Widget demo example script (#36)

* Add demo of all available widgets

* Add file picker widget to widget_demo.py

examples/widget_demo.py

@@ -0,0 +1,43 @@
+"""Widget demonstration of magicgui."""
+
+from enum import Enum
+from datetime import datetime
+import math
+from pathlib import Path
+
+from magicgui import event_loop, magicgui
+
+
+class Medium(Enum):
+    """Enum for various media and their refractive indices."""
+
+    Glass = 1.520
+    Oil = 1.515
+    Water = 1.333
+    Air = 1.0003
+
+
+@magicgui(call_button="Calculate", result={"disabled": True, "fixedWidth": 100})
+def widget_demo(
+    boolean=True,
+    integer=1,
+    float=3.14159,
+    string="Text goes here",
+    dropdown=Medium.Glass,
+    datetime=datetime.now(),
+    filename=Path.home()
+):
+    """Run some computation."""
+    print("Running some computation...")
+    return "Result!"
+
+
+with event_loop():
+    # the original function still works
+    # we can create a gui
+    gui = widget_demo.Gui(show=True)
+    # we can list for changes to parameters in the gui
+    # ... these also trigger for direct parameter access on the gui object
+    gui.string_changed.connect(print)
+    # we can connect a callback function to the __call__ event on the function
+    gui.called.connect(lambda x: gui.set_widget("result", str(x), position=-1))