Component decorators (#354)

Add decorator variants for decorator components

Add the "pipe" operator for components, similar to what was done for Elements.
We are able to put something like:
```
Button(...) | Maybe(&show_button)
```

Add decorators for:
- `Maybe`
- `CatchEvent`
- `Renderer`

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

include/ftxui/component/component.hpp

@@ -4,6 +4,7 @@
 #include <functional>  // for function
 #include <memory>      // for make_shared, shared_ptr
 #include <string>      // for wstring
+#include <utility>     // for forward
 #include <vector>      // for vector
 
 #include "ftxui/component/component_base.hpp"     // for Component, Components
@@ -26,6 +27,14 @@ std::shared_ptr<T> Make(Args&&... args) {
   return std::make_shared<T>(std::forward<Args>(args)...);
 }
 
+// Pipe operator to decorate components.
+using ComponentDecorator = std::function<Component(Component)>;
+using ElementDecorator = std::function<Element(Element)>;
+Component operator|(Component component, ComponentDecorator decorator);
+Component operator|(Component component, ElementDecorator decorator);
+Component& operator|=(Component& component, ComponentDecorator decorator);
+Component& operator|=(Component& component, ElementDecorator decorator);
+
 namespace Container {
 Component Vertical(Components children);
 Component Vertical(Components children, int* selector);
@@ -38,38 +47,55 @@ Component Tab(Components children, int* selector);
 Component Button(ConstStringRef label,
                  std::function<void()> on_click,
                  Ref<ButtonOption> = {});
+
 Component Checkbox(ConstStringRef label,
                    bool* checked,
                    Ref<CheckboxOption> option = {});
+
 Component Input(StringRef content,
                 ConstStringRef placeholder,
                 Ref<InputOption> option = {});
+
 Component Menu(ConstStringListRef entries,
                int* selected_,
                Ref<MenuOption> = {});
+
 Component MenuEntry(ConstStringRef label, Ref<MenuEntryOption> = {});
+
 Component Dropdown(ConstStringListRef entries, int* selected);
+
 Component Radiobox(ConstStringListRef entries,
                    int* selected_,
                    Ref<RadioboxOption> option = {});
+
 Component Toggle(ConstStringListRef entries,
                  int* selected,
                  Ref<ToggleOption> option = {});
+
 template <class T>  // T = {int, float, long}
 Component Slider(ConstStringRef label, T* value, T min, T max, T increment);
+
 Component ResizableSplitLeft(Component main, Component back, int* main_size);
 Component ResizableSplitRight(Component main, Component back, int* main_size);
 Component ResizableSplitTop(Component main, Component back, int* main_size);
 Component ResizableSplitBottom(Component main, Component back, int* main_size);
+
 Component Renderer(Component child, std::function<Element()>);
 Component Renderer(std::function<Element()>);
 Component Renderer(std::function<Element(bool /* focused */)>);
+ComponentDecorator Renderer(ElementDecorator);
+
 Component CatchEvent(Component child, std::function<bool(Event)>);
+ComponentDecorator CatchEvent(std::function<bool(Event)> on_event);
+
 Component Maybe(Component, const bool* show);
+Component Maybe(Component, std::function<bool()>);
+ComponentDecorator Maybe(const bool* show);
+ComponentDecorator Maybe(std::function<bool()>);
+
 Component Collapsible(ConstStringRef label,
                       Component child,
                       Ref<bool> show = false);
-
 }  // namespace ftxui
 
 // Include component using the old deprecated wstring.

include/ftxui/component/deprecated.hpp

@@ -5,9 +5,10 @@
 
 namespace ftxui {
 
-Component Input(WideStringRef content,
-                ConstStringRef placeholder,
-                Ref<InputOption> option = {});
+[[deprecated("use Input with normal std::string instead.")]] Component Input(
+    WideStringRef content,
+    ConstStringRef placeholder,
+    Ref<InputOption> option = {});
 }  // namespace ftxui
 
 #endif /* FTXUI_COMPONENT_DEPRECATED_HPP */

include/ftxui/dom/elements.hpp

@@ -28,6 +28,7 @@ enum class GaugeDirection { Left, Up, Right, Down };
 // -> text("ftxui") | bold | underlined
 // -> underlined(bold(text("FTXUI")))
 Element operator|(Element, Decorator);
+Element& operator|=(Element&, Decorator);
 Elements operator|(Elements, Decorator);
 Decorator operator|(Decorator, Decorator);