1.给NSString建一个分类
.h文件

/**
 判断该字符串是不是一个有效的URL
 
 @return YES：是一个有效的URL or NO
 */
- (BOOL)isValidUrl;

/** 根据图片名 判断是否是gif图 */
- (BOOL)isGifImage;

/** 根据图片data 判断是否是gif图 */
+ (BOOL)isGifWithImageData: (NSData *)data;

/**
 根据image的data 判断图片类型
 
 @param data 图片data
 @return 图片类型(png、jpg...)
 */
+ (NSString *)contentTypeWithImageData: (NSData *)data;

.m文件

- (BOOL)isValidUrl {
    NSString *regex =@"[a-zA-z]+://[^\\s]*";
    NSPredicate *urlTest = [NSPredicate predicateWithFormat:@"SELF MATCHES %@",regex];
    return [urlTest evaluateWithObject:self];
}

- (BOOL)isGifImage {
    
    NSString *ext = self.pathExtension.lowercaseString;
    
    if ([ext isEqualToString:@"gif"]) {
        return YES;
    }
    return NO;
}

+ (BOOL)isGifWithImageData: (NSData *)data {
    if ([[self contentTypeWithImageData:data] isEqualToString:@"gif"]) {
        return YES;
    }
    return NO;
}

+ (NSString *)contentTypeWithImageData: (NSData *)data {
    
    uint8_t c;
    
    [data getBytes:&c length:1];
    
    switch (c) {
            
        case 0xFF:
            
            return @"jpeg";
            
        case 0x89:
            
            return @"png";
            
        case 0x47:
            
            return @"gif";
            
        case 0x49:
            
        case 0x4D:
            
            return @"tiff";
            
        case 0x52:
            
            if ([data length] < 12) {
                
                return nil;
                
            }
            
            NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];
            
            if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {
                
                return @"webp";
                
            }
            
            return nil;
            
    }
    
    return nil;
}