Xamarin.Forms：事后更改RelativeLayout约束

Question

是否可以在设置一次后更改RelativeLayout的约束？

在文档中我看到像GetBoundsConstraint(BindableObject)这样的方法，但我认为只有拥有XAML文件时才能使用它们。现在我试图在代码中这样做。我得到null如果我尝试

RelativeLayout.GetBoundsConstraint(this.label); 

我发现的唯一方法是从布局中删除的孩子，以新的约束重新添加。

例子：

public class TestPage : ContentPage 
{ 
    private RelativeLayout relativeLayout; 
    private BoxView view; 
    private bool moreWidth = false; 

    public TestPage() 
    { 
     this.view = new BoxView 
     { 
      BackgroundColor = Color.Yellow, 
     }; 

     Button button = new Button 
     { 
      Text = "change", 
      TextColor = Color.Black, 
     }; 

     button.Clicked += Button_Clicked; 

     this.relativeLayout = new RelativeLayout(); 

     this.relativeLayout.Children.Add(this.view, 
      Constraint.Constant(0), 
      Constraint.Constant(0), 
      Constraint.Constant(80), 
      Constraint.RelativeToParent((parent) => 
      { 
       return parent.Height; 
      })); 

     this.relativeLayout.Children.Add(button, 
      Constraint.RelativeToParent((parent) => 
      { 
       return parent.Width/2; 
      }), 
      Constraint.RelativeToParent((parent) => 
      { 
       return parent.Height/2; 
      })); 

     this.Content = this.relativeLayout; 
    } 

    private void Button_Clicked(object sender, EventArgs e) 
    { 
     double width = 0; 
     if(this.moreWidth) 
     { 
      width = 120; 
     } 
     else 
     { 
      width = 80; 
     } 
     var c= BoundsConstraint.FromExpression((Expression<Func<Rectangle>>)(() => new Rectangle(0, 0, width, this.Content.Height)), new View[0]); 
     RelativeLayout.SetBoundsConstraint(this.view, c); 
     this.relativeLayout.ForceLayout(); 
     this.moreWidth = !this.moreWidth; 
    } 
} 

Answer 1

它不正式可能与Xamarin形式的当前版本。 RelativeLayout容器仅在从其子集合中添加/删除项目（缓存解决的约束 - 为性能假设）时才会重新计算约束条件。即使各种约束条件都作为Bindable属性实现，但它们在更改时仍不会重新计算。

我假设意图是有朝一日尊重约束更新，这对于动画比较有用，但现在看起来并不这样。

无论其，我看了一下为RelativeLayout的反编译源，它是可以破解一起变通的办法 - 但它可能无法满足您的需求，这取决于你有多少功能要求和多么复杂你的约束定义是。

参见本实施例中的代码（关键部分是设置使用SetBoundsConstraint，它覆盖所添加的视图的内部计算的边界约束 - ，然后调用ForceLayout()）：

public partial class App : Application 
{ 
    public App() 
    { 
     var label = new Label { 
      Text = "Test", 
      HorizontalTextAlignment = TextAlignment.Center, 
      VerticalTextAlignment = TextAlignment.Center, 
      BackgroundColor = Color.Silver 
     }; 
     var layout = new RelativeLayout(); 
     layout.Children.Add (label, 
      Constraint.Constant (50), 
      Constraint.Constant (100), 
      Constraint.Constant (260), 
      Constraint.Constant (30)); 
     MainPage = new ContentPage { 
      Content = layout 
     }; 

     var fwd = true; 
     layout.Animate ("bounce", 
      (delta) => { 
       var d = fwd ? delta : 1.0 - delta; 
       var y = 100.0 + (50.0 * d); 
       var c = BoundsConstraint.FromExpression ((Expression<Func<Rectangle>>)(() => new Rectangle (50, y, 260, 30)), new View [0]); 
       RelativeLayout.SetBoundsConstraint(label, c); 
       layout.ForceLayout(); 
      }, 16, 800, Easing.SinInOut, (f, b) => { 
       // reset direction 
       fwd = !fwd; 
      },() => { 
       // keep bouncing 
       return true; 
      }); 
    } 
} 

Answer 2

如果我正确理解你，我想尝试以下

为RelativeLayout的

像实施SizeChanged事件： -

var relativeLayout = new RelativeLayout(); 

relativeLayout.SizeChanged += someEventForTesting; 

并且每当你想调整相对布局或某个特定子项时调用该事件。

public void someEventForTesting(Object sender , EventArgs) 
{ 
var layout = (RelativeLayout)sender ; 
//here you have access to layout and every child , you can add them again with new constraint . 
}