progress bar code in vb net 2010 6: Model-View-Controller in Xcode in Objective-C

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CHAPTER 6: Model-View-Controller in Xcode
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Figure 6 4. MVC and message flow in the TemperatureConverter application
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A consequence of this design is that the data and the logic for the conversion are stored in the Model. If you wanted to change to a different algorithm for the conversion, or to introduce a different conversion (say, to and from degrees Kelvin), that work would happen almost entirely within the Model. The Controller takes the values from the View (the user interface) and feeds those values into the Model. The resulting value from the conversion is then placed back in the View to update the converted temperature value. Open Converter.h and make it look like Listing 6 2.
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Listing 6 2. Converter.h #import <Cocoa/Cocoa.h> @interface Converter : NSObject { float originalTemp; } @property(readwrite) float originalTemp; - (float)convertCToF; - (float)convertFToC; @end
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Now open Converter.m and add code to make it look like Listing 6 3.
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CHAPTER 6: Model-View-Controller in Xcode
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Listing 6 3. Converter.m #import "Converter.h" @implementation Converter @synthesize originalTemp; - (float)convertCToF { return ((self.originalTemp * 1.8) + 32.0); } - (float)convertFToC { return ((self.originalTemp - 32.0) / 1.8); } @end
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This is the Model for the application. The Model will receive an original temperature value, either in centigrade or Fahrenheit, and then the appropriate method will be called (by the Controller, as you will see shortly). After transforming it the Model returns the new temperature value. Now let s move to the Controller. Open TemperatureConverterAppDelegate.h and add code to it as shown in Listing 6 4.
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Listing 6 4. TemperatureConverterAppDelegate.h #import <Cocoa/Cocoa.h> #import "Converter.h" @interface TemperatureConverterAppDelegate : NSObject <NSApplicationDelegate> { IBOutlet NSTextField *centigradeField; IBOutlet NSTextField *fahrenheitField; Converter *temperatureConverter; } - (IBAction)convertTemperature:(id)sender; @end
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Note that the Controller imports Converter.h this makes the Controller aware of the Model. The other declarations in this file are for the outlets that you will be building shortly in Interface Builder, and the method that is invoked by changes in the user interface. Now open the implementation file TemperatureConverterAppDelegate.m and update it to look like Listing 6 5.
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Listing 6 5. TemperatureConverterAppDelegate.m #import "TemperatureConverterAppDelegate.h" @implementation TemperatureConverterAppDelegate @synthesize window; - (void)awakeFromNib { fahrenheitField.floatValue = 32.0; centigradeField.floatValue = 0.0; tempConverter = Converter.new;
CHAPTER 6: Model-View-Controller in Xcode
} - (IBAction)convertTemperature:(id)sender { float temperatureF = fahrenheitField.floatValue; float temperatureC = centigradeField.floatValue; if(sender == fahrenheitField) { tempConverter.originalTemp = temperatureF; temperatureC = tempConverter.convertFToC; centigradeField.floatValue = temperatureC; } else if (sender == centigradeField) { tempConverter.originalTemp = temperatureC; temperatureF = tempConverter.convertCToF; fahrenheitField.floatValue = temperatureF; } } @end
The awakeFromNib method simply initializes some objects with starting values. The convertTemperature method takes the value from one Text Field, depending on which is the sender, sets the original temperature value and calls the appropriate method in the Model object, takes back the converted value and places it in the other Text Field. Notice that the Controller is not aware of exactly how the conversion takes place that is the concern of the Model.
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Experienced Cocoa developers will note (perhaps with horror) that there is no alloc and no dealloc in TemperatureConverterAppDelegate.m. Normally, this is a recipe for disaster conventional Cocoa memory management practice dictates that you actively manage memory allocation. In practice, this means that you allocate memory for objects and then later, when the objects are no longer needed, you release the memory. If you don t do this, the result is memory leakage and possible crashes. Objective-C v2.0 introduces garbage collection as an option. This monitors object allocation and automatically releases object memory when an object goes out of scope. The TemperatureConverter project uses garbage collection. If you want to set up garbage collection for a project, choose the Project Edit Project Settings menu. The Build tab in this window has dozens of options, but you can zero in on the Garbage Collection option using the search field (see Figure 6 7). Change the value for Objective-C Garbage Collection to Required. Also, change the Configuration: popup to All Configurations. In principle garbage collection (which is a common technology in other application development systems such as Java and C #) offers greater simplicity to the developer. However, it is fair to say that for ObjectiveC, garbage collection is a new and hot topic of discussion at the time of writing, and for many developers there is no alternative to the conventional alloc-retain-release mechanism. Now let s turn to the View. Open MainMenu.xib and add two Text Field controls and two Label controls. Your Controller is already there (that s the Temperature Converter App Delegate). Make it look like Figure 6 2. Notice that there is no button you aren t going to need one in this application.
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