ios - 如何在屏幕中央呈现按钮？

Question

我正在使用 Google Sign In对于我的应用程序，默认情况下呈现时，它看起来像附加的一样

我环顾四周，看看如何以编程方式添加约束，以便根据屏幕的大小，按钮落在屏幕的中央，所以我的代码看起来像

- (void)signInWithGooglePlus {
    GPPSignInButton *signInButton = [[GPPSignInButton alloc] init];
    [signInButton setTranslatesAutoresizingMaskIntoConstraints:NO];
    [signInButton setStyle:kGPPSignInButtonStyleWide];
    [signInButton setColorScheme:kGPPSignInButtonColorSchemeDark];

    NSLayoutConstraint *vConstraint = [NSLayoutConstraint constraintWithItem:signInButton attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1 constant:0];
    NSLayoutConstraint *hConstraint = [NSLayoutConstraint constraintWithItem:signInButton attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1 constant:0];
    [self.view addConstraint:vConstraint];
    [self.view addConstraint:hConstraint];

    [self.view addSubview:signInButton];

    GPPSignIn *signIn = [GPPSignIn sharedInstance];
    signIn.shouldFetchGooglePlusUser = YES;
    signIn.shouldFetchGoogleUserEmail = YES;

    signIn.clientID = kClientId;
    signIn.scopes = @[@"profile"];
    signIn.delegate = self;
    [signIn trySilentAuthentication];
}

并被调用

- (void)viewDidLoad {
    [super viewDidLoad];
    [self signInWithGooglePlus];
}

当我运行它时，它失败了

2015-03-14 16:50:11.265 myapp-ios[24332:1661610] The view hierarchy is not prepared for the constraint: <NSLayoutConstraint:0x7ff4c0c1b490 GPPSignInButton.centerY == UIView:0x7ff4c0f54930.centerY   (Names: GPPSignInButton:0x7ff4c0f55da0 )>
    When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView _viewHierarchyUnpreparedForConstraint:] to debug.
2015-03-14 16:50:11.267 myapp-ios[24332:1661610] View hierarchy unprepared for constraint.
    Constraint: <NSLayoutConstraint:0x7ff4c0c1b490 GPPSignInButton.centerY == UIView:0x7ff4c0f54930.centerY   (Names: GPPSignInButton:0x7ff4c0f55da0 )>
    Container hierarchy: 
<UIView: 0x7ff4c0f54930; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0x7ff4c0f52d20>>
    View not found in container hierarchy: <GPPSignInButton: 0x7ff4c0f55da0; baseClass = UIButton; frame = (0 0; 226 48); opaque = NO; layer = <CALayer: 0x7ff4c0f542b0>>
    That view's superview: NO SUPERVIEW
2015-03-14 16:50:11.279 myapp-ios[24332:1661610] *** Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to install constraint on view.  Does the constraint reference something from outside the subtree of the view?  That's illegal. constraint:<NSLayoutConstraint:0x7ff4c0c1b490 GPPSignInButton.centerY == UIView:0x7ff4c0f54930.centerY   (Names: GPPSignInButton:0x7ff4c0f55da0 )> view:<UIView: 0x7ff4c0f54930; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0x7ff4c0f52d20>>'
*** First throw call stack:
(
    0   CoreFoundation                      0x000000010bb93a75 __exceptionPreprocess + 165
    1   libobjc.A.dylib                     0x000000010b828bb7 objc_exception_throw + 45
    2   CoreFoundation                      0x000000010bb939ad +[NSException raise:format:] + 205
    3   Foundation                          0x00000001094d5689 -[NSLayoutConstraint _addToEngine:integralizationAdjustment:mutuallyExclusiveConstraints:] + 187
    4   UIKit                               0x000000010a21bed5 __57-[UIView(AdditionalLayoutSupport) _switchToLayoutEngine:]_block_invoke_2 + 474
    5   Foundation                          0x00000001094e32fe -[NSISEngine withBehaviors:performModifications:] + 155
    6   UIKit                               0x000000010a21bcdb __57-[UIView(AdditionalLayoutSupport) _switchToLayoutEngine:]_block_invoke + 452
    7   UIKit                               0x000000010a21baee -[UIView(AdditionalLayoutSupport) _switchToLayoutEngine:] + 197
    8   UIKit                               0x000000010a21b761 -[UIView(AdditionalLayoutSupport) _initializeHostedLayoutEngine] + 404
    9   UIKit                               0x000000010a21c281 -[UIView(AdditionalLayoutSupport) _layoutEngineCreateIfNecessary] + 53
    10  UIKit                               0x000000010a210bfa -[UIView(UIConstraintBasedLayout) _layoutEngine_didAddLayoutConstraint:roundingAdjustment:mutuallyExclusiveConstraints:] + 156
    11  UIKit                               0x000000010a210f94 -[UIView(UIConstraintBasedLayout) _tryToAddConstraintWithoutUpdatingConstraintsArray:roundingAdjustment:mutuallyExclusiveConstraints:] + 30
    12  UIKit                               0x000000010a2110bc -[UIView(UIConstraintBasedLayout) _tryToAddConstraint:roundingAdjustment:mutuallyExclusiveConstraints:] + 243
    13  myapp-ios                        0x0000000108f0fd5d -[GooglePlusLoginViewController signInWithGooglePlus] + 557
    14  myapp-ios                        0x0000000108f0fa7b -[GooglePlusLoginViewController handleAuthentication] + 43
    15  myapp-ios                        0x0000000108f0fa44 -[GooglePlusLoginViewController viewDidLoad] + 228
    16  UIKit                               0x0000000109cba580 -[UIViewController loadViewIfRequired] + 738
    17  UIKit                               0x0000000109cba77e -[UIViewController view] + 27
    18  UIKit                               0x0000000109bd9509 -[UIWindow addRootViewControllerViewIfPossible] + 58
    19  UIKit                               0x0000000109bd98a1 -[UIWindow _setHidden:forced:] + 247
    20  UIKit                               0x0000000109be5f8c -[UIWindow makeKeyAndVisible] + 42
    21  myapp-ios                        0x0000000108f0f483 -[AppDelegate application:didFinishLaunchingWithOptions:] + 627
    22  UIKit                               0x0000000109b8f458 -[UIApplication _handleDelegateCallbacksWithOptions:isSuspended:restoreState:] + 248
    23  UIKit                               0x0000000109b90002 -[UIApplication _callInitializationDelegatesForMainScene:transitionContext:] + 2540
    24  UIKit                               0x0000000109b92e3e -[UIApplication _runWithMainScene:transitionContext:completion:] + 1349
    25  UIKit                               0x0000000109b91d35 -[UIApplication workspaceDidEndTransaction:] + 179
    26  FrontBoardServices                  0x000000010f7a7243 __31-[FBSSerialQueue performAsync:]_block_invoke + 16
    27  CoreFoundation                      0x000000010bac8c7c __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    28  CoreFoundation                      0x000000010babe9c5 __CFRunLoopDoBlocks + 341
    29  CoreFoundation                      0x000000010babe785 __CFRunLoopRun + 2389
    30  CoreFoundation                      0x000000010babdbc6 CFRunLoopRunSpecific + 470
    31  UIKit                               0x0000000109b917a2 -[UIApplication _run] + 413
    32  UIKit                               0x0000000109b94580 UIApplicationMain + 1282
    33  myapp-ios                        0x0000000108f0f1e3 main + 115
    34  libdyld.dylib                       0x000000010d104145 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException

问题
如何将我的 GPSSignInButton 放在屏幕中央？

Answer 1

在创建约束之前，您必须将 signInButton 添加为 subview ；这就是您收到该错误的原因。只需更改语句的顺序即可。

[self.view addSubview:signInButton];
NSLayoutConstraint *vConstraint = [NSLayoutConstraint constraintWithItem:signInButton attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1 constant:0];
NSLayoutConstraint *hConstraint = [NSLayoutConstraint constraintWithItem:signInButton attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1 constant:0];
[self.view addConstraint:vConstraint];
[self.view addConstraint:hConstraint];