A major benefit of using Blockly is that it ensures the generated code is syntactically correct.
The Connection Checker is the object that enforces Blockly's rules about which connections are compatible. By default the checker applies a simple type system, as described in the Type Checks page.
The Checks
Blockly applies three levels of checks: safety, type, and drag checks.
Safety checks
Safety checks ensure that the blocks are on the same workspace, that the connections are on different blocks, and so on. These ensure that Blockly does not get into a bad state.
Safety checks also block nonsensical combinations, such as connecting two next connections.
Type checks
Developers can label connections with type information. Type checks use this information to enforce a type system--e.g. by blocking the connection of a string where a number is expected.
Drag checks
Drag checks are applied only when connecting blocks by dragging, rather than programmatically. For example, only connections within a certain radius should be considered during a drag.
Overriding the Connection Checker
Developers who wish to provide their own logic for type checks or drag checks
can register a substitute connection checker object that implements the
IConnectionChecker interface.
To implement your own safety checks: override doSafetyChecks
on your connection checker.
To implement your own type checks: override doTypeChecks
on your connection checker.
To implement your own drag checks: override doDragChecks
on your connection checker.
Example code
The strict connection checker plugin is a simple example of a custom checker.