Example of Lightning Components for handling Apex Callback Errors

ExampleCallback.js

action.setCallback(this, function(a){
  if (a.getState() === "SUCCESS") {                        
    //happy path stuff
  } else if (a.getState() === "ERROR"){
    var appEvent = $A.get("e.c:handleCallbackError");
    appEvent.setParams({
        "errors" : a.getError(),
        "errorComponentName" : "someUniqueName"
    });
    appEvent.fire();                        
  }
});
$A.enqueueAction(doUpdate);      

exampleComponent.cmp

<aura:component controller="myApex">   

...
    <!--whichever component is going to throw an apex callback error needs to register to use the error event-->
    <aura:registerEvent name="handleCallbackError" type="c:handleCallbackError"/>
    
    <!--add the errorHandler anywhere in the component, preferably at the outermost level-->
    <c:LightningErrorHandler errorHandlerName="someUniqueName"/>

...

</aura:component/>

lightningErrorHandler.cmp

<aura:component >
	<aura:attribute name="errorHandlerName" type="string" required="false" description="if set, only toasts errors when the name matches" access="global"/>
	<aura:handler event="c:handleCallbackError" action="{!c.makeToast}"   />	
</aura:component>

lightningErrorHandlerController.js

({
	makeToast : function(component, event, helper) {

		//should we skip this event?
		if (event.getParam("errorComponentName") && (component.get("v.errorHandlerName") !== event.getParam("errorComponentName"))){ 
			return; 
		}  

		var errors = event.getParam("errors");

		if (errors) {
			errors.forEach( function (error){

				//top-level error.  there can be only one
				if (error.message){
					helper.toastThis(error.message);					
				}

				//page-level errors (validation rules, etc)
				if (error.pageErrors){
					error.pageErrors.forEach( function(pageError) {
						helper.toastThis(pageError.message)						
					});					
				}

				if (error.fieldErrors){
					//field specific errors--we'll say what the field is					
					for (var fieldName in error.fieldErrors) {
						//each field could have multiple errors
						error.fieldErrors[fieldName].forEach( function (errorList){	
							helper.toastThis(errorList.message, "Field Error on " + fieldName + " : ")							
						});                                
					};  //end of field errors forLoop					
				} //end of fieldErrors if
			}); //end Errors forEach
		}
	}
})

LightningErrorHandlerHelper.js

({
	toastThis : function(message, title) {
		var toastEvent = $A.get("e.force:showToast");
		toastEvent.setParams({
			"title": title || "Error:",
			"message": message,
			"type": "error",
			"mode": "sticky"
		});
		toastEvent.fire();
	}
})

Do you by chance have code that shows how to throw or handle an exception in Apex, so that the resulting message shows up in action.getError()[0].pageErrors or some other predictable property?

I like to throw exceptions from Apex as AuraHandledException like below
try { // do stuff } catch (DomainSpecificException e) { throw new AuraHandledException('Some error message making sense for the receiving controller'); }

somehow this got some attention on twitter. If you're reading this recently, use https://github.com/mshanemc/lightningErrorHandler instead. It's got some more useful features built in.

HI,
I am using this component to throw error message from Apex callbacks.
But My Component is embedded in a Visualforce page.
Will the toast still show up ?