QML：有条件地设置属性组的不同属性

Question

如何一次性有条件地设置属性组的不同属性？

示例：假设有一个上下文属性_context.condition 可用。鉴于该值，我想为 qml 项目设置不同的锚点。

// Some item...
Rectangle {
    id: square
    width: 50
    height: 50

    // For simple properties this should work:
    color: { if (_context.condition) "blue"; else "red" }

    // But how to do it for complex properties like 'anchors'?
    // Note that I set different properties for different values of the condition.
    // Here is how I would do it, but this does not work:
    anchors: { 
        if (_context.condition) {
            // Anchors set 1:
            horizontalCenter: parent.horizontalCenter
            bottom: parent.bottom
            bottomMargin: 20
        } else {
            // Anchors set 2:
            verticalCenter: parent.verticalCenter
            right: parent.right
            rightMargin: 20
        }
    }
}

我在 Qt 5.3 中使用 QtQuick 2.0。谢谢！

Answer 1

你可以试试这个（未测试）：

anchors {
        horizontalCenter: _context.condition ? parent.horizontalCenter : undefined;
        bottom: _context.condition ? parent.bottom : undefined;
        bottomMargin: _context.condition ? 20 : undefined;
        verticalCenter: _context.condition ? undefined : parent.verticalCenter;    
        right: _context.condition ? undefined : parent.right;
        rightMargin: _context.condition ? undefined : 20;
        }

Resetting properties values

另外，根据this空花括号可以用来重置属性的值：

Item {
    property var first:  {}   // nothing = undefined
    property var second: {{}} // empty expression block = undefined
    property var third:  ({}) // empty object
}