Widget

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• AppWidgetProvider publishes app widget in the homescreen. It publishes app widget by allowing us (i.e. developers ) to interface with the app widget programmatically, based on broadcast events.It provides the broadcast when the App Widget is updated.

Activity和appwidget的区别

•  We must understand the difference between normal Activity and app widget. A normal Activity individual controls can draw outside of their own bounds, and even those of their parent layouts if they allow it via their clipChildren attribute. However doing this on the home screen could interfere with other widgets, and to prevent this kind of bad behaviour from app widgets, they do not get direct access to the app widget layout and it’s child views.

限制如下

• Instead they must use a RemoteViews object to update these views. RemoteViews is a proxy which provides us with limited access to these views, and consequently we have a limited subset of widgets that we can use in our app widget layouts such as : AnalogClock, Button, Chronometer, ImageButton, ImageView, ProgressBar, TextView, ViewFlipper, ListView, GridView, StackView, AdapterViewFlipper. We are also limited to the following layout types: FrameLayout, LinearLayout, RelativeLayout, GridLayout.

使用静态处理多实例

• the most important factor is saving the widget id of a particular widget.To be able to distinguish between multiple instances of the same AppWidgetProvider, when registering the “onClick” event (intent) you must add an extra value with the widget ID.

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加载流程

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• 它可以被嵌入到其它应用程序中，比如桌面，并接收周期性的更新。你可以通过一个 App Widget Provider 来发布一个 Widget。可以容纳 Widget 的应用叫做 App Widget Host
• 系统最多30分钟才更新挂件的内容，为了实时更新时钟我们必须自己发送广播来实现挂件内容的更新，其方法就是覆写父类AppWidgetProvider的onReceive()方法，并在注册的广播接收的intent-fliter里添加接受自己的广播。
• The AppWidgetProvider receives only the event broadcasts that are relevant to the App Widget, such as when the App Widget is updated, deleted, enabled, and disabled onUpdate()This is called to update the App Widget at intervals defined by the updatePeriodMillis attribute in the AppWidgetProviderInfo his method is also called when the user adds the App Widget, so it should perform the essential setup, such as define event handlers for Views and start a temporary Service, if necessary. However, if you have declared a configuration Activity, this method is not called when the user adds the App Widget, but is called for the subsequent updates. It is the responsibility of the configuration Activity to perform the first update when configuration is done.onEnabled(Context)This is called when an instance the App Widget is created for the first time. For example, if the user adds two instances of your App Widget, this is only called the first time. If you need to open a new database or perform other setup that only needs to occur once for all App Widget instances, then this is a good place to do it.which is an array of IDs that identify each App Widget created by this provider. In this way, if the user creates more than one instance of the App Widget, then they are all updated simultaneously. However, only one updatePeriodMillis schedule will be managed for all instances of the App Widget. For example, if the update schedule is defined to be every two hours, and a second instance of the App Widget is added one hour after the first one, then they will both be updated on the period defined by the first one and the second update period will be ignored (they'll both be updated every two hours, not every hour).