add options feature on apps package and add a SetHost option to force host on multiple subdomains that should mean the same exact application, so any handler should run based on the same Host field

apps/switch_options.go

@@ -0,0 +1,76 @@
+package apps
+
+import "net/http"
+
+type (
+	// SwitchOptions holds configuration
+	// for the switcher application.
+	SwitchOptions struct {
+		// RequestModifiers holds functions to run
+		// if and only if at least one Filter passed.
+		// They are used to modify the request object
+		// of the matched Application, e.g. modify the host.
+		//
+		// See `SetHost` option too.
+		RequestModifiers []func(*http.Request)
+		// Note(@kataras): I though a lot of API designs for that one and the current is the safest to use.
+		// I skipped the idea of returning a wrapped Application to have functions like app.UseFilter
+		// or the idea of accepting a chain of Iris Handlers here because the Context belongs
+		// to the switcher application and a new one is acquired on the matched Application level,
+		// so communication between them is not possible although
+		// we can make it possible but lets not complicate the code here, unless otherwise requested.
+	}
+
+	// SwitchOption should be implemented by all options
+	// passed to the `Switch` package-level last variadic input argument.
+	SwitchOption interface {
+		Apply(*SwitchOptions)
+	}
+
+	// SwitchOptionFunc provides a functional way to pass options
+	// to the `Switch` package-level function's last variadic input argument.
+	SwitchOptionFunc func(*SwitchOptions)
+)
+
+// Apply completes the `SwitchOption` interface.
+func (f SwitchOptionFunc) Apply(opts *SwitchOptions) {
+	f(opts)
+}
+
+// DefaultSwitchOptions returns a fresh SwitchOptions
+// struct value with its fields set to their defaults.
+func DefaultSwitchOptions() SwitchOptions {
+	return SwitchOptions{
+		RequestModifiers: nil,
+	}
+}
+
+// Apply completes the `SwitchOption` interface.
+// It does copies values from "o" to "opts" when necessary.
+func (o SwitchOptions) Apply(opts *SwitchOptions) {
+	if v := o.RequestModifiers; len(v) > 0 {
+		opts.RequestModifiers = v // override, not append.
+	}
+}
+
+// SetHost is a SwitchOption.
+// It force sets a Host field for the matched Application's request object.
+// Extremely useful when used with Hosts SwitchProvider.
+// Usecase: www. to root domain without redirection (SEO reasons)
+// and keep the same internal request Host for both of them so
+// the root app's handlers will always work with a single host no matter
+// what the real request Host was.
+func SetHost(hostField string) SwitchOptionFunc {
+	if hostField == "" {
+		return nil
+	}
+
+	setHost := func(r *http.Request) {
+		r.Host = hostField
+		r.URL.Host = hostField // note: the URL.String builds the uri based on that.
+	}
+
+	return func(opts *SwitchOptions) {
+		opts.RequestModifiers = append(opts.RequestModifiers, setHost)
+	}
+}