Apple Newton Utilities User's Guide download pdf (Page 515)

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101

CHAPTER 12

Special-Purpose Objects for Data Storage and Retrieval

Using Special-Purpose Data Storage Objects 12-9

The NewVBO and NewCompressedVBO store methods create virtual binary

objects. Both methods require that you specify the class of the binary object to be

created, as well as the store on which VBO data is to reside.

The following code fragment uses the store method

NewVBO to create a new,

uncompressed, “blank” virtual binary object on the default store:

// create new uncompressed VBO of size 5 KB and class 'samples

local binData := GetDefaultStore():NewVBO('samples,5000);

Another way to create an uncompressed VBO is to pass nil as the

values of the

companderName and companderData parameters to the

NewCompressedVBO method, as the following code fragment shows:

// create new uncompressed VBO of size 5 KB and class 'samples

local binData := GetDefaultStore():NewCompressedVBO('samples, 5000,

nil, nil);

When you create a compressed VBO, you need to specify how the system is to

expand and compress data moved to and from the store associated with the VBO.

The system provides two compressor-expanders (also known as companders),

which compress and expand raw binary data on demand. The

companderName

parameter to the

NewCompressedVBO method indicates the compander to be used

for that particular VBO’s data.

The Lempel-Ziv compander is a suitable for most data types; its use is speciﬁed by

passing the string

"TLZStoreCompander" as the value of the companderName

parameter to the

NewCompressedVBO method. The pixel map compander is

specialized for use with pixel map data; its use is speciﬁed by passing the string

"TPixelMapCompander" as the value of the companderName parameter to the

NewCompressedVBO method.

▲ W ARNING

The pixel map compander makes certain assumptions about the

data passed to it; do not use it for any kind of data other than pixel

maps. For more information, see the description of the

NewCompressedVBO method (page 9-75) in

Newton

Programmer’s Reference

. ▲

Because both of the companders provided by the current implementation of the

system initialize themselves automatically, you must always pass

nil as the value

of the

companderArgs parameter to the NewCompressedVBO method.

To create a new compressed VBO, specify a compander and a store in the arguments

to the

NewCompressedVBO method, as shown in the following example:

// create new compressed VBO of size 5 KB and class 'pixMap

local binData := GetDefaultStore():NewCompressedVBO('pixMap,

5000,"TPixelMapCompander", nil);

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