Adding hooks "beforeTooltipDraw" and "afterTooltipDraw" to plugins

[JewelsJLF]

Add a call to plugins.notify before tooltip is drawn. This will allow tooltip to show up on top of a plugin when using "beforeTooltipDraw" inside the plugin.

E.g, Use "beforeTooltipDraw" instead of "afterDraw" in this watermark plugin: https://github.com/JewelsJLF/chartjs-plugin-watermark

[simonbrunel]

Wouldn't beforeTooltipDraw be more explicit and thus durable (e.g. if we decide to add beforeTooltipUpdate or whatever related to the tooltip)? Also, since there is only one managed tooltip, not sure it should be plural.

[etimberg]

Good point. I'd agree with this change

[JewelsJLF]

Sounds good, I'll update it.

[simonbrunel]

What about making this new hook consistent with *DatasetDraw:

• make it cancelable: if the hook return false, the tooltip should not be drawn
• use the new hook signature that accepts only one argument: hook(chart, args, options)

We might want to move that code in a new function, similar to drawDataset:

/**
 * @private
 */
_drawTooltip: function(easingValue) {
    var me = this;
    var tooltip = me.tooltip;
    var args = {
        tooltip: tooltip ,
        easingValue: easingValue
    };

    if (plugins.notify(me, 'beforeTooltipDraw', [args]) === false) {
        return;
    }

    tooltip .draw();

    plugins.notify(me, 'afterTooltipDraw', [args]);
},

Then in draw():

    // ...
    me.drawDatasets(easingValue);
    me._drawTooltip(easingValue);
    
    plugins.notify(me, 'afterDraw', [easingValue]);
    // ...

Thoughts?

[JewelsJLF]

easingValue doesn't seem to be used when drawing the tooltip... any reason to pass it as an arument to the drawTooltip function anyway? Also, any particular reason to call it _drawTooltip instead of just drawTooltip?

[simonbrunel]

easingValue to be consistent with other *Draw hooks (it's more or less the animation progress). We can still add it later on demand if you think it's useless right now. _drawTooltip because it's a private method (there are many other private methods not prefixed because we switched to that convention too late).

[simonbrunel]

Could you also update the unit tests with these new hooks (I think it's simply add them before afterDraw)?

[simonbrunel]

and move options after args.easingValue :)

[simonbrunel]

Thanks @JewelsJLF

@etimberg I think this PR is safe enough to be included in 2.7.1, what do you think?

[etimberg]

@simonbrunel I agree. Let's merge this :)