第一种简便的方法

unsigned long long size = 0;
    
    NSString * cachePath = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES).lastObject;
    NSString * dirpath = [cachePath stringByAppendingPathComponent:@"default"];
    
    NSFileManager * mgr = [NSFileManager defaultManager];
    
    NSArray * subpaths = [mgr subpathsAtPath:dirpath];

//  NSDirectoryEnumerator * subpaths = [mgr enumeratorAtPath:dirpath];      遍历器 / 迭代器  

    for (NSString * subpath in subpaths) {
        NSString * fullsubpath = [dirpath stringByAppendingPathComponent:subpath];
         size += [mgr attributesOfItemAtPath:fullsubpath error:nil].fileSize;
    }
    NSLog(@"%llu",size);
    NSLog(@"%f",size*1.0/1000/1000);

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第二种 可以获取文件或者文件夹的大小

.h

#import <Foundation/Foundation.h>

@interface NSString (XMGExtension)
- (unsigned long long)fileSize;
@end

.m

#import "NSString+XMGExtension.h"

@implementation NSString (XMGExtension)

- (unsigned long long)fileSize
{
    // 总大小
    unsigned long long size = 0;
    
    // 文件管理者
    NSFileManager *mgr = [NSFileManager defaultManager];
    
    // 是否为文件夹
    BOOL isDirectory = NO;
    // 路径是否存在
    BOOL exists = [mgr fileExistsAtPath:self isDirectory:&isDirectory];
    if (!exists) return size;
    if (isDirectory) { // 文件夹
        // 获得文件夹的大小  == 获得文件夹中所有文件的总大小
        NSDirectoryEnumerator *enumerator = [mgr enumeratorAtPath:self];
        for (NSString *subpath in enumerator) {
            // 全路径
            NSString *fullSubpath = [self stringByAppendingPathComponent:subpath];
            // 累加文件大小
            size += [mgr attributesOfItemAtPath:fullSubpath error:nil].fileSize;
        }
    } else { // 文件
        size = [mgr attributesOfItemAtPath:self error:nil].fileSize;
    }
    return size;
}
@end