Improved error messages

Author: mweststrate

src/core/action.ts

@@ -89,9 +89,18 @@ export function isStrictModeEnabled(): boolean {
 }
 
 export function allowStateChanges<T>(allowStateChanges: boolean, func: () => T): T {
+	// TODO: deprecate / refactor this function in next major
+	// Currently only used by `@observer`
+	// Proposed change: remove first param, rename to `forbidStateChanges`,
+	// require error callback instead of the hardcoded error message now used
+	// Use `inAction` instead of allowStateChanges in derivation.ts to check strictMode
 	const prev = allowStateChangesStart(allowStateChanges);
-	const res = func();
-	allowStateChangesEnd(prev);
+	let res;
+	try {
+		res = func();
+	} finally {
+		allowStateChangesEnd(prev);
+	}
 	return res;
 }
 

src/core/derivation.ts

@@ -109,7 +109,7 @@ export function checkIfStateModificationsAreAllowed(atom: IAtom) {
 		fail(getMessage("m031") + atom.name);
 	// Should not be possible to change observed state outside strict mode, except during initialization, see #563
 	if (!globalState.allowStateChanges && atom.observers.length > 0)
-		fail(getMessage(globalState.strictMode ? "m030" : "m030") + atom.name);
+		fail(getMessage(globalState.strictMode ? "m030a" : "m030b") + atom.name);
 }
 
 /**

src/utils/messages.ts

@@ -28,6 +28,8 @@ const messages = {
 "m028" : "It is not allowed to set `useStrict` when a derivation is running" ,
 "m029" : "INTERNAL ERROR only onBecomeUnobserved shouldn't be called twice in a row" ,
 "m030" : "Since strict-mode is enabled, changing observable values outside actions is not allowed. Please wrap the code in an `action` if this change is intended. Tried to modify: " ,
+"m030a" : "Since strict-mode is enabled, changing observed observable values outside actions is not allowed. Please wrap the code in an `action` if this change is intended. Tried to modify: " ,
+"m030b" : "Side effects like changing state are not allowed at this point. Are you trying to modify state from, for example, the render function of a React component? Tried to modify: " ,
 "m031" : "Computed values are not allowed to not cause side effects by changing observables that are already being observed. Tried to modify: ",
 "m032" : "* This computation is suspended (not in use by any reaction) and won't run automatically.\n	Didn't expect this computation to be suspended at this point?\n	  1. Make sure this computation is used by a reaction (reaction, autorun, observer).\n	  2. Check whether you are using this computation synchronously (in the same stack as they reaction that needs it).",
 "m033" : "`observe` doesn't support the fire immediately property for observable maps." ,

test/strict-mode.js

@@ -1,7 +1,7 @@
 var test = require('tape');
 var mobx = require('../');
 
-var strictError = /Since strict-mode is enabled, changing observable values outside actions is not allowed. Please wrap the code in an `action` if this change is intended. Tried to modify: /;
+var strictError = /Since strict-mode is enabled, changing observed observable values outside actions is not allowed. Please wrap the code in an `action` if this change is intended. Tried to modify: /;
 
 test('strict mode should not allow changes outside action', t => {
 	var a = mobx.observable(2);
@@ -189,7 +189,7 @@ test('strict mode checks', function(t) {
         mobx.extras.allowStateChanges(false, function() {
             x.set(4);
         });
-    });
+    }, /Side effects like changing state are not allowed at this point/);
 
 	mobx.extras.resetGlobalState();
 	d()