# DisplayItemList

[DisplayItemList](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/display_item_list.h;l=50;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6) is a container of paint operations represented as [PaintOp](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/paint_op.h;l=107;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6) such as ClipRectOp, DrawColorOp ...
Let's see what it does.

## How to use DisplayItemList
It starts painting from [StartPaint](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/display_item_list.h;l=68;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6).
Then [push](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/display_item_list.h;l=80;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6) the operations to the list.
After pushing all of the operations, end painting with [EndPaintOfPairedBegin](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/display_item_list.h;l=108;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6).

context_->list_->StartPaint();
context_->list_->push<cc::SaveOp>();
context_->list_->push<cc::ClipRectOp>(gfx::RectToSkRect(clip_rect),
                                      SkClipOp::kIntersect, antialias);
context_->list_->EndPaintOfPairedBegin();

Actually StartPaint implementation is empty for production build, so it's just used as a marker.
(Only implemented meaningfully on debug build)

## PaintOpBuffer
[PaintOpBuffer](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/paint_op_buffer.h;l=101;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6) is a buffer to carry PaintOp elements.
It's stored as [`data_`](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/paint_op_buffer.h;l=362;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6).

To run registered operations, [PaintOpBuffer::Playback](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/paint_op_buffer.cc;l=186;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6) is called.
Playback calls each operation's Raster [here](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/paint_op_buffer.cc;l=239-254;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6).
[PaintOp::Raster](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/paint_op.cc;l=1661;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6) calls raster functions corresponds to the operation type registered as [`type`](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/paint_op.h;l=109;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6).
Each types of operation is implemented like [ClipRectOp](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/paint_op.h;l=342;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6). Raster implementaiton of ClipRectOp is [ClipRectOp::Raster](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/paint_op.cc;l=1069;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6) below:

void ClipRectOp::Raster(const ClipRectOp* op,
                        SkCanvas* canvas,
                        const PlaybackParams& params) {
  canvas->clipRect(op->rect, op->op, op->antialias);
}

Other looks relatively the same, so what PaintOp does is basically a bypass to gfx::Canvas which goes to cc.

[DisplayItemList::Raster](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/display_item_list.cc;l=81;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6) gets the clip bounds via [GetCanvasClipBounds](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/display_item_list.cc;l=29;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6) and start rasterization.
Raster starts operation from `offset` in the buffer, and [Playback](https://source.chromium.org/chromium/chromium/src/+/main:cc/paint/display_item_list.cc;l=91;drc=88d87ae50728261cfdbf77ae493a8d506eaf42e6).

Operation types are

#define TYPES(M)      \
  M(AnnotateOp)       \
  M(ClipPathOp)       \
  M(ClipRectOp)       \
  M(ClipRRectOp)      \
  M(ConcatOp)         \
  M(CustomDataOp)     \
  M(DrawColorOp)      \
  M(DrawDRRectOp)     \
  M(DrawImageOp)      \
  M(DrawImageRectOp)  \
  M(DrawIRectOp)      \
  M(DrawLineOp)       \
  M(DrawOvalOp)       \
  M(DrawPathOp)       \
  M(DrawRecordOp)     \
  M(DrawRectOp)       \
  M(DrawRRectOp)      \
  M(DrawSkottieOp)    \
  M(DrawSlugOp)       \
  M(DrawTextBlobOp)   \
  M(NoopOp)           \
  M(RestoreOp)        \
  M(RotateOp)         \
  M(SaveOp)           \
  M(SaveLayerOp)      \
  M(SaveLayerAlphaOp) \
  M(ScaleOp)          \
  M(SetMatrixOp)      \
  M(SetNodeIdOp)      \
  M(TranslateOp)