Android offers a sophisticated and powerful componentized model for building your UI, based on the fundamental layout classes:
ViewGroup. To start with, the platform includes a variety of prebuilt View and ViewGroup subclasses — called widgets and layouts, respectively — that you can use to construct your UI.
A partial list of available widgets includes
Spinner, and the more special-purpose
If none of the prebuilt widgets or layouts meets your needs, you can create your own View subclass. If you only need to make small adjustments to an existing widget or layout, you can simply subclass the widget or layout and override its methods.
Creating your own View subclasses gives you precise control over the appearance and function of a screen element. To give an idea of the control you get with custom views, here are some examples of what you could do with them:
The sections below explain how to create custom Views and use them in your application. For detailed reference information, see the
Here is a high level overview of what you need to know to get started in creating your own View components:
Viewclass or subclass with your own class.
on', for example,
onKeyDown(). This is similar to the
ListActivitythat you override for lifecycle and other functionality hooks.
Tip: Extension classes can be defined as inner classes inside the activities that use them. This is useful because it controls access to them but isn't necessary (perhaps you want to create a new public View for wider use in your application).
Fully customized components can be used to create graphical components that appear however you wish. Perhaps a graphical VU meter that looks like an old analog gauge, or a sing-a-long text view where a bouncing ball moves along the words so you can sing along with a karaoke machine. Either way, you want something that the built-in components just won't do, no matter how you combine them.
Fortunately, you can easily create components that look and behave in any way you like, limited perhaps only by your imagination, the size of the screen, and the available processing power (remember that ultimately your application might have to run on something with significantly less power than your desktop workstation).
To create a fully customized component:
View, so you will usually start by extending this to create your new super component.
onMeasure()and are also likely to need to override
onDraw()if you want the component to show something. While both have default behavior, the default
onDraw()will do nothing, and the default
onMeasure()will always set a size of 100x100 — which is probably not what you want.
on...methods may also be overridden as required.
onDraw() method delivers you a
Canvas upon which you can implement anything you want: 2D graphics, other standard or custom components, styled text, or anything else you can think of.
Note: This does not apply to 3D graphics. If you want to use 3D graphics, you must extend
SurfaceView instead of View, and draw from a separate thread. See the GLSurfaceViewActivity sample for details.
onMeasure() is a little more involved.
onMeasure() is a critical piece of the rendering contract between your component and its container.
onMeasure() should be overridden to efficiently and accurately report the measurements of its contained parts. This is made slightly more complex by the requirements of limits from the parent (which are passed in to the
onMeasure() method) and by the requirement to call the
setMeasuredDimension() method with the measured width and height once they have been calculated. If you fail to call this method from an overridden
onMeasure() method, the result will be an exception at measurement time.
At a high level, implementing
onMeasure() looks something like this:
onMeasure()method is called with width and height measure specifications (
heightMeasureSpecparameters, both are integer codes representing dimensions) which should be treated as requirements for the restrictions on the width and height measurements you should produce. A full reference to the kind of restrictions these specifications can require can be found in the reference documentation under
View.onMeasure(int, int)(this reference documentation does a pretty good job of explaining the whole measurement operation as well).
onMeasure()method should calculate a measurement width and height which will be required to render the component. It should try to stay within the specifications passed in, although it can choose to exceed them (in this case, the parent can choose what to do, including clipping, scrolling, throwing an exception, or asking the
onMeasure()to try again, perhaps with different measurement specifications).
setMeasuredDimension(int width, int height)method must be called with the calculated measurements. Failure to do this will result in an exception being thrown.
Here's a summary of some of the other standard methods that the framework calls on views:
|Creation||Constructors||There is a form of the constructor that are called when the view is created from code and a form that is called when the view is inflated from a layout file. The second form should parse and apply any attributes defined in the layout file.|
||Called after a view and all of its children has been inflated from XML.|
||Called to determine the size requirements for this view and all of its children.|
||Called when this view should assign a size and position to all of its children.|
||Called when the size of this view has changed.|
||Called when the view should render its content.|
||Called when a new key event occurs.|
||Called when a key up event occurs.|
||Called when a trackball motion event occurs.|
||Called when a touch screen motion event occurs.|
||Called when the view gains or loses focus.|
||Called when the window containing the view gains or loses focus.|
||Called when the view is attached to a window.|
||Called when the view is detached from its window.|
||Called when the visibility of the window containing the view has changed.|
The CustomView sample in the API Demos provides an example of a customized View. The custom View is defined in the LabelView class.
The LabelView sample demonstrates a number of different aspects of custom components:
setTextColor()and so on.
onMeasuremethod to determine and set the rendering size of the component. (Note that in LabelView, the real work is done by a private
onDraw()method to draw the label onto the provided canvas.
You can see some sample usages of the LabelView custom View in custom_view_1.xml from the samples. In particular, you can see a mix of both
android: namespace parameters and custom
app: namespace parameters. These
app: parameters are the custom ones that the LabelView recognizes and works with, and are defined in a styleable inner class inside of the samples R resources definition class.
If you don't want to create a completely customized component, but instead are looking to put together a reusable component that consists of a group of existing controls, then creating a Compound Component (or Compound Control) might fit the bill. In a nutshell, this brings together a number of more atomic controls (or views) into a logical group of items that can be treated as a single thing. For example, a Combo Box can be thought of as a combination of a single line EditText field and an adjacent button with an attached PopupList. If you press the button and select something from the list, it populates the EditText field, but the user can also type something directly into the EditText if they prefer.
To create a compound component:
onMeasure()methods since the layout will have default behavior that will likely work just fine. However, you can still override them if you need to.
onKeyDown(), to perhaps choose certain default values from the popup list of a combo box when a certain key is pressed.
To summarize, the use of a Layout as the basis for a Custom Control has a number of advantages, including:
onMeasure()methods (plus most of the other
on...methods) will likely have suitable behavior so you don't have to override them.
In the API Demos project that comes with the SDK, there are two List examples — Example 4 and Example 6 under Views/Lists demonstrate a SpeechView which extends LinearLayout to make a component for displaying Speech quotes. The corresponding classes in the sample code are
There is an even easier option for creating a custom View which is useful in certain circumstances. If there is a component that is already very similar to what you want, you can simply extend that component and just override the behavior that you want to change. You can do all of the things you would do with a fully customized component, but by starting with a more specialized class in the View hierarchy, you can also get a lot of behavior for free that probably does exactly what you want.
For example, the SDK includes a NotePad application in the samples. This demonstrates many aspects of using the Android platform, among them is extending an EditText View to make a lined notepad. This is not a perfect example, and the APIs for doing this might change from this early preview, but it does demonstrate the principles.
If you haven't done so already, import the NotePad sample into Eclipse (or just look at the source using the link provided). In particular look at the definition of
MyEditText in the NoteEditor.java file.
Some points to note here
The class is defined with the following line:
public static class MyEditText extends EditText
NoteEditoractivity, but it is public so that it could be accessed as
NoteEditor.MyEditTextfrom outside of the
NoteEditorclass if desired.
static, meaning it does not generate the so-called "synthetic methods" that allow it to access data from the parent class, which in turn means that it really behaves as a separate class rather than something strongly related to
NoteEditor. This is a cleaner way to create inner classes if they do not need access to state from the outer class, keeps the generated class small, and allows it to be used easily from other classes.
EditText, which is the View we have chosen to customize in this case. When we are finished, the new class will be able to substitute for a normal
As always, the super is called first. Furthermore, this is not a default constructor, but a parameterized one. The EditText is created with these parameters when it is inflated from an XML layout file, thus, our constructor needs to both take them and pass them to the superclass constructor as well.
In this example, there is only one method to be overridden:
onDraw() — but there could easily be others needed when you create your own custom components.
For the NotePad sample, overriding the
onDraw() method allows us to paint the blue lines on the
EditText view canvas (the canvas is passed into the overridden
onDraw() method). The super.onDraw() method is called before the method ends. The superclass method should be invoked, but in this case, we do it at the end after we have painted the lines we want to include.
We now have our custom component, but how can we use it? In the NotePad example, the custom component is used directly from the declarative layout, so take a look at
note_editor.xml in the
<view class="com.android.notepad.NoteEditor$MyEditText" id="@+id/note" android:layout_width="fill_parent" android:layout_height="fill_parent" android:background="@android:drawable/empty" android:padding="10dip" android:scrollbars="vertical" android:fadingEdge="vertical" />
NoteEditor$MyEditTextnotation which is a standard way to refer to inner classes in the Java programming language.
If your custom View component is not defined as an inner class, then you can, alternatively, declare the View component with the XML element name, and exclude the
class attribute. For example:
<com.android.notepad.MyEditText id="@+id/note" ... />
Notice that the
MyEditText class is now a separate class file. When the class is nested in the
NoteEditor class, this technique will not work.
And that's all there is to it. Admittedly this is a simple case, but that's the point — creating custom components is only as complicated as you need it to be.
A more sophisticated component may override even more
on... methods and introduce some of its own helper methods, substantially customizing its properties and behavior. The only limit is your imagination and what you need the component to do.