Fix text crash (#2442)

* [iOS] Fix crash that _view may be deallocated in other threads but still used while drawing text to temp context. The _view is unused at all!

* [iOS] Fix crash that self is not properly used in block.

ios/sdk/WeexSDK/Sources/Component/WXTextComponent.mm

@@ -377,10 +377,9 @@ - (UIImage *)drawRect:(CGRect)rect;
 {
     CGContextRef context = UIGraphicsGetCurrentContext();
     if (_isCompositingChild) {
-        [self drawTextWithContext:context bounds:rect padding:_padding view:nil];
+        [self drawTextWithContext:context bounds:rect padding:_padding];
     } else {
-        WXTextView *textView = (WXTextView *)_view;
-        [self drawTextWithContext:context bounds:rect padding:_padding view:textView];
+        [self drawTextWithContext:context bounds:rect padding:_padding];
     }
 
     return nil;
@@ -741,7 +740,7 @@ - (void)_updateAttributesOnComponentThread:(NSDictionary *)attributes
     [self syncTextStorageForView];
 }
 
-- (void)drawTextWithContext:(CGContextRef)context bounds:(CGRect)bounds padding:(UIEdgeInsets)padding view:(WXTextView *)view
+- (void)drawTextWithContext:(CGContextRef)context bounds:(CGRect)bounds padding:(UIEdgeInsets)padding
 {
     if (bounds.size.width <= 0 || bounds.size.height <= 0) {
         return;

ios/sdk/WeexSDK/Sources/Display/WXComponent+Display.m

@@ -105,19 +105,26 @@ - (void)didFinishDrawingLayer:(BOOL)success
 
 - (WXDisplayBlock)_displayBlock
 {
+    __weak WXComponent* wself = self;
     WXDisplayBlock displayBlock = ^UIImage *(CGRect bounds, BOOL(^isCancelled)(void)) {
         if (isCancelled()) {
             return nil;
         }
 
-        UIGraphicsBeginImageContextWithOptions(bounds.size, [self _bitmapOpaqueWithSize:bounds.size] , 0.0);
-        UIImage *image = [self drawRect:bounds];
-        if (!image) {
-            image = UIGraphicsGetImageFromCurrentImageContext();
+        __strong WXComponent* sself = wself;
+        if (sself) {
+            UIGraphicsBeginImageContextWithOptions(bounds.size, [sself _bitmapOpaqueWithSize:bounds.size] , 0.0);
+            UIImage *image = [sself drawRect:bounds];
+            if (!image) {
+                image = UIGraphicsGetImageFromCurrentImageContext();
+            }
+            UIGraphicsEndImageContext();
+
+            return image;
+        }
+        else {
+            return nil;
         }
-        UIGraphicsEndImageContext();
-
-        return image;
     };
 
     return displayBlock;