在 SwiftUI 中使渐变视图动画颜色向一个方向旋转

Question

So I have a linear gradient view and I have the colors animating with the .animation() modifier. Right now, the colors swirl back and forth as I adjust the start and end points of the gradient and use an .easeInAndOut() modifier to animate. I want the colors to swirl in one direction, clockwise, repeatedly, but instead it swirls back and forth.

struct AnimatedBackground: View {
   @State var start = UnitPoint.leading
   @State var end = UnitPoint.trailing
   
 
   
   let timer = Timer.publish(every: 1, on: .main, in: .default).autoconnect()
   let colors: [Color] = [ .blue, .red ]
   
   
   var body: some View {
       
       
       LinearGradient(gradient: Gradient(colors: colors), startPoint: start, endPoint: end)
           .animation(Animation.easeInOut(duration: 3).repeatForever())
           .onReceive(timer, perform: { _ in
               
              
   
                self.start = .topLeading
               self.end = .bottomTrailing
               
           
               self.start = .bottom
               self.end = .top
           
            
               self.start = .bottomLeading
               self.end = .topTrailing
               
               
               self.start = .leading
               self.end = .trailing
               
        
               self.start = .topLeading
               self.end =  .bottomTrailing
               
               
               self.start = .top
               self.end = .bottom
               
               
               self.start = .bottomLeading
               self.end = .topTrailing
               
            
               self.start = .leading
               self.end = .trailing
               
            
               
           }).edgesIgnoringSafeArea(.all)
   }
}

The gradient colors swirl back and forth . I want the colors to swirl in one clockwise direction, smoothly and repeatedly.

Answer 1

Let's debug! First, before and after your huge sequence of self.start = ... , try adding print statements.

print("Before: \(start), \(end)")
self.start = .topLeading
self.end = .bottomTrailing
self.start = .bottom
self.end = .top
self.start = .bottomLeading
self.end = .topTrailing
self.start = .leading
self.end = .trailing
self.start = .topLeading
self.end =  .bottomTrailing
self.start = .top
self.end = .bottom
self.start = .bottomLeading
self.end = .topTrailing
self.start = .leading
self.end = .trailing
print("After: \(start), \(end)")

You'll notice that both print s happen at pretty much the exact same time.

Before: UnitPoint(x: 0.0, y: 0.5), UnitPoint(x: 1.0, y: 0.5)
After: UnitPoint(x: 0.0, y: 0.5), UnitPoint(x: 1.0, y: 0.5)

This is because Swift code executes instantly, one line after another in an extremely small amount of time. As a result, start always becomes (x: 0.0, y: 0.5) ( .leading ), and end always becomes (x: 1.0, y: 0.5) ( .trailing ). Your gradient probably doesn't even animate at all!

To fix, you should simply get the current value of start and end , then manually calculate the next value.

struct AnimatedBackground: View {
    @State var start = UnitPoint.leading
    @State var end = UnitPoint.trailing

    let timer = Timer.publish(every: 1, on: .main, in: .default).autoconnect()
    let colors: [Color] = [.blue, .red]

    var body: some View {
        LinearGradient(gradient: Gradient(colors: colors), startPoint: start, endPoint: end)
            .animation(Animation.easeInOut(duration: 3).repeatForever(), value: start) /// don't forget the `value`!
            .onReceive(timer) { _ in
                
                self.start = nextPointFrom(self.start)
                self.end = nextPointFrom(self.end)

            }
            .edgesIgnoringSafeArea(.all)
    }
    
    /// cycle to the next point
    func nextPointFrom(_ currentPoint: UnitPoint) -> UnitPoint {
        switch currentPoint {
        case .top:
            return .topLeading
        case .topLeading:
            return .leading
        case .leading:
            return .bottomLeading
        case .bottomLeading:
            return .bottom
        case .bottom:
            return .bottomTrailing
        case .bottomTrailing:
            return .trailing
        case .trailing:
            return .topTrailing
        case .topTrailing:
            return .top
        default:
            print("Unknown point")
            return .top
        }
    }
}

Result:

Note: To get an even smoother animation, maybe check out rotationEffect .