mirror of https://github.com/status-im/metro.git
61 lines
5.4 KiB
Markdown
61 lines
5.4 KiB
Markdown
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---
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id: bundling
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title: Bundling
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---
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When bundling, each of the modules gets assigned a numeric id, meaning no dynamic requires are supported. Requires are changed by its numeric version, and modules are stored in different possible formats. Three different formats of bundling are supported:
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## Plain bundle
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This is the standard bundling format. In this format, all files are wrapped with a function call, then added to the global file. This is useful for environments that expect a JS only bundle (e.g. a browser). Just requiring the entry point with the `.bundle` extension should trigger a build of it.
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## Indexed RAM bundle
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This format composes the bundle as a binary file, which format has the following parts (all numbers are expressed in Little Endian):
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* A magic number: a `uint32` must be located at the beginning of the file, with the value '0xFB0BD1E5'. This is used to verify the file.
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* An offset table: the table is a sequence of `uint32` pairs, with a header
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* For the header, two `uint32`s can be found: the length of the table, and the length of the startup code.
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* For the pairs, they represent the offset in the file and the length of code module, in bytes.
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* Each of the modules, finished by a null byte (`\0`).
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```
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` 0 1 2 3 4 5 6
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Magic number | Header size |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Startup code size | Module 0 offset |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Module 0 length | |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
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+ ... +
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+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| | Module n offset |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Module n length | Module 0 code | Module 0 code | ... | \0 |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Module 1 code | Module 1 code | ... | \0 | |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
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+ ... +
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+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| | Module n code | Module n code | ... | \0 |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+`
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```
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This structure is optimal for an environment that is able to load all code in memory at once:
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* By using the offset table, one can load any module in constant time, where the code for module `x` is located at `file[(x + 3) * sizeof(uint32)]`. Since there is a null character (`\0`) separating all modules, usually length does not even need to be used, and the module can be loaded directly as an ASCIIZ string.
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* Startup code is always found at `file[sizeof(uint32)]`.
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This bundling is usually used by iOS.
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## File RAM bundle
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Each module is stored as a file, with the name `js-modules/${id}.js`, plus an extra file called `UNBUNDLE` is created, which its only content is the magic number, `0xFB0BD1E5`. Note that the `UNBUNDLE` file is created at the root.
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This bundling is usually used by Android, since package contents are zipped, and access to a zipped file is much faster. If the indexed format was used instead, all the bundled should be unzipped at once to get the code for the corresponding module.
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