监听输入框的删除按钮！
et_key.setOnKeyListener(new View.OnKeyListener() {
    @Override
    public boolean onKey(View v, int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_DEL) {
            String content = et_key.getText().toString();
            int length = content.length();
            if(content.equals("")){
                return true;
            }
            content.endsWith("-");

        }