42 lines
2.4 KiB
JavaScript
42 lines
2.4 KiB
JavaScript
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let EXPORTED_SYMBOLS = ['hookFunction'];
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/**
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* Add hooks to a function to execute before and after it. The function to modify is functionContext[functionName]. Call only once per function - modification is not supported.
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*
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* Other addons wishing to access the original function may do so using the .originalFunction member of the replacement function. This member can also be set if required, to insert a new function replacement into the chain rather than appending.
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*
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* @param functionContext The object on which the function is a property
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* @param functionName The name of the property containing the function (on functionContext)
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* @param onBeforeFunction A function to be called before the hooked function is executed. It will be passed the same parameters as the hooked function. It's return value will be passed on to onAfterFunction
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* @param onAfterFunction A function to be called after the hooked function is executed. The parameters passed to it are: onBeforeFunction return value, arguments object from original hooked function, return value from original hooked function. It's return value will be returned in place of that of the original function.
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* @returns A function which can be called to safely un-hook the hook
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*/
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function hookFunction(functionContext, functionName, onBeforeFunction, onAfterFunction) {
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let originalFunction = functionContext[functionName];
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if (!originalFunction) {
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throw new Error("Could not find function " + functionName);
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}
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let replacementFunction = function() {
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let onBeforeResult = null;
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if (onBeforeFunction) {
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onBeforeResult = onBeforeFunction.apply(this, arguments);
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}
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let originalResult = replacementFunction.originalFunction.apply(this, arguments);
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if (onAfterFunction) {
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return onAfterFunction.call(this, onBeforeResult, arguments, originalResult);
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} else {
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return originalResult;
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}
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}
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replacementFunction.originalFunction = originalFunction;
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functionContext[functionName] = replacementFunction;
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return function () {
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// Not safe to simply assign originalFunction back again, as something else might have chained onto this function, which would then break the chain
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// Unassigning these variables prevent any effects of the hook, though the function itself remains in place.
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onBeforeFunction = null;
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onAfterFunction = null;
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};
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}
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