咩係Design patterns？佢到底有咩用？點解你要知Design patterns？

其實我地平時寫code果時好多時侯用緊既framework，Library或者Engine 都用左好多5同既Design patterns，你不知不覺間其實已經用緊佢地。

||設計模式並不直接用來完成程式碼的編寫，而是描述在各種不同情況下，要怎麼解決問題的一種方案。物件導向設計模式通常以類別或物件來描述其中的關係和相互作用，但不涉及用來完成應用程式的特定類別或物件。設計模式能使不穩定依賴於相對穩定、具體依賴於相對抽象，避免會引起麻煩的緊耦合，以增強軟件設計面對並適應變化的能力。

並非所有的軟件模式都是設計模式，設計模式特指軟件「設計」層次上的問題。還有其他非設計模式的模式，如架構模式。同時，演算法不能算是一種設計模式，因為演算法主要是用來解決計算上的問題，而非設計上的問題。

by wikipedia||

如果你係用緊react js 開發web app，你點都會用到佢既state，而呢樣其實已經係叫做一個finite state machine 既design patterns。

拎state pattern黎做例子，去整一個售貨機logic既code

```

public interface IVendingMachineState

{

void InsertCoin(VendingMachine machine);

void EjectCoin(VendingMachine machine);

void SelectProduct(VendingMachine machine);

void Dispense(VendingMachine machine);

}

public class ReadyState : IVendingMachineState

{

public void InsertCoin(VendingMachine machine)

{

Console.WriteLine("Coin accepted!");

machine.SetState(new HasCoinState());

}

public void EjectCoin(VendingMachine machine)

{

Console.WriteLine("No coin to return");

}

public void SelectProduct(VendingMachine machine)

{

Console.WriteLine("Please insert a coin first");

}

public void Dispense(VendingMachine machine)

{

Console.WriteLine("Please insert a coin first");

}

}

public class HasCoinState : IVendingMachineState

{

public void InsertCoin(VendingMachine machine)

{

Console.WriteLine("Coin already inserted");

}

public void EjectCoin(VendingMachine machine)

{

Console.WriteLine("Coin returned");

machine.SetState(new ReadyState());

}

public void SelectProduct(VendingMachine machine)

{

Console.WriteLine("Product selected");

machine.ReduceInventory();

machine.Dispense();

if (machine.GetInventoryCount() > 0)

machine.SetState(new ReadyState());

else

machine.SetState(new SoldOutState());

}

public void Dispense(VendingMachine machine)

{

Console.WriteLine("Please select a product first");

}

}

public class SoldOutState : IVendingMachineState

{

public void InsertCoin(VendingMachine machine)

{

Console.WriteLine("Machine is sold out. Cannot accept coins");

}

public void EjectCoin(VendingMachine machine)

{

Console.WriteLine("No coin to return");

}

public void SelectProduct(VendingMachine machine)

{

Console.WriteLine("Machine is sold out");

}

public void Dispense(VendingMachine machine)

{

Console.WriteLine("Machine is sold out");

}

}

public class VendingMachine

{

private IVendingMachineState currentState;

private int inventory;

public VendingMachine(int inventory)

{

this.inventory = inventory;

if (inventory > 0)

currentState = new ReadyState();

else

currentState = new SoldOutState();

}

public void SetState(IVendingMachineState state)

{

currentState = state;

}

public void InsertCoin()

{

currentState.InsertCoin(this);

}

public void EjectCoin()

{

currentState.EjectCoin(this);

}

public void SelectProduct()

{

currentState.SelectProduct(this);

}

public void Dispense()

{

currentState.Dispense(this);

}

public void ReduceInventory()

{

if (inventory > 0)

inventory--;

}

public int GetInventoryCount()

{

return inventory;

}

}

public class Program

{

public static void Main()

{

VendingMachine machine = new VendingMachine(2);

// Test the machine with different operations

Console.WriteLine("--- First Transaction ---");

machine.SelectProduct(); // Should fail - no coin

machine.InsertCoin();

machine.SelectProduct();

Console.WriteLine("\n--- Second Transaction ---");

machine.InsertCoin();

machine.EjectCoin();

machine.SelectProduct();

Console.WriteLine("\n--- Third Transaction ---");

machine.InsertCoin();

machine.SelectProduct();

Console.WriteLine("\n--- Machine Sold Out ---");

machine.InsertCoin();

machine.SelectProduct();

}

}

```

run段code既result

||

--- First Transaction ---

Please insert a coin first

Coin accepted!

Product selected

--- Second Transaction ---

Coin accepted!

Coin returned

Please insert a coin first

--- Third Transaction ---

Coin accepted!

Product selected

--- Machine Sold Out ---

Machine is sold out. Cannot accept coins

Machine is sold out

||

咁可能你會問同就咁hardcode哂d係同一個class出黎效果一樣有咩分別，其實就係個修改靈活上面就差好多，而家sample就可能得3個情況，但如果之後再需要加上去既時候，你整多個state specify 有咩behavior 就得，5洗再係同一個class 加成堆if else，無論係後人或者你之後幾年後睇返都冇咁辛苦同。

每個5同design pattern都係解決緊5同問題，所以一d common 既design pattern 例如observe pattern 就係解決緊點樣描述一個物件有改動果時會通知觀察者呢個事情。呢d都會係一d工具會比到方向你去點處理件問題。

淺談: Design patterns

Comments