Sandbox:Roy 043

Description
creates an App Link ( class), a type of object that references or links to another object, in the same document, or in another document. It is especially designed to efficiently duplicate a single object multiple times, which helps with the creation of complex assemblies from smaller subassemblies, and from multiple reusable components like screws, nuts, and similar fasteners.

The App Link object was newly introduced in v0.19; in the past, simple duplication of objects could be achieved with, but this is a less efficient solution due to its implementation which essentially creates a copy of the internal Shape of the source object. On the other hand, a Link references directly the original Shape, so it is more memory efficient.

By itself the Link object can behave like an array, duplicating its base object many times; this can be done by setting its property to  or larger. This "Link Array" object can also be created with the different array tools of the Draft Workbench, for example,, , and.

When used with the PartDesign Workbench, Links are intended to be used with, so it is recommended to set  to  to select the features of the entire Body, and not the individual features. To create arrays of the internal PartDesign Features, use, , and.

The tool is not defined by a particular workbench, but by the base system, thus it is found in the  that is available in all workbenches. The Link object, used in conjunction with to group various objects, forms the basis of the  Assembly3 and  Assembly4 Workbenches.

Same document with selection

 * 1) Select an object in the tree view or 3D view for which you wish to create a Link.
 * 2) Press the  button. The produced object has the same icon as the original object, but has an arrow overlay indicating it is a Link.

Same document without selection

 * 1) If no object is selected, press the  button to create an empty Link.svg Link.
 * 2) Go to the property editor, then click on the  property to open the Link selection dialog to choose an object, then press.
 * 3) Instead of choosing an entire object in the tree view, you can also pick subelements (vertices, edges, or faces) of a single object in the 3D view. In this case, the Link will duplicate only these subelements, and the arrow overlay will be different. This can also be done with.



External document

 * 1) Start with a document that has at least one object which will be the source of the Link.
 * 2) Open a new document or an existing document. For easier handling, use  to show both documents in the tree view. Before you proceed, save both documents. The Link won't be able to find its source and target unless both documents are saved on disk.
 * 3) In the first document, select the object that you wish to link; then switch tabs in the main view area to switch to the second document.
 * 4) Press . The produced object has the same icon as the original object, but has an additional arrow overlay indicating it is a Link coming from an external document.


 * When saving the document with the Link, it will also ask to save the source document which contains the original object.
 * To include the original object in the document with the Link, use or.
 * can be used on an existing Link object, in order to create a Link to a Link which ultimately resolves to the original object in the source document. This can be used with to pick only certain subelements as well.



Drag and drop
Instead of switching document tabs, you can create Links by performing a drag and drop operation in the tree view: select the source object from the first document, drag it, then drop it into the second document's name while holding the key in the keyboard.

Dragging and dropping results in different actions depending on the modifier key that is held.
 * Without modifier key it simply moves the object from one document to the other; an inclined arrow is shown in the cursor.
 * Holding the key copies the object; a plus sign is shown in the cursor.
 * Holding the key creates a Link; a pair of chain links is shown in the cursor.

For the and  modifiers, dragging and dropping can also be done with a single document. That is, dragging an object and dropping it into the same document's name can be used to create multiple copies or multiple Links to it.

Link Array
Draft OrthoArray.

When a Link is created, by default its is, so only a single Link object will be visible in the tree view.

Given that is  by default, when  is set to  or more, automatically more Links will be created below the first one; each new Link can be placed in the desired position by changing its own  property.

In similar way, each element of the array can have its own appearance changed, either by the and  properties, or by using the  menu on the entire array and then selecting individual faces; this is described in Overriding appearance.



Once you are satisfied with the placement and properties of the Link elements in the array, you may change to  in order hide the individual Links in the tree view; this has the benefit of making the system more responsive, particularly if you have many objects in the document.

When creating this type of Link array, you must place each of the elements manually; however, if you would like to use specific patterns to place the copies, you may use the array tools of the Draft Workbench, like, , and ; these commands can create normal copies or Link copies depending on the options at creation time.

Link to Std Part
can be used on in order to quickly duplicate groups of objects positioned in space, that is, assemblies. A Link to a Part will keep the visibility of its child objects synchronized with that Part. If you hide a child object in the Link, it will be hidden in the original Part, as well as in all other Links to that Part.



Link to Std Group
A regular does not possess a  property, so it cannot control the position of the objects inside of it. However, when is used with, the resulting Link behaves essentially like a , and can also be moved in space. A Link to a Group allows independent control of the visibility of its child objects.



Override material
When a Link is created, by default the is, so the Link will have the same appearance as the original.

When is set to, the  property will now control the appearance of the Link.

Regardless of the state of, it is possible to individually set the appearance of the subelements (vertices, edges, faces) of a Link.
 * 1) Select the Link in the tree view. Open the context menu (right-click), and pick.
 * 2) Now pick the individual subelements that you want in the 3D view, press, and change the properties including transparency.
 * 3) To remove the custom attributes, select the elements in the list, and press.
 * 4) When you are satisfied with the result, press  to close the dialog.

as of v0.19, the coloring of the subelements is subject to the topological naming problem so it should be done as the last modelling step, when the model is not subject to change any more.



Override colors
When is  and individual elements are listed in the tree view in a Link Array, each Link can be shown or hidden by pressing the  bar in the keyboard.

Another way to hide the individual elements is using the menu.
 * 1) Select the array, open the  menu (right click).
 * 2) In the 3D view, pick any subelement from any Link in the array.
 * 3) Press . An icon of an eye Invisible.svg should appear, indicating that this element has been hidden from the 3D view. The object will temporarily show itself when the cursor hovers over the Invisible.svg icon.
 * 4) You can click  to confirm the operation and close the dialog. The Link will remain hidden even if it is shown as visible in the tree view.



If you wish to restore the visibility of this array element, enter the dialog once more, pick the eye icon, then click on to remove the hidden status, and click  to confirm and close the dialog. The element will be visible in the 3D view again.

When the Link is for a or a, the  menu works in similar way as with arrays; it allows controlling the face color, entire object color, and visibility of the objects in the group.



Properties
The App Link ( class) is derived from the basic App DocumentObject ( class) and inherits all its properties.

The following are the specific properties available in the property editor. The App Link will additionally show the properties of the original. Hidden properties can be shown by using the command in the context menu of the property editor.

Data

 * : it indicates the source object of the App Link; this can be an entire object, or a subelement of it (vertex, edge, or face).
 * : it defaults to, in which case the Link will override the 's own placement. If it is set to , the Link will be placed in the same position as the , and its placement will be relative to the 's placement. This can also be achieved with.
 * : it is an offset applied on top of the of the . This property is normally hidden but appears if  is set to ; in this case,  now becomes hidden.
 * : the placement of the Link in absolute coordinates.
 * : it defaults to, in which case the tree view will show the individual Link copies, as long as is  or larger.
 * : it defaults to . If it is or larger, the App Link will behave like an array, and will duplicate the same  many times. If  is, each element in the array will be displayed in the tree view, and each can have its own  modified. Each Link copy will have a name based on the Link's Name, augmented by , where  is a number starting from . For example, with a single , the copies will be named , , , etc.
 * : name of the execute function that will run for this particular Link object. It defaults to . Set it to to disable it.
 * : list of Link elements that have had their color overriden.
 * : or.
 * : it defaults to . It is a factor for uniform scaling in each direction, , and . For example, a cube of x  x , that is scaled by , will result in a shape with dimensions  x  x.
 * : the scale factor for each component for all Link elements when  is  or larger. If  is other than, this same value will be used in the three components.
 * : the scale factor for each Link element.
 * : the visibility state of each Link element, either  or.
 * : the placement for each Link element.
 * : the list of Link elements.


 * : a custom class associated with this object. This only exists for the Python version. See Scripting.

View

 * : it defaults to ; it can be, , ; defines the style of the edges in the 3D view.
 * : a float that determines the width in pixels of the edges in the 3D view. It defaults to.
 * : it defaults to ; if set to it will override the 's material, and it will display the colors defined in.
 * : similar to, defines the size of the vertices.
 * : if it is, the object can be picked with the pointer in the 3D view. Otherwise, the object cannot be selected until this option is set to.


 * : this property includes sub-properties that describe the appearance of the object.
 * , it defaults to, which is displayed as on base 255, light blue.
 * , it defaults to, which is displayed as on base 255, dark gray.
 * , it defaults to, which is displayed as on base 255, black.
 * , it defaults to, which is displayed as on base 255, black.
 * , it defaults to
 * , it defaults to.


 * : if individual materials have been added, they will be listed here.
 * : if the individual faces or edges of the link have been overridden they will be listed here.
 * : if the individual materials of the link have been overridden they will be listed here.
 * : a custom viewprovider class associated with this object. This only exists for the Python version. See Scripting.
 * : a custom viewprovider class associated with this object. This only exists for the Python version. See Scripting.


 * : or.
 * : see the information in App FeaturePython.
 * : see the information in App FeaturePython.


 * : see the information in App FeaturePython.
 * : see the information in App FeaturePython.

Scripting
See also: FreeCAD Scripting Basics and scripted objects.

See Part Feature for the general information on adding objects to the document.

An App Link is created with the method of the document. its can be defined by changing its  property, or by using its  method.

The basic doesn't have a Proxy object so it can't be fully used for sub-classing.

For Python subclassing you should create a object.