Some small corrections

Added example expression (hardcoded for development)
Added example change for expressions (hardcoded for development)

MathPad/MPExpressionLayout.m

@@ -10,6 +10,7 @@
 #import "MPExpressionStorage.h"
 #import "MPFunctionLayout.h"
 #import "MPModel.h"
+#import "MPExpressionView.h"
 
 @implementation MPExpressionLayout
 
@@ -71,15 +72,20 @@
 
 #pragma mark Cache Methods
 
+// TODO: Return nil from caching with illegal index
+
 - (void)invalidate
 {
     _valid = NO;
     [self.parent invalidate];
+    [self.expressionView setNeedsDisplay:YES];
 }
 
-- (void)expressionEditedInRange:(NSRange)range replacementLength:(NSUInteger)length
+- (void)editedExpressionInRange:(NSRange)range replacementLength:(NSUInteger)length
 {
-    while (_symbolCache.count < self.expression.numberOfSymbols) {
+    // TODO: New symbols may also be inserted in the middle or at the beginning
+    NSInteger changeInLength = length - range.length;
+    while (_symbolCache.count < (self.expression.numberOfSymbols + changeInLength)) {
         [_symbolCache addObject:[NSNull null]];
     }
     NSMutableArray *newPlaceholders = [[NSMutableArray alloc] initWithCapacity:length];
@@ -101,7 +107,11 @@
 - (MPFunctionLayout *)functionLayoutForFunctionAtIndex:(NSUInteger)index;
 {
     if ([self hasCacheForSymbolAtIndex:index]) {
-        return _symbolCache[index];
+        id cacheObject = _symbolCache[index];
+        if ([cacheObject isKindOfClass:[NSValue class]]) {
+            return nil;
+        }
+        return cacheObject;
     }
      MPFunctionLayout *layout = [MPFunctionLayout functionLayoutForFunctionAtIndexPath:[self.expressionPath indexPathByAddingIndex:index] parent:self];
     while (index >= _symbolCache.count) {
@@ -177,6 +187,8 @@
                   atPoint:(NSPoint)point
 {
     id symbol = [self.expression symbolAtIndex:index];
+    // point.x = point.y = 0;
+    NSLog(@"draw Symbol: %@ at Point: (x: %f, y: %f)", symbol, point.x, point.y);
     if ([symbol isString]) {
         [self.textStorage setString:symbol];
         NSRange glyphRange = [self.layoutManager glyphRangeForTextContainer:self.textContainer];
@@ -184,6 +196,7 @@
                                             atPoint:point];
     } else {
         MPFunctionLayout *layout = [self functionLayoutForFunctionAtIndex:index];
+        NSLog(@"layout: %@, index: %ld", layout, index);
         [layout drawFunctionAtPoint:point];
     }
 }