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本节讲Objective-C的强引用，对象在什么情况下被释放，已经对象之间的关系与对象释放的相关性。

Manage the Object Graph through Ownership and Responsibility

As you’ve already seen, memory for Objective-C objects is allocated dynamically (on the heap), which means you need to use pointers to keep track of an object’s address. Unlike scalar values, it’s not always possible to determine an object’s lifetime by the
scope of one pointer variable. Instead, an object must be kept active in memory for as long as it is needed by other objects.

Rather than trying to worry about managing the lifecycle of each object manually, you should instead think about the relationships between objects.

In the case of an XYZPerson object, for example, the two string properties for firstName and lastName are effectively “owned” by the XYZPerson instance. This means they should stay in memory as long as the XYZPerson object stays in memory.

When one object relies on other objects in this way, effectively taking ownership of those other objects, the first object is said to have strong references to the other objects. In Objective-C, an object is kept alive as long as it has at least one strong
reference to it from another object. The relationships between the XYZPerson instance and the two NSString objects is shown in Figure 3-2.

Figure 3-2  Strong Relationships



When an XYZPerson object is deallocated from memory, the two string objects will also be deallocated, assuming there aren’t any other strong references left to them.

To add a little more complexity to this example, consider the object graph for an application like that shown in Figure 3-3.

Figure 3-3  The Name Badge Maker application



When the user clicks the Update button, the badge preview is updated with the relevant name information.

The first time a person’s details are entered and the update button clicked, the simplified object graph might look like Figure 3-4.

Figure 3-4  Simplified object graph for initial XYZPerson creation



When the user modifies the person’s first name, the object graph changes to look like Figure 3-5.

Figure 3-5  Simplified object graph while changing the person’s first name



The badge display view maintains a strong relationship to the original @"John" string object, even though the XYZPerson object now has a different firstName. This means the @"John" object stays in memory, used by the badge view to print the name.

Once the user clicks the Update button a second time, the badge view is told to update its internal properties to match the person object, so the object graph looks like Figure 3-6.

Figure 3-6  Simplified object graph after updating the badge view



At this point, the original @"John" object no longer has any strong references to it, so it is removed from memory.

By default, both Objective-C properties and variables maintain strong references to their objects. This is fine for many situations, but it does cause a potential problem with strong reference cycles.