Add Window.takeScreenshot API (#225)

* Integrate Window.takeScreenshot into examples

* Add Window.takeScreenshot

* Change screen capture example

* Fix formatting

examples/Examples.re

@@ -40,6 +40,7 @@ let state: state = {
     {name: "Input", render: w => InputExample.render(w)},
     {name: "Radio Button", render: _ => RadioButtonExample.render()},
     {name: "Game Of Life", render: _ => GameOfLife.render()},
+    {name: "Screen Capture", render: w => ScreenCapture.render(w)},
     {name: "Tree View", render: w => TreeView.render(w)},
   ],
   selectedExample: "Animation",
@@ -95,10 +96,9 @@ type action =
   | SelectExample(string);
 
 let reducer = (s: state, a: action) => {
-  let SelectExample(name) = a;
-
-  let ret: state = {...s, selectedExample: name};
-  ret;
+  switch (a) {
+  | SelectExample(name) => {...s, selectedExample: name}
+  };
 };
 
 let init = app => {

examples/ScreenCapture.re

@@ -0,0 +1,78 @@
+open Revery.UI;
+open Revery.UI.Components;
+open Revery.Core;
+
+let backgroundColor = Color.hex("#212733");
+let activeBackgroundColor = Color.hex("#2E3440");
+let inactiveBackgroundColor = Color.hex("#272d39");
+let selectionHighlight = Color.hex("#90f7ff");
+
+module ActionButton = {
+  let component = React.component("ActionButton");
+
+  let make = (~name, ~onClick, ()) =>
+    component((_slots: React.Hooks.empty) => {
+      let wrapperStyle =
+        Style.[
+          backgroundColor(selectionHighlight),
+          border(~width=4, ~color=activeBackgroundColor),
+        ];
+      let textHeaderStyle =
+        Style.[
+          color(Colors.black),
+          fontFamily("Roboto-Regular.ttf"),
+          fontSize(14),
+          margin(16),
+        ];
+      <Clickable style=wrapperStyle onClick>
+        <Text style=textHeaderStyle text=name />
+      </Clickable>;
+    });
+
+  let createElement = (~children as _, ~name, ~onClick, ()) =>
+    React.element(make(~name, ~onClick, ()));
+};
+
+module CaptureArea = {
+  let component = React.component("Capture Area");
+
+  let make = w =>
+    component(slots => {
+      let (count, setCount, slots) = React.Hooks.state(0, slots);
+      let (file, setFile, _slots: React.Hooks.empty) =
+        React.Hooks.state(None, slots);
+
+      let capture = () => {
+        let exed = Environment.getExecutingDirectory();
+        let filename = Printf.sprintf("Scrot_%d.tga", count);
+        let fullname = exed ++ filename;
+        Window.takeScreenshot(w, fullname);
+        setCount(count + 1);
+        setFile(Some(filename));
+      };
+
+      let viewStyle =
+        Style.[
+          position(`Absolute),
+          left(0),
+          right(0),
+          top(0),
+          bottom(0),
+          flexDirection(`Column),
+        ];
+
+      let imageStyle = Style.[width(400), height(300)];
+
+      <View style=viewStyle>
+        <ActionButton name="Take a screenshot!" onClick=capture />
+        {switch (file) {
+         | None => <View />
+         | Some(src) => <Image style=imageStyle src />
+         }}
+      </View>;
+    });
+
+  let createElement = (~w, ~children as _, ()) => React.element(make(w));
+};
+
+let render = w => <CaptureArea w />;

src/Core/Window.re

@@ -323,6 +323,26 @@ let getDevicePixelRatio = (w: t) => {
   /. float_of_int(windowSizeInScreenCoordinates.width);
 };
 
+let takeScreenshot = (w: t, filename: string) => {
+  open Glfw;
+
+  let width = w.framebufferWidth;
+  let height = w.framebufferHeight;
+
+  let image = Image.create(~width, ~height, ~numChannels=4, ~channelSize=1);
+  let buffer = Image.getBuffer(image);
+
+  /* WebGL is weird in that we can't capture with glReadPixels during
+     a render operation. Instead, we want to wait till it's over (we
+     can force this by triggering a new render) and then taking the
+     screenshot */
+  render(w);
+  glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
+
+  Image.save(image, filename);
+  Image.destroy(image);
+};
+
 let destroyWindow = (w: t) => Glfw.glfwDestroyWindow(w.glfwWindow);
 
 let shouldClose = (w: t) => Glfw.glfwWindowShouldClose(w.glfwWindow);