Delegate focus methods from SkiaLayer to backedLayer (#543)

* Delegate focus methods from SkiaLayer to backedLayer

Similar to the other methods

(see the comment "We need to delegate all event listeners..." why we do that)

* Add a comment

skiko/src/awtMain/kotlin/org/jetbrains/skiko/SkiaLayer.awt.kt

 package org.jetbrains.skiko
 
 import org.jetbrains.skia.*
+import org.jetbrains.skia.Canvas
 import org.jetbrains.skiko.redrawer.Redrawer
+import java.awt.*
 import java.awt.Color
-import java.awt.Component
-import java.awt.Graphics
 import java.awt.event.*
 import java.awt.im.InputMethodRequests
-import java.awt.Window
 import java.util.concurrent.CancellationException
 import javax.accessibility.Accessible
 import javax.swing.JComponent
@@ -83,6 +82,22 @@ actual open class SkiaLayer internal constructor(
             override fun getInputMethodRequests(): InputMethodRequests? {
                 return this@SkiaLayer.inputMethodRequests
             }
+
+            // check isRequestFocusEnabled manually, because it isn't implemented for Canvas.
+            // But it should be implemented, how it is implemented for JComponent.
+            // See Component.setRequestFocusEnabled description.
+            override fun requestFocus(cause: FocusEvent.Cause?) {
+                if (canReceiveFocus(cause)) {
+                    super.requestFocus(cause)
+                }
+            }
+
+            override fun requestFocusInWindow(cause: FocusEvent.Cause?): Boolean {
+                return canReceiveFocus(cause) && super.requestFocusInWindow(cause)
+            }
+
+            private fun canReceiveFocus(cause: FocusEvent.Cause?) = cause != FocusEvent.Cause.MOUSE_EVENT ||
+                    isRequestFocusEnabled
         }
         @Suppress("LeakingThis")
         add(backedLayer)
@@ -366,6 +381,30 @@ actual open class SkiaLayer internal constructor(
         backedLayer.doProcessInputMethodEvent(e)
     }
 
+    override fun addFocusListener(l: FocusListener?) {
+        backedLayer.addFocusListener(l)
+    }
+
+    override fun removeFocusListener(l: FocusListener?) {
+        backedLayer.removeFocusListener(l)
+    }
+
+    override fun setFocusable(focusable: Boolean) {
+        backedLayer.isFocusable = focusable
+    }
+
+    override fun isFocusable(): Boolean {
+        return backedLayer.isFocusable
+    }
+
+    override fun hasFocus(): Boolean {
+        return backedLayer.hasFocus()
+    }
+
+    override fun isFocusOwner(): Boolean {
+        return backedLayer.isFocusOwner
+    }
+
     override fun requestFocus() {
         backedLayer.requestFocus()
     }
@@ -374,6 +413,22 @@ actual open class SkiaLayer internal constructor(
         backedLayer.requestFocus(cause)
     }
 
+    override fun requestFocusInWindow(): Boolean {
+        return backedLayer.requestFocusInWindow()
+    }
+
+    override fun requestFocusInWindow(cause: FocusEvent.Cause?): Boolean {
+        return backedLayer.requestFocusInWindow(cause)
+    }
+
+    override fun setFocusTraversalKeysEnabled(focusTraversalKeysEnabled: Boolean) {
+        backedLayer.focusTraversalKeysEnabled = focusTraversalKeysEnabled
+    }
+
+    override fun getFocusTraversalKeysEnabled(): Boolean {
+        return backedLayer.focusTraversalKeysEnabled
+    }
+
     override fun addInputMethodListener(l: InputMethodListener) {
         super.addInputMethodListener(l)
         backedLayer.addInputMethodListener(l)
@@ -416,10 +471,6 @@ actual open class SkiaLayer internal constructor(
         backedLayer.removeKeyListener(l)
     }
 
-    override fun setFocusTraversalKeysEnabled(focusTraversalKeysEnabled: Boolean) {
-        backedLayer.focusTraversalKeysEnabled = focusTraversalKeysEnabled
-    }
-
     /**
      * Redraw on the next animation Frame (on vsync signal if vsync is enabled).
      */