比較目標C與Swift中使用NSXMLParser的性能,存在較大的性能差異。目標C中僅註冊didStartElement
,didEndElement
和foundCharacters
的性能爲〜17 MB/s,但Swift中的低至〜1.4 MB/s(當轉換爲String時,請參見下文)。代碼以發佈(優化)模式運行。使用NSXMLParser的迅速性能
目標C:
#import <Foundation/Foundation.h>
@interface MyDelegate: NSObject <NSXMLParserDelegate> {
@public
int didStartElement;
int didEndElement;
int foundCharacters;
}
@end
@implementation MyDelegate
-(MyDelegate *)init {
didStartElement = 0;
didEndElement = 0;
foundCharacters = 0;
return self;
}
-(void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName attributes:(NSDictionary *)attributeDict {
didStartElement += 1;
}
-(void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName {
didEndElement += 1;
}
-(void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string {
foundCharacters += 1;
}
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
NSURL *input = [NSURL fileURLWithPath: [[NSProcessInfo processInfo] arguments][1]];
NSError *error;
if (![input checkResourceIsReachableAndReturnError:&error]) {
NSLog(error.description);
abort();
}
NSXMLParser *parser = [[NSXMLParser alloc] initWithContentsOfURL:input];
MyDelegate *delegate = [[MyDelegate alloc] init];
parser.delegate = delegate;
NSDate *start = [NSDate new];
if (![parser parse]) {
NSLog(parser.parserError.description);
}
NSDate *end = [NSDate new];
NSLog(@"Done. #didStartElement: %d, #didEndElement: %d, #foundCharacters: %d", delegate->didStartElement, delegate->didEndElement, delegate->foundCharacters);
NSDictionary *attrs = [[NSFileManager defaultManager] attributesOfItemAtPath:input.path error:&error];
// Determine MB/s
if (error != nil) {
NSLog(@"%@", error);
abort();
}
double throughput = ((NSNumber *)[attrs valueForKey:NSFileSize]).doubleValue/[end timeIntervalSinceDate:start]/1e6;
NSLog(@"Throughput %f MB/s", throughput);
}
return 0;
}
斯威夫特:
import Foundation
var input = NSURL(fileURLWithPath: Process.arguments[1])!
var error: NSError?
if !input.checkResourceIsReachableAndReturnError(&error) {
println(error)
abort()
}
class MyDelegate: NSObject, NSXMLParserDelegate {
var didStartElement = 0
var didEndElement = 0
var foundCharacters = 0
func parser(parser: NSXMLParser, didStartElement elementName: String, namespaceURI: String?, qualifiedName qName: String?, attributes attributeDict: [NSObject : AnyObject]) {
didStartElement += 1
}
func parser(parser: NSXMLParser, didEndElement elementName: String, namespaceURI: String?, qualifiedName qName: String?) {
didEndElement += 1
}
func parser(parser: NSXMLParser, foundCharacters string: String) {
foundCharacters += 1
}
}
var parser = NSXMLParser(contentsOfURL: input)!
println(input)
var delegate = MyDelegate()
parser.delegate = delegate
var start = NSDate()
parser.parse()
var end = NSDate()
println("Done. #didStartElement: \(delegate.didStartElement), #didEndElement \(delegate.didEndElement), #foundCharacters \(delegate.foundCharacters)")
// Determine MB/s
var attrs = NSFileManager.defaultManager().attributesOfItemAtPath(input.path!, error: &error)
if error != nil {
println(error!)
abort()
}
var throughput = Double(attrs![NSFileSize]! as Int)/end.timeIntervalSinceDate(start)/1e6
println("Throughput \(throughput) MB/s")
鑄造時大量的性能損失;看到在didStartElement
爲用於attributes
參數的類型定義的區別:
NSDictionary
:19 MB/s的
[NSObject: AnyObject]
:8.5百萬字節/秒
[String: String]
:1.4百萬字節/秒
使用計數器儀表,顯然36%的時間花費在將字典轉換爲Swift上(使用[NSObject: AnyObject]
):
由於節點的屬性與進一步處理相關,因此c將它們轉移到Swift字符串是無法避免的。如何仍然在Swift中獲得不錯的處理性能?
更新
當用C直接使用的libxml2的SAX解析器,性能大約是110 MB /秒。所以這裏確實存在一些性能問題。
使用libxml的SAX解析器需要定義回調。目前無法從Swift函數創建C函數指針。那麼這甚至可能繞過Objective C和橋接? – bouke 2014-10-19 15:33:36
我擔心,傳遞一個指向Swift函數的指針在C代碼中被調用尚不支持。這個問題需要調查。有一個有趣的觀察:http://stackoverflow.com/questions/24107099/function-callback-from-c-to-swift。 – CouchDeveloper 2014-10-19 15:43:53
謝謝!雖然目前不可能直接從Swift實現libxml的sax解析器,但可以使用它的ObjC包裝器。通過將字符指針傳遞給Swift,從ObjC到Swift沒有太多橋接開銷。 – bouke 2014-11-01 09:44:38