docs: add RequirmentSpec.md

docs/RequirementSpec.md

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+# The Spec
+
+- [Requirement and Spec](#requirement-and-spec)
+  - [Definitions](#definitions)
+- [Use Cases](#use-cases)
+  - [A.1 Creating a UML Object](#a1-creating-a-uml-object)
+  - [B.1 Creating a UML Connection Line](#b1-creating-a-uml-connection-line)
+  - [C.1 Select/Unselect a Single Object](#c1-selectunselect-a-single-object)
+  - [D.1 Group Objects](#d1-group-objects)
+  - [D.2 UnGroup Objects](#d2-ungroup-objects)
+  - [E.1 Move Objects](#e1-move-objects)
+  - [F.1 Change Object Name](#f1-change-object-name)
+
+This requirement document is also the basic functionality that your project must accomplish by the end of this course—a UML editor.
+
+The UML editor is divided into three parts:
+
+- The first part is the editing area known as the Canvas area.
+- To the left of the editing area is a row of buttons. You must complete at least the following six buttons as shown in the diagram:
+  - `select`
+  - `association`
+  - `generalization`
+  - `composition`
+  - `class`
+  - `use case`
+
+Besides the `select` button, all other buttons are used to create a UML object within the editing area. This UML editor should be used similarly to the general drawing software you are familiar with, although slight operational differences (for more user-friendliness) may lead to complex implementation differences. The specification is written specifically to reduce implementation confusion; please follow it to complete the described functionalities.
+
+## Requirement and Spec
+
+- **Use Case Format**: The requirement document uses UML's Use Case to describe system requirements and specifications.
+
+### Definitions
+
+- **Basic Object**: Such as class or use case objects.
+- **Connection Object**: Such as various association lines.
+- **Composite Object**: A composite object is made up of multiple basic objects combined using a group function. A composite object is a tree-like container, meaning it can contain other composite objects. The scope of a composite object is defined as the minimum square area that completely contains all its constituent objects.
+- **Object Depth (0-99)**: Each object has a depth value relative to other objects. An object with a lower depth value should cover other objects and first receive and intercept mouse events falling on it. When two objects overlap, the top-most object receives the mouse event.
+
+## Use Cases
+
+### A.1 Creating a UML Object
+
+- **Precondition**: This use case applies to the `class` and `use case` buttons.
+- **Steps**:
+  1. After pressing the button, its color turns black to inform the user of the current object creation mode.
+  2. The user moves the cursor to the editing area.
+  3. At coordinates (x,y), the user presses the left mouse button to create the selected object at the top left corner (a blank object is drawn in the editing area).
+  4. The user can repeatedly perform steps 2 and 3 to continue creating the same object in the editing area until the mode is changed.
+- **Alternatives**:
+  1. If the user presses another button, the mode switches to that of the other button.
+
+### B.1 Creating a UML Connection Line
+
+- **Precondition**: This use case applies to `association line`, `generalization line`, and `composition line`.
+- **Steps**:
+  1. The user presses the left mouse button within the boundary of a class or use case object in the editing area but does not release it (mouse pressed).
+  2. The user drags without releasing the left button.
+  3. The user releases the mouse button over another class or use case object area (mouse released).
+  4. A line object is created in the editing area, connecting the two objects. Depending on the type of connection line, various arrows are drawn at the endpoint object.
+- **Alternatives**:
+  1. If the mouse-pressed coordinates are not on any class or use case object, then from mouse pressed to mouse drag to mouse released, no action occurs.
+  2. If the mouse-released coordinates are not on any class or use case object, then no connection line object is created.
+
+### C.1 Select/Unselect a Single Object
+
+- **Precondition**: This use case applies when the `select` button is pressed.
+- **Steps**:
+  1. The user clicks on a basic object.
+  2. If other objects are in a selected state, their selected state is canceled.
+  3. The four connection ports of this basic object are explicitly displayed.
+- **Alternatives**:
+  1. If the user clicks on coordinates not inside any basic object, then perform steps 2 as described above.
+
+### D.1 Group Objects
+
+- **Precondition**: More than one basic object is selected.
+- **Steps**:
+  1. The user goes to the Edit Menu and selects the "Group" option.
+  2. The selected basic objects are merged into one composite object.
+
+### D.2 UnGroup Objects
+
+- **Precondition**: Only one composite object is selected.
+- **Steps**:
+  1. The user goes to the Edit Menu and selects the "UnGroup" option.
+  2. The composite object is decomposed one layer.
+
+### E.1 Move Objects
+
+- **Precondition**: The 'select' button is pressed.
+- **Steps**:
+  1. The user presses and holds the mouse left button over a basic object (including composite objects) within the editing area.
+  2. The user drags without releasing the left mouse button.
+  3. The user releases the mouse button at another coordinate (x, y).
+  4. The basic object is moved to the new coordinate (x, y).
+  5. All connection lines linked to this basic object are redrawn.
+- **Note**: If the (x, y) coordinates overlap with another object's area, the basic object at (x, y) will overlap other objects. The objects should be drawn according to their depth order.
+
+### F.1 Change Object Name
+
+- **Precondition**: A basic object is in a selected state.
+- **Steps**:
+  1. The user goes to the Edit Menu and selects the "Change Object Name" option.
+  2. A small window appears with a text area and OK and Cancel buttons.
+  3. The user can enter a new name in the text area and press OK to change the name of the basic object. The window then disappears.
+- **Alternatives**:
+  1. If the user presses the Cancel button, the window disappears and the name change is canceled.