Introduce tester feature flag

devtools/src/tester/null.rs

@@ -0,0 +1,51 @@
+use crate::Program;
+use crate::core::window;
+use crate::core::{Element, Theme};
+use crate::runtime::Task;
+use crate::widget::horizontal_space;
+
+use std::marker::PhantomData;
+
+pub struct Tester<P: Program> {
+    _type: PhantomData<P::Message>,
+}
+
+#[derive(Debug, Clone)]
+pub enum Message {}
+
+#[allow(missing_debug_implementations)]
+pub struct Tick<P: Program> {
+    _type: PhantomData<P::Message>,
+}
+
+impl<P: Program> Tester<P> {
+    pub fn new() -> Self {
+        Self { _type: PhantomData }
+    }
+
+    pub fn is_busy(&self) -> bool {
+        false
+    }
+
+    pub fn update(&mut self, _program: &P, _message: Message) -> Task<Tick<P>> {
+        Task::none()
+    }
+
+    pub fn tick(&mut self, _program: &P, _tick: Tick<P>) -> Task<Tick<P>> {
+        Task::none()
+    }
+
+    pub fn view<'a, T: 'static>(
+        &'a self,
+        _program: &P,
+        _window: window::Id,
+        _current: impl FnOnce() -> Element<'a, T, Theme, P::Renderer>,
+        _emulate: impl Fn(Tick<P>) -> T + 'a,
+    ) -> Element<'a, T, Theme, P::Renderer> {
+        horizontal_space().into()
+    }
+
+    pub fn controls(&self) -> Element<'_, Message, Theme, P::Renderer> {
+        horizontal_space().into()
+    }
+}