Android Studio屏幕方向以及UI界面状态的保存代码详解

项目：Orientation

package com.example.orientation;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
/*
  = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
  本实例主要学习，屏幕翻转时，界面如何自适应，创建横屏布局
  1.禁止切换横屏：在 AndroidManifest.xml-->application->activity->中设置如下代码（android:screenOrientation="portrait"）
   <activity android:name=".MainActivity" android:screenOrientation="portrait" >
  2. 创建 Landscape 布局，横屏时，会自动加载 Landscape 的布局界面（清单文件中，注意去掉 android:screenOrientation="portrait" ）
  3. 翻转屏幕时，保存窗口控件的状态值;

  = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
 */
  Button button;
  TextView textView;

  String TAG = "myTag";

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    button = findViewById(R.id.button );
    textView = findViewById(R.id.textView);

    //如果State中的值不为空，如果有相应的这个组件的值，则读取出来赋值上去
    if(savedInstanceState !=null)
    {
      String s = savedInstanceState.getString("key");
      textView.setText(s);
    }

    button.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View view) {
        textView.setText(button.getText());
      }
    });
  }

  @Override
  protected void onDestroy() {
    super.onDestroy();
    Log.d(TAG,"onDestroy:");
  }

  @Override
  //将 textView 中的值，先保存到 outState 中（键值对）
  public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putString("key",textView.getText().toString());
  }
}

扩展学习：

UI界面设计

TextView

<TextView
    android:id="@+id/textview"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:gravity="center"
    android:text="This is a TextView"
    android:textColor="#00ff00"
    android:textSize="24sp" />

要想使得文字居中，需要添加属性android:gravity="center"，可选择的选项还有top、bottom、left、right、center等，center相当于center_vertical|center_horizontal。
使用android:textSize="24sp"指定文字大小，android:textColor="#00ff00"指定文字颜色。

Button

<Button
    android:id="@+id/button"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="Button"
    android:textAllCaps="false"/>

在Android中，Button上面的文字默认英文全部大写，可以通过设置android:textAllCaps="false"改变

EditText

<EditText
    android:id="@+id/edittext"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:hint="HelloWorld"
    android:maxLength="20"
    android:maxLines="1" />

通过设置hint属性可以得到提示文字，设置maxLines使得输入框中最大输入行数。

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