Objective-C if语句评估

Question

I want to check this statement:

 if([optionsDictionary objectForKey:@"showTimes"] 
     && [[optionsDictionary objectForKey:@"showTimes"]isEqualToString:@"1"])

App crashes. The question is - does Objective-C check all conditions? Because in C# if 1 condition fails then second is not checked at all

Answer 1

Use [[optionsDictionary objectForKey:@"showTimes"] boolValue] to determine whether it's 0 or 1.

Modify your condition as

if([optionsDictionary objectForKey:@"showTimes"] 
 && (1 == [[optionsDictionary objectForKey:@"showTimes"] boolValue]))

Answer 2

(1) If statements and the && and || operators are evaluated exactly the same in Objective-C as they are evaluated in C, C++ or Java.

(2) If you send any message to an object that is nil, the result will be 0 / NO / nil. Therefore, the && is pointless. You could have just written

if ([[optionsDictionary objectForKey:@"showTimes"]isEqualToString:@"1"])

By the way, it is less confusing to yourself and everyone else to check whether an object is nil by comparing it to nil.

(3) Sending a method isEqualToString to an object that is not an NSString will crash. The crash will tell you what message you sent to what kind of object. NSCFBoolean is an NSNumber with a boolean value, so it crashes.

You need to either know what's in your dictionary, or check it. If you didn't put the values into the dictionary yourself, you don't know what's in there. For example, if you used a parser to parse a JSON document, your current code will crash again if the JSON document contained a null value. Or a dictionary.

The best thing to do, because this will happen to you again and again, is to write an NSDictionary category with a method boolValueForKey: which does all the checking for you.