如何在passcode textview android中为新角色的输入设置动画？

Question

目前我正在为密码输入实现自定义textView。我使用自定义转换来显示最后输入的数字，如下所示：

public class PinCodePasswordTransformationMethod extends PasswordTransformationMethod {

char passwordBullet = '\u26aa';

@Override
public CharSequence getTransformation(CharSequence source, View view) {
    return new PasswordCharSequence(source);
}

private class PasswordCharSequence implements CharSequence {
    private CharSequence mSource;

    public PasswordCharSequence(CharSequence source) {
        mSource = source; // Store char sequence
    }
    public char charAt(int index) {
        if(index != mSource.length()-1)
            return passwordBullet;
        else
            return mSource.charAt(index);

    }
    public int length() {
        return mSource.length(); // Return default
    }
    public CharSequence subSequence(int start, int end) {
        return mSource.subSequence(start, end); // Return default
    }
}
}

并将其应用于textview，如下所示：

mTextView.setTransformationMethod(new PinCodePasswordTransformationMethod());

现在每个字母在输入时立即出现。如何为更改设置动画，以便为新的密码输入和密码子弹字符的倒数第二个更改动画？