Beginning Android 4 中 Demo Basic/Switch 的问题.

作者的版本:

layout (main.xml):

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >
    <Switch
        android:id="@+id/switchdemo"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="WTF?" />
</LinearLayout>

java代码:

public class SwitchActivity extends Activity
  implements CompoundButton.OnCheckedChangeListener {
  Switch sw;

  @Override
  public void onCreate(Bundle icicle) {
    super.onCreate(icicle);
    setContentView(R.layout.main);

    sw=(Switch)findViewById(R.id.switchdemo);
    sw.setOnCheckedChangeListener(this);
  }

//  @Override
  public void onCheckedChanged(CompoundButton buttonView,
                                 boolean isChecked) {
    if (isChecked) {
      sw.setTextOn("This switch is: on");
    }
    else {
      sw.setTextOff("This switch is: off");
    }
  }
}

作者提供的 java 代码有问题. 原因参考(http://stackoverflow.com/questions/29811646/android-settexton-not-working-in-oncheckchanged-method)

I don't think your calls to setTextOn and setTextOff need to be in an if - they just define how the toggle appears when on or off, so they don't need to be set conditionally. Ref: API – Simon MᶜKenzie Apr 23 '15 at 1:01

---

The setTextOn and setTextOff functions are to used to set the labels depending on the state of the Switch.

The text "The switch is: On" is just the label of your Switch and does not convey the state of your Switch.

To achieve the result that you want, you need to call setShowText(true):

sw = (Switch)findViewById(R.id.swish);
sw.setShowText(true);

or you can add it in your XML.

<Switch
        android:layout_width="453dp"
        android:layout_height="100dp"
        android:id="@+id/swish"
        android:layout_gravity="center_vertical"
        android:layout_alignParentTop="true"
        android:showText="true"/>

---

As observed by @Simon M, this xml and java snippet produce consistent output as shown by the screen below.

<Switch
    android:layout_width="453dp"
    android:layout_height="100dp"
    android:id="@+id/swish"
    android:layout_gravity="center_vertical"
    android:textOn="ON!!"
    android:textOff="OFF!"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"/>

    sw = (Switch)findViewById(R.id.swish);
    sw.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
        @Override
        public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
          // absolutely nothing
        });

答案说的很清楚了. 只需要在 onCreate 里面调用 setTextOn/Off 即可. onCheckedChanged 什么都不用做. 或者根本步调用 setTextOn/Off, 直接在 main.xml 中用 `android::textOn/Off`即可.

下面是修改后的 java 实现, 没有修改 main.xml, 也展示了 setText 与 setTextOn/Off 的区别.

public class SwitchActivity extends ActionBarActivity implements CompoundButton.OnCheckedChangeListener {

    Switch sw;
    @Override
    public void onCreate(Bundle icicle) {
        super.onCreate(icicle);
        setContentView(R.layout.main);
        sw=(Switch)findViewById(R.id.switchdemo);
        sw.setOnCheckedChangeListener(this);
        sw.setTextOff("#OFF#");
        sw.setTextOn("#ON#");
    }

    public void onCheckedChanged(CompoundButton buttonView,
                                 boolean isChecked) {
        if (isChecked) {
            sw.setText("I'm On.");
        } else {
            sw.setText("I'm Off.");
        }
    }
}