amp-bind: Support intermediary vars in AMP.setState()

[dreamofabear]

Benefits

• Reduces document markup due to code duplication.
• Improves performance by caching expression fragments.

Possible solution

One approach is to allow chained actions. E.g. instead of...

on="tap:AMP.setState({foo: 1 + 2, bar: (1 + 2) + 3})"

...do this:

on="tap:AMP.setState({onePlusTwo: 1 + 2}), 
        AMP.setState({foo: onePlusTwo, bar: onePlusTwo + 3})"

This doesn't work today because the second action doesn't wait for the first to complete.

Only really painful in more complicated use cases though, e.g. examples/bind/turing.amp.html.

[kmh287]

Could setState take a variable number of object literals and then call bind's setState on them in order?

[kmh287]

Also, couldn't this be used to get around our expression complexity limit?

[dreamofabear]

Could setState take a variable number of object literals and then call bind's setState on them in order?

Yes, though it's a slightly more complex change since we'd have to change the action parser.

Also, couldn't this be used to get around our expression complexity limit?

Expressions in setState should go through the same code path.

[kmh287]

I'm thinking though that if we allow multi-step calculations, then someone could just break up a larger calculation into multiple steps to circumvent the complexity limit.

For instance if my expression is A + B where A and B are complex expressions and (A + B) has an AST size of 51 (one over the current limit) then I could do.

AMP.setState({x: A, y: B}), AMP.setState({z: x + y})

or

AMP.setState({x:A}), AMP.setState({y:B}), AMP.setState({z:x+y})

[dreamofabear]

Good point, we'd have to have a more sophisticated constraint to avoid that.

[kmh287]

We could put a limit on the number of sequential setStates, but even then each one we allow multiplies our effective complexity limit.

[dreamofabear]

Another option is to add a new custom element:

<amp-variable id="onePlusOne" expression="1 + 1">

Slightly less powerful and more explicit than chaining AMP.setState actions.

[kmh287]

Once #8678 is in, won't users be able to chain setState to create intermediary vars?

[dreamofabear]

Fixed with #12285.