Defining and inflating a layout_Android Application Development Cookbook（Second Edition）-QQ阅读女频幻言网

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Defining and inflating a layout

When using the Android Studio wizard to create a new project, it automatically creates the res/layout/activity_main.xml file (as shown in the following screenshot). It then inflates the XML file in the onCreate() callback with setContentView(R.layout.activity_main).

For this recipe, we will create two, slightly different layouts and switch between them with a button.

Getting ready

Create a new project in Android Studio and call it InflateLayout. Once the project is created, expand the res/layout folder so we can edit the activity_main.xml file.

How to do it...

Edit the res/layout/activity_main.xml file so it includes a button as defined here:

<Button
    android:id="@+id/buttonLeft"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Left Button"
    android:layout_centerVertical="true"
    android:layout_alignParentLeft="true"
    android:onClick="onClickLeft"/>

Now make a copy of activity_main.xml and call it activity_main2.xml. Change the button so it matches the following:

<Button
    android:id="@+id/buttonRight"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Right Button"
    android:layout_centerVertical="true"
    android:layout_alignParentRight="true"
    android:onClick="onClickRight"/>

Open MainActivity.java and add the following two methods to handle the button clicks:

public void onClickLeft(View view) {
    setContentView(R.layout.activity_main2);
}

public void onClickRight(View view) {
    setContentView(R.layout.activity_main);
}

Run this application on a device or emulator to see it in action.

How it works...

The key here is the call to setContentView(), which we have come across before in the autogenerated onCreate() code. Just pass a layout ID to setContentView() and it automatically inflates the layout.

This code is meant to make the concept easy to understand but would be overkill for simply changing the property of a Button (in this example, we could just change the alignment on the button click). Inflating the layout is usually needed once, in the onCreate() method, but there are times when you may want to manually inflate a layout, as we did here. (If you were manually handling orientation changes, it would be a good example.)

There's more...

As well as identifying a layout using a resource ID, as we did here, setContentView()can also take a View as an argument, for example:

findViewById(R.id.myView)
setContentView(myView);