Silverlight入门系列应用MVVM模式10BackgroundWorkder和MVVM.docx
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Silverlight入门系列应用MVVM模式10BackgroundWorkder和MVVM
Silverlight下这个BackgroundWorker我们应当很熟悉了，以前在WinForms下面就有;当我们需要在当前UI线程之外的线程处理耗时的操作而不阻塞UI线程，就可以用这个BackgroundWorker.用起来也是很方便，需要注意的是在DoWork事件中不要操作UI线程的内容，把UI操作都放到ReportProgress事件处理和WorkCompleted事件处理中。
记住这两点就可以了。
今天讲的是Silverlight如何在MVVM模式下用BackgroundWorker。
总体原则就是把后台耗时业务处理和界面逻辑分开，既实现多线程，又实现界面和逻辑分离。
主要BackgroundWorker代码：
ViewModel代码：
（里面有个Command启动耗时任务，任务中ReportProgress事件处理和WorkCompleted事件通知UI）
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usingSystem;
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usingSystem.ComponentModel;
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usingSystem.Windows.Input;
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namespaceBackgroundWorkerTest
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{
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publicclassViewModel:
INotifyPropertyChanged
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{
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publicICommandStartTaskCommand{get;privateset;}
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privateint_counter=0;
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publicViewModel（）
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{
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StartTaskCommand=newDelegateCommand（StartTask,CanStartTask）;
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}
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privatevoidStartTask（objectparam）
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{
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varbackgroundWorker=newBackgroundWorker（）;
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backgroundWorker.WorkerSupportsCancellation=true;
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backgroundWorker.WorkerReportsProgress=true;
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backgroundWorker.DoWork+=（s,e）=>
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{
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for（inti=0;i<10;i++）
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{
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System.Threading.Thread.Sleep（1000）;
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_counter++;
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backgroundWorker.ReportProgress（i*10,string.Format（"ExecutetoNo.{0}",_counter.ToString（）））;
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}
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};
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backgroundWorker.ProgressChanged+=（s,e）=>
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{
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CurrentProgress=e.ProgressPercentage;
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CurrentProgressText=e.UserState!
=null?
e.UserState.ToString（）:
String.Empty;
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};
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backgroundWorker.RunWorkerCompleted+=（s,e）=>
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{
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CurrentProgress=100;
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CurrentProgressText="Completed!
";
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};
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backgroundWorker.RunWorkerAsync（）;
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}
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privateboolCanStartTask（objectparam）
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{
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returntrue;
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}
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privatestring_currentProgressText;
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publicstringCurrentProgressText
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{
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get{return_currentProgressText;}
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set
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{
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if（_currentProgressText!
=value）
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{
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_currentProgressText=value;
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if（PropertyChanged!
=null）
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{
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PropertyChanged（this,newPropertyChangedEventArgs（"CurrentProgressText"））;
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}
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}
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}
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}
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privatedouble_currentProgress;
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publicdoubleCurrentProgress
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{
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get{return_currentProgress;}
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set
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{
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if（_currentProgress!
=value）
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{
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_currentProgress=value;
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if（PropertyChanged!
=null）
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{
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PropertyChanged（this,newPropertyChangedEventArgs（"CurrentProgress"））;
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}
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}
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}
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}
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publiceventPropertyChangedEventHandlerPropertyChanged;
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}
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publicclassDelegateCommand:
ICommand
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{
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///
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///Occurswhenchangesoccurthataffectwhetherthecommandshouldexecute.
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///
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publiceventEventHandlerCanExecuteChanged;
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FunccanExecute;
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Action
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boolcanExecuteCache;
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///
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///Initializesanewinstanceoftheclass.
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///
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///Theexecuteaction.
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///Thecanexecute.
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publicDelegateCommand（Action
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FunccanExecute）
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{
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this.executeAction=executeAction;
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this.canExecute=canExecute;
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}
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#regionICommandMembers
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///
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///Definesthemethodthatdetermineswhetherthecommand
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///canexecuteinitscurrentstate.
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///
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///
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///Datausedbythecommand.
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///Ifthecommanddoesnotrequiredatatobepassed,
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///thisobjectcanbesettonull.
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///
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///
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///trueifthiscommandcanbeexecuted;otherwise,false.
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///
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publicboolCanExecute（objectparameter）
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{
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booltempCanExecute=canExecute（parameter）;
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if（canExecuteCache!
=tempCanExecute）
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{
175:
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canExecuteCache=tempCanExecute;
177:
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if（CanExecuteChanged!
=null）
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{
180:
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CanExecuteChanged（this,newEventArgs（））;
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}
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}
186:
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returncanExecuteCache;
190:
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}
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///
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///Definesthemethodtobecalledwhenthecommandisinvoked.
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///
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///
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///Datausedbythecommand.
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///Ifthecommanddoesnotrequiredatatobepassed,
206:
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///thisobjectcanbesettonull.
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///
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publicvoidExecute（objectparameter）
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{
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executeAction（parameter）;
215:
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}
217:
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#endregion
219:
220:
}
221:
}
View（Xaml.cs）代码：
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usingSystem.Windows.Controls;
2:
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namespaceBackgroundWorkerTest
4:
{
5:
publicpartialclassMainPage:
UserControl
6:
{
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publicMainPage（）
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{
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InitializeComponent（）;
10:
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this.DataContext=newViewModel（）;
12:
}
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}
14:
}
View（Xaml）代码：
1:
Class="BackgroundWorkerTest.MainPage"
2:
xmlns="
3:
xmlns:
x="
4:
xmlns:
d="
5:
xmlns:
mc="http:
//schemas.openxmlformats.org/markup-compatibility/2006"
6:
mc:
Ignorable="d"
7:
d:
DesignHeight="300"d:
DesignWidth="400">
8:
9:
Name="LayoutRoot"Background="White">
10:
VerticalAlignment="Top"Width="87"Command="{BindingStartTaskCommand}"/>
11:
Text="{BindingCurrentProgressText}"VerticalAlignment="Top"Width="359"/>
12:
Margin="18,75,0,0"Name="progressBar1"VerticalAlignment="Top"Width="361"/>
13:
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Dispatcher
如果没有使用BackgroundWorker在非UI线程需要调用UI回调的时候，需要调用当前Application的Dispatcher来完成，比如：
1:
privatevoidUpdateUI（）
2:
{
3:
Dispatcher.BeginInvoke（（）=>
4:
{
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ProgressBar.Value=50;
6:
}）;
7:
}
如果你在一个Class里面（非UserControl），也就是无法获得Application对象的情况下，情况要负责一些，你可以使用一个假的控件对象来构造一个Dummy控件，例如：
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//以下代码在非UserControl的Class里面：
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TextBlock_dispatcherObject=newTextBlock（）;
3:
privatevoidUpdateUINasty（）//avoidthisapproachunlessit'sallyou'vegot
4:
{
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_dispatcherObject.Dispatcher.BeginInvoke（（）=>
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{
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SomePage.ProgressBar.Value=50;
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}）;
9:
}
反之，如果运行在UI线程里面，就不必像上面那样做，最好使用Deployment.Current.Dispatcher对象：
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privatevoidUpdateUINotSoNasty（）
2:
{
3:
Deployment.Current.Dispatcher.BeginInvoke（（）=>
4:
{
5:
SomePage.ProgressBar.Value=50;
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}）;
7:
}
当然，最保险的方法是先检查当前线程是否是UI线程，如果不是，那么进行UI操作需要用BeginInvoke：
1:
Actionmyaction=（）=>DoSomething（）;
2:
vardispatch=Deployment.Current.Dispatcher;
3:
if（dispatch.CheckedAccess）
4:
{
5:
myaction（）;
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}
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else
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{
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dispatch.BeginInvoke（myaction（））;
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}
代码截图：
使用定时器：
DispatcherTimer
1:
privatevoidStartTimer（）
2:
{
3:
DispatcherTimertimer=newDispatcherTimer（）;
4:
5:
timer.Tick+=（s,e）=>
6:
{
7:
//dosomeveryquickworkhere
8:
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//updatetheUI
10:
StatusText.Text=DateTime.Now.Second.ToString（）;
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};
12:
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timer.Interval=TimeSpan.FromSeconds
（1）;
14:
timer.Start（）;
15:
}
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