Add info after scale bar has been drawn (#67)

Author: ppinard

README.md

@@ -108,6 +108,35 @@ ax.add_artist(scalebar)
 fig.savefig("original_resolution.png", dpi=dpi)
 ```
 
+### Information about the drawn scale bar
+
+After the scale bar has been drawn, the `info` property returns the following dataclass.
+
+```python
+@dataclasses.dataclass
+class ScaleBarInfo:
+    length_px: int
+    value: float
+    units: str
+    scale_text: str
+    window_extent: matplotlib.transforms.Bbox
+```
+
+Note that the `info` property returns a `ValueError` exception if the scale bar has not been drawn.
+
+```python
+fig, ax = plt.subplots()
+
+scalebar = ScaleBar(0.08, "cm", length_fraction=0.25)
+ax.add_artist(scalebar)
+
+print(scalebar.info) # raises a ValueError exception
+
+fig.canvas.draw()
+
+print(scalebar.info) # works
+```
+
 ## ScaleBar arguments
 
 Here are arguments of the **ScaleBar** class constructor and examples how to use them.

matplotlib_scalebar/scalebar.py

@@ -40,6 +40,7 @@
 # Standard library modules.
 import bisect
 import warnings
+import dataclasses
 
 # Third party modules.
 import matplotlib
@@ -142,6 +143,15 @@ def _validate_legend_loc(loc):
 }
 
 
+@dataclasses.dataclass
+class ScaleBarInfo:
+    length_px: int
+    value: float
+    units: str
+    scale_text: str
+    window_extent: matplotlib.transforms.Bbox
+
+
 class ScaleBar(Artist):
     zorder = 6
 
@@ -368,6 +378,7 @@ def __init__(
         self.rotation = rotation
         self.bbox_to_anchor = bbox_to_anchor
         self.bbox_transform = bbox_transform
+        self._info = None
 
     def _calculate_best_length(self, length_px):
         dx = self.dx
@@ -393,6 +404,8 @@ def _calculate_exact_length(self, value, units):
         return newvalue / self.dx
 
     def draw(self, renderer, *args, **kwargs):
+        self._info = None
+
         if not self.get_visible():
             return
         if self.dx == 0:
@@ -555,6 +568,10 @@ def _get_value(attr, default):
         box.patch.set_alpha(box_alpha)
         box.draw(renderer)
 
+        self._info = ScaleBarInfo(
+            length_px, value, units, scale_text, box.get_window_extent(renderer)
+        )
+
     def get_dx(self):
         return self._dx
 
@@ -841,3 +858,9 @@ def set_bbox_transform(self, bbox_transform):
         self._bbox_transform = bbox_transform
 
     bbox_transform = property(get_bbox_transform, set_bbox_transform)
+
+    @property
+    def info(self):
+        if self._info is None:
+            raise ValueError("Scale bar has not been drawn. Call figure.canvas.draw()")
+        return self._info

tests/test_scalebar.py

@@ -369,3 +369,32 @@ def test_bbox_transform(scalebar):
     scalebar.bbox_transform = scalebar.axes.transAxes
     assert scalebar.get_bbox_transform() == scalebar.axes.transAxes
     assert scalebar.bbox_transform == scalebar.axes.transAxes
+
+
+def test_info():
+    fig = plt.figure()
+    ax = fig.add_subplot(111)
+
+    data = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
+    ax.imshow(data)
+
+    scalebar = ScaleBar(0.5)
+    ax.add_artist(scalebar)
+
+    with pytest.raises(ValueError):
+        scalebar.info
+
+    plt.draw()
+
+    info = scalebar.info
+    assert info.length_px == pytest.approx(0.4, 1e-4)
+    assert info.value == pytest.approx(2, 1e-4)
+    assert info.units == "dm"
+    assert info.scale_text == "2 dm"
+    assert info.window_extent.x0 == pytest.approx(456.5755555555555, 1e-4)
+    assert info.window_extent.y0 == pytest.approx(390.81511111111104, 1e-4)
+    assert info.window_extent.x1 == pytest.approx(511.4111111111111, 1e-4)
+    assert info.window_extent.y1 == pytest.approx(421.01111111111106, 1e-4)
+
+    plt.close()
+    del fig