feat(doc): add doc about how to use popover

frontend/app_flowy/packages/appflowy_popover/README.md

@@ -1,38 +1,107 @@
-<!--
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-this README's contents appear on the landing page for your package.
-
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-[creating packages](https://dart.dev/guides/libraries/create-library-packages)
-and the Flutter guide for
-[developing packages and plugins](https://flutter.dev/developing-packages).
--->
-
 # AppFlowy Popover
 
 A Popover can be used to display some content on top of another.
 
-## Features
+It can be used to display a dropdown menu.
 
 > A popover is a transient view that appears above other content onscreen when you tap a control or in an area. Typically, a popover includes an arrow pointing to the location from which it emerged. Popovers can be nonmodal or modal. A nonmodal popover is dismissed by tapping another part of the screen or a button on the popover. A modal popover is dismissed by tapping a Cancel or other button on the popover.
 
 Source: [Human Interface Guidelines](https://developer.apple.com/design/human-interface-guidelines/ios/views/popovers/).
 
+## Features
+
 - Basic popover style
+- Follow the target automatically
 - Nested popover support
-- Exclusive popover API
+- Exclusive API
+
+![](./screenshot.png)
 
 ## Example
 
 ```dart
 Popover(
+  // Define how to trigger the popover
   triggerActions: PopoverTriggerActionFlags.click,
   child: TextButton(child: Text("Popover"), onPressed: () {}),
+  // Define the direction of the popover
+  direction: PopoverDirection.bottomWithLeftAligned,
   popupBuilder(BuildContext context) {
     return PopoverMenu();
   },
 );
 ```
+
+### Trigger the popover manually
+
+Sometimes, if you want to trigger the popover manually, you can use a `PopoverController`.
+
+```dart
+class MyWidgetState extends State<GridDateCell> {
+  late PopoverController _popover;
+
+  @override
+  void initState() {
+    _popover = PopoverController();
+    super.initState();
+  }
+
+  // triggered by another widget
+  _onClick() {
+    _popover.show();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Popover(
+      controller: _popover,
+      ...
+    )
+  }
+}
+```
+
+### Make several popovers exclusive
+
+The popover has a mechanism to make sure there are only one popover is shown in a group of popovers.
+It's called `PopoverMutex`.
+
+If you pass the same mutex object to the popovers, there will be only one popover is triggered.
+
+```dart
+class MyWidgetState extends State<GridDateCell> {
+  final _popoverMutex = PopoverMutex();
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      children: [
+        Popover(
+          mutex: _popoverMutex,
+          ...
+        ),
+        Popover(
+          mutex: _popoverMutex,
+          ...
+        ),
+        Popover(
+          mutex: _popoverMutex,
+          ...
+        ),
+      ]
+    )
+  }
+}
+```
+
+## API
+
+| Param          | Description                                                      | Type                                    |
+| -------------- | ---------------------------------------------------------------- | --------------------------------------- |
+| offset         | The offset between the popover and the child                     | `Offset`                                |
+| popupBuilder   | The function used to build the popover                           | `Widget Function(BuildContext context)` |
+| triggerActions | Define the actions about how to trigger the popover              | `int`                                   |
+| mutex          | If multiple popovers are exclusive, pass the same mutex to them. | `PopoverMutex`                          |
+| direction      | The direction where the popover should be placed                 | `PopoverDirection`                      |
+| onClose        | The callback will be called after the popover is closed          | `void Function()`                       |
+| child          | The child to trigger the popover                                 | `Widget`                                |

frontend/app_flowy/packages/appflowy_popover/example/lib/example_button.dart

@@ -13,15 +13,36 @@ class _PopoverMenuState extends State<PopoverMenu> {
 
   @override
   Widget build(BuildContext context) {
-    return Container(
+    return Material(
+        type: MaterialType.transparency,
+        child: Container(
           width: 200,
           height: 200,
-      decoration: const BoxDecoration(color: Colors.yellow),
+          decoration: BoxDecoration(
+            color: Colors.white,
+            borderRadius: const BorderRadius.all(Radius.circular(8)),
+            boxShadow: [
+              BoxShadow(
+                color: Colors.grey.withOpacity(0.5),
+                spreadRadius: 5,
+                blurRadius: 7,
+                offset: const Offset(0, 3), // changes position of shadow
+              ),
+            ],
+          ),
           child: ListView(children: [
-        const Text("App"),
+            Container(
+              margin: const EdgeInsets.all(8),
+              child: const Text("Popover",
+                  style: TextStyle(
+                      fontSize: 14,
+                      color: Colors.black,
+                      fontStyle: null,
+                      decoration: null)),
+            ),
             Popover(
-          triggerActions:
+              triggerActions: PopoverTriggerActionFlags.hover |
-              PopoverTriggerActionFlags.hover | PopoverTriggerActionFlags.click,
+                  PopoverTriggerActionFlags.click,
               mutex: popOverMutex,
               offset: const Offset(10, 0),
               popupBuilder: (BuildContext context) {
@@ -33,8 +54,8 @@ class _PopoverMenuState extends State<PopoverMenu> {
               ),
             ),
             Popover(
-          triggerActions:
+              triggerActions: PopoverTriggerActionFlags.hover |
-              PopoverTriggerActionFlags.hover | PopoverTriggerActionFlags.click,
+                  PopoverTriggerActionFlags.click,
               mutex: popOverMutex,
               offset: const Offset(10, 0),
               popupBuilder: (BuildContext context) {
@@ -46,7 +67,7 @@ class _PopoverMenuState extends State<PopoverMenu> {
               ),
             ),
           ]),
-    );
+        ));
   }
 }