有两种方法可以创建，都是全局对象Qt提供的方法
一：用Qt.createComponent加载一个qml文件并创建Component
二：用Qt.createQmlObject从一个qml字符串创建Component
注意，以上两种方法返回的是Component，Component在QML中是一种类型，参考文档：
http://qt-project.org/doc/qt-5/qml-qtqml-component.html#details
因此还要调用Component的createObject来创建真正可用的对象。createObject第一个参数是指定挂在谁的下方，也就是父窗口是谁。如果传递null,则代表暂时不显示。
如果要销毁，只需要调用对象的destroy方法即可。
调用createObject方法需要注意qml文件已经被加载完成才行，因为这种机制允许远程加载qml文件，所以需要检查Component的status属性：
This property holds the status of component loading. The status can be one of the following:
Component.Null - no data is available for the component
Component.Ready - the component has been loaded, and can be used to create instances.
Component.Loading - the component is currently being loaded
Component.Error - an error occurred while loading the component. Calling errorString() will provide a human-readable description of any errors.
This property holds the status of component loading. The status can be one of the following:
Component.Null - no data is available for the component
Component.Ready - the component has been loaded, and can be used to create instances.
Component.Loading - the component is currently being loaded
Component.Error - an error occurred while loading the component. Calling errorString() will provide a human-readable description of any errors.
下面是例子代码：
function createItem() {
if (itemComponent.status == Component.Ready && draggedItem == null) {
draggedItem = itemComponent.createObject(
window,
{
"image": paletteItem.image,
"x": posnInWindow.x,
"y": posnInWindow.y,
"z": 3
}
);
// make sure created item is above the ground layer
} else if (itemComponent.status == Component.Error) {
draggedItem = null;
console.log("error creating component");
console.log(itemComponent.errorString());
}
}
注意在JavaScript中没有真正的类型，类型也是由对象模拟的。所以Qml 的Component在JavaScript代码中也表现为一个对象，比如上面代码的itemComponent就是Qml的Component，但也是一个对象。用它的createObject再创建真正可用的对象挂在window对象下面。
官方文档参考：
http://qt-project.org/doc/qt-4.8/qdeclarativedynamicobjects.html
http://qt-project.org/doc/qt-5/qml-qtqml-qt.html#createComponent-method
http://qt-project.org/doc/qt-5/qml-qtqml-qt.html#createQmlObject-method
这就和web开发用JavaScript动态创建HTML的tag并插入到DOM模型中很像了。