examples/calculator: intro the calculator abstract UI

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --git a/examples/calculator/src/calculator.nit b/examples/calculator/src/calculator.nit
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+# This file is part of NIT ( http://www.nitlanguage.org ).
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Portable calculator UI
+module calculator is
+app_name "app.nit Calc."
+       app_version(0, 1, git_revision)
+       app_namespace "org.nitlanguage.calculator"
+
+       # Lock in portrait mode
+       android_manifest_activity """android:screenOrientation="portrait""""
+       android_api_target 15
+end
+
+import app::ui
+import android::aware
+
+import calculator_logic
+
+redef class App
+       redef fun on_create
+       do
+               # Create the main window
+               window = new CalculatorWindow
+               super
+       end
+end
+
+# The main (and only) window of this calculator
+class CalculatorWindow
+       super Window
+
+       # Calculator context, our business logic
+       private var context = new CalculatorContext
+
+       # Main window layout
+       private var layout = new VerticalLayout(parent=self)
+
+       # Main display, at the top of the screen
+       private var display = new TextInput(parent=layout)
+
+       # Maps operators as `String` to their `Button`
+       private var buttons = new HashMap[String, Button]
+
+       init
+       do
+               # All the button labels, row by row
+               var rows = [["7", "8", "9", "+"],
+                           ["4", "5", "6", "-"],
+                           ["1", "2", "3", "*"],
+                           ["0", ".", "C", "/"],
+                           ["="]]
+
+               for row in rows do
+                       var row_layout = new HorizontalLayout(parent=layout)
+
+                       for op in row do
+                               var but = new Button(parent=row_layout, text=op)
+                               but.observers.add self
+                               buttons[op] = but
+                       end
+               end
+       end
+
+       redef fun on_event(event)
+       do
+               if event isa ButtonPressEvent then
+
+                       var op = event.sender.text
+                       if op == "." then
+                               event.sender.enabled = false
+                               context.switch_to_decimals
+                       else if op.is_numeric then
+                               var n = op.to_i
+                               context.push_digit n
+                       else
+                               buttons["."].enabled = true
+                               context.push_op op.chars.first
+                       end
+
+                       display.text = context.display_text
+               end
+       end
+end