如何省略"Binding loop detected for property"警告？

Question

我决定重写这个问题。我试图让它清晰而简短，但没有成功。

我想要实现的是一个我可以放在任何东西上的物体(Rectangle，Image等等(，并让它对触摸手势做出反应。不幸的是，我遇到了一个我无法解决的问题。我找不到帮助，所以我在这里尝试。

这是简单的main.qml：

import QtQuick 2.5
import QtQuick.Controls 1.3
import QtQuick.Window 2.2
import QtQuick.Dialogs 1.2
ApplicationWindow {
    title: qsTr("Touch surface test")
    width: 640
    height: 480
    visible: true

    Rectangle {
        width: 200
        height: 60
        color: "blue"
        MyTouchSurface {
            id: myTouchSurface
            anchors.fill: parent
        }
    }
}

这里是MyTouchSurface.qml

import QtQuick 2.5
MultiPointTouchArea {
    id: myTouchSurface
    // this is object I want to respond to touch gestures
    property Item target: parent
    // this is object I want to keep target in. I won't let target get out of it
    property Item container: parent.parent
    property double targetX: target.x
    property double targetY: target.y
    property double centerX: target.width / 2
    property double centerY: target.height / 2
    property double lastTouchX
    property double lastTouchY
    property double lastXDrag
    property double lastYDrag
    // here I calculate received touches and move target
    onPressed: {
        lastTouchX = touchPoints[0].sceneX
        lastTouchY = touchPoints[0].sceneY
        if (slidingAnimation.running)
            slidingAnimation.stop()
    }
    onTouchUpdated: {
        if (touchPoints.length) {
            target.x += touchPoints[0].sceneX - lastTouchX
            target.y += touchPoints[0].sceneY - lastTouchY
            lastXDrag = touchPoints[0].sceneX - lastTouchX
            lastYDrag = touchPoints[0].sceneY - lastTouchY
            lastTouchX = touchPoints[0].sceneX
            lastTouchY = touchPoints[0].sceneY
        }
        else
            startSliding()
    }
    // when lifting fingers off screen I want to slide target across it's container
    function startSliding() {
        slidingAnimation.toX = target.x + lastXDrag * 10
        slidingAnimation.toY = target.y + lastYDrag * 10
        slidingAnimation.restart()
    }
    // This is animation responsible for sliding the target
    ParallelAnimation {
        id: slidingAnimation
        property double toX
        property double toY
        property int animationDuration: 250
        NumberAnimation {
            target: myTouchSurface.target
            property: "x"
            to: slidingAnimation.toX
            duration: slidingAnimation.animationDuration
            easing.type: Easing.OutQuad
        }
        NumberAnimation {
            target: myTouchSurface.target
            property: "y"
            to: slidingAnimation.toY
            duration: slidingAnimation.animationDuration
            easing.type: Easing.OutQuad
        }
    }
    // this is how I keep target object within container
    onTargetXChanged: {
        if (container != null) {
            if (target.x + centerX < 0)
                target.x = -centerX
            else if (target.x + centerX > container.width)
                target.x = container.width - centerX
        }
    }
    onTargetYChanged: {
        if (container != null) {
            if (target.y + centerY < 0)
                target.y = -centerY
            else if (target.y + centerY > container.height)
                target.y = container.height - centerY
        }
    }
}

我希望将所有计算和函数都包含在MyTouchSurface中，以便轻松应用于其他对象并在另一个项目中使用它。

我遇到的问题是我不知道如何防止目标对象移出容器。好吧，这里的代码运行得很好，但它也产生了关于绑定循环的非常丑陋的错误。

这是怎么回事：

• 当目标发生变化时，我会检查目标的 x 和 y
• 如果 x 或 y 超出指定区域，我将它们移回
• 我再次收到信号"x 更改"或"y 更改">
• 我收到有关循环的错误，因为我的函数导致自己再次执行

所以我在问：有没有其他简单的方法可以防止对象飞离屏幕并同时不接收绑定循环错误？

我知道我可以使用Timer不时检查目标的x和y，并将它们带回来。

我知道我可以更改动画的easing和duration，使其停止在容器边界本身，但看起来它已停止。

认真地。难道就没有简单的办法吗？

---

不起作用的事情：

• 在容器外检测到对象后停止动画(如果它移动得非常快，它将停止退出屏幕(
• 停止动画，然后更改目标的x或y

---

感谢到目前为止帮助我的每个人。很高兴知道我并不孤单:)

Answer 1

解决方案很简单Connections：

import QtQuick 2.5
MultiPointTouchArea {
    id: myTouchSurface
    // this is object I want to respond to touch gestures
    property Item target: parent
    // this is object I want to keep target in. I won't let target get out of it
    property Item container: parent.parent
    property double targetX: target.x
    property double targetY: target.y
    property double centerX: target.width / 2
    property double centerY: target.height / 2
    property double lastTouchX
    property double lastTouchY
    property double lastXDrag
    property double lastYDrag
    // here I calculate received touches and move target
    onPressed: {
        lastTouchX = touchPoints[0].sceneX
        lastTouchY = touchPoints[0].sceneY
        if (slidingAnimation.running)
            slidingAnimation.stop()
    }
    onTouchUpdated: {
        if (touchPoints.length) {
            target.x += touchPoints[0].sceneX - lastTouchX
            target.y += touchPoints[0].sceneY - lastTouchY
            lastXDrag = touchPoints[0].sceneX - lastTouchX
            lastYDrag = touchPoints[0].sceneY - lastTouchY
            lastTouchX = touchPoints[0].sceneX
            lastTouchY = touchPoints[0].sceneY
        }
        else
            startSliding()
    }
    // when lifting fingers off screen I want to slide target across it's container
    function startSliding() {
        slidingAnimation.toX = target.x + lastXDrag * 10
        slidingAnimation.toY = target.y + lastYDrag * 10
        slidingAnimation.restart()
    }
    // This is animation responsible for sliding the target
    ParallelAnimation {
        id: slidingAnimation
        property double toX
        property double toY
        property int animationDuration: 250
        NumberAnimation {
            target: myTouchSurface.target
            property: "x"
            to: slidingAnimation.toX
            duration: slidingAnimation.animationDuration
            easing.type: Easing.OutQuad
        }
        NumberAnimation {
            target: myTouchSurface.target
            property: "y"
            to: slidingAnimation.toY
            duration: slidingAnimation.animationDuration
            easing.type: Easing.OutQuad
        }
    }
    Connections{
        target:myTouchSurface.target
        onXChanged:{
            if (container != null) {
                if (target.x + centerX < 0)
                    target.x = -centerX
                else if (target.x + centerX > container.width)
                    target.x = container.width - centerX
            }
        }
        onYChanged:{
            if (container != null) {
                if (target.y + centerY < 0)
                    target.y = -centerY
                else if (target.y + centerY > container.height)
                    target.y = container.height - centerY
            }
        }
    }
    // this is how I keep target object within container
//    onTargetXChanged: {
//    }
//    onTargetYChanged: {
//    }
}

Answer 2

我认为您的问题是您在属性更新处理程序(onTargetXChanged()(内更新targetX或target.x(y

(。

这可能会被 QML 引擎拾取为竞价循环，就好像您将target.x从onTargetXChanged()更改信号onTargetXChanged()因为它在属性double targetX: target.x中绑定到它时将再次触发。

你可以尝试这样的事情：

TouchArea {
  property Item target  //  target is the Image object being moved
  property double targetX: target.x
  property double targetY: target.y
  onTargetXChanged: {
    /* if object is out of specified area move x respectively */
    performPropertyUpdate(args)
  }
  onTargetYChanged: {
    /* if object is out of specified area move y respectively */
    performPropertyUpdate(args)
  }
  function performPropertyUpdate(args) {
    /* actual setting of target.x or target.y */
  }
}

Answer 3

要解决这个问题myX应该是x的别名，

property alias myX: rectangle2.x

你遇到这个问题是因为myX正在更改x的值，x正在更改myX的值：一个循环。