Add initial pixel buffer caching usage

Leverages `QGraphicsItem.cacheMode` to speed up interactivity via
less `.paint()` calls (on mouse interaction) and redraws of the
underlying path when there are no transformations (other then a shift).
In order to keep the "flat bar on new time period" UX, a couple special
methods have to be triggered to get a redraw of the pixel buffer when
appending new data.

Use `QPainterPath.controlPointRect()` over `.boundingRect()` since
supposedly it's a lot faster. Drop all use of `QPicture` (since it seems
to conflict with the pixel buffer stuff?) and it doesn't give any
measurable speedup when drawing the "last bar" lines.

Oh, and add some profiling for now.

piker/ui/_graphics.py

@@ -27,7 +27,7 @@ from numba import jit, float64, int64  # , optional
 from PyQt5 import QtCore, QtGui
 from PyQt5.QtCore import QLineF, QPointF
 
-from .._profile import timeit
+# from .._profile import timeit
 # from ..data._source import numba_ohlc_dtype
 from ._style import (
     _xaxis_at,
@@ -355,7 +355,7 @@ class CrossHair(pg.GraphicsObject):
                 # update all subscribed curve dots
                 # first = plot._ohlc[0]['index']
                 for cursor in opts.get('cursors', ()):
-                    cursor.setIndex(ix) # - first)
+                    cursor.setIndex(ix)
 
             # update the label on the bottom of the crosshair
             self.xaxis_label.update_label(
@@ -495,10 +495,16 @@ def gen_qpath(
     w,
 ) -> QtGui.QPainterPath:
 
+    profiler = pg.debug.Profiler(disabled=False)
+
     x, y, c = path_arrays_from_ohlc(data, start, bar_gap=w)
+    profiler("generate stream with numba")
 
     # TODO: numba the internals of this!
-    return pg.functions.arrayToQPath(x, y, connect=c)
+    path = pg.functions.arrayToQPath(x, y, connect=c)
+    profiler("generate path with arrayToQPath")
+
+    return path
 
 
 class BarItems(pg.GraphicsObject):
@@ -520,10 +526,21 @@ class BarItems(pg.GraphicsObject):
     ) -> None:
         super().__init__()
 
-        self.last_bar = QtGui.QPicture()
+        # NOTE: this prevents redraws on mouse interaction which is
+        # a huge boon for avg interaction latency.
 
+        # TODO: one question still remaining is if this makes trasform
+        # interactions slower (such as zooming) and if so maybe if/when
+        # we implement a "history" mode for the view we disable this in
+        # that mode?
+        self.setCacheMode(QtGui.QGraphicsItem.DeviceCoordinateCache)
+
+        # not sure if this is actually impoving anything but figured it
+        # was worth a shot:
+        # self.path.reserve(int(100e3 * 6))
+
+        # self.last_bar = QtGui.QPicture()
         self.path = QtGui.QPainterPath()
-        # self._h_path = QtGui.QGraphicsPathItem(self.path)
 
         self._pi = plotitem
 
@@ -570,7 +587,7 @@ class BarItems(pg.GraphicsObject):
 
         # create pics
         # self.draw_history()
-        self.draw_last_bar()
+        # self.draw_last_bar()
 
         # trigger render
         # https://doc.qt.io/qt-5/qgraphicsitem.html#update
@@ -587,17 +604,16 @@ class BarItems(pg.GraphicsObject):
     # ) -> None:
     #     ...
 
+    # def draw_last_bar(self) -> None:
+    #     """Currently this draws lines to a cached ``QPicture`` which
+    #     is supposed to speed things up on ``.paint()`` calls (which
+    #     is a call to ``QPainter.drawPicture()`` but I'm not so sure.
 
-    def draw_last_bar(self) -> None:
+    #     """
-        """Currently this draws lines to a cached ``QPicture`` which
+    #     p = QtGui.QPainter(self.last_bar)
-        is supposed to speed things up on ``.paint()`` calls (which
+    #     # p.setPen(self.bars_pen)
-        is a call to ``QPainter.drawPicture()`` but I'm not so sure.
+    #     p.drawLines(*tuple(filter(bool, self._last_bar_lines)))
-
+    #     p.end()
-        """
-        p = QtGui.QPainter(self.last_bar)
-        p.setPen(self.bars_pen)
-        p.drawLines(*tuple(filter(bool, self._last_bar_lines)))
-        p.end()
 
     # @timeit
     def update_from_array(
@@ -642,12 +658,19 @@ class BarItems(pg.GraphicsObject):
             # update path
             old_path = self.path
             self.path = prepend_path
+            # self.path.reserve(int(100e3 * 6))
             self.path.addPath(old_path)
 
+            # trigger redraw despite caching
+            self.prepareGeometryChange()
+
         if append_length:
             # generate new lines objects for updatable "current bar"
             self._last_bar_lines = lines_from_ohlc(array[-1], self.w)
-            self.draw_last_bar()
+
+            # self.draw_last_bar()
+            # self.update()
+
 
             # generate new graphics to match provided array
             # path appending logic:
@@ -659,11 +682,14 @@ class BarItems(pg.GraphicsObject):
             self.path.moveTo(float(istop - self.w), float(new_bars[0]['open']))
             self.path.addPath(append_path)
 
+            # trigger redraw despite caching
+            self.prepareGeometryChange()
+
         self._xrange = first_index, last_index
 
-        if just_history:
+        # if just_history:
-            self.update()
+        #     self.update()
-            return
+        #     return
 
         # last bar update
         i, o, h, l, last, v = array[-1][
@@ -685,31 +711,32 @@ class BarItems(pg.GraphicsObject):
                 # update body
                 body.setLine(i, l, i, h)
 
-        # XXX: pretty sure this is causing an issue where the bar has
+            # XXX: pretty sure this is causing an issue where the bar has
-        # a large upward move right before the next sample and the body
+            # a large upward move right before the next sample and the body
-        # is getting set to None since the next bar is flat but the shm
+            # is getting set to None since the next bar is flat but the shm
-        # array index update wasn't read by the time this code runs. Iow
+            # array index update wasn't read by the time this code runs. Iow
-        # we're doing this removal of the body for a bar index that is
+            # we're doing this removal of the body for a bar index that is
-        # now out of date / from some previous sample. It's weird
+            # now out of date / from some previous sample. It's weird
-        # though because i've seen it do this to bars i - 3 back?
+            # though because i've seen it do this to bars i - 3 back?
 
         # else:
         #     # XXX: h == l -> remove any HL line to avoid render bug
         #     if body is not None:
-        #         body = self.lines[index - 1][0] = None
+        #         self._last_bar_lines = (None, larm, rarm)
+        #         # body = self.lines[index - 1][0] = None
 
-        self.draw_last_bar()
+        # self.draw_last_bar()
+        self.resetTransform()
+        self.setTransform(self.transform())
         self.update()
 
     # @timeit
     def paint(self, p, opt, widget):
 
-        # profiler = pg.debug.Profiler(disabled=False, delayed=False)
+        profiler = pg.debug.Profiler(disabled=False) #, delayed=False)
-
-        # TODO: use to avoid drawing artefacts?
-        # self.prepareGeometryChange()
 
         # p.setCompositionMode(0)
+        p.setPen(self.bars_pen)
 
         # TODO: one thing we could try here is pictures being drawn of
         # a fixed count of bars such that based on the viewbox indices we
@@ -717,10 +744,12 @@ class BarItems(pg.GraphicsObject):
         # as is necesarry for what's in "view". Not sure if this will
         # lead to any perf gains other then when zoomed in to less bars
         # in view.
-        p.drawPicture(0, 0, self.last_bar)
+        # p.drawPicture(0, 0, self.last_bar)
+        p.drawLines(*tuple(filter(bool, self._last_bar_lines)))
+        profiler('draw last bar')
 
-        p.setPen(self.bars_pen)
         p.drawPath(self.path)
+        profiler('draw history path')
 
     # @timeit
     def boundingRect(self):
@@ -738,16 +767,31 @@ class BarItems(pg.GraphicsObject):
         # bounding rect for us).
 
         # compute aggregate bounding rectangle
-        lb = self.last_bar.boundingRect()
+        # lb = self.last_bar.boundingRect()
-        hb = self.path.boundingRect()
+
+        # hb = self.path.boundingRect()
+
+        # apparently this a lot faster says the docs?
+        # https://doc.qt.io/qt-5/qpainterpath.html#controlPointRect
+        hb = self.path.controlPointRect()
+        hb_size = hb.size()
+        # print(f'hb_size: {hb_size}')
+
+        w = hb_size.width() + 1
+        h = hb_size.height() + 1
+
+        br = QtCore.QRectF(
 
-        return QtCore.QRectF(
             # top left
             QtCore.QPointF(hb.topLeft()),
+
             # total size
-            QtCore.QSizeF(QtCore.QSizeF(lb.size()) + hb.size())
+            # QtCore.QSizeF(QtCore.QSizeF(lb.size()) + hb.size())
+            QtCore.QSizeF(w, h)
             # QtCore.QSizeF(lb.size() + hb.size())
         )
+        # print(f'bounding rect: {br}')
+        return br
 
 
 # XXX: when we get back to enabling tina mode for xb