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Flattenables
============
Many objects in Skia, such as `SkShaders` and other effects on `SkPaint`, need to be
flattened into a data stream for either transport or as part of the key to the
font cache. Classes for these objects should derive from `SkFlattenable` or one of
its subclasses. If you create a new flattenable class, you need to make sure you
do a few things so that it will work on all platforms:
1: Override the method `flatten` (the default scope is protected):
<!--?prettify?-->
~~~~
virtual void flatten(SkFlattenableWriteBuffer& buffer) const override {
this->INHERITED::flatten(buffer);
// Write any private data that needs to be stored to recreate this object
}
~~~~
2: Override the (protected) constructor that creates an object from an
`SkFlattenableReadBuffer`:
<!--?prettify?-->
~~~~
SkNewClass(SkFlattenableReadBuffer& buffer)
: INHERITED(buffer) {
// Read the data from the buffer in the same order as it was written to the
// SkFlattenableWriteBuffer and construct the new object
}
~~~~
3: Declare a set of deserialization procs for your object in the class declaration:
We have a macro for this:
<!--?prettify?-->
~~~~
public:
SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkNewClass)
~~~~
4: If your class is declared in a `.cpp` file or in a private header file, create a
function to register its group:
This occurs in cases where the classes are hidden behind a factory, like many effects
and shaders are. Then in the parent class header file (such as `SkGradientShader`) you
need to add:
<!--?prettify?-->
~~~~
public:
SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
~~~~
Then in the cpp file you define all the members of the group together:
<!--?prettify?-->
~~~~
SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkGroupClass)
SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkMemberClass1)
SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkMemberClass2)
// etc
SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
~~~~
5: Register your flattenable with the global registrar:
You need to add one line to `SkFlattenable::InitalizeFlattenables()`. To register the
flattenable in a Skia build, that function is defined in `SkGlobalInitialization_default.cpp`.
For Chromium, it is in `SkGlobalInitialization_chromium.cpp`.
For a single flattenable add
<!--?prettify?-->
~~~~
SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkNewClass)
~~~~
For a group, add
<!--?prettify?-->
~~~~
SkGroupClass::InitializeFlattenables();
~~~~
|
