vintasoft barcode .net sdk 17: NSPredicate in Objective-C

Generating QR Code in Objective-C 17: NSPredicate

CHAPTER 17: NSPredicate
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to your own objects). You can think of NSPredicate as yet another means of indirection. You can use a predicate object that does your checking, rather than asking explicitly in code, Are these the droids I m looking for By swapping predicate objects around, you can have common code sift through your data without hard-coding the conditions you re looking for. This is another application of the Open/Closed Principle that you met back in 3.
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Creating a Predicate
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Before you can use an NSPredicate against one of your objects, you need to create it, which you can do in two fundamental ways. One involves creating a lot of objects and assembling them. This requires a lot of code and is handy if you re building a general user interface for specifying searches. The other way involves query strings you put into your code. These are much easier to deal with when just getting started, so we ll concentrate on query strings in this book. The usual caveats with string-oriented APIs apply here, especially lack of error checking by the compiler and, sometimes, curious runtime errors. There is no escape from CarParts we ll be basing this chapter s examples on the garage of cars built in the last chapter. You can find everything in the 17.01 Car-Part-Predicate project. To start, we ll look at just one car:
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Car *car; car = makeCar (@"Herbie", @"Honda", @"CRX", 1984, 2, 110000, 58); [garage addCar: car];
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Recall that we wrote the makeCar function to build up a car and give it an engine and some tires. In this case, we have Herbie, a two-door 1984 Honda CRX with a 58-horespower engine with 110,000 miles on it. Now, let s make a predicate:
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NSPredicate *predicate; predicate = [NSPredicate predicateWithFormat: @"name == 'Herbie'"];
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Let s pull this apart. predicate is one of our usual Objective-C object pointers, which will point to an NSPredicate object. We use the NSPredicate class method +predicateWithFormat: to actually create the predicate. We give it a string, and +predicateWithFormat: takes that string and builds a tree of objects behind the scenes that will be used to evaluate the predicate.
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predicateWithFormat sounds a lot like stringWithFormat, provided by NSString, which lets you plug in stuff using printf-style format specifiers. As you ll see later, you will be able to do the same thing with predicateWithFormat. Cocoa has consistent naming schemes it s nice like that.
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CHAPTER 17: NSPredicate
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This predicate string looks like a standard C expression. On the left-hand side is a key path, name. Next comes an operator for equality, ==, and a quoted string on the right-hand side. If a chunk of text in the predicate string is not quoted, it is treated as a key path. If it s quoted, it s treated as a literal string. You can use single quotes or double quotes (as long as they re balanced). Usually, you ll use single quotes; otherwise, you ll have to escape each double quote in the string.
Evaluate the Predicate
OK, so we ve got a predicate. What now We evaluate it against an object!
BOOL match = [predicate evaluateWithObject: car]; NSLog (@"%s", (match) "YES" : "NO"); -evaluateWithObject: tells the receiving object (the predicate) to evaluate itself with the
given object. In this case, it takes the car, applies valueForKeyPath: using name as the key path to get the name. Then, it compares it for equality to Herbie . If the name and Herbie are the same, -evaluateWithObject: returns YES, otherwise NO. The NSLog uses the ternary operator to convert the numerical BOOL to a human-readable string. Here s another predicate:
predicate = [NSPredicate predicateWithFormat: @"engine.horsepower > 150"]; match = [predicate evaluateWithObject: car];
The predicate string has a key path on the left-hand side. This key path digs into the car, finds the engine, and then finds the horsepower of the engine. Next, it compares that value with 150 to see if it s larger. After we evaluate this against Herbie, match has the value of NO, because little Herbie s horsepower (58) is not greater than 150. Checking an object against a particular predicate s truth is all well and good, but things get more interesting when you have collections of objects. Say we wanted to see which cars in our garage are the most powerful. We can loop through the cars and test each one with this predicate:
NSArray *cars = [garage cars]; for (Car *car in [garage cars]) { if ([predicate evaluateWithObject: car]) { NSLog (@"%@", car.name); } }
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