SwiftUI View属性willSet& didSet属性观察器不起作用 [英] SwiftUI View Property willSet & didSet property observers not working

问题描述

我有一个带有附加ViewModel的SwiftUI(测试版5)视图.我想通过navigationLink导航到它并传递一个简单的参数(在本例中为FSAC)

I have a SwiftUI (Beta 5) view with an attached ViewModel. I want to navigate to it via a navigationLink and pass in a simple parameter (called FSAC in this case)

我使用

NavigationLink("Next", destination: MyTestView(FSAC: "testFsac"))

该视图具有一个带有willSet和didSet属性观察器的FSAC属性

The view has an FSAC Property with willSet and didSet property observer

struct MyTestView: View {
    @ObservedObject var vm = MyTestViewModel()

var FSAC: String {
    willSet {
        print("will set fsac")
    }
    didSet {
        print("did set fsac")
        vm.FSAC = FSAC
    }
}

var body: some View {
    VStack {
        Text("FSAC: \(FSAC)")
        Text("VM FSAC: \(vm.FSAC)")
    }
}

}

从不调用print语句.第一个文本框正确显示参数；第二个是空白.

The print statements are never called. The first text box displays the parameter correctly; the second is blank.

如何触发财产观察员?

更一般地说，是否存在使用NavigationLink将参数传递给具有ViewModel的View的正确"方法?

More generally, is there a "correct" way to use a navigationLink to pass parameters to a View that has a ViewModel?

推荐答案

感谢arsenius对为何财产观察员在这种情况下未触发的解释.

Thanks to arsenius for an explanation as to why the Property Observers did not fire in this instance.

作为修复，我删除了视图中的属性，并使用带有签名的init函数替换了该属性，该签名包含要从NavigationLink调用传递的必需数据.在初始化过程中，我直接在ViewModel上调用了一个函数.

As a fix, I removed the property in the view, and replaced it with an init function with a signature which included the required data to be passed from the NavigationLink call. Within the init, I called a function on the ViewModel directly.

struct MyTestView: View {

    @ObservedObject var vm = MyTestViewModel()

    init(FSAC: String) {
        if(FSAC != "") {
            vm.SetFsac(FSAC: FSAC)
        }
    }
...

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