Custom Shape

Written by Administrator

Wednesday, 07 April 2010

Article Index
Custom Shape
Optimising DefiningGeometry
A custom shape example
Repositioning a shape
Choosing a method

Page 5 of 5

Best practice?

So which method should we use?

It all depends. Recreating the geometry is easy and it isn't too inefficient if the geometry is quick to generate. There is a theoretical satisfaction in applying a manual shift to the co-ordinates of the stored geometry but in a general case it could become complex. Setting the geometries transform seems like overkill but it is extremely general - you can implement scaling or rotation in the same way. Given that hardware accelerated graphics can apply transformations efficiently this seems like the best general method.

Notice however that all of the examples in this article duck the issue of implementing the Shape properties as dependency properties.

There is one last big problem with the custom Shape class as it stands. Currently there is only a constructor with parameters and this means that we cannot instantiate a Tree using XAML as XAML needs a class to have a parameterless constructor.

We could just add a parameterless constructor and then allow the properties to be set after the creation of the instance. However, while it is reasonable to allow positional and even scale parameters to be set after the instance has been created, there is a sense in which L, s, t and dt determine the type of tree and these should be set when the tree is created.

There is also the problem that you can't generate a tree until all of these parameters have values. One easy way around the problem is to simply give each one a default value and generate the geometry each time one of them is set. This approach is fine if you are happy with regenerating the geometry each time a parameter is changed. However, if you want to generate the geometry once when the instance is created and then apply transformations to move, scale and rotate then it is more difficult to allow a parameterless constructor.

The solution is to use the ISupportInitilize interface and a batch parameter initialisation approach. This puts off the creation of the geometry until everything is defined and its honoured by XAML.

Coming soon - parameterless constructors with the help of ISupportInitialize.

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