Xamarin.Forms：如何使用相对布局居中视图？ `Width`和`Height`返回-1

Question

当试图在Xamarin.Forms中使用控件的Height和Width属性时，都返回-1，这会导致相对布局在屏幕上偏离中心。

var mainLayout = new RelativeLayout();

//Add the Switch to the center of the screen
mainLayout.Children.Add(mySwitch,
    Constraint.RelativeToParent(parent => parent.Width / 2 - mySwitch.Width / 2),
    Constraint.RelativeToParent(parent => parent.Height / 2 - mySwitch.Height / 2));

//Add a Label below the switch
mainLayout.Children.Add(switchLabel,
    Constraint.RelativeToParent(parent => parent.Width / 2 - switchLabel.Width / 2),
    Constraint.RelativeToView(mySwitch, (parent, view) => view.Y + mySwitch.Height + 10));

Content = mainLayout;

Answer 1

Why does Xamarin.Forms return -1 for Height and Width?

Xamarin.Forms返回-1作为这些属性的默认值，并且它保持为-1，直到Xamarin.Forms创建本机控件，例如UIButton，并将原生控件添加到布局中。

在此链接中，您可以看到将-1作为默认值返回的Xamarin.Forms源代码：https://github.com/xamarin/Xamarin.Forms/blob/master/Xamarin.Forms.Core/VisualElement.cs

Best Way to Use Relative Layouts to Constrain Views

选项1：本地功能（需要C＃7.0或更高版本）

使用Local Function动态检索Width和Height属性

var mainLayout = new RelativeLayout();

//Add the Switch to the center of the screen
mainLayout.Children.Add(mySwitch,
    Constraint.RelativeToParent(parent => parent.Width / 2 - getSwitchWidth(parent)/ 2),
    Constraint.RelativeToParent(parent => parent.Height / 2 - getSwitchHeight(parent) / 2));

//Add a Label below the switch
mainLayout.Children.Add(switchLabel,
    Constraint.RelativeToParent(parent => parent.Width / 2 - getLabelWidth(parent) / 2),
    Constraint.RelativeToView(mySwitch, (parent, view) => view.Y + getSwitchHeight(parent) + 10));

Content = mainLayout;

double getSwitchWidth(RelativeLayout parent) => mySwitch.Measure(parent.Width, parent.Height).Request.Width;
double getSwitchHeight(RelativeLayout parent) => mySwitch.Measure(parent.Width, parent.Height).Request.Height;
double getLabelWidth(RelativeLayout parent) => switchLabel.Measure(parent.Width, parent.Height).Request.Width;
double getLabelHeight(RelativeLayout parent) => switchLabel.Measure(parent.Width, parent.Height).Request.Height;

选项2：Func<RelativeLayout, double>

使用Func动态检索Width和Height属性

var mainLayout = new RelativeLayout();

Func<RelativeLayout, double> getSwitchWidth = (parent) => mySwitch.Measure(parent.Width, parent.Height).Request.Width;
Func<RelativeLayout, double> getSwitchHeight = (parent) => mySwitch.Measure(parent.Width, parent.Height).Request.Height;
Func<RelativeLayout, double> getLabelWidth = (parent) => switchLabel.Measure(parent.Width, parent.Height).Request.Width;
Func<RelativeLayout, double> getLabelHeight = (parent) => switchLabel.Measure(parent.Width, parent.Height).Request.Height;

//Add the Switch to the center of the screen
mainLayout.Children.Add(mySwitch,
    Constraint.RelativeToParent(parent => parent.Width / 2 - getSwitchWidth(parent)/ 2),
    Constraint.RelativeToParent(parent => parent.Height / 2 - getSwitchHeight(parent) / 2));

//Add a Label below the switch
mainLayout.Children.Add(switchLabel,
    Constraint.RelativeToParent(parent => parent.Width / 2 - getLabelWidth(parent) / 2),
    Constraint.RelativeToView(mySwitch, (parent, view) => view.Y + getSwitchHeight(parent) + 10));

Content = mainLayout;

感谢@BrewMate教我这招！