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What this is and what this not is
---------------------------------
The marshaling code has been designed to specifically address the needs for
XobotOS, it is a general managed <-> native marshaler for .NET applications.
One important design principle is correcness and robustness over performance
or beauty of the generated code. The generated code may look very ugly and
it's also a bit bloated - but it also makes it very difficult to make any
mistakes.
Four different layers
---------------------
In XobotOS, we have four different layers:
(a) The Java source code that will be automatically converted to C#
(b) An intermediate layer of generated C code - this should be regarded as
strictly private, an implementation detail that may change at any time.
(c) A C++ library.
Android's JNI glue code contains both the Java <-> native marshaling code
as well as some added functionality on top of it.
In XobotOS, I separated this additional functionality from the marshaling
code and put it into a separate C++ library.
All the code has been manually written and this library could also be
re-used by other projects.
(d) The Skia / Android native libraries.
From a user's point of view, the public APIs in (a) simply call into (c)/(d),
the intermediate layer is just an internal implementation detail.
Marshaling classes and instance pointers
----------------------------------------
The native Java API is a huge mess and also very inconsistent:
* They use the 32-bit 'int' type for pointers everywhere.
* A native instance method may or may not take the native 'this' pointer;
sometimes they use native JNI reflection to get it.
Which means that the type 'int' in a method signature could mean three
different things: a pointer, an enum or an integer.
Therefor, an XML entry is required.
A special word about 'structs'
------------------------------
The term 'struct' is misleading since a valuetype struct does not exist in Java.
Several 'structs' that you would expect to be valuetypes are in fact classes
in XobotOS - for instance android.graphics.Rect. These cannot be turned into
valuetypes as this would chance semantics in a way that neither the converter
nor the compiler would be able to detect.
Consider this:
void hello (Foo foo)
{
foo.hello = 9;
foo = new Foo ();
foo.hello = 15;
}
int test ()
{
Foo foo = new Foo ();
foo.hello = 3;
hello (foo);
return foo.hello;
}
What's the result of test() ?
Well, it's 9 is Foo is a class, 3 if it's a struct - and 15 if it's a struct
and you add the 'ref' modifier to hello().
An additional difficulty is that 'structs' like android.graphics.Rect may
sometimes be null - and the corresponding C++ Skia function also allows a NULL
argument.
Different marshaling modes
--------------------------
Marshaling info for arrays, strings and structs is created automatically, XML
entries are only used to specify special marshaling modes.
There are three different marshaling modes:
* The default is "read-only" - 'const Foo&' is used as native type.
Specifying 'flags="ALLOW_NULL' in the XML turns it into 'const Foo*' and
null checks are added both on the native and the managed side.
("read-only" means accidentally passing this to a function that expects a
non-const reference/pointer would yield a compilation error; there is no
protection against the native code explicitly casting this to non-const and
writing into it).
* "writable" (mode="REF" in the XML) means that the native code may modify and
the modification will be automatically marshaled back into managed code.
This mode is not allowed for strings.
For arrays, it means that the native code may modify its elements - but it
must not touch the length or address of the array.
For structs, each of its members will be marshaled according to the rules in
this section. However, a different marshaling mode may be specified for
individual members.
'Foo&' is used as native type - or 'Foo*' for ALLOW_NULL.
* "out" (mode="OUT" in the XML).
The native code creates and returns a new instance of the type - 'Foo&' is
used for structs and 'Foo*' for arrays, strings and when used as a return value.
This mode may be used to return complex data structures such as strings or
arrays of arrays from native code.
When looking at the C++ API:
* 'const Foo&' - no special marshal flags have been used, parameter is passed
"IN" and may not be null.
* 'Foo&' - native code may modify this.
* 'const Foo*' - may be null
* 'Foo*' - native code may modify this, may be null.
Special handling of arrays
--------------------------
When marshaling an array of a blittable type, the marshaling code passes a
pointer to the actual managed array elements to the native code using
GCHandle.AddrOfPinnedObject().
C++ API for arrays and structures
---------------------------------
For structures, the C++ API must define the data type - the marshaling code
simply takes care of marshaling it to the managed side and vice-versa.
For arrays, there's a template type 'NativeArray<T>' in <MarshalHelper.h>,
it provides a non-copyable representation of a managed array with an API to
access its elemements, automatically checking bounds.
For strings, 'NativeString' derives from 'NativeElement<char16_t>' and
provides some extra functionality like conversion to and from
android::String16.
Intermediate representation of structs
--------------------------------------
Marshaling structs is a little bit difficult because they may contain
non-blittable elements such as arrays, strings or other structs.
Internally, arrays and strings are turned into a structure containing their
length and address and when these types are used as struct members, special
marshaling code must be generated.
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