如何在 Android 上的 Java Canvas 对象上使用 SKIA C++ API 进行绘制？

Question

I am experimenting with native drawing using the Java Canvas API on Android. As far as I can understand, Canvas is built as an abstraction on top of SKCanvas from the Skia graphics engine. I'm willing to pass the Canvas object obtained in the OnDraw() method of a view to native code and draw using C++ and the Skia API.

What would be the best way to gain access to the SkCanvas pointer after passing the java Canvas object to native code using JNI ?

Here is the information I have been able to assimilate, which ultimately boils down to accessing a private member from the instance a native platform object .

Source code of Canvas.java from https://cs.android.com/android/platform/superproject/+/master:frameworks/base/graphics/java/android/graphics/Canvas.java;l=59 shows that it has private variable named mNativeCanvasWrapper (defined in frameworks/base/graphics/java/android/graphics/BaseCanvas.java ) which can be accessed via reflection through the function declared below.

/** @hide */
@UnsupportedAppUsage
public long getNativeCanvasWrapper() {
    return mNativeCanvasWrapper;
}

mNativeCanvasWrapper , among other things, is passed to all functions responsible for drawing. Following the trail of one such function ( getWidth() ), I can see it's JNI registration in https://cs.android.com/android/platform/superproject/+/master:frameworks/base/libs/hwui/jni/android_graphics_Canvas.cpp;l=692

{"nGetWidth","(J)I", (void*) CanvasJNI::getWidth},

Definition of CanvasJNI::getWidth()

static jint getWidth(CRITICAL_JNI_PARAMS_COMMA jlong canvasHandle) {
    return static_cast<jint>(get_canvas(canvasHandle)->width());
}

Definition of get_canvas(canvasHandle)

static Canvas* get_canvas(jlong canvasHandle) {
    return reinterpret_cast<Canvas*>(canvasHandle);
}

The above Canvas* pointer actually points to a class called SkiaCanvas , which inherits from the Canvas base class. Definition for the class can be found at frameworks/base/libs/hwui/SkiaCanvas.cpp . This is the class that contains the actual pointer to the SKCanvas object that I need (marked private), as can be seen at https://cs.android.com/android/platform/superproject/+/master:frameworks/base/libs/hwui/SkiaCanvas.h;l=219 .

How can I obtain the SKCanvas pointer from a SkiaCanvas pointer, given that it's marked private and the only getter is marked protected?

Answer 1

The answer is in two parts.
First, getting the Canvas* out of the JNI object:

jclass cls_Canvas = env->GetObjectClass(obj);
jmethodID mid_Canvas_getNativeCanvasWrapper = env->getMethodID("getNativeCanvasWrapper", "()J");
jlong canvas_ptr = env->CallLongMethod(obj, mid_Canvas_getNativeCanvasWrapper);

The second part goes from there:

class MyCanvas : public SkiaCanvas { public: SkCanvas* getCanvas() { return asSkCanvas(); };
MyCanvas* skc = reinterpret_cast<MyCanvas*>(canvas_ptr);
SkCanvas* s = myc->getCanvas();