Add lazy props feature

[reinink]

This PR implements the new "lazy props" feature, as discussed here: inertiajs/inertia#287

This feature allows you to define optional (lazy) properties that are not included on the initial visit to a page. These properties are only ever included when using partial reloads (the only option).

You mark a prop as lazy using the new Inertia::lazy() method, which behind the scenes wraps the prop in a new LazyProp class:

return Inertia::render('Users/Index', [
    'users' => User::paginate('id', 'name', 'email'),
    'companies' => Inertia::lazy(fn () => Company::get('id', 'name')),
]);

Then, to load this prop later on, you perform a partial reload from your page component:

<inertia-link class="hover:underline" href="/users" :only="['companies']">Load companies</inertia-link>

Or programatically:

Inertia.reload({ only: ['companies'] })

[gajosadrian]

What's the difference between Lazily evaluated and Lazy prop?

Lazily evaluated

return Inertia::render('Users/Index', [
    // Lazily evaluated (only run if required)
    'companies' => function () {
        return Company::orderBy('name')
            ->get('id', 'name');
    },
    // Evaluated immediately
    'users' => User::orderBy('name')
        ->select('id', 'name')
        ->paginate(),
]);

Lazy prop

return Inertia::render('Users/Index', [
    'users' => User::paginate('id', 'name', 'email'),
    'companies' => Inertia::lazy(fn () => Company::get('id', 'name')),
]);

[pkboom]

What's the difference between Lazily evaluated and Lazy prop?

Lazily evaluated

return Inertia::render('Users/Index', [
    // Lazily evaluated (only run if required)
    'companies' => function () {
        return Company::orderBy('name')
            ->get('id', 'name');
    },
    // Evaluated immediately
    'users' => User::orderBy('name')
        ->select('id', 'name')
        ->paginate(),
]);

Lazy prop

return Inertia::render('Users/Index', [
    'users' => User::paginate('id', 'name', 'email'),
    'companies' => Inertia::lazy(fn () => Company::get('id', 'name')),
]);

The difference is 'This feature allows you to define optional (lazy) properties that are not included on the initial visit to a page.'

[gajosadrian]

And the first one also Lazily evaluated (only run if required). So I assume that Lazily evaluated is always required on initial visit.

[reinink]

@gajosadrian It works like this:

return Inertia::render('Users/Index', [
    // ALWAYS included on first load
    // OPTIONALLY included on partial reloads
    // ALWAYS evaluated
    'users' => User::get(),

    // ALWAYS included on first load
    // OPTIONALLY included on partial reloads
    // ONLY evaluated when needed
    'users' => fn () => User::get(),

    // NEVER included on first load
    // OPTIONALLY included on partial reloads
    // ONLY evaluated when needed
    'users' => Inertia::lazy(fn () => User::get()),
]);