flowchart TB
    subgraph 多次获取
        A1["s1 = Singleton()"]
        A2["s2 = Singleton()"]
    end

    A1 -->|__new__| C{_instance 存在?}
    A2 -->|__new__| C

    C -->|否| N["创建新实例<br/>_instance = new"]
    C -->|是| R["返回 _instance"]

    N --> R
    R --> S["s1 is s2 = True<br/>✅ 同一个实例"]

flowchart LR
    subgraph 客户端
        C["notification =<br/>Factory.create('email')"]
    end

    subgraph 工厂
        F["NotificationFactory"]
        F --> D{"type = ?"}
    end

    subgraph 产品
        D -->|email| E["EmailNotification()"]
        D -->|sms| S["SMSNotification()"]
        D -->|push| P["PushNotification()"]
    end

    C --> F
    E --> R["notification.send('Hi!')"]
    S --> R
    P --> R

classDiagram
    class Application {
        <<abstract>>
        +new_document() str
        +create_document() Document*
    }

    class PDFApplication {
        +create_document() PDFDocument
    }

    class WordApplication {
        +create_document() WordDocument
    }

    class Document {
        <<interface>>
        +open() str
        +save() str
    }

    class PDFDocument {
        +open() str
        +save() str
    }

    class WordDocument {
        +open() str
        +save() str
    }

    Application <|-- PDFApplication : 继承
    Application <|-- WordApplication : 继承
    Document <|.. PDFDocument : 实现
    Document <|.. WordDocument : 实现
    PDFApplication --> PDFDocument : 创建
    WordApplication --> WordDocument : 创建

flowchart TB
    subgraph 建造过程
        B["Builder"] --> S1["size(12)"]
        S1 --> S2["crust('薄脆')"]
        S2 --> S3["sauce('番茄')"]
        S3 --> S4["cheese('马苏里拉')"]
        S4 --> S5["add_topping('培根')"]
        S5 --> S6["well_done()"]
        S6 --> S7["build() → Pizza"]
    end

    subgraph Director（可选）
        D["PizzaChef"] --> T1["make_margherita()"]
        D --> T2["make_pepperoni()"]
        T1 -->|使用 Builder| P1["玛格丽特披萨"]
        T2 -->|使用 Builder| P2["辣香肠披萨"]
    end

    S7 --> P["🍕 定制披萨"]

flowchart LR
    subgraph 原型
        P["基础合同模板"]
        P --> V1["变量: {title, party_a, ...}"]
    end

    subgraph 克隆
        C1["service = base.clone()"]
        C2["nba = base.clone()"]
    end

    subgraph 定制
        C1 --> M1["修改变量: 服务合同"]
        C2 --> M2["修改变量: 球员合同"]
    end

    P --> C1
    P --> C2

flowchart TB
    subgraph 连接池
        POOL["ConnectionPool"]
        IDLE["空闲连接池: [Conn#1, Conn#2]"]
        IN_USE["使用中: {Conn#3}"]
        MAX["最大连接数: 5"]
    end

    T1["线程 1"] -->|acquire| POOL
    T2["线程 2"] -->|acquire| POOL
    POOL -->|复用空闲| T1
    POOL -->|创建新连接| T2

    T1 -->|release| POOL
    POOL -->|归还到空闲池| IDLE

classDiagram
    class ConfigManager {
        -_config: dict
        -_frozen: bool
        -_readonly_keys: set
        -_listeners: list
        -_history: list
        +get(key, default) any
        +set(key, value) ConfigManager
        +freeze() ConfigManager
        +on_change(cb) ConfigManager
        +to_json() str
        +stats: dict
    }

    class ConfigBuilder {
        -_data: dict
        +set(key, value) ConfigBuilder
        +set_database() ConfigBuilder
        +set_redis() ConfigBuilder
        +set_app() ConfigBuilder
        +build() ConfigManager
        +build_and_freeze() ConfigManager
    }

    class ConfigFactory {
        +from_json_file(path) ConfigManager
        +from_json_str(str) ConfigManager
        +from_dict(dict) ConfigManager
        +from_defaults() ConfigManager
    }

    ConfigManager <.. ConfigBuilder : 建造
    ConfigManager <.. ConfigFactory : 工厂创建