3.8 Block

Blocks are like mini drawings. The figure shows a block, inserted at two different places in the drawing.

Inserting a block does not copy the objects of the block. A reference to the block is created in the form of a block insertion object. The block insertion object (or Block insert for short) has a number of attributes

• a reference to the block that it inserts. If there are any changes in the block, all insertion objects that reference the block will change with it
• a Linear transformation matrix that describes a possible translation, rotation, scaling, mirroring, shear or combination of those operations. In the figure, one of the block inserts is mirrored and both inserts are translated, but still they are insertions of the same block
• an insert line color. The default value is kCADColorUncolored, which retains individual line colors. Setting it to another color, recursively overrules line colors of objects in the block
• an insert fill color. The default value is kCADColorUncolored, which retains individual fill colors. Setting it to another color, recursively overrules fill colors of objects in the block
• a layer. The insertion object is placed on a layer and individual objects of the blocks are also placed on layers. When these layers have different attribute values, the weakest value wins. For example, when the block insert’s layer is dimmed, all objects in the placed block will also be dimmed, unless a block object is put on an invisible layer (and therefore invisible in each insertion).

A block can be inserted not only in a drawing but also in another block. DemoCAD lets you edit blocks recursively with option double-click, the code is worth studying.

In fact, there is little difference between a block and the true drawing, except that the true drawing has a drawing plane attached and is immediately visible. The true drawing can be considered the root block in the drawing’s block hierarchy and for programming purposes the true drawing is considered a block with a special block ID assigned, kCADRootBlockID.

A block has the following attributes

• the block’s ID, a number (of type longint) that uniquely identifies it
• the block’s name. The name of a block need not be unique. When pasting objects from other areas, not only the block’s name, attributes and unmasked keys identify the block, but also its content in the form of objects
• the block’s DXF flag, kCADTrue if the block was imported from a DXF file, kCADFalse if not (see Importing DXF files)
• the block’s DXF dimension flag, kCADTrue if the block is a dimension in the DXF file from which it was imported (see Importing DXF files)
• the block’s keys can be used to further identify the block, for example to distinguish between user blocks and application specific blocks, see Keys.

Methods
CADAddBlock, CADRemoveBlock, CADGetBlocks, CADGetBlockInfo, CADSetBlockName, CADGetBlockKey, CADSetBlockKey, CADAddInsertXY, CADAddInsert, CADObjInsert, CADGetInsertID, CADGetInsert, CADGetInsertXY, CADSetInsertID, CADSetInsert, CADSetInsertXY, CADGetInsertColor, CADSetInsertColor, CADClear.