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大家知道在使用RadioGroup的radiobutton时使用android:backgroud方法，背景图片会变形。而使用android:drawableTop图片是靠右侧的。
而网上说的
android:button="@null"
android:drawableTop="@drawable/rate_selector" 
android:layout_margin="1dip"
这个方法试过了，没用，而且Activity里改变drawableTop也比较麻烦。
于是只能自己构建RadioButton类
RadioButton的父类CompoundButton里的onDraw方法，draw出来的内容，只有默认的水平靠左，没有水平居中的 ，其源代码如下：
 
@Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        final Drawable buttonDrawable = mButtonDrawable;
        if (buttonDrawable != null) {
            final int verticalGravity = getGravity() & Gravity.VERTICAL_GRAVITY_MASK;
            final int height = buttonDrawable.getIntrinsicHeight();

            int y = 0;

            switch (verticalGravity) {
                case Gravity.BOTTOM:
                    y = getHeight() - height;
                    break;
                case Gravity.CENTER_VERTICAL:
                    y = (getHeight() - height) / 2;
                    break;
            }

            buttonDrawable.setBounds(0, y, buttonDrawable.getIntrinsicWidth(), y + height);
            buttonDrawable.draw(canvas);
        }
    }
 
继承RadioButton构建自己的MyRadioButton类：
public class MyRadioButton extends RadioButton{
  private Context context;
  
  private Drawable mButtonDrawable;
  private int mButtonResource;
  
  public MyRadioButton(Context context) {
    super(context);
    this.context = context;
  }
  
  public MyRadioButton(Context context, AttributeSet attrs) {
    super(context, attrs);
    this.context = context;
  }
  
  @Override
  public void setButtonDrawable(int resid) {
    if (resid != 0 && resid == mButtonResource) {
      return;
    }

    mButtonResource = resid;

    Drawable d = null;
    if (mButtonResource != 0) {
      d = getResources().getDrawable(mButtonResource);
    }
    setButtonDrawable(d);
  }

  @Override
  public void setButtonDrawable(Drawable d) {
    if (d != null) {
      if (mButtonDrawable != null) {
        mButtonDrawable.setCallback(null);
        unscheduleDrawable(mButtonDrawable);
      }
      d.setCallback(this);
      d.setState(getDrawableState());
      d.setVisible(getVisibility() == VISIBLE, false);
      mButtonDrawable = d;
      mButtonDrawable.setState(null);
      setMinHeight(mButtonDrawable.getIntrinsicHeight());
    }

    refreshDrawableState();
  }
  
  // 核心代码部分
  @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    final Drawable buttonDrawable = mButtonDrawable;
    if (buttonDrawable != null) {
      final int verticalGravity = getGravity()
          & Gravity.VERTICAL_GRAVITY_MASK;
      final int height = buttonDrawable.getIntrinsicHeight();
      final int width = buttonDrawable.getIntrinsicWidth();

      int y = 0;

      switch (verticalGravity) {
      case Gravity.BOTTOM:
        y = getHeight() - height;
        break;
      case Gravity.CENTER_VERTICAL:
        y = (getHeight() - height) / 2;
        break;
      }

      int x = 0;
      x = (getWidth() - width) / 2;

      buttonDrawable.setBounds(x, y, x + width, y + height);
      buttonDrawable.draw(canvas);
    }
  }
}
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