关于transitionFromViewController的一些用法

很多时候一个UIViewController有很多的小的子view，这些view很多时候被覆盖在最后，我们一般在最外层addSubview 添加view这些view大多不会一直处于在界面上显示，一般只有在某些时候才会显示，虽然这些view很少出现，但却常常把他们放在内存中，当内存警告时，只能删除他们

这个时候用transitionFromViewController

对于那些暂时不需要显示的subview，通过addchildviewcontroller 把subviewcontroller加进去，同时调用self addchildviewcontroller：需要显示时再调用transitionFromViewController方法。将其添加进入底层的ViewController中。

相应的view对应相应的controller

当某个子view没有显示的时候，将不会load，减少内存的使用，

当内存紧张的时候，没有被load的view辉被释放，优化了内存，

[self
transitionFromViewController:_currentViewController
toViewController:third
duration:0.1
options:UIViewAnimationOptionTransitionNone
animations:^{

}
completion:^(BOOL finished) {

if (finished) {

_currentViewController =
third;
}
else {

_currentViewController = oldViewController;
}
}];
参数1，当前的viewcontroller ，参数二，显示的controller 参数三，过度时间，参数4，过度效果， block语句块，
过度完成后设置当前的controller

简单的使用如下：

- (void)viewDidLoad {

[super
viewDidLoad];

first = [[FirstViewController
alloc]init];

[self
addChildViewController:first];

second = [[SecondViewController
alloc]init];

[self
addChildViewController:second];

third =[[ThirdViewController
alloc]init];

[self
addChildViewController:third];

[_AddViews
addSubview:first.view];

_currentViewController =
first;

}
- (IBAction)ThreeBtn:(id)sender {

NSLog(@"%@",_currentViewController);

if (_currentViewController ==
third ) {

return;
}

UIViewController *oldViewController =
_currentViewController;

[self
transitionFromViewController:_currentViewController
toViewController:third
duration:0.1
options:UIViewAnimationOptionTransitionNone
animations:^{

}
completion:^(BOOL finished) {

if (finished) {

_currentViewController =
third;
}
else {

_currentViewController = oldViewController;
}
}];

}

- (IBAction)SecondBtn:(id)sender {

NSLog(@"%@",_currentViewController);

if (_currentViewController ==
second ) {

return;
}

UIViewController *oldViewController =
_currentViewController;

[self
transitionFromViewController:_currentViewController
toViewController:second
duration:0.1
options:UIViewAnimationOptionTransitionNone
animations:^{

}
completion:^(BOOL finished) {

if (finished) {

_currentViewController =
second;
}
else {

_currentViewController = oldViewController;
}
}];

}

- (IBAction)OneBtn:(id)sender {

NSLog(@"%@",_currentViewController);

if (_currentViewController ==
first ) {

return;
}

UIViewController *oldViewController =
_currentViewController;

[self
transitionFromViewController:_currentViewController
toViewController:first
duration:0.1
options:UIViewAnimationOptionTransitionNone
animations:^{

}
completion:^(BOOL finished) {

if (finished) {

_currentViewController =
first;
}
else {

_currentViewController = oldViewController;
}
}];

}