英文:
Why does Go's encoding/xml.Decoder.Token() not produce xml.Attr tokens as it should?
问题
使用encoding/xml.Decoder,我正在尝试手动解析从http://www.khronos.org/files/collada_schema_1_4加载的XML文件。
为了测试目的,我只是遍历文档并打印出遇到的任何令牌类型:
func Test (r io.Reader) {
var t xml.Token
var pa *xml.Attr
var a xml.Attr
var co xml.Comment
var cd xml.CharData
var se xml.StartElement
var pi xml.ProcInst
var ee xml.EndElement
var is bool
var xd = xml.NewDecoder(r)
for i := 0; i < 24; i++ {
if t, err = xd.Token(); (err == nil) && (t != nil) {
if a, is = t.(xml.Attr); is { print("ATTR\t"); println(a.Name.Local) }
if pa, is = t.(*xml.Attr); is { print("*ATTR\t"); println(pa) }
if co, is = t.(xml.Comment); is { print("COMNT\t"); println(co) }
if cd, is = t.(xml.CharData); is { print("CDATA\t"); println(cd) }
if pi, is = t.(xml.ProcInst); is { print("PROCI\t"); println(pi.Target) }
if se, is = t.(xml.StartElement); is { print("START\t"); println(se.Name.Local) }
if ee, is = t.(xml.EndElement); is { print("END\t\t"); println(ee.Name.Local) }
}
}
}
现在这是输出:
PROCI xml
CDATA [1/64]0xf84004e050
START schema
CDATA [2/129]0xf84004d090
COMNT [29/129]0xf84004d090
CDATA [2/129]0xf84004d090
START annotation
CDATA [3/129]0xf84004d090
START documentation
CDATA [641/1039]0xf840061000
END documentation
CDATA [2/1039]0xf840061000
END annotation
CDATA [2/1039]0xf840061000
COMNT [37/1039]0xf840061000
CDATA [2/1039]0xf840061000
START import
END import
CDATA [2/1039]0xf840061000
COMNT [14/1039]0xf840061000
CDATA [2/1039]0xf840061000
START element
CDATA [3/1039]0xf840061000
START annotation
注意,即使在最后(第24行),已经传递了许多属性,但没有输出ATTR或*ATTR行。
这是在Windows 7 64位下的Go 1.0.3 64位中进行的。我是做错了什么还是应该提交一个Go软件包错误报告?
[附注:当对正确准备的结构进行正常的xml.Unmarshal时,xml包可以很好地捕获和映射已知命名和映射的属性。但我还需要收集根元素中的“未知”属性(为了收集此用例的命名空间信息,该用例是http://github.com/metaleap/go-xsd),因此我尝试使用Decoder.Token()。]
英文:
Using encoding/xml.Decoder I'm attempting to manually parse an XML file loaded from http://www.khronos.org/files/collada_schema_1_4
For test purposes, I'm just iterating over the document printing out whatever token type is encountered:
func Test (r io.Reader) {
var t xml.Token
var pa *xml.Attr
var a xml.Attr
var co xml.Comment
var cd xml.CharData
var se xml.StartElement
var pi xml.ProcInst
var ee xml.EndElement
var is bool
var xd = xml.NewDecoder(r)
for i := 0; i < 24; i++ {
if t, err = xd.Token(); (err == nil) && (t != nil) {
if a, is = t.(xml.Attr); is { print("ATTR\t"); println(a.Name.Local) }
if pa, is = t.(*xml.Attr); is { print("*ATTR\t"); println(pa) }
if co, is = t.(xml.Comment); is { print("COMNT\t"); println(co) }
if cd, is = t.(xml.CharData); is { print("CDATA\t"); println(cd) }
if pi, is = t.(xml.ProcInst); is { print("PROCI\t"); println(pi.Target) }
if se, is = t.(xml.StartElement); is { print("START\t"); println(se.Name.Local) }
if ee, is = t.(xml.EndElement); is { print("END\t\t"); println(ee.Name.Local) }
}
}
}
Now here's the output:
PROCI xml
CDATA [1/64]0xf84004e050
START schema
CDATA [2/129]0xf84004d090
COMNT [29/129]0xf84004d090
CDATA [2/129]0xf84004d090
START annotation
CDATA [3/129]0xf84004d090
START documentation
CDATA [641/1039]0xf840061000
END documentation
CDATA [2/1039]0xf840061000
END annotation
CDATA [2/1039]0xf840061000
COMNT [37/1039]0xf840061000
CDATA [2/1039]0xf840061000
START import
END import
CDATA [2/1039]0xf840061000
COMNT [14/1039]0xf840061000
CDATA [2/1039]0xf840061000
START element
CDATA [3/1039]0xf840061000
START annotation
Notice no ATTR or *ATTR lines are output even though by the last (24th) line many attributes have been passed both in the root xs:schema element as well as in xs:import and xs:element elements.
This is in Go 1.0.3 64-bit under Windows 7 64-bit. Am I doing something wrong or should I file a Go package bug report?
[Side note: when doing a normal xml.Unmarshal into properly prepared structs, known-named-and-mapped attributes are captured and mapped by the xml package just fine. But I also need to collect "unknown" attributes in the root element (to collect namespace information for this use-case, the use-case being http://github.com/metaleap/go-xsd ), hence my attempts to use Decoder.Token().]
答案1
得分: 5
是的,这种行为是预期的。属性被解析,但不作为xml.Token返回。属性只是不是Tokens。参见:http://golang.org/pkg/encoding/xml/#Token
可以通过Token StartElement中的Attr字段访问属性。参见:http://golang.org/pkg/encoding/xml/#StartElement
(一些一般提示:
a)不要使用print或println。
b)a, ok := t.(SomeType)的习惯用法被称为“逗号好”,因为布尔值通常被命名为“ok”,而不是“is”。请遵循这些约定。
c)习惯用法可能是这样的
switch t := t.(type) {
case xml.StartElement: ...
case xml.EndElement: ...
}
而不是你的“if a, is = t.(xml.Attr) ...”列表。
d)所有这些“var se xml.StartElement”都是噪音(杂乱)。使用
if se, ok := t.(xml.StartElement); ok { ... }
这将使您的代码更易读。)
英文:
Yes, this behavior is expected. The attributes are parsed, but
not returned as a xml.Token. Attributes simply arn't Tokens.
See: http://golang.org/pkg/encoding/xml/#Token
The attributes are accessible through the Attr field in
the Token StartElement.
See: http://golang.org/pkg/encoding/xml/#StartElement
(( Some general hints:
a) Do not use print or println.
b) The a, ok := t.(SomeType) idioma is called "comma okay", because the boolean is normaly named "ok", not "is". Please stick to these conventions.
c) Idiomatic would be something like
switch t := t.(type) {
case xml.StartElement: ...
case xml.EndElement: ...
}
instead of your list of "if a, is = t.(xml.Attr) ..."
d) All this "var se xml.StartElement" is noise (clutter). Use
if se, ok := t.(xml.StartElement); ok { ... }
This would make your code much readable. ))
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