PageUtils

Description

Class containing utilities that you don’t have to write from scratch. (But you knew that.)

Main class is PageUtils.

Starting from the top, then, with redirect()

global static PageReference redirect(String url){
        PageReference pageRef = new PageReference(url);
        pageRef.setRedirect(true);
        return pageRef;
}

When you need to send someone to a new page dynamically, you need to get a PageReference and set their page to it. Hence, redirect();

Posting messages - when someone clicks Submit() or does something that causes your page to do some work, you often need to give them information w/o necessarily redirecting them to a new page; addMessages() does that for you, and PageUtils has methods to set specific messages w/o (you guessed it) writing it from scratch:

    global static void addConfirm(String message){
        ApexPages.addMessage(new ApexPages.message(ApexPages.severity.CONFIRM,message));
    }
    
    global static void addInfo(String message){
        ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,message));
    }
    
    global static void addWarning(String message){
        ApexPages.addMessage(new ApexPages.message(ApexPages.severity.WARNING,message));
    }
    
    global static void addError(String message){
        ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,message));
    }
    
    global static void addFatal(String message){
        ApexPages.addMessage(new ApexPages.message(ApexPages.severity.FATAL,message));
    }

You might also want to check to see whether your page has accumulated any errors(perhaps in a Unit Test); PageUtils() has methods for that

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    global static ApexPages.Message[] getErrorMessages(){
        return PageUtils.getMessagesAtLevel(ApexPages.Severity.ERROR, true);
    }

global static Boolean hasErrorMessages(){
    final ApexPages.Message[] errorMessages = getErrorMessages();
    return errorMessages != null && errorMessages.size() > 0;
}

global static ApexPages.Message[] getErrorMessagesOnly(){
    return PageUtils.getMessagesAtLevel(ApexPages.Severity.ERROR, false);
}

global static Boolean hasErrorMessagesOnly(){
	final ApexPages.Message[] errorMessagesOnly = getErrorMessagesOnly();
    return errorMessagesOnly != null && errorMessagesOnly.size() > 0;
}   

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You might very much want to know what those messages are, perhaps to write to the system log for reference

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    global static String messagesToString(){
        return messagesToString(ApexPages.getMessages());
    }

global static String messagesToString(){
    return messagesToString(ApexPages.getMessages());
}

global static String messagesToString(ApexPages.Message[] messages){
    if(messages == null){
        return 'null';
    }
    if( messages.size() <= 0){
        return '{empty}';
    }
    String returnString = '';
    for(Integer i = 0; i < messages.size(); i++){
        if(i != 0){ returnString += '; '; }
        returnString += 'ApexPages.Message[';
        returnString += i;
        returnString += ']: (summary=';
        returnString += messages[i].getSummary(); 
        returnString += ',componentLabel=';
        returnString += messages[i].getComponentLabel(); 
        returnString += ',severity=';
        returnString += messages[i].getSeverity(); 
        returnString += ',detail=';
        returnString += messages[i].getDetail(); 
        returnString += ')';
    }
    return '{' + returnString + '}';
}

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If you only want to determine the level of a particular message, isMessageAtLevelOrHigher(message, level) does that as well

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global static Boolean isMessageAtLevelOrHigher(ApexPages.Message message, ApexPages.Severity level){
    if(level == null || message == null){
        return false;
    }
    ApexPages.Severity msgLevel = message.getSeverity();
    while(true){
        if(level == msgLevel){
            return true;    
        }
        if(msgLevel == ApexPages.Severity.CONFIRM){msgLevel = ApexPages.Severity.WARNING;}
        else if(msgLevel == ApexPages.Severity.INFO){msgLevel = ApexPages.Severity.WARNING;}
        else if(msgLevel == ApexPages.Severity.WARNING){msgLevel = ApexPages.Severity.ERROR;}
        else if(msgLevel == ApexPages.Severity.ERROR){msgLevel = ApexPages.Severity.FATAL;}
        else { break; }
    }
    return false;
}

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(This ^ looks like it would be really useful for Unit Tests…)