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

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Creating a View Class

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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:

Conform to Android standardsProvide custom styleable attributes that work with Android XML layoutsSend accessibility eventsBe compatible with multiple Android platforms. The Android framework provides a set of base classes and XML tags to help you create a view that meets all of these requirements. This lesson discusses how to use the Android framework to create the core functionality of a view class.Subclass a View

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.

Define Custom Attributes

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:

Define custom attributes for your view in a resource element 自定义attributes属性 Specify values for the attributes in your XML layout 在xml中指定属性值 Retrieve attribute values at runtime 在runtime 取回属性Apply the retrieved attribute values to your view 在自定义view上应用取回的属性值 This section discusses how to define custom attributes and specify their values. The next section deals with retrieving and applying the values at runtime.

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.

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

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.

Apply Custom Attributes 应用自定义属性

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:

Resource references within attribute values are not resolvedStyles are not applied Instead, pass the AttributeSet to obtainStyledAttributes(). This method passes back a TypedArray array of values that have already been dereferenced and styled.

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.

Add Properties and Events 添加属性和事件

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.

Design For Accessibility(易接近)

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:

Label your input fields using the android:contentDescription attributeSend accessibility events by calling sendAccessibilityEvent() when appropriate.Support alternate controllers, such as D-pad and trackball For more information on creating accessible views, see Making Applications Accessible in the Android Developers Guide.

Android 四种点击事件 Android四种点击事件xml文件代码如下:(1--3均可用)Buttonandroid:id=@id/button1android:layout_width=wrap_contentandroid:layout_height=wrap_contentandroid:text=Button1/Buttonandroid:id=@id

activity通信 上节说到,Activity是什么;这次来了解了解Activity通信相关内容。都知道,人与人交流产生许多不可预知的美。因为交流,信息得意传递;因为交流,任

android轻量级开源缓存框架——ASimpleCache(ACache)源码分析 转载请注明出处:

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