Android Button 狀態圖片 Selector 封裝：自定義屬性實現複用避免重複 XML

痛點

每個 Button 都需寫 selector XML (btn_background.xml)，項目中按鈕眾多時極其繁瑣，維護成本高。

解決方案：自定義 Button 組件 + 自定義屬性

1. 定義自定義屬性

2. 實現自定義 Button

public class StateButton extends AppCompatButton {
    private Drawable normalDrawable, pressedDrawable, focusedDrawable, disabledDrawable;
    private StateListDrawable stateListDrawable;

    public StateButton(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context, attrs);
    }

    private void init(Context context, AttributeSet attrs) {
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.StateButton);
        normalDrawable = a.getDrawable(R.styleable.StateButton_normalDrawable);
        pressedDrawable = a.getDrawable(R.styleable.StateButton_pressedDrawable);
        focusedDrawable = a.getDrawable(R.styleable.StateButton_focusedDrawable);
        disabledDrawable = a.getDrawable(R.styleable.StateButton_disabledDrawable);
        a.recycle();

        buildStateListDrawable();
        setBackground(stateListDrawable); // 背景而非 src
    }

    private void buildStateListDrawable() {
        stateListDrawable = new StateListDrawable();
        // 順序重要：具體狀態在前，默認在後
        if (pressedDrawable != null) 
            stateListDrawable.addState(new int[]{android.R.attr.state_pressed}, pressedDrawable);
        if (focusedDrawable != null) 
            stateListDrawable.addState(new int[]{android.R.attr.state_focused}, focusedDrawable);
        if (disabledDrawable != null) 
            stateListDrawable.addState(new int[]{-android.R.attr.state_enabled}, disabledDrawable);
        if (normalDrawable != null) 
            stateListDrawable.addState(StateSet.WILD_CARD, normalDrawable); // 默認
    }

    // 運行時動態修改
    public void setNormalDrawable(Drawable d) {
        normalDrawable = d;
        buildStateListDrawable();
    }
    // ... 其他 setter
}

3. 佈局中直接使用

進階：統一風格主題屬性 (更優雅)

所有 Button 自動套用主題風格，單個覆蓋只需在佈局加 style="@style/Widget.App.Button.Secondary"。

現代方案：MaterialButton (Material Components)

優勢：內建 Ripple、ShapeAppearance、狀態屬性映射、無需自定義類、符合 Material Design 規範。

方案對比總結