemplateLayoutCellHeightCacheAbsentValue);
            }
        }
    }];
}

- (BOOL)hasCachedHeightAtIndexPath:(NSIndexPath *)indexPath
{
    [self buildHeightCachesAtIndexPathsIfNeeded:@[indexPath]];
    NSNumber *cachedNumber = self.sections[indexPath.section][indexPath.row];
    return ![cachedNumber isEqualToNumber:@(_FDTemplateLayoutCellHeightCacheAbsentValue)];
}

- (void)cacheHeight:(CGFloat)height byIndexPath:(NSIndexPath *)indexPath
{
    [self buildHeightCachesAtIndexPathsIfNeeded:@[indexPath]];
    self.sections[indexPath.section][indexPath.row] = @(height);
}

- (CGFloat)cachedHeightAtIndexPath:(NSIndexPath *)indexPath
{
    [self buildHeightCachesAtIndexPathsIfNeeded:@[indexPath]];
#if CGFLOAT_IS_DOUBLE
    return [self.sections[indexPath.section][indexPath.row] doubleValue];
#else
    return [self.sections[indexPath.section][indexPath.row] floatValue];
#endif
}

@end

2. 接下來是UITableView的一個擴展UITableView + FDTemplateLayoutCellPrivate

• 第一個方法fd_templateCellForReuseIdentifier:identifier，這個方法主要是通過你傳入的一個identifier(就是復用的id)獲取cell。

　　　　第一句是這樣的 NSMutableDictionary *templateCellsByIdentifiers = objc_getAssociatedObject(self, _cmd);

　　　　OC中的 _cmd 代表的就是本方法，objc_getAssociatedObject 獲取一個關聯對象的屬性。

• 接下來提供了一個方法來獲取管理Cache的_FDTemplateLayoutCellHeightCache的對象fd_cellHeightCache。

• 屬性：fd_autoCacheInvalidationEnabled 記錄是否自動緩存高度

• 屬性：fd_precacheEnabled

這是一個私有類，下面給出這個類的完整代碼：

@interface UITableView (FDTemplateLayoutCellPrivate)

/// Returns a template cell created by reuse identifier, it has to be registered to table view.
/// Lazy getter, and associated to table view.
- (id)fd_templateCellForReuseIdentifier:(NSString *)identifier;

/// A private height cache data structure.
@property (nonatomic, strong, readonly) _FDTemplateLayoutCellHeightCache *fd_cellHeightCache;

/// This is a private switch that I don‘t think caller should concern.
/// Auto turn on when you use "-fd_heightForCellWithIdentifier:cacheByIndexPath:configuration".
@property (nonatomic, assign) BOOL fd_autoCacheInvalidationEnabled;

/// It helps to improve scroll performance by "pre-cache" height of cells that have not
/// been displayed on screen. These calculation tasks are collected and performed only
/// when "RunLoop" is in "idle" time.
///
/// Auto turn on when you use "-fd_heightForCellWithIdentifier:cacheByIndexPath:configuration".
@property (nonatomic, assign) BOOL fd_precacheEnabled;

/// Debug log controlled by "fd_debugLogEnabled".
- (void)fd_debugLog:(NSString *)message;

@end

@implementation UITableView (FDTemplateLayoutCellPrivate)

- (id)fd_templateCellForReuseIdentifier:(NSString *)identifier
{
    NSAssert(identifier.length > 0, @"Expects a valid identifier - %@", identifier);
    
    NSMutableDictionary *templateCellsByIdentifiers = objc_getAssociatedObject(self, _cmd);
    if (!templateCellsByIdentifiers) {
        templateCellsByIdentifiers = @{}.mutableCopy;
        objc_setAssociatedObject(self, _cmd, templateCellsByIdentifiers, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    
    UITableViewCell *templateCell = templateCellsByIdentifiers[identifier];
    
    if (!templateCell) {
        templateCell = [self dequeueReusableCellWithIdentifier:identifier];
        NSAssert(templateCell != nil, @"Cell must be registered to table view for identifier - %@", identifier);
        templateCell.fd_isTemplateLayoutCell = YES;
        templateCellsByIdentifiers[identifier] = templateCell;
        [self fd_debugLog:[NSString stringWithFormat:@"layout cell created - %@", identifier]];
    }
    
    return templateCell;
}

- (_FDTemplateLayoutCellHeightCache *)fd_cellHeightCache {
    _FDTemplateLayoutCellHeightCache *cache = objc_getAssociatedObject(self, _cmd);
    if (!cache) {
        cache = [_FDTemplateLayoutCellHeightCache new];
        objc_setAssociatedObject(self, _cmd, cache, OBJC_ASSOCIATION_RETAIN);
    }
    return cache;
}

- (BOOL)fd_autoCacheInvalidationEnabled
{
    return [objc_getAssociatedObject(self, _cmd) boolValue];
}

- (void)setFd_autoCacheInvalidationEnabled:(BOOL)enabled
{
    objc_setAssociatedObject(self, @selector(fd_autoCacheInvalidationEnabled), @(enabled), OBJC_ASSOCIATION_RETAIN);
}

- (BOOL)fd_precacheEnabled
{
    return [objc_getAssociatedObject(self, _cmd) boolValue];
}

- (void)setFd_precacheEnabled:(BOOL)precacheEnabled
{
    objc_setAssociatedObject(self, @selector(fd_precacheEnabled), @(precacheEnabled), OBJC_ASSOCIATION_RETAIN);
}

- (void)fd_debugLog:(NSString *)message
{
    if (!self.fd_debugLogEnabled) {
        return;
    }
    NSLog(@"** FDTemplateLayoutCell ** %@", message);
}

@end

3. 下面又是一個分類，（這個是重點計算高度，調用緩存管理方法的分類）UITableView + FDTemplateLayoutCellPrecache

這個裏面的方法在他blog中也有提到就是在NSDefaultRunLoopMode下當狀態將要進入休眠的時候把計算方法分解成多個RunLoop Source任務（source0）

- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(id)arg waitUntilDone:(BOOL)wait modes:(NSArray *)array;

這個方法將創建一個 Source 0 任務，分發到指定線程的 RunLoop 中，在給定的 Mode 下執行，若指定的 RunLoop 處於休眠狀態，則喚醒它處理事件.

主要邏輯就是先通過遍歷所有section和row找到還沒有緩存的row，然後加入到待緩存數組 ，創建一個observer去監聽Runloop的狀態 ，如果空閑了去創建source0任務，執行計算方法並緩存起來。如果預緩存任務完成了就把監聽的Observer移除了。

下面給出這個類的代碼：

@implementation UITableView (FDTemplateLayoutCellPrecache)

- (void)fd_precacheIfNeeded
{
    if (!self.fd_precacheEnabled) {
        return;
    }
    
    // Delegate could use "rowHeight" rather than implements this method.
    if (![self.delegate respondsToSelector:@selector(tableView:heightForRowAtIndexPath:)]) {
        return;
    }
    
    CFRunLoopRef runLoop = CFRunLoopGetCurrent();
    
    // This is a idle mode of RunLoop, when UIScrollView scrolls, it jumps into "UITrackingRunLoopMode"
    // and won‘t perform any cache task to keep a smooth scroll.
    CFStringRef runLoopMode = kCFRunLoopDefaultMode;
    
    // Collect all index paths to be precached.
    NSMutableArray *mutableIndexPathsToBePrecached = self.fd_allIndexPathsToBePrecached.mutableCopy;
    
    // Setup a observer to get a perfect moment for precaching tasks.
    // We use a "kCFRunLoopBeforeWaiting" state to keep RunLoop has done everything and about to sleep
    // (mach_msg_trap), when all tasks finish, it will remove itself.
    CFRunLoopObserverRef observer = CFRunLoopObserverCreateWithHandler
    (kCFAllocatorDefault, kCFRunLoopBeforeWaiting, true, 0, ^(CFRunLoopObserverRef observer, CFRunLoopActivity _) {
        // Remove observer when all precache tasks are done.
        if (mutableIndexPathsToBePrecached.count == 0) {
            CFRunLoopRemoveObserver(runLoop, observer, runLoopMode);
            return;
        }
        // Pop first index path record as this RunLoop iteration‘s task.
        NSIndexPath *indexPath = mutableIndexPathsToBePrecached.firstObject;
        [mutableIndexPathsToBePrecached removeObject:indexPath];
        
        // This method creates a "source 0" task in "idle" mode of RunLoop, and will be
        // performed in a future RunLoop iteration only when user is not scrolling.
        [self performSelector:@selector(fd_precacheIndexPathIfNeeded:)
                     onThread:[NSThread mainThread]
                   withObject:indexPath
                waitUntilDone:NO
                        modes:@[NSDefaultRunLoopMode]];
    });
    
    CFRunLoopAddObserver(runLoop, observer, runLoopMode);
}

- (void)fd_precacheIndexPathIfNeeded:(NSIndexPath *)indexPath
{
    if (![self.fd_cellHeightCache hasCachedHeightAtIndexPath:indexPath]) {
        CGFloat height = [self.delegate tableView:self heightForRowAtIndexPath:indexPath];
        [self.fd_cellHeightCache cacheHeight:height byIndexPath:indexPath];
        [self fd_debugLog:[NSString stringWithFormat:
                           @"precached - [%@:%@] %@",
                           @(indexPath.section),
                           @(indexPath.row),
                           @(height)]];
    }
}

- (NSArray *)fd_allIndexPathsToBePrecached
{
    NSMutableArray *allIndexPaths = @[].mutableCopy;
    for (NSInteger section = 0; section < [self numberOfSections]; ++section) {
        for (NSInteger row = 0; row < [self numberOfRowsInSection:section]; ++row) {
            NSIndexPath *indexPath = [NSIndexPath indexPathForRow:row inSection:section];
            if (![self.fd_cellHeightCache hasCachedHeightAtIndexPath:indexPath]) {
                [allIndexPaths addObject:indexPath];
            }
        }
    }
    return allIndexPaths.copy;
}

@end

4. 下面又是一個分類UITableView + FDTemplateLayoutCellAutomaticallyCacheInvalidation　

　　因為我們會有一些操作導致cell的改變，所以這裏作者要保證在每次cell改變的時候把sections數組改掉，然後如果新增或者修改了 需要重新計算高度。用到了methodSwizzle 黑魔法。這裏作者把swizzle放在了UITableView的load類方法中。需要使用methodSwizzle的方法有：

  SEL selectors[] = {
        @selector(reloadData),
        @selector(insertSections:withRowAnimation:),
        @selector(deleteSections:withRowAnimation:),
        @selector(reloadSections:withRowAnimation:),
        @selector(moveSection:toSection:),
        @selector(insertRowsAtIndexPaths:withRowAnimation:),
        @selector(deleteRowsAtIndexPaths:withRowAnimation:),
        @selector(reloadRowsAtIndexPaths:withRowAnimation:),
        @selector(moveRowAtIndexPath:toIndexPath:)
    };

這個類的代碼：

@implementation UITableView (FDTemplateLayoutCellAutomaticallyCacheInvalidation)

+ (void)load
{
    // All methods that trigger height cache‘s invalidation
    SEL selectors[] = {
        @selector(reloadData),
        @selector(insertSections:withRowAnimation:),
        @selector(deleteSections:withRowAnimation:),
        @selector(reloadSections:withRowAnimation:),
        @selector(moveSection:toSection:),
        @selector(insertRowsAtIndexPaths:withRowAnimation:),
        @selector(deleteRowsAtIndexPaths:withRowAnimation:),
        @selector(reloadRowsAtIndexPaths:withRowAnimation:),
        @selector(moveRowAtIndexPath:toIndexPath:)
    };
    
    for (NSUInteger index = 0; index < sizeof(selectors) / sizeof(SEL); ++index) {
        SEL originalSelector = selectors[index];
        SEL swizzledSelector = NSSelectorFromString([@"fd_" stringByAppendingString:NSStringFromSelector(originalSelector)]);
        
        Method originalMethod = class_getInstanceMethod(self, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(self, swizzledSelector);
        
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

- (void)fd_reloadData
{
    if (self.fd_autoCacheInvalidationEnabled) {
        [self.fd_cellHeightCache.sections removeAllObjects];
    }
    [self fd_reloadData]; // Primary call
    [self fd_precacheIfNeeded];
}

- (void)fd_insertSections:(NSIndexSet *)sections withRowAnimation:(UITableViewRowAnimation)animation
{
    if (self.fd_autoCacheInvalidationEnabled) {
        [sections enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {
            [self.fd_cellHeightCache.sections insertObject:@[].mutableCopy atIndex:idx];
        }];
    }
    [self fd_insertSections:sections withRowAnimation:animation]; // Primary call
    [self fd_precacheIfNeeded];
}

- (void)fd_deleteSections:(NSIndexSet *)sections withRowAnimation:(UITableViewRowAnimation)animation
{
    if (self.fd_autoCacheInvalidationEnabled) {
        [sections enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {
            [self.fd_cellHeightCache.sections removeObjectAtIndex:idx];
        }];
    }
    [self fd_deleteSections:sections withRowAnimation:animation]; // Primary call
}

- (void)fd_reloadSections:(NSIndexSet *)sections withRowAnimation:(UITableViewRowAnimation)animation
{
    if (self.fd_autoCacheInvalidationEnabled) {
        [sections enumerateIndexesUsingBlock: ^(NSUInteger idx, BOOL *stop) {
            NSMutableArray *rows = self.fd_cellHeightCache.sections[idx];
            for (NSInteger row = 0; row < rows.count; ++row) {
                rows[row] = @(_FDTemplateLayoutCellHeightCacheAbsentValue);
            }
        }];
    }
    [self fd_reloadSections:sections withRowAnimation:animation]; // Primary call
    [self fd_precacheIfNeeded];
}

- (void)fd_moveSection:(NSInteger)section toSection:(NSInteger)newSection
{
    if (self.fd_autoCacheInvalidationEnabled) {
        [self.fd_cellHeightCache.sections exchangeObjectAtIndex:section withObjectAtIndex:newSection];
    }
    [self fd_moveSection:section toSection:newSection]; // Primary call
}

- (void)fd_insertRowsAtIndexPaths:(NSArray *)indexPaths withRowAnimation:(UITableViewRowAnimation)animation
{
    if (self.fd_autoCacheInvalidationEnabled) {
        [indexPaths enumerateObjectsUsingBlock:^(NSIndexPath *indexPath, NSUInteger idx, BOOL *stop) {
            NSMutableArray *rows = self.fd_cellHeightCache.sections[indexPath.section];
            [rows insertObject:@(_FDTemplateLayoutCellHeightCacheAbsentValue) atIndex:indexPath.row];
        }];
    }
    [self fd_insertRowsAtIndexPaths:indexPaths withRowAnimation:animation]; // Primary call
    [self fd_precacheIfNeeded];
}

- (void)fd_deleteRowsAtIndexPaths:(NSArray *)indexPaths withRowAnimation:(UITableViewRowAnimation)animation
{
    if (self.fd_autoCacheInvalidationEnabled) {
        [indexPaths enumerateObjectsUsingBlock:^(NSIndexPath *indexPath, NSUInteger idx, BOOL *stop) {
            [self.fd_cellHeightCache.sections[indexPath.section] removeObjectAtIndex:indexPath.row];
        }];
    }
    [self fd_deleteRowsAtIndexPaths:indexPaths withRowAnimation:animation]; // Primary call
}

- (void)fd_reloadRowsAtIndexPaths:(NSArray *)indexPaths withRowAnimation:(UITableViewRowAnimation)animation
{
    if (self.fd_autoCacheInvalidationEnabled) {
        [indexPaths enumerateObjectsUsingBlock:^(NSIndexPath *indexPath, NSUInteger idx, BOOL *stop) {
            NSMutableArray *rows = self.fd_cellHeightCache.sections[indexPath.section];
            rows[indexPath.row] = @(_FDTemplateLayoutCellHeightCacheAbsentValue);
        }];
    }
    [self fd_reloadRowsAtIndexPaths:indexPaths withRowAnimation:animation]; // Primary call
    [self fd_precacheIfNeeded];
}

- (void)fd_moveRowAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath *)destinationIndexPath
{
    if (self.fd_autoCacheInvalidationEnabled) {
        NSMutableArray *sourceRows = self.fd_cellHeightCache.sections[sourceIndexPath.section];
        NSMutableArray *destinationRows = self.fd_cellHeightCache.sections[destinationIndexPath.section];
        
        NSNumber *sourceValue = sourceRows[sourceIndexPath.row];
        NSNumber *destinationValue = destinationRows[destinationIndexPath.row];
        
        sourceRows[sourceIndexPath.row] = destinationValue;
        destinationRows[destinationIndexPath.row] = sourceValue;
    }
    [self fd_moveRowAtIndexPath:sourceIndexPath toIndexPath:destinationIndexPath]; // Primary call
}

@end

5. 下面還有一個分類UITableView + FDTemplateLayoutCell，這個類提供外界獲取cell高度的方法

• fd_heightForCellWithIdentifier:configuration:configuration
• fd_heightForCellWithIdentifier:cacheByIndexPath:configuration:configuration

　　這個類的方法如下：

@implementation UITableView (FDTemplateLayoutCell)

- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier configuration:(void (^)(id))configuration
{
    if (!identifier) {
        return 0;
    }
    
    // Fetch a cached template cell for `identifier`.
    UITableViewCell *cell = [self fd_templateCellForReuseIdentifier:identifier];
    
    // Manually calls to ensure consistent behavior with actual cells (that are displayed on screen).
    [cell prepareForReuse];
    
    // Customize and provide content for our template cell.
    if (configuration) {
        configuration(cell);
    }
    
    // Add a hard width constraint to make dynamic content views (like labels) expand vertically instead
    // of growing horizontally, in a flow-layout manner.
    NSLayoutConstraint *tempWidthConstraint =
    [NSLayoutConstraint constraintWithItem:cell.contentView
                                 attribute:NSLayoutAttributeWidth
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:nil
                                 attribute:NSLayoutAttributeNotAnAttribute
                                multiplier:1.0
                                  constant:CGRectGetWidth(self.frame)];
    [cell.contentView addConstraint:tempWidthConstraint];
    
    // Auto layout engine does its math
    CGSize fittingSize = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
    
    [cell.contentView removeConstraint:tempWidthConstraint];
    
    // Add 1px extra space for separator line if needed, simulating default UITableViewCell.
    if (self.separatorStyle != UITableViewCellSeparatorStyleNone) {
        fittingSize.height += 1.0 / [UIScreen mainScreen].scale;
    }
    
    return fittingSize.height;
}

- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier cacheByIndexPath:(NSIndexPath *)indexPath configuration:(void (^)(id))configuration
{
    if (!identifier || !indexPath) {
        return 0;
    }
    
    // Enable auto cache invalidation if you use this "cacheByIndexPath" API.
    if (!self.fd_autoCacheInvalidationEnabled) {
        self.fd_autoCacheInvalidationEnabled = YES;
    }
    // Enable precache if you use this "cacheByIndexPath" API.
    if (!self.fd_precacheEnabled) {
        self.fd_precacheEnabled = YES;
        // Manually trigger precache only for the first time.
        [self fd_precacheIfNeeded];
    }
    
    // Hit the cache
    if ([self.fd_cellHeightCache hasCachedHeightAtIndexPath:indexPath]) {
        [self fd_debugLog:[NSString stringWithFormat:
                           @"hit cache - [%@:%@] %@",
                           @(indexPath.section),
                           @(indexPath.row),
                           @([self.fd_cellHeightCache cachedHeightAtIndexPath:indexPath])]];
        return [self.fd_cellHeightCache cachedHeightAtIndexPath:indexPath];
    }
    
    // Do calculations
    CGFloat height = [self fd_heightForCellWithIdentifier:identifier configuration:configuration];
    [self fd_debugLog:[NSString stringWithFormat:
                       @"calculate - [%@:%@] %@",
                       @(indexPath.section),
                       @(indexPath.row),
                       @(height)]];
    
    // Cache it
    [self.fd_cellHeightCache cacheHeight:height byIndexPath:indexPath];
    
    return height;
}

- (BOOL)fd_debugLogEnabled
{
    return [objc_getAssociatedObject(self, _cmd) boolValue];
}

- (void)setFd_debugLogEnabled:(BOOL)debugLogEnabled
{
    objc_setAssociatedObject(self, @selector(fd_debugLogEnabled), @(debugLogEnabled), OBJC_ASSOCIATION_RETAIN);
}

@end

開源庫UITableView+FDTemplateLayoutCell學習