<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical"
 android:background="@color/colorLogin"

 >

 <!-- Login progress -->
 <ProgressBar
 android:id="@+id/login_progress"
 style="?android:attr/progressBarStyleLarge"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_marginBottom="8dp"
 android:visibility="gone" />
 <RelativeLayout
 android:layout_width="match_parent"
 android:layout_height="180dp"

 android:id="@+id/head_img"
 >

 <de.hdodenhof.circleimageview.CircleImageView
  android:layout_width="80dp"
  android:layout_height="80dp"
  android:src="@mipmap/nav_head"
  android:layout_alignParentBottom="true"
  android:layout_centerHorizontal="true"
  android:layout_marginBottom="25dp" />
 </RelativeLayout>
 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:paddingBottom="20dp"
 android:orientation="vertical">
 <EditText
  android:id="@+id/et_user"
  android:layout_width="match_parent"
  android:layout_height="60dp"
  android:hint="@string/userName"
  android:background="@color/colorLoginForm"
  android:layout_marginBottom="5dp"
  />
 <TextView
  android:id="@+id/tv_tip"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:textColor="@color/error"
  />
 <EditText
  android:id="@+id/et_pass"
  android:layout_width="match_parent"
  android:layout_height="60dp"
  android:background="@color/colorLoginForm"
  android:hint="@string/passWord"
  android:paddingTop="1dp"
  />
 </LinearLayout>
 <Button
 android:id="@+id/button"
 android:layout_width="match_parent"
 android:layout_height="50dp"
 android:background="@color/loginButton"
 android:text="@string/loginButton"
 android:textColor="@color/colorLoginForm"
 />

</LinearLayout>

public class MainActivity extends AppCompatActivity {
 EditText etUser;
 EditText etPassWord;
 TextView tvTip;
 Button button;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_main);
 //初始化View控件
 findView();
 //用于检测输入的用户名操作
 checkLength();
 }

 private void checkLength() {
 //为etUser设置焦点改变监听事件
 etUser.setOnFocusChangeListener(new View.OnFocusChangeListener(){
  @Override
  public void onFocusChange(View v, boolean hasFocus) {
  //如果失去焦点则进行用户名的检测
  if(etUser.hasFocus()==false){
   //如果用户名长度小于3或者大于9，则提示用户名错误且登陆不可点击
   if(etUser.getText().toString().length()>9||etUser.getText().toString().length()<3){
   tvTip.setText("用户名不合法！");
   button.setClickable(false);
   }else{
   //如果用户名合法且密码不为空，设置提示字体消失按钮可点击
   if(etPassWord.getText().toString()!=""){
   button.setClickable(true);
   tvTip.setText("");
   }
  }
  }


  }
 });
 }

 private void findView() {
 etUser= (EditText) findViewById(R.id.et_user);
 etPassWord= (EditText) findViewById(R.id.et_pass);
 tvTip= (TextView) findViewById(R.id.tv_tip);
 button= (Button) findViewById(R.id.button);
 }
}