为什么保留财产可能无法保留在Objective-C上？

Question

Why the following code crashes? Commented code doesn't crash.

@property (retain) NSDate *lastCurrentDate;

...

@synthesize lastCurrentDate;

- (void)viewWillAppear:(BOOL)animated {
    BOOL crash = [lastCurrentDate isEqualToDate:[NSDate date]]);
}

- (void)viewDidDisappear:(BOOL)animated {
    //lastCurrentDate = [[NSDate date] retain];
    lastCurrentDate = [NSDate date];
}

So, why retain property may not retain on Objective-C?

Answer 1

When you write @synthesize lastCurrentDate - you also create variable named ' lastCurrentState ', and when you write lastCurrentDate = [NSDate date]; you directly access this variable. Properties should be accessed via dot: self.lastCurrentDate = ....;

In last xCodes you don't need to write synthesize - it do it automatically, but creates variable named with '_' prefix. It equals to: @synthesize variable = _variable;

Answer 2

Use self.lastCurrentDate = [NSDate date] . Because when you use self.lastCurrentDate , it will assign via setter method. You declare vaiable via retain property, So your setter method will do two operation, assign and retain .

Answer 3

因为您直接分配给实例变量，所以没有使用属性访问器方法：

self.lastCurrentDate = [NSDate date];