自定义View继承自View类，需要重写onDraw方法，通过canvas和paint进行绘制

@Overrideprotected void onDraw(Canvas canvas) {super.onDraw(canvas);canvas.drawLine();}

 

public void drawLine(float startX, float startY, float stopX, float stopY,@NonNull Paint paint) {super.drawLine(startX, startY, stopX, stopY, paint);}

 framework/base/graphics/java/android/graphics/BaseCanvas.java

public void drawLine(float startX, float startY, float stopX, float stopY,@NonNull Paint paint) {throwIfHasHwFeaturesInSwMode(paint);nDrawLine(mNativeCanvasWrapper, startX, startY, stopX, stopY, paint.getNativeInstance());}

private static native void nDrawLine(long nativeCanvas, float startX, float startY, float stopX,float stopY, long nativePaint);

framework/base/libs/hwui/jni/android_graphics_Canvas.cpp

// If called from Canvas these are regular JNI
// If called from DisplayListCanvas they are @FastNative
static const JNINativeMethod gDrawMethods[] = {{"nDrawColor","(JII)V", (void*) CanvasJNI::drawColor},{"nDrawColor","(JJJI)V", (void*) CanvasJNI::drawColorLong},{"nDrawPaint","(JJ)V", (void*) CanvasJNI::drawPaint},{"nDrawPoint", "(JFFJ)V", (void*) CanvasJNI::drawPoint},{"nDrawPoints", "(J[FIIJ)V", (void*) CanvasJNI::drawPoints},{"nDrawLine", "(JFFFFJ)V", (void*) CanvasJNI::drawLine},

static void drawLine(JNIEnv* env, jobject, jlong canvasHandle, jfloat startX, jfloat startY,jfloat stopX, jfloat stopY, jlong paintHandle) {Paint* paint = reinterpret_cast<Paint*>(paintHandle);get_canvas(canvasHandle)->drawLine(startX, startY, stopX, stopY, *paint);
}

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

framework/base/libs/hwui/hwui/Canvas.cpp

 virtual void drawLine(float startX, float startY, float stopX, float stopY,const Paint& paint) = 0;

frameworks\base\libs\hwui\SkiaCanvas.cpp

void SkiaCanvas::drawLine(float startX, float startY, float stopX, float stopY,const Paint& paint) {applyLooper(&paint,[&](const SkPaint& p) { mCanvas->drawLine(startX, startY, stopX, stopY, p); });
}

    void applyLooper(const Paint* paint, Proc proc, void (*preFilter)(SkPaint&) = nullptr) {BlurDrawLooper* looper = paint ? paint->getLooper() : nullptr;Paint pnt = paint ? *paint : Paint();if (preFilter) {preFilter(pnt);}this->onFilterPaint(pnt);if (looper) {looper->apply(pnt, [&](SkPoint offset, const Paint& modifiedPaint) {mCanvas->save();mCanvas->translate(offset.fX, offset.fY);proc(modifiedPaint);mCanvas->restore();});} else {proc(pnt);}}

external/skia/src/core/SkCanvas.cpp

void SkCanvas::drawLine(SkScalar x0, SkScalar y0, SkScalar x1, SkScalar y1, const SkPaint& paint) {SkPoint pts[2];pts[0].set(x0, y0);pts[1].set(x1, y1);this->drawPoints(kLines_PointMode, 2, pts, paint);
}

void SkCanvas::drawPoints(PointMode mode, size_t count, const SkPoint pts[], const SkPaint& paint) {TRACE_EVENT0("skia", TRACE_FUNC);this->onDrawPoints(mode, count, pts, paint);
}

void SkCanvas::onDrawPoints(PointMode mode, size_t count, const SkPoint pts[],const SkPaint& paint) {if ((long)count <= 0 || paint.nothingToDraw()) {return;}SkASSERT(pts != nullptr);SkRect bounds;// Compute bounds from points (common for drawing a single line)if (count == 2) {bounds.set(pts[0], pts[1]);} else {bounds.setBounds(pts, SkToInt(count));}// Enforce paint style matches implicit behavior of drawPointsSkPaint strokePaint = paint;strokePaint.setStyle(SkPaint::kStroke_Style);if (this->internalQuickReject(bounds, strokePaint)) {return;}auto layer = this->aboutToDraw(this, strokePaint, &bounds);if (layer) {this->topDevice()->drawPoints(mode, count, pts, layer->paint());}
}

external\skia\src\core\SkDevice.h

  virtual void drawPaint(const SkPaint& paint) = 0;virtual void drawPoints(SkCanvas::PointMode mode, size_t count,const SkPoint[], const SkPaint& paint) = 0;

external\skia\src\gpu\v1\Device.cpp

void Device::drawPaint(const SkPaint& paint) {ASSERT_SINGLE_OWNERGR_CREATE_TRACE_MARKER_CONTEXT("skgpu::v1::Device", "drawPaint", fContext.get());GrPaint grPaint;if (!SkPaintToGrPaint(this->recordingContext(), fSurfaceDrawContext->colorInfo(), paint,this->asMatrixProvider(), &grPaint)) {return;}fSurfaceDrawContext->drawPaint(this->clip(), std::move(grPaint), this->localToDevice());
}void Device::drawPoints(SkCanvas::PointMode mode,size_t count,const SkPoint pts[],const SkPaint& paint) {ASSERT_SINGLE_OWNERGR_CREATE_TRACE_MARKER_CONTEXT("skgpu::v1::Device", "drawPoints", fContext.get());SkScalar width = paint.getStrokeWidth();if (width < 0) {return;}GrAA aa = fSurfaceDrawContext->chooseAA(paint);if (count == 2 && mode == SkCanvas::kLines_PointMode) {if (paint.getPathEffect()) {// Probably a dashed line. Draw as a path.GrPaint grPaint;if (SkPaintToGrPaint(this->recordingContext(),fSurfaceDrawContext->colorInfo(),paint,this->asMatrixProvider(),&grPaint)) {SkPath path;path.setIsVolatile(true);path.moveTo(pts[0]);path.lineTo(pts[1]);fSurfaceDrawContext->drawPath(this->clip(),std::move(grPaint),aa,this->localToDevice(),path,GrStyle(paint, SkPaint::kStroke_Style));}return;}if (!paint.getMaskFilter() &&paint.getStrokeWidth() > 0 &&  // drawStrokedLine doesn't support hairlines.paint.getStrokeCap() != SkPaint::kRound_Cap) { // drawStrokedLine doesn't do round caps.// Simple stroked line. Bypass path rendering.GrPaint grPaint;if (SkPaintToGrPaint(this->recordingContext(),fSurfaceDrawContext->colorInfo(),paint,this->asMatrixProvider(),&grPaint)) {fSurfaceDrawContext->drawStrokedLine(this->clip(),std::move(grPaint),aa,this->localToDevice(),pts,SkStrokeRec(paint, SkPaint::kStroke_Style));}return;}}SkScalar scales[2];bool isHairline = (0 == width) ||(1 == width && this->localToDevice().getMinMaxScales(scales) &&SkScalarNearlyEqual(scales[0], 1.f) && SkScalarNearlyEqual(scales[1], 1.f));// we only handle non-coverage-aa hairlines and paints without path effects or mask filters,// else we let the SkDraw call our drawPath()if (!isHairline ||paint.getPathEffect() ||paint.getMaskFilter() ||fSurfaceDrawContext->chooseAAType(aa) == GrAAType::kCoverage) {SkRasterClip rc(this->devClipBounds());SkDraw draw;draw.fDst = SkPixmap(SkImageInfo::MakeUnknown(this->width(), this->height()), nullptr, 0);draw.fMatrixProvider = this;draw.fRC = &rc;draw.drawPoints(mode, count, pts, paint, this);return;}GrPrimitiveType primitiveType = point_mode_to_primitive_type(mode);const SkMatrixProvider* matrixProvider = this;GrPaint grPaint;if (!SkPaintToGrPaint(this->recordingContext(), fSurfaceDrawContext->colorInfo(), paint,*matrixProvider, &grPaint)) {return;}static constexpr SkVertices::VertexMode kIgnoredMode = SkVertices::kTriangles_VertexMode;sk_sp<SkVertices> vertices = SkVertices::MakeCopy(kIgnoredMode, SkToS32(count), pts, nullptr,nullptr);fSurfaceDrawContext->drawVertices(this->clip(), std::move(grPaint), *matrixProvider,std::move(vertices), &primitiveType);
}