DCHECK when loading images that do not exist.

ios/chrome/app/startup/setup_debugging.mm

Index: ios/chrome/app/startup/setup_debugging.mm
diff --git a/ios/chrome/app/startup/setup_debugging.mm b/ios/chrome/app/startup/setup_debugging.mm
index d918ccb836971567d2034d6cf317a53de7340921..e620d4d8131a1693e0f4927125cc524a5289447e 100644
--- a/ios/chrome/app/startup/setup_debugging.mm
+++ b/ios/chrome/app/startup/setup_debugging.mm
@@ -4,6 +4,8 @@
 
 #include "ios/chrome/app/startup/setup_debugging.h"
 
+#include <objc/runtime.h>
+
 #include "base/logging.h"
 #include "components/crash/core/common/objc_zombie.h"
 
@@ -11,6 +13,46 @@
 #error "This file requires ARC support."
 #endif
 
+namespace {
+
+#ifndef NDEBUG
+
+void swizzleUIImageImageNamed() {
+  NSMutableSet* whiteList = [NSMutableSet set];

[marq (ping after 24h), 2017/05/09 09:35:30]

Add a comment that the whitelist will be retained by the block.

+
+  // TODO(crbug.com/): Add missing image.
+  [whiteList addObject:@"card_close_button_pressed_incognito"];
+  // TODO(crbug.com/): Add missing image.
+  [whiteList addObject:@"glif-mic-to-dots-large_37"];
+
+  // The original implementation of [UIImage imageNamed:].

[lpromero, 2017/05/05 15:24:37]

Super tiny nit: Add a dash: "of -[".

[marq (ping after 24h), 2017/05/09 09:35:31]

I don't think that's needed, since imageNamed: is a class method.

[sdefresne, 2017/05/09 10:07:27]

Shouldn't it be "of +[UIImage imageNamed:." then?

[marq (ping after 24h), 2017/05/10 12:59:11]

Either:

[UIImage imageNamed:] 

or

UIImage's +imageNamed: method.

[jif, 2017/05/10 13:34:46]

Done.

+  // Called by the new implementation.
+  static IMP originalImp;
+  IMP* originalImpPtr = &originalImp;
+
+  id swizzleBlock = ^(id self, NSString* imageName) {
+    // Call the original [UIImage imageNamed:] method.
+    IMP imp = *originalImpPtr;
+    Class klass = objc_getClass("UIImage");

[marq (ping after 24h), 2017/05/09 09:35:30]

Don't use the Java 'klass' naming convention. 'class' isn't an Obj-C reserved
word, so just use that (or inline the getClass() call).

[sdefresne, 2017/05/09 10:07:27]

But "class" is a C++ reserved name, and cannot be used in a Obj-C++ file.

[marq (ping after 24h), 2017/05/10 12:59:11]

Then, 'aClass'.

[jif, 2017/05/10 13:34:46]

Done.

+    UIImage* image = imp(klass, @selector(imageNamed:), imageName);
+
+    if (![whiteList containsObject:imageName]) {
+      DCHECK(image);
+    }
+    return image;
+  };
+
+  Method method = class_getClassMethod([UIImage class], @selector(imageNamed:));
+  DCHECK(method);
+
+  IMP blockImp = imp_implementationWithBlock(swizzleBlock);
+  originalImp = method_setImplementation(method, blockImp);
+}
+
+#endif
+
+}  // namespace
+
 @implementation SetupDebugging
 
 + (void)setUpDebuggingOptions {
@@ -19,6 +61,11 @@ + (void)setUpDebuggingOptions {
 #if TARGET_IPHONE_SIMULATOR
   DCHECK(ObjcEvilDoers::ZombieEnable(true, 10000));
 #endif
+
+// Enable the detection of missing image assets.
+#ifndef NDEBUG
+  swizzleUIImageImageNamed();
+#endif
 }
 
 @end