Allow components to accept children elements

[jstarry]

Fixes: #537

Terminology

• (B) Base component that renders components nested inside each other
• (P) Parent component that has a children property and can render those children
• (C) Child component that is nested inside parent and included inside the Parent's children

Todo

• Add example for nested components
• Support arbitrary html nested inside component tags
• Support nested components inside component tags
• Allow modifying & accessing (C) props when rendering (P)
• Allow filtering (C) components when rendering (P)
• Children prop be required or optional
• Clean up nested component example
• Fix parser for generic component type
• Write tests
• Update documentation and README
• Investigate passing required properties from (P) -> (C)
• Allow sending messages from (C) -> (B)

[therustmonk]

That's very important change! Thank you! 👍

[jstarry]

@hgzimmerman could you please review this when you have time?

[hgzimmerman]

I don't have extensive experience in writing (or reading for that matter) macros in Rust, which makes up the majority of the delicate parts of this PR. Given my limited understanding, I didn't see anything suspect.

I thought that the code involving Variants in the nested_list example is really clever and cool, but exceptionally non-obvious to someone who hasn't seen that pattern before. I didn't think you could have different child components before seeing this. When a guide/book is written, some special attention will need to be given towards this feature.

Thanks for implementing this feature!

[jstarry]

@hgzimmerman yes, totally agreed about the complexity with the Variants. I think if this causes confusion it could be abstracted away in a macro at some point but I don't think it will be a common pattern. For now, a page in the book would be great. I think I'll start on that soon (mostly about the ins and outs of the macros)

[gtors]

It would be great if Children will have is_empty method. It useful in case you need to add additional html elements/css styles in case children exist.

Example:

impl MenuItem {
    fn render_caret(&self) -> Html<Self> {
        if self.props.children.is_empty() {
            html! {}
        } else {
            html! {
                <b class="caret"></b>
            }
        }
    }
}

[jstarry]

@gtors done!

[jstarry]

bors r+

[bors]

Build succeeded

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