A button consists of text or an icon (or both text and an icon) that communicates what action occurs when the user touches it.
Depending on whether you want a button with text, an icon, or both, you can create the button in your layout in three ways:
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_text"
... />
<ImageButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/button_icon"
... />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_text"
android:drawableLeft="@drawable/button_icon"
... />
When the user clicks a button, the {@link android.widget.Button} object receives an on-click event.
To define the click event handler for a button, add the {@link android.R.attr#onClick android:onClick} attribute to the {@code <Button>} element in your XML layout. The value for this attribute must be the name of the method you want to call in response to a click event. The {@link android.app.Activity} hosting the layout must then implement the corresponding method.
For example, here's a layout with a button using {@link android.R.attr#onClick android:onClick}:
<?xml version="1.0" encoding="utf-8"?>
<Button xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/button_send"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_send"
android:onClick="sendMessage" />
Within the {@link android.app.Activity} that hosts this layout, the following method handles the click event:
/** Called when the user touches the button */
public void sendMessage(View view) {
// Do something in response to button click
}
The method you declare in the {@link android.R.attr#onClick android:onClick} attribute must have a signature exactly as shown above. Specifically, the method must:
You can also declare the click event handler programmatically rather than in an XML layout. This might be necessary if you instantiate the {@link android.widget.Button} at runtime or you need to declare the click behavior in a {@link android.app.Fragment} subclass.
To declare the event handler programmatically, create an {@link android.view.View.OnClickListener} object and assign it to the button by calling {@link android.view.View#setOnClickListener}. For example:
Button button = (Button) findViewById(R.id.button_send);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
// Do something in response to button click
}
});
The appearance of your button (background image and font) may vary from one device to another, because devices by different manufacturers often have different default styles for input controls.
You can control exactly how your controls are styled using a theme that you apply to your entire application. For instance, to ensure that all devices running Android 4.0 and higher use the Holo theme in your app, declare {@code android:theme="@android:style/Theme.Holo"} in your manifest's {@code <application>} element. Also read the blog post, Holo Everywhere for information about using the Holo theme while supporting older devices.
To customize individual buttons with a different background, specify the {@link android.R.attr#background android:background} attribute with a drawable or color resource. Alternatively, you can apply a style for the button, which works in a manner similar to HTML styles to define multiple style properties such as the background, font, size, and others. For more information about applying styles, see Styles and Themes.
One design that can be useful is a "borderless" button. Borderless buttons resemble basic buttons except that they have no borders or background but still change appearance during different states, such as when clicked.
To create a borderless button, apply the {@link android.R.attr#borderlessButtonStyle} style to the button. For example:
<Button
android:id="@+id/button_send"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_send"
android:onClick="sendMessage"
style="?android:attr/borderlessButtonStyle" />
If you want to truly redefine the appearance of your button, you can specify a custom background. Instead of supplying a simple bitmap or color, however, your background should be a state list resource that changes appearance depending on the button's current state.
You can define the state list in an XML file that defines three different images or colors to use for the different button states.
To create a state list drawable for your button background:
To ensure that your images fit buttons of various sizes, create the bitmaps as Nine-patch bitmaps.
res/drawable/ directory of
your project. Be sure each bitmap is named properly to reflect the button state that they each
represent, such as {@code button_default.9.png}, {@code button_pressed.9.png}, and {@code
button_focused.9.png}.res/drawable/ directory (name it something like
button_custom.xml). Insert the following XML:
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:drawable="@drawable/button_pressed"
android:state_pressed="true" />
<item android:drawable="@drawable/button_focused"
android:state_focused="true" />
<item android:drawable="@drawable/button_default" />
</selector>
This defines a single drawable resource, which will change its image based on the current state of the button.
<item> defines the bitmap to use when the button is
pressed (activated).<item> defines the bitmap to use when the button is
focused (when the button is highlighted using the trackball or directional
pad).<item> defines the bitmap to use when the button is in the
default state (it's neither pressed nor focused).Note: The order of the <item> elements is
important. When this drawable is referenced, the <item> elements are traversed
in-order to determine which one is appropriate for the current button state. Because the default
bitmap is last, it is only applied when the conditions android:state_pressed and
android:state_focused have both evaluated as false.
This XML file now represents a single drawable resource and when referenced by a {@link android.widget.Button} for its background, the image displayed will change based on these three states.
<Button
android:id="@+id/button_send"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_send"
android:onClick="sendMessage"
android:background="@drawable/button_custom" />
For more information about this XML syntax, including how to define a disabled, hovered, or other button states, read about State List Drawable.