iOS设计模式之访问者模式

作者: 点滴86 | 来源:发表于2024-07-01 23:34 被阅读0次

访问者模式

电脑上有很多资源文件，它们的格式有三种：PDF、PPT、Word。现在要开发一个工具来处理这批资源文件。这个工具的其中一个功能是，把这些资源文件中的文本内容抽取出来放到txt文件中。

@protocol DMResourceFileDelegate <NSObject>

- (void)extractTxt;

@end

@interface DMPDFFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMPDFFile

- (void)extractTxt
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

@end

@interface DMPPTFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMPPTFile

- (void)extractTxt
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

@end

@interface DMWordFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMWordFile

- (void)extractTxt
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

@end

@interface DMToolApplication : NSObject

@end

@implementation DMToolApplication

- (void)test
{
    NSMutableArray<DMResourceFileDelegate> *tmpArray = [[NSMutableArray<DMResourceFileDelegate> alloc] init];
    DMPDFFile *pdfFile = [[DMPDFFile alloc] init];
    [tmpArray addObject:pdfFile];
    
    DMPPTFile *pptFile = [[DMPPTFile alloc] init];
    [tmpArray addObject:pptFile];
    
    DMWordFile *wordFile = [[DMWordFile alloc] init];
    [tmpArray addObject:wordFile];
    
    for (id<DMResourceFileDelegate> resourceFile in tmpArray) {
        [resourceFile extractTxt];
    }
}

@end

如果工具的功能不停地扩展，不仅要能抽取文本内容，还要支持压缩、提取元文件信息（文件名、大小、更新时间等等）构建索引等一系列的功能，继续按照上面代码的实现思路，就会存在几个问题：

违背开闭原则，添加一个新的功能，所有类的代码都要修改
虽然功能增多，每个类的代码都不断膨胀，可读性和可维护性都变差了
把所有比较上层的业务逻辑都耦合到DMPDFFile、DMPPTFile、DMWordFile类中，导致这些类的职责不够单一，变成了大杂烩

针对上面的问题，常用的解决方法就是拆分解耦，把业务操作跟具体的数据结构解耦，设计成独立的类。重构之后的代码如下：

@interface DMPDFFile : NSObject

@end

@implementation DMPDFFile

@end

@interface DMPPTFile : NSObject

@end

@implementation DMPPTFile

@end

@interface DMWordFile : NSObject

@end

@implementation DMWordFile

@end

@interface DMExtractor : NSObject

- (void)extractPDF:(DMPDFFile *)pdfFile;

- (void)extractPPT:(DMPPTFile *)pptFile;

- (void)extractWord:(DMWordFile *)wordFile;

@end

@implementation DMExtractor

- (void)extractPDF:(DMPDFFile *)pdfFile
{
    
}

- (void)extractPPT:(DMPPTFile *)pptFile
{
    
}

- (void)extractWord:(DMWordFile *)wordFile
{
    
}

@end

@interface DMToolApplication : NSObject

@end

@implementation DMToolApplication

- (void)test
{
    DMExtractor *extractor = [[DMExtractor alloc] init];
    
    NSMutableArray *tmpArray = [[NSMutableArray alloc] init];
    DMPDFFile *pdfFile = [[DMPDFFile alloc] init];
    [tmpArray addObject:pdfFile];
    
    DMPPTFile *pptFile = [[DMPPTFile alloc] init];
    [tmpArray addObject:pptFile];
    
    DMWordFile *wordFile = [[DMWordFile alloc] init];
    [tmpArray addObject:wordFile];
    
    for (id resourceFile in tmpArray) {
        if ([resourceFile isKindOfClass:[DMPDFFile class]]) {
            DMPDFFile *pdfFile = (DMPDFFile *)resourceFile;
            [extractor extractPDF:pdfFile];
        } else if ([resourceFile isKindOfClass:[DMPPTFile class]]) {
            DMPPTFile *pptFile = (DMPPTFile *)resourceFile;
            [extractor extractPPT:pptFile];
        } else if ([resourceFile isKindOfClass:[DMWordFile class]]) {
            DMWordFile *wordFile = (DMWordFile *)resourceFile;
            [extractor extractWord:wordFile];
        }
    }
}

@end

遍历tmpArray获取resourceFile，需要手动转换类型，调用对应的方法，继续优化这部分

@class DMExtractor;

@protocol DMResourceFileDelegate <NSObject>

- (void)accept:(DMExtractor *)extractor;

@end

@interface DMPDFFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMPDFFile

- (void)accept:(DMExtractor *)extractor
{
    [extractor extractPDF:self];
}

@end

@interface DMPPTFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMPPTFile

- (void)accept:(DMExtractor *)extractor
{
    [extractor extractPPT:self];
}

@end

@interface DMWordFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMWordFile

- (void)accept:(DMExtractor *)extractor
{
    [extractor extractWord:self];
}

@end

@interface DMExtractor : NSObject

- (void)extractPDF:(DMPDFFile *)pdfFile;

- (void)extractPPT:(DMPPTFile *)pptFile;

- (void)extractWord:(DMWordFile *)wordFile;

@end

@implementation DMExtractor

- (void)extractPDF:(DMPDFFile *)pdfFile
{
    
}

- (void)extractPPT:(DMPPTFile *)pptFile
{
    
}

- (void)extractWord:(DMWordFile *)wordFile
{
    
}

@end

@interface DMToolApplication : NSObject

@end

@implementation DMToolApplication

- (void)test
{
    DMExtractor *extractor = [[DMExtractor alloc] init];
    
    NSMutableArray<DMResourceFileDelegate> *tmpArray = [[NSMutableArray<DMResourceFileDelegate> alloc] init];
    DMPDFFile *pdfFile = [[DMPDFFile alloc] init];
    [tmpArray addObject:pdfFile];
    
    DMPPTFile *pptFile = [[DMPPTFile alloc] init];
    [tmpArray addObject:pptFile];
    
    DMWordFile *wordFile = [[DMWordFile alloc] init];
    [tmpArray addObject:wordFile];
    
    for (id<DMResourceFileDelegate> resourceFile in tmpArray) {
        [resourceFile accept: extractor];
    }
}

@end

现在，如果要继续添加新的功能，比如前面提到的压缩功能，根据不同的文件类型，使用不同的压缩算法来压缩资源文件。代码实现如下：

@class DMExtractor;
@class DMCompressor;

@protocol DMResourceFileDelegate <NSObject>

- (void)accept:(DMExtractor *)extractor;

- (void)acceptCompressor:(DMCompressor *)compressor;

@end

@interface DMPDFFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMPDFFile

- (void)accept:(DMExtractor *)extractor
{
    [extractor extractPDF:self];
}

- (void)acceptCompressor:(DMCompressor *)compressor
{
    [compressor compressPDF:self];
}

@end

@interface DMPPTFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMPPTFile

- (void)accept:(DMExtractor *)extractor
{
    [extractor extractPPT:self];
}

- (void)acceptCompressor:(DMCompressor *)compressor
{
    [compressor compressPPT:self];
}

@end

@interface DMWordFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMWordFile

- (void)accept:(DMExtractor *)extractor
{
    [extractor extractWord:self];
}

- (void)acceptCompressor:(DMCompressor *)compressor
{
    [compressor compressWord:self];
}

@end

@interface DMExtractor : NSObject

- (void)extractPDF:(DMPDFFile *)pdfFile;

- (void)extractPPT:(DMPPTFile *)pptFile;

- (void)extractWord:(DMWordFile *)wordFile;

@end

@implementation DMExtractor

- (void)extractPDF:(DMPDFFile *)pdfFile
{
    
}

- (void)extractPPT:(DMPPTFile *)pptFile
{
    
}

- (void)extractWord:(DMWordFile *)wordFile
{
    
}

@end

@interface DMCompressor : NSObject

- (void)compressPDF:(DMPDFFile *)pdfFile;

- (void)compressPPT:(DMPPTFile *)pptFile;

- (void)compressWord:(DMWordFile *)wordFile;

@end

@implementation DMCompressor

- (void)compressPDF:(DMPDFFile *)pdfFile
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

- (void)compressPPT:(DMPPTFile *)pptFile
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

- (void)compressWord:(DMWordFile *)wordFile
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

@end

@interface DMToolApplication : NSObject

@end

@implementation DMToolApplication

- (void)test
{
    NSLog(@"重构三");
    DMExtractor *extractor = [[DMExtractor alloc] init];
    DMCompressor *compressor = [[DMCompressor alloc] init];
    
    NSMutableArray<DMResourceFileDelegate> *tmpArray = [[NSMutableArray<DMResourceFileDelegate> alloc] init];
    DMPDFFile *pdfFile = [[DMPDFFile alloc] init];
    [tmpArray addObject:pdfFile];
    
    DMPPTFile *pptFile = [[DMPPTFile alloc] init];
    [tmpArray addObject:pptFile];
    
    DMWordFile *wordFile = [[DMWordFile alloc] init];
    [tmpArray addObject:wordFile];
    
    for (id<DMResourceFileDelegate> resourceFile in tmpArray) {
        [resourceFile accept:extractor];
    }
    
    for (id<DMResourceFileDelegate> resourceFile in tmpArray) {
        [resourceFile acceptCompressor:compressor];
    }
}
@end

上面的代码还存在一些问题，新添加一个新的业务，还是需要修改每个资源文件，违反了开闭原则。针对这个问题，做如下修改，
协议DMVisitor，三个方法- (void)visitPDF:(DMPDFFile *)pdfFile、- (void)visitPPT:(DMPPTFile *)pptFile、- (void)visitWord:(DMWordFile *)wordFile 分别处理三种不同类型的资源文件。具体做什么业务，由遵守这个协议的具体类来决定，比如DMExtractor负责抽取文本内容、DMCompressor负责压缩。当添加一个新的业务功能的时候，资源文件类不需要做任务修改，只需要新的业务功能遵守DMVisitor协议，实现对应的功能，就可以了。然后再修改下DMToolApplication的使用代码，如下：


@class DMPDFFile;
@class DMPPTFile;
@class DMWordFile;

@protocol DMVisitor <NSObject>

- (void)visitPDF:(DMPDFFile *)pdfFile;

- (void)visitPPT:(DMPPTFile *)pptFile;

- (void)visitWord:(DMWordFile *)wordFile;

@end

@interface DMExtractor : NSObject <DMVisitor>

@end

@implementation DMExtractor

- (void)visitPDF:(DMPDFFile *)pdfFile
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

- (void)visitPPT:(DMPPTFile *)pptFile
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

- (void)visitWord:(DMWordFile *)wordFile
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

@end

@interface DMCompressor : NSObject <DMVisitor>

@end

@implementation DMCompressor

- (void)visitPDF:(DMPDFFile *)pdfFile
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

- (void)visitPPT:(DMPPTFile *)pptFile
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

- (void)visitWord:(DMWordFile *)wordFile
{
    NSLog(@"%@:%@", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

@end

@protocol DMResourceFileDelegate <NSObject>

- (void)acceptVisitor:(id<DMVisitor>)visitor;

@end

@interface DMPDFFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMPDFFile

- (void)acceptVisitor:(id<DMVisitor>)visitor
{
    [visitor visitPDF:self];
}

@end

@interface DMPPTFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMPPTFile

- (void)acceptVisitor:(id<DMVisitor>)visitor
{
    [visitor visitPPT:self];
}

@end

@interface DMWordFile : NSObject <DMResourceFileDelegate>

@end

@implementation DMWordFile

- (void)acceptVisitor:(id<DMVisitor>)visitor
{
    [visitor visitWord:self];
}

@end

@interface DMToolApplication : NSObject

@end

@implementation DMToolApplication

- (void)test
{
    DMExtractor *extractor = [[DMExtractor alloc] init];
    DMCompressor *compressor = [[DMCompressor alloc] init];
    
    NSMutableArray<DMResourceFileDelegate> *tmpArray = [[NSMutableArray<DMResourceFileDelegate> alloc] init];
    DMPDFFile *pdfFile = [[DMPDFFile alloc] init];
    [tmpArray addObject:pdfFile];
    
    DMPPTFile *pptFile = [[DMPPTFile alloc] init];
    [tmpArray addObject:pptFile];
    
    DMWordFile *wordFile = [[DMWordFile alloc] init];
    [tmpArray addObject:wordFile];
    
    for (id<DMResourceFileDelegate> resourceFile in tmpArray) {
        [resourceFile acceptVisitor:extractor];
    }
    
    for (id<DMResourceFileDelegate> resourceFile in tmpArray) {
        [resourceFile acceptVisitor:compressor];
    }
}

@end

iOS设计模式之访问者模式

访问者模式

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