一娶牌、問題背景

在我們實(shí)際卡法需求中可能會要修改tablayout下方的indicator的樣式與屬性，比如indicator的長度鹤盒，或者給tablayout加個(gè)背景但是需要indicator滑動的效果国章，然而原生的tablayout卻僅僅提供了tablayout的height即厚度的屬性，自定義化的程度很低芙盘。

二驯用、問題探索

通過查閱源碼，發(fā)現(xiàn)tablayout的下劃線是由其內(nèi)部類SlidingTabStrip實(shí)現(xiàn)何陆，其繼承自LinearLayout布局的viewgroup晨汹。源碼如下

 private class SlidingTabStrip extends LinearLayout {
        private int mSelectedIndicatorHeight;
        private final Paint mSelectedIndicatorPaint;
        int mSelectedPosition = -1;
        float mSelectionOffset;
        private int mLayoutDirection = -1;
        private int mIndicatorLeft = -1;
        private int mIndicatorRight = -1;
        private ValueAnimator mIndicatorAnimator;
        SlidingTabStrip(Context context) {
            super(context);
            setWillNotDraw(false);
            mSelectedIndicatorPaint = new Paint();
        }
        ...
 @Override
 public void draw(Canvas canvas) {
   super.draw(canvas);
   // Thick colored underline below the current selection
   if (mIndicatorLeft >= 0 && mIndicatorRight > mIndicatorLeft) {
     canvas.drawRect(mIndicatorLeft, getHeight() - 
      mSelectedIndicatorHeight,mIndicatorRight, 
      getHeight(), mSelectedIndicatorPaint);
  }
}

省略部分細(xì)節(jié)代碼，查看其ondraw方法會發(fā)現(xiàn)其繪制過程和三個(gè)變量相關(guān)：mSelectedIndicatorHeight贷盲，mIndicatorLeft 和mIndicatorRight 淘这，由于mSelectedIndicatorHeight有提供接口可以改變可忽略，實(shí)際控制其繪制過程的為剩下倆個(gè)變量巩剖。

三铝穷、問題解決方案

正如上述縮分析的，控制SlidingTabStrip的繪制過程的為mIndicatorLeft 和mIndicatorRight 倆個(gè)變量佳魔，這倆個(gè)變量同時(shí)也是控制其位置顯示的變量曙聂，只要獲取到這倆個(gè)變量其實(shí)就可以進(jìn)行我們自定義Indicator的封裝，那么如何獲取這倆個(gè)變量呢鞠鲜？——反射

//自定義drawable
public class IndicatorDrawable extends Drawable{
    View view;
    Paint paint;

    float paddingLeft;
    float paddingTop;

    public IndicatorDrawable(View view) {
        this.view = view;
        paint = new Paint();
        paint.setColor(Color.parseColor("#0f0"));
        float density = view.getResources().getDisplayMetrics().density;
        //這兩個(gè)留白可以根據(jù)需求更改
        paddingLeft = 0 * density;
        paddingTop =  5 * density;
    }

    @Override
    public void draw(@NonNull Canvas canvas) {
        //自定義背景
        int mIndicatorLeft = getIntValue("mIndicatorLeft");
        int mIndicatorRight = getIntValue("mIndicatorRight");
        int height = view.getHeight();
        int radius = height / 2;
        if (mIndicatorLeft >= 0 && mIndicatorRight > mIndicatorLeft) {
            //canvas.drawRoundRect(new RectF(mIndicatorLeft + (int)paddingLeft, (int)paddingTop, mIndicatorRight - (int)paddingLeft, height - (int)paddingTop), radius, radius, paint);
            //縮減indicator的寬度    
            canvas.drawRect(mIndicatorLeft+40, height - 4,
                        mIndicatorRight-40, height, paint);
        }
    }

    int getIntValue(String name) {
        try {
            Field f = view.getClass().getDeclaredField(name);
            f.setAccessible(true);
            Object obj = f.get(view);
            return (Integer) obj;
        } catch (Exception e) {
            e.printStackTrace();
        }

        return 0;
    }

    @Override
    public void setAlpha(@IntRange(from = 0, to = 255) int alpha) {

    }

    @Override
    public void setColorFilter(@Nullable ColorFilter colorFilter) {

    }

    @Override
    public int getOpacity() {
        return PixelFormat.UNKNOWN;
    }
}