Moved canvas creation and size management into a new Canvas class.
This is the first step towards a more object-oriented architecture.
Since we create multiple canvases, and have to maintain several
module-global variables to track their properties, they are the ideal
place to start.
This commit also removes sizing code that was duplicated between
makeCanvas and resizeCanvas.
These methods provide a common way to draw HTML text above a canvas.
The getTextInfo method generates div HTML for text with a given font
style/class and angle, measures the element's dimensions, and saves
everything in a cache. The drawText method takes the resulting entry,
finishes generating the inline styles necessary to position the div, and
adds the result to a buffer. The render method dumps the buffer into an
overlay and expires unused cache entries.
Instead of giving the axis its own font property, we simply look at its
options, where the font comes from in the first place. A separate
property is unnecessary and inconsistent with the way other axis options
This significantly improves performance, since we already create the
elements when measuring them, and that effort is now no longer wasted.
We must take care to detach, rather than remove, when clearing the text
layer, so we can add the elements back later if necessary.
Every cache element now contains the actual text element instead of just
its HTML, plus a flag indicating whether it is visible. The addText and
removeText methods control the state of this flag, and the render method
uses it to manage elements within the text container. So where we
previously used drawText to actually render text, now we add each string
once, then let the render method take care of drawing them as necessary.
This dramatically improves performance by eliminating the need to clear
and re-populate HTML text on every drawing cycle. Since the elements
are now static between add/remove calls, this also allows users to add
interactivity, as they could in 0.7. Finally, it eliminates the need
for a separate 'hot' cache.
I also removed the unnecessary 'dimensions' object; it's easier and
faster to store the width and height at the top level of the info
The getTextInfo method previously added new text to the top-level
container when measuring it. Now it adds the text to the text layer,
just as it will be when rendered, so that parent-child CSS rules can
This also avoids having to safe a reference to the top-level container,
since it wasn't used anywhere else.
This lets users 'namespace' text more naturally, i.e. placing x-axis
labels in a different container from y-axis labels, providing more
flexibility when it comes to styling and interactivity.
Internally the text cache now has a second tier: layers > text > info.
Previously the cache was divided only by layer, with entries keyed on a
string built from the text and style. Now the style has its own tier in
the cache, i.e. layers > styles > text > info.
This introduces some complexity, since the nested for loops are ugly,
but at the same time we avoid having to create the cache-key strings.
More importantly it solves the problem of uniqueness that exists when we
try to join strings that may contain arbitrary text. It also allows a
further optimization in the canvas plugin, which can now set text style
and color just once per distinct style, instead of with every string.