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

Question

我决定重写这个问题。我试图让它变得清晰和简短，但没有成功。

我想要实现的是一个我可以放在任何东西(Rectangle、Image 等等)上并让它响应触摸手势的对象。不幸的是我遇到了一个我无法解决的问题。我找不到帮助，所以我在这里试试。

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
        }
    }
}

我想在 MyTouchSurface 中包含所有计算和函数，以便很容易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，这里的代码工作得很好，但它也会生成关于绑定(bind)循环的非常难看的错误。

发生的事情是:

• 我检查目标的 x 和 y 当它们改变时
• 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 changed”或“y changed”
• 我收到有关循环的错误消息，因为我的函数导致其自身再次执行

所以我想问:是否有任何其他简单的方法可以防止对象飞出屏幕并且不会同时收到绑定(bind)循环错误？

我知道我可以使用 Timer 不时地检查目标的 x 和 y，然后将它们带回来。

我知道我可以更改动画的 easing 和 duration 使其在容器边界上停止，但看起来它已停止。

说真的。有没有简单的方法？

---

无法正常工作的事情:

• 在容器外检测到对象后停止动画(如果它移动得非常快，它将停止在屏幕之外)
• 停止动画然后改变目标的x或y

---

感谢迄今为止帮助过我的所有人。很高兴知道我并不孤单:)

Answer 1

解决方案是简单地使用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: {

//    }
}