Add support for macros when using the multi command

docs/multi.md

@@ -43,6 +43,55 @@ To take a screenshot of just the area of a page defined by a CSS selector, add `
   selector: "#bighead"
 ```
 
+Use `--macro` to run JavaScript before every shot is taken:
+
+    shot-scraper multi shots.yml --macro macros.yaml
+
+Macros can be a `script` or a `function`:
+
+* `script` is executed immediately
+* `function` exposes a function that can be called in your `javascript` block:
+
+Sample `macros.yaml`:
+
+```yaml
+- type: script
+  javascript: |
+    function (){
+        console.log("This runs before the shot is taken")
+    }
+
+- type: function
+  name: sayHello
+  javascript: |
+    function (){
+        console.log("Hello World")
+    }
+
+```
+
+Sample `shots.yaml`:
+
+```yaml
+- output: demo.png
+  url: https://example.com
+  width: 1920
+  height: 1080
+  javascript: |
+    function(){
+      console.log("Taking a shot");
+      sayHello();
+    }
+```
+
+Expected output (if using `--log-console`):
+
+```
+This runs before the shot is taken
+Taking a shot
+Hello World
+```
+
 You can pass more than one selector using a `selectors:` list. You can also use `padding:` to specify additional padding:
 
 ```yaml

shot_scraper/cli.py

@@ -389,6 +389,11 @@ def _browser_context(
     help="Just take shots matching these output files",
     multiple=True,
 )
+@click.option(
+    "--macro",
+    type=click.File("r"),
+    help="Apply the macros contained in this file to all shots",
+)
 @browser_option
 @user_agent_option
 @reduced_motion_option
@@ -403,6 +408,7 @@ def multi(
     fail_on_error,
     noclobber,
     outputs,
+    macro,
     browser,
     user_agent,
     reduced_motion,
@@ -426,6 +432,14 @@ def multi(
 
     https://shot-scraper.datasette.io/en/stable/multi.html
     """
+    # Load macros config
+    macroConfig = []
+    if macro:
+        macroConfig = yaml.safe_load(macro)
+    if not isinstance(macroConfig, list):
+        raise click.ClickException("Macro file must contain a list")
+
+    # Load shots config
     shots = yaml.safe_load(config)
     if shots is None:
         shots = []
@@ -451,6 +465,7 @@ def multi(
             if outputs and shot.get("output") not in outputs:
                 continue
             try:
+                shot['macros'] = macroConfig
                 take_shot(
                     context,
                     shot,
@@ -1004,6 +1019,17 @@ def on_response(response):
     if wait:
         time.sleep(wait / 1000)
 
+    macros = shot.get("macros")
+    if macros:
+        for macro in macros:
+            if macro['type'] == 'script':
+                _evaluate_js(page, macro['javascript'])
+            elif macro['type'] == 'function':
+                _evaluate_js(page, "function(){{ window.{} = {} }}".format(macro['name'], macro['javascript']))
+            else:
+                raise click.ClickException("Invalid macro type: '{}'".format(macro['type']))
+
+
     javascript = shot.get("javascript")
     if javascript:
         _evaluate_js(page, javascript)