Kevin Dangoor Reporter
	Description • 12 years ago

This was a suggestion that I received (alas, I forget where I saw it!)

It would be nice if the debugger could break when a variable's value changed. That would make it easier to spot unexpected changes. Apparently, Visual Studio supports this for unmanaged code[1]. I don't know if this is common in other environments.

1: http://stackoverflow.com/questions/160045/visual-studio-debugger-break-when-a-value-changes

	Panos Astithas (he/him) [:past] (please ni?)
	Comment 1 • 12 years ago

This is in our plans, however the required SpiderMonkey API isn't there, yet.

In terms of user interface, what would be the best way to expose this?

a) Context menu on the variable in the property view, with the option "Break on change"?
b) In the future Watch Expression view, have an expression with a single variable name create a watchpoint?
c) A separate Watch Variables view?
d) ?

	Kevin Dangoor Reporter
	Comment 2 • 12 years ago

(In reply to Panos Astithas [:past] from comment #1)
> In terms of user interface, what would be the best way to expose this?
> 
> a) Context menu on the variable in the property view, with the option "Break
> on change"?
> b) In the future Watch Expression view, have an expression with a single
> variable name create a watchpoint?
> c) A separate Watch Variables view?
> d) ?

Context menu in the property view makes sense.

I don't know what the Watch Expression view will look like... is it a list of conditional breakpoints where the expressions are displayed? (From looking at bug 727429, I'm not clear on whether "watch expressions" are conditional breakpoints or something different...)

	Panos Astithas (he/him) [:past] (please ni?)
	Comment 3 • 12 years ago

(In reply to Kevin Dangoor from comment #2)
> (In reply to Panos Astithas [:past] from comment #1)
> > In terms of user interface, what would be the best way to expose this?
> > 
> > a) Context menu on the variable in the property view, with the option "Break
> > on change"?
> > b) In the future Watch Expression view, have an expression with a single
> > variable name create a watchpoint?
> > c) A separate Watch Variables view?
> > d) ?
> 
> Context menu in the property view makes sense.
> 
> I don't know what the Watch Expression view will look like... is it a list
> of conditional breakpoints where the expressions are displayed? (From
> looking at bug 727429, I'm not clear on whether "watch expressions" are
> conditional breakpoints or something different...)

Watch expressions are expressions the debugger evaluates each time it enters the paused state. I envision a list of (expression,result) tuples as the UI, same as Firebug and WebKit. But now that I think of it, it wouldn't work right with the watchpoint API, so we probably need a different UI. So, context menu to set and an additional view (group in the accordion-like UI we have) to display the list of watched objects and allow watchpoint removal? Or a decorator icon on the variable to toggle watchpoint functionality?

Honza, does Firebug expose a UI for using Object.prototype.watch to inspect changes to variables?

	Panos Astithas (he/him) [:past] (please ni?)
	Comment 4 • 12 years ago

(In reply to Kevin Dangoor from comment #2)
> I don't know what the Watch Expression view will look like... is it a list
> of conditional breakpoints where the expressions are displayed? (From
> looking at bug 727429, I'm not clear on whether "watch expressions" are
> conditional breakpoints or something different...)

This reminded me that I hadn't filed a bug for implementing conditional breakpoints, so bug 740825 will cover that work.

	David Bruant
	Comment 5 • 12 years ago

This would be so awesome! I've discussed this topic with Jorendorff and Jimb (adding them in copy) back in December. They mentioned that since variables are heavily optimized in SpiderMonkey, it could require generating a different machine code.

	Mike Ratcliffe [:miker] [:mratcliffe] [:mikeratcliffe]
	Comment 7 • 11 years ago

Firebug has this feature so it must be possible.

	Sylvestre Ledru [:Sylvestre]
	Comment 8 • 6 years ago

We have it now:
https://developer.mozilla.org/en-US/docs/Tools/Debugger/How_to/Set_Watch_Expressions