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package templ
import (
"context"
"io"
"slices"
)
// RenderFragments renders the specified fragments to w.
func RenderFragments(ctx context.Context, w io.Writer, c Component, ids ...any) error {
ctx = context.WithValue(ctx, fragmentContextKey, &FragmentContext{
W: w,
IDs: ids,
})
return c.Render(ctx, io.Discard)
}
type fragmentContextKeyType int
const fragmentContextKey fragmentContextKeyType = iota
// FragmentContext is used to control rendering of fragments within a template.
type FragmentContext struct {
W io.Writer
IDs []any
Active bool
}
// Fragment defines a fragment within a template that can be rendered conditionally based on the id.
// You can use it to render a specific part of a page, e.g. to reduce the amount of HTML returned from a HTMX-initiated request.
// Any non-matching contents of the template are rendered, but discarded by the FramentWriter.
func Fragment(id any) Component {
return &fragment{
ID: id,
}
}
type fragment struct {
ID any
}
func (f *fragment) Render(ctx context.Context, w io.Writer) (err error) {
// If not in a fragment context, if we're a child fragment, or in a mismatching fragment context, render children normally.
fragmentCtx := getFragmentContext(ctx)
if fragmentCtx == nil || fragmentCtx.Active || !slices.Contains(fragmentCtx.IDs, f.ID) {
return GetChildren(ctx).Render(ctx, w)
}
// Instruct child fragments to render their contents normally, because the writer
// passed to them is already the FragmentContext's writer.
fragmentCtx.Active = true
defer func() {
fragmentCtx.Active = false
}()
return GetChildren(ctx).Render(ctx, fragmentCtx.W)
}
// getFragmentContext retrieves the FragmentContext from the provided context. It returns nil if no
// FragmentContext is found or if the context value is of an unexpected type.
func getFragmentContext(ctx context.Context) *FragmentContext {
ctxValue := ctx.Value(fragmentContextKey)
if ctxValue == nil {
return nil
}
v, ok := ctxValue.(*FragmentContext)
if !ok {
return nil
}
return v
}
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