Adding New Box Classes

Adding New Box Classes

1. Paste the following into the python source code file (either iso.py or non-iso.py) and edit <boxtype> to match the box type you are writing a class definition for (refer to the box_factory() or box_factory_non_iso() functions to understand why naming the class using a specific convention is important). Also choose from <Mp4Box|Mp4FullBox> to inherit from the appropriate superclass depending on whether the box incorporates version and header fields

    `class <BoxType>Box(<Mp4Box|Mp4FullBox>):`
   
    `def __init__(self, fp, header, parent):`
        `super().__init__(fp, header, parent)`
        `try:`
            `# At the point of entry into the try-block, the pointer fp will` 
            `# be pointing to the start of the box payload`
            `# i.e. immediately after the header (and version + flag information if derived from FullBox)`
   
            `# replace the pass statement with statements that actually read the data`
            `# e.g. self.box_info['my_field'] = read_u32(fp)`
            `pass`
        `finally:`
            `fp.seek(self.start_of_box + self.size)`
   

2. Edit the code inside the try block of the class definition of step 1 to read the data fields appropriate for the box type.
   Note: try-finally has been adopted as a project convention even though it does not really do anything at run-time as there is no except clause. The reasons for using it are:

   • it emphasises that the file pointer must be located at the end of the box on leaving __init__.
   • the mp4 file should parse successfully even when the class definition is incomplete.
   • it is easy to cast your eye over the source code file and verify that all __init__ end with the line fp.seek(self.start_of_box + self.size)