3
我實現了一個Xamarin.Forms控件。我目前正在嘗試的問題是,重寫的自定義渲染器的Draw()
方法會阻止UI(至少在iOS平臺上)。我google了,但沒有成功。是否可以在不阻擋用戶界面的情況下在後臺執行繪圖?重寫ViewRenderer的繪製方法阻止UI
下面是iOS平臺的簡單渲染器的代碼,它演示了這個問題。
public class MyCustomRenderer : ViewRenderer
{
protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
{
base.OnElementPropertyChanged(sender, e);
SetNeedsDisplay();
}
public override void Draw(CoreGraphics.CGRect rect)
{
var myControl = (MyControl)this.Element;
if (!myControl.IsRendered)
{
using (var context = UIGraphics.GetCurrentContext())
{
var token = CancellationToken.None;
var task = Task.Factory.StartNew(() => TimeConsumingRendering(context, token), token);
// task.Wait() blocks the UI but draws the desired graphics.
// When task.Wait() is commented out = the desired graphics doesn't get drawn and it doesn't block the UI
task.Wait();
}
}
}
private void TimeConsumingRendering(CGContext context, CancellationToken token)
{
try
{
for (int i = 0; i <= 100; i++)
{
token.ThrowIfCancellationRequested();
var delay = Task.Delay(50);
delay.Wait();
}
context.ScaleCTM(1f, -1f);
context.TranslateCTM(0, -Bounds.Height);
context.SetTextDrawingMode(CGTextDrawingMode.FillStroke);
context.SelectFont("Helvetica-Bold", 16f, CGTextEncoding.MacRoman);
context.SetFillColor(new CoreGraphics.CGColor(1f, 0f, 0f));
context.ShowTextAtPoint(0, 0, "Finished");
}
catch
{ }
}
}