android 自定义view之（一） Creating a View Class(Android 自定义view炫酷动画)

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A well-designed custom view is much like any other well-designed class. It encapsulates a specific set of functionality with an easy to use interface, it uses CPU and memory efficiently, and so forth. In addition to being a well-designed class, though, a custom view should:

All of the view classes defined in the Android framework extend View. Your custom view can also extend View directly, or you can save time by extending one of the existing view subclasses, such as Button.

To allow the Android Developer Tools to interact with your view, at a minimum you must provide a constructor that takes a Context and an AttributeSet object as parameters. This constructor allows the layout editor to create and edit an instance of your view.

To add a built-in View to your user interface, you specify it in an XML element and control its appearance and behavior with element attributes. Well-written custom views can also be added and styled via XML. To enable this behavior in your custom view, you must:

To define custom attributes, add <declare-styleable> resources to your project. It’s customary to put these resources into ares/values/attrs.xml file. Here’s an example of an attrs.xml file:

This code declares two custom attributes, showText and labelPosition, that belong to a styleable entity named PieChart. The name of the styleable entity is, by convention, the same name as the name of the class that defines the custom view. Although it’s not strictly necessary to follow this convention, many popular code editors depend on this naming convention to provide statement completion.

Once you define the custom attributes, you can use them in layout XML files just like built-in attributes. The only difference is that your custom attributes belong to a different namespace. Instead of belonging to the they belong package name]. For example, here’s how to use the attributes defined for PieChart:

In order to avoid having to repeat the long namespace URI, the sample uses an xmlns directive. This directive assigns the alias custom to the namespace You can choose any alias you want for your namespace.

Notice the name of the XML tag that adds the custom view to the layout. It is the fully qualified name of the custom view class. If your view class is an inner class, you must further qualify it with the name of the view’s outer class. further. For instance, the PieChart class has an inner class called PieView. To use the custom attributes from this class, you would use the tag com.example.customviews.charting.PieChart$PieView.

When a view is created from an XML layout, all of the attributes in the XML tag are read from the resource bundle and passed into the view’s constructor as an AttributeSet. Although it’s possible to read values from the AttributeSet directly, doing so has some disadvantages:

The Android resource compiler does a lot of work for you to make calling obtainStyledAttributes() _easier. For each <declare-styleable> resource in the res directory, the generated R.java defines both an array of attribute ids and a set of constants that define the index for each attribute in the array. You use the predefined constants to read the attributes from the _TypedArray. Here’s how the PieChart class reads its attributes:

Note that TypedArray objects are a shared resource and must be recycled after use.

Attributes are a powerful way of controlling the behavior and appearance of views, but they can only be read when the view is initialized. To provide dynamic behavior, expose a property getter and setter pair for each custom attribute. The following snippet shows how PieChart exposes a property called showText:

Notice that setShowText calls invalidate() and requestLayout() (记住setter方法里调用 invalidate() 跟 requstLayout()方法). These calls are crucial to ensure that the view behaves reliably. You have to invalidate the view after any change to its properties that might change its appearance, so that the system knows that it needs to be redrawn. Likewise, you need to request a new layout if a property changes that might affect the size or shape of the view. Forgetting these method calls can cause hard-to-find bugs.

Custom views should also support event listeners to communicate important events. （只是事件监听）For instance, PieChart exposes a custom event called OnCurrentItemChanged to notify listeners that the user has rotated the pie chart to focus on a new pie slice.

It’s easy to forget to expose properties and events（不要忘记暴露属性跟事件）, especially when you’re the only user of the custom view. Taking some time to carefully define your view’s interface reduces future maintenance costs. A good rule to follow is to always expose any property that affects the visible appearance or behavior of your custom view.

Your custom view should support the widest range of users. This includes users with disabilities that prevent them from seeing or using a touchscreen. To support users with disabilities, you should:

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