iOS: Run-time error when using a class category in a different static library

When you are calling a member function defined in a class category (https://developer.apple.com/library/ios/#documentation/cocoa/conceptual/objectivec/chapters/occategories.html) from a different static library, you might see a run-time exception of "selector not recognized".

This is a well-know issue with Xcode and iOS. There is a technical note from Apple to describe the problem and the workaround by using the -all_load flag or the -force_load flag followed by the archive path:

https://developer.apple.com/library/mac/#qa/qa1490/_index.html

There is also another workaround without defining the flag. In the file that define the class category, add a C function to the .h and .m, and call this function anywhere in the project that you call the member function of the class category. For example, if you have a class category called UIImage (A) in UIImage+A.h and UIImage+A.m, you can add a function like this:


UIImage+A.h
------------------
// This function needs to be called in the file that uses UIImage (A)
void forceLoadUIImageCategoryA(void);

@interface UIImage (A)
...
@end


UIImage+A.m
------------------
void forceLoadUIImageCategoryA(void)
{
    // Do nothing
}

@implementation UIImage (A)
...
@end


In the file that uses any member functions of UIImage (A), call forceLoadUIImageCategoryA(); anywhere. Calling this C function will make Xcode link the .h and .m.

Xcode warning: no previous prototype for function ...

This warning appears when you define a C-style function like this:

In .h:
void foo();

In .m:
void foo()
{
}

However, according to C standard, you should define the function with the void keyword if there is no parameters in your function:

In .h:
void foo(void);

In .m:
void foo(void)
{
}

Why UIView's autoresizingMask does not work

I was trying to use to autoresizingMask in a UILabel to resize the width when the device is rotated. Following my first instinct, I wrote label.autoresizingMask = UIViewAutoresizingFlexibleWidth. However, the label didn't resize. Why?

There are a few values for the mask such as UIViewAutoresizingNone, UIViewAutoresizingFlexibleRightMargin, UIViewAutoresizingFlexibleLeftMargin, and UIViewAutoresizingFlexibleHeight. I tried to use UIViewAutoresizingFlexibleRightMargin or UIViewAutoresizingFlexibleLeftMargin but there was no difference.

Wait a minute. If we need to set those values to enable flexible margins, width, and height, what does it mean if we leave them unset? If I only set the autoresizingMask property to UIViewAutoresizingFlexibleWidth, it actually means Flexible Width AND Fixed Left Margin AND Fixed Right Margin. Even though the width is flexible, the fixed left and right margins do not allow the flexible width to expand or shrink.

Now, it is clear that when we need to set flexible width or height, we must also set  at least a corresponding margin to be flexible at the same time. For example, if we want the label to expand the end of the text (assuming that the text goes from left to right) when the width changes, we should set autoresizingMask to UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleRightMargin so that the label can expand it's width at the right margin.

Use of undeclared identifier 'kUTTypeImage'

This constant is declared in UTCoreTypes.h in the MobileCoreServices framework. In the .m files that you use the constant kUTTypeImage, import the above .h file:

   #import <MobileCoreServices/UTCoreTypes.h>

In the application project, you also need to include the MobileCoreServices framework.