Add TagHelperContext.Items bag to enable passing values between related tag helpers

[DamianEdwards]

Add a new property public IDictionary<string, object> Items { get; } to the TagHelperContext class to facilitate the passing of arbitrary items between Tag Helpers running on the same element.

e.g.

[HtmlElementName("my-tag")]
public class MyTagHelper : TagHelper {
    public override void Process(TagHelperContext context, TagHelperOutput output) {
        context.Items["message"] = "This came from the MyTagHelper class!";
    }
}

[HtmlElementName("my-tag")]
public class MyOtherTagHelper : TagHelper {
    public override void Process(TagHelperContext context, TagHelperOutput output) {
        var message = context.Items["message"];
    }
}

[NTaylorMullen]

I don't feel like a single bag to communicate between multiple TagHelpers on the same elment has a whole lot of value and doesn't help solve the major problem of communicating between child => parent and vice versa.

Ultimately it will be less common for TagHelper authors to write multiple TagHelpers that communicate among each other on the same element which is why this item should probably be re-thought to provide greater value.

[dougbu]

in an offline discussion, thought we ended with agreement some of the existing property bags (FormContext.FormData, ViewContext.ViewData, likely a few others) had reasonable semantics but not quite the correct scope. this led us to a proposed design where public IDictionary<string, object> Items { get; } is backed by a CopyOnWriteDictionary and each TagHelperContext would initialize that dictionary to its parent's dictionary.

this design provides

• communication of arbitrary information between tag helpers targeting the same element (no harm though likely not a primary scenario, agreed)
• communication of arbitrary information between tag helpers and any tag helper nested within
• worry-free cleanup of per-helper content -- parent and sibling scopes never see a tag helper's changes to its Items content
• communication of arbitrary information from nested tag helpers back to their parent; all that's required is the parent creating its (well-known) Items entry before executing its childen

[NTaylorMullen]

The way this item is worded leads me to believe the Items bag is only accessible to TagHelpers running on the same element. If that Items bag is communicated down scope-wise via CopyOnWriteDictionary then I'm 100% on board.

[DamianEdwards]

👍

-----Original Message-----
From: "N. Taylor Mullen" notifications@github.com
Sent: ‎1/‎27/‎2015 11:19 AM
To: "aspnet/Razor" Razor@noreply.github.com
Cc: "Damian Edwards" damian@damianedwards.com
Subject: Re: [Razor] Add TagHelperContext.Items bag to enable passing valuesbetween tag helpers and content (#238)

The way this item is worded leads me to believe the Items bag is only accessible to TagHelpers running on the same element. If that Items bag is communicated down scope-wise via CopyOnWriteDictionary then I'm 100% on board.
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