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

Question

尝试在 Xamarin.Forms 中使用控件的Height和Width属性时，两者都返回 -1，这会导致相对布局在屏幕上偏心显示。

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

为什么 Xamarin.Forms 为Height和Width返回 -1 ？

Xamarin.Forms 返回 -1 作为这些属性的默认值，并保持 -1，直到 Xamarin.Forms 创建本机控件（例如 UIButton）并将该本机控件添加到布局中。

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

使用相对布局约束视图的最佳方式

选项 1：本地函数（需要 C# 7.0 或更高版本）

使用局部函数动态检索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 - getWidth(parent, mySwitch)/ 2),
    Constraint.RelativeToParent(parent => parent.Height / 2 - getHeight(parent, mySwitch) / 2));

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

Content = mainLayout;

static double getWidth(RelativeLayout parent, View view) => view?.Measure(parent.Width, parent.Height).Request.Width ?? -1;
static double getHeight(RelativeLayout parent, View view) => view?.Measure(parent.Width, parent.Height).Request.Height ?? -1;

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教我这个技巧！