refactor Use and Defs element with native code support(iOS)

ios/Text/RNSVGText.m

@@ -81,7 +81,7 @@ static void RNSVGFreeTextFrame(RNSVGTextFrame frame)
         }
         // We should consider snapping this shift to device pixels to improve rendering quality
         // when a line has subpixel width.
-        CGAffineTransform offset = CGAffineTransformMakeTranslation(-shift, frame.baseLine + frame.lineHeight * i + (self.path == NULL ? 0 : -frame.lineHeight));
+        CGAffineTransform offset = CGAffineTransformMakeTranslation(-shift, frame.baseLine + frame.lineHeight * i + (self.path ? -frame.lineHeight : 0));
         CGPathAddPath(path, &offset, [self setLinePath:frame.lines[i]]);
     }
     
@@ -115,11 +115,11 @@ static void RNSVGFreeTextFrame(RNSVGTextFrame frame)
         for(CFIndex j = 0; j < runGlyphCount; ++j, ++glyphIndex) {
             CGPathRef letter = [cache pathForGlyph:glyphs[j] fromFont:runFont];
             CGPoint point = positions[j];
-            if (letter != NULL) {
+            if (letter) {
                 CGAffineTransform transform;
                 
                 // draw glyphs along path
-                if (self.path != NULL) {
+                if (self.path) {
                     CGPoint slope;
                     CGRect bounding = CGPathGetBoundingBox(letter);
                     UIBezierPath* path = [UIBezierPath bezierPathWithCGPath:self.path];

ios/Text/RnSVGGlyphCache.m

@@ -13,7 +13,7 @@
 -(id)init
 {
     self = [super init];
-    if(self != nil)
+    if(self)
     {
         cache = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, NULL, &kCFTypeDictionaryValueCallBacks);
     }
@@ -29,7 +29,7 @@
 {
     // First we lookup the font to get to its glyph dictionary
     CFMutableDictionaryRef glyphDict = (CFMutableDictionaryRef)CFDictionaryGetValue(cache, font);
-    if(glyphDict == NULL)
+    if(!glyphDict)
     {
         // And if this font hasn't been seen before, we'll create and set the dictionary for it
         glyphDict = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, NULL, &kCFTypeDictionaryValueCallBacks);
@@ -38,11 +38,11 @@
     }
     // Next we try to get a path for the given glyph from the glyph dictionary
     CGPathRef path = (CGPathRef)CFDictionaryGetValue(glyphDict, (const void *)(uintptr_t)glyph);
-    if(path == NULL)
+    if(!path)
     {
         // If the path hasn't been seen before, then we'll create the path from the font & glyph and cache it.
         path = CTFontCreatePathForGlyph(font, glyph, NULL);
-        if(path == NULL)
+        if(!path)
         {
             // If a glyph does not have a path, then we need a placeholder to set in the dictionary
             path = (CGPathRef)kCFNull;