傳送門:鏈式編程小Demohtml
這篇文章是 Masonry 框架源碼的解析和筆記。學習Masonry以前,先了解這個框架設計的初衷---傳統的利用系統API進行純代碼佈局的不足。而後,根據Masonry常見的幾個鏈式語法中,順藤摸瓜地瞭解Masonry的調用棧。最後,學習並思考這個框架用到的設計模式和鏈式編程思想。git
+(instancetype)constraintWithItem:(id)view1
attribute:(NSLayoutAttribute)attr1
relatedBy:(NSLayoutRelation)relation
toItem:(nullable id)view2
attribute:(NSLayoutAttribute)attr2
multiplier:(CGFloat)multiplier
constant:(CGFloat)c;
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- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
self.view.backgroundColor = [UIColor yellowColor];
UIView *subView = [[UIView alloc] init];
subView.backgroundColor = [UIColor redColor];
// 在設置約束前,先將子視圖添加進來
[self.view addSubview:subView];
// 使用autoLayout約束,禁止將AutoresizingMask轉換爲約束
[subView setTranslatesAutoresizingMaskIntoConstraints:NO];
// 設置subView相對於VIEW的上左下右各40像素
NSLayoutConstraint *constraintTop = [NSLayoutConstraint constraintWithItem:subView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:40];
NSLayoutConstraint *constraintLeft = [NSLayoutConstraint constraintWithItem:subView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:40];
// 因爲iOS座標系的原點在左上角,因此設置下,右邊距使用負值
NSLayoutConstraint *constraintBottom = [NSLayoutConstraint constraintWithItem:subView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0 constant:-40];
NSLayoutConstraint *constraintRight = [NSLayoutConstraint constraintWithItem:subView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-40];
// 將四條約束加進數組中
NSArray *array = [NSArray arrayWithObjects:constraintTop, constraintLeft, constraintBottom, constraintRight, nil];
// 把約束條件設置到父視圖的Contraints中
[self.view addConstraints:array];
}
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可見,系統傳統的代碼佈局有點繁瑣。爲了簡化上述傳統佈局代碼,被普遍應用的第三方框架 Masonry 對AutoLayout 進行了封裝,Swift版則是 SnapKit。這篇文章就是針對 Masonry 源代碼的解析與學習筆記。在這以前,以下圖所示,是 Masonry 源代碼的結構圖:github
mas_makeConstraints
:外部調用#import "Masonry.h"
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[self.containerView addSubview:self.bannerView];
[self.bannerView mas_makeConstraints:^(MASConstraintMaker *make) {
make.leading.equalTo(self.containerView.mas_leading);
make.top.equalTo(self.containerView.mas_top);
make.trailing.equalTo(self.containerView.mas_trailing);
make.height.equalTo(@(kViewWidth(131.0)));
}];
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mas_makeConstraints
:實現原理,經過導入的頭文件分析#import <Foundation/Foundation.h>
//! Project version number for Masonry.
FOUNDATION_EXPORT double MasonryVersionNumber;
//! Project version string for Masonry.
FOUNDATION_EXPORT const unsigned char MasonryVersionString[];
#import "MASUtilities.h"
#import "View+MASAdditions.h"
#import "View+MASShorthandAdditions.h"
#import "ViewController+MASAdditions.h"
#import "NSArray+MASAdditions.h"
#import "NSArray+MASShorthandAdditions.h"
#import "MASConstraint.h"
#import "MASCompositeConstraint.h"
#import "MASViewAttribute.h"
#import "MASViewConstraint.h"
#import "MASConstraintMaker.h"
#import "MASLayoutConstraint.h"
#import "NSLayoutConstraint+MASDebugAdditions.h"
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其中
View+MASAdditions
分類爲UIView
添加了mas_makeConstraints
方法編程
- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block {
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];
block(constraintMaker);
return [constraintMaker install];
}
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@interface MASConstraintMaker () <MASConstraintDelegate>
@property (nonatomic, weak) MAS_VIEW *view;
@property (nonatomic, strong) NSMutableArray *constraints;
@end
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- (id)initWithView:(MAS_VIEW *)view {
self = [super init];
if (!self) return nil;
self.view = view;
self.constraints = NSMutableArray.new;
return self;
}
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.top
:經過MASConstraintMaker
類源碼分析先分析設置 第一個約束屬性 的狀況(且惟一一個):例如設計模式
make.top.equalTo(self.containerView.mas_top);
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- (MASConstraint *)top {
return [self addConstraintWithLayoutAttribute:NSLayoutAttributeTop];
}
- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
return [self constraint:nil addConstraintWithLayoutAttribute:layoutAttribute];
}
- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
MASViewAttribute *viewAttribute = [[MASViewAttribute alloc] initWithView:self.view layoutAttribute:layoutAttribute];
MASViewConstraint *newConstraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:viewAttribute];
if ([constraint isKindOfClass:MASViewConstraint.class]) {
//replace with composite constraint
NSArray *children = @[constraint, newConstraint];
MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];
compositeConstraint.delegate = self;
[self constraint:constraint shouldBeReplacedWithConstraint:compositeConstraint];
return compositeConstraint;
}
if (!constraint) {
newConstraint.delegate = self;
[self.constraints addObject:newConstraint];
}
return newConstraint;
}
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該方法返回的newConstraint
是一個MASViewConstraint
類的示例,而MASViewConstraint
類又是MASConstraint
的子類,返回類型寫成MASConstraint
沒毛病。數組
代碼較多,暫時能夠只先看if (!constraint)
裏面的代碼。可見,最後設置 newConstraint
對象代理爲self
(即 MASConstraintMaker
),並添加到一開始準備好的 self.constraints 數組中,返回。bash
其中,設置 MASViewConstraint
類 newConstraint
對象的 MASConstraintDelegate
代理爲self
(即 MASConstraintMaker
),其做用就是爲了可以同時設置多個約束屬性!即鏈式語法。數據結構
@protocol MASConstraintDelegate <NSObject>
/**
* Notifies the delegate when the constraint needs to be replaced with another constraint. For example
* A MASViewConstraint may turn into a MASCompositeConstraint when an array is passed to one of the equality blocks
*/
- (void)constraint:(MASConstraint *)constraint shouldBeReplacedWithConstraint:(MASConstraint *)replacementConstraint;
- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute;
@end
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第2.3.1節的MASConstraintMaker.m
代碼中,先是初始化了 MASViewAttribute
對象並保存了 view、item以及 NSLayoutAttribute
三個屬性。框架
- (id)initWithView:(MAS_VIEW *)view layoutAttribute:(NSLayoutAttribute)layoutAttribute {
self = [self initWithView:view item:view layoutAttribute:layoutAttribute];
return self;
}
- (id)initWithView:(MAS_VIEW *)view item:(id)item layoutAttribute:(NSLayoutAttribute)layoutAttribute {
self = [super init];
if (!self) return nil;
_view = view;
_item = item;
_layoutAttribute = layoutAttribute;
return self;
}
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而後又初始化了 MASViewConstraint
對象,內部配置了些默認參數並保存瞭如上的第一個約束參數 MASViewAttribute
。less
- (id)initWithFirstViewAttribute:(MASViewAttribute *)firstViewAttribute {
self = [super init];
if (!self) return nil;
_firstViewAttribute = firstViewAttribute;
self.layoutPriority = MASLayoutPriorityRequired;
self.layoutMultiplier = 1;
return self;
}
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.equalTo
:經過基類MASConstraint
及其子類MASViewConstraint
分析第一個約束屬性 設置完後,走到.equalTo
時,前面返回已是一個 MASViewConstraint
(繼承自MASConstraint
) 對象了,於是調用的是在基類MASConstraint
中聲明並實現的block屬性getter方法。
- (MASConstraint * (^)(id))equalTo {
return ^id(id attribute) {
return self.equalToWithRelation(attribute, NSLayoutRelationEqual);
};
}
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其中,基類 MASConstraint
僅僅聲明,並無實現equalToWithRelation
抽象方法。可是,如2.3節中的鏈式語法.top
,該方法返回的newConstraint
實際是其子類--MASViewConstraint
類的實例,故而可調用子類MASViewConstraint
實現的equalToWithRelation
方法:
- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation {
return ^id(id attribute, NSLayoutRelation relation) {
if ([attribute isKindOfClass:NSArray.class]) {
NSAssert(!self.hasLayoutRelation, @"Redefinition of constraint relation");
NSMutableArray *children = NSMutableArray.new;
for (id attr in attribute) {
MASViewConstraint *viewConstraint = [self copy];
viewConstraint.layoutRelation = relation;
viewConstraint.secondViewAttribute = attr;
[children addObject:viewConstraint];
}
MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];
compositeConstraint.delegate = self.delegate;
[self.delegate constraint:self shouldBeReplacedWithConstraint:compositeConstraint];
return compositeConstraint;
} else {
NSAssert(!self.hasLayoutRelation || self.layoutRelation == relation && [attribute isKindOfClass:NSValue.class], @"Redefinition of constraint relation");
self.layoutRelation = relation;
self.secondViewAttribute = attribute;
return self;
}
};
}
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代碼較多,暫時可先看else {
裏面的代碼。
首先是 self.layoutRelation
保存了約束關係且重寫了 set
方法,在裏面用 self.hasLayoutRelation
這個 BOOL
標識已經有約束關係。
- (void)setLayoutRelation:(NSLayoutRelation)layoutRelation {
_layoutRelation = layoutRelation;
self.hasLayoutRelation = YES;
}
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而後一樣是重寫了 self.secondViewAttribute
的 set
方法,這裏會根據不一樣的狀況作不一樣的操做。
- (void)setSecondViewAttribute:(id)secondViewAttribute {
if ([secondViewAttribute isKindOfClass:NSValue.class]) {
[self setLayoutConstantWithValue:secondViewAttribute];
} else if ([secondViewAttribute isKindOfClass:MAS_VIEW.class]) {
_secondViewAttribute = [[MASViewAttribute alloc] initWithView:secondViewAttribute layoutAttribute:self.firstViewAttribute.layoutAttribute];
} else if ([secondViewAttribute isKindOfClass:MASViewAttribute.class]) {
MASViewAttribute *attr = secondViewAttribute;
if (attr.layoutAttribute == NSLayoutAttributeNotAnAttribute) {
_secondViewAttribute = [[MASViewAttribute alloc] initWithView:attr.view item:attr.item layoutAttribute:self.firstViewAttribute.layoutAttribute];;
} else {
_secondViewAttribute = secondViewAttribute;
}
} else {
NSAssert(NO, @"attempting to add unsupported attribute: %@", secondViewAttribute);
}
}
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其中,第1種狀況對應的是:
make.height.equalTo(@20.0f)
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傳入 NSValue
的時, 會直接設置 constraint
的 offset
, centerOffset
, sizeOffset
, 或者 insets
。調用棧以下:
//MASViewConstraint.m
if ([secondViewAttribute isKindOfClass:NSValue.class]) {
[self setLayoutConstantWithValue:secondViewAttribute];
}
//MASConstraint.m
- (void)setLayoutConstantWithValue:(NSValue *)value {
if ([value isKindOfClass:NSNumber.class]) {
self.offset = [(NSNumber *)value doubleValue];
} else if (strcmp(value.objCType, @encode(CGPoint)) == 0) {
CGPoint point;
[value getValue:&point];
self.centerOffset = point;
} else if (strcmp(value.objCType, @encode(CGSize)) == 0) {
CGSize size;
[value getValue:&size];
self.sizeOffset = size;
} else if (strcmp(value.objCType, @encode(MASEdgeInsets)) == 0) {
MASEdgeInsets insets;
[value getValue:&insets];
self.insets = insets;
} else {
NSAssert(NO, @"attempting to set layout constant with unsupported value: %@", value);
}
}
//MASViewConstraint.m
- (void)setOffset:(CGFloat)offset {
self.layoutConstant = offset;
}
//MASViewConstraint.m
- (void)setLayoutConstant:(CGFloat)layoutConstant {
_layoutConstant = layoutConstant;
#if TARGET_OS_MAC && !(TARGET_OS_IPHONE || TARGET_OS_TV)
if (self.useAnimator) {
[self.layoutConstraint.animator setConstant:layoutConstant];
} else {
self.layoutConstraint.constant = layoutConstant;
}
#else
self.layoutConstraint.constant = layoutConstant;
#endif
}
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第2種狀況,通常是直接傳入一個視圖:
make.top.equalTo(self)
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這時, 就會初始化一個 layoutAttribute
屬性與 firstViewArribute
相同的 MASViewAttribute
, 上面的代碼就會使視圖與 view 頂部對齊。
第3種狀況,會傳入一個視圖的 MASViewAttribute
:
make.top.equalTo(view.mas_bottom);
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使用這種寫法時, 通常是由於約束的方向不一樣. 這行代碼會使視圖的頂部與 view 的底部對齊。
.height.width
:Masonry的鏈式語法特性make.height.width.equalTo(@20);
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其中,.height
設置第一個約束屬性時,調用的是 MASConstraintMaker.m
中的 .height
, addConstraintWithLayoutAttribute
,以及- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute
。
- (MASConstraint *)height {
return [self addConstraintWithLayoutAttribute:NSLayoutAttributeHeight];
}
- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
return [self constraint:nil addConstraintWithLayoutAttribute:layoutAttribute];
}
- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
MASViewAttribute *viewAttribute = [[MASViewAttribute alloc] initWithView:self.view layoutAttribute:layoutAttribute];
MASViewConstraint *newConstraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:viewAttribute];
if ([constraint isKindOfClass:MASViewConstraint.class]) {
//replace with composite constraint
NSArray *children = @[constraint, newConstraint];
MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];
compositeConstraint.delegate = self;
[self constraint:constraint shouldBeReplacedWithConstraint:compositeConstraint];
return compositeConstraint;
}
if (!constraint) {
newConstraint.delegate = self;
[self.constraints addObject:newConstraint];
}
return newConstraint;
}
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該方法調用棧返回的是一個MASViewConstraint
(父類是 MASConstraint
) 對象。
所以,經過 .width
設置第二個約束屬性的時候,調用的先是基類 MASConstraint.m
中的.width
,而後調用由子類MASViewConstraint
實現的addConstraintWithLayoutAttribute
方法。這時候的調用棧爲:
- (MASConstraint *)width {
return [self addConstraintWithLayoutAttribute:NSLayoutAttributeWidth];
}
- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute __unused)layoutAttribute {
MASMethodNotImplemented();
}
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- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
NSAssert(!self.hasLayoutRelation, @"Attributes should be chained before defining the constraint relation");
return [self.delegate constraint:self addConstraintWithLayoutAttribute:layoutAttribute];
}
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這其中,self.delegate
是什麼呢?如2.3.1節所述,MASConstraintMaker.m 中設置了 MASViewConstraint
類 newConstraint
對象的 MASConstraintDelegate
代理爲「self」
(即 MASConstraintMaker
),其做用就是爲了可以同時設置多個約束屬性,即鏈式語法。因此,第二個設置約束屬性跟第一個設置約束屬性最終 調用的方法同樣(都是MASConstraintMaker.m中實現的addConstraintWithLayoutAttribute
)。
- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
MASViewAttribute *viewAttribute = [[MASViewAttribute alloc] initWithView:self.view layoutAttribute:layoutAttribute];
MASViewConstraint *newConstraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:viewAttribute];
if ([constraint isKindOfClass:MASViewConstraint.class]) {
//replace with composite constraint
NSArray *children = @[constraint, newConstraint];
MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];
compositeConstraint.delegate = self;
[self constraint:constraint shouldBeReplacedWithConstraint:compositeConstraint];
return compositeConstraint;
}
if (!constraint) {
newConstraint.delegate = self;
[self.constraints addObject:newConstraint];
}
return newConstraint;
}
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當設置 第二次約束屬性 並執行完以後,咱們還能夠發現 constraint
不爲 nil
,而是一個 MASViewConstraint
對象 ,因此該方法調用棧返回的不是 MASViewConstraint
對象,而是 MASCompositeConstraint
這個對象了,下面咱們來看看這個類。
MASCompositeConstraint
MASCompositeConstraint
是約束的集合,它裏面有個私有的數組用來存放多個 MASViewAttribute
對象。
make.height.width.equalTo(@20)
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當設置 第二個約束屬性,走到 .width
時,最終走的是:
- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
MASViewAttribute *viewAttribute = [[MASViewAttribute alloc] initWithView:self.view layoutAttribute:layoutAttribute];
MASViewConstraint *newConstraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:viewAttribute];
if ([constraint isKindOfClass:MASViewConstraint.class]) {
//replace with composite constraint
NSArray *children = @[constraint, newConstraint];
MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];
compositeConstraint.delegate = self;
[self constraint:constraint shouldBeReplacedWithConstraint:compositeConstraint];
return compositeConstraint;
}
....
}
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其中,能夠成功的走進 if
判讀裏面,將 .height
.wight
兩條約束 MASViewConstraint
對象塞到數組裏,建立 MASCompositeConstraint
對象,而且一樣設置了 delegate
,最後還把 self.constraints
裏面事先添加好的約束 MASViewConstraint
對象替換成了 MASCompositeConstraint
對象。
#pragma mark - MASConstraintDelegate
- (void)constraint:(MASConstraint *)constraint shouldBeReplacedWithConstraint:(MASConstraint *)replacementConstraint {
NSUInteger index = [self.childConstraints indexOfObject:constraint];
NSAssert(index != NSNotFound, @"Could not find constraint %@", constraint);
[self.childConstraints replaceObjectAtIndex:index withObject:replacementConstraint];
}
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另外,咱們能夠點擊 MASCompositeConstraint
初始化方法裏看看,它內部會經過 for
循環,把數組裏面的全部 MASViewConstraint
對象一樣設置了 delegate
。
- (id)initWithChildren:(NSArray *)children {
self = [super init];
if (!self) return nil;
_childConstraints = [children mutableCopy];
for (MASConstraint *constraint in _childConstraints) {
constraint.delegate = self;
}
return self;
}
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這麼作的目的同時是爲了可以繼續鏈式調用,好比咱們再設置第三個約束屬性 .left
make.height.width.left.equalTo(@20);
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這時候的調用棧以下:
- (MASConstraint *)left {
return [self addConstraintWithLayoutAttribute:NSLayoutAttributeLeft];
}
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- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
[self constraint:self addConstraintWithLayoutAttribute:layoutAttribute];
return self;
}
- (MASConstraint *)constraint:(MASConstraint __unused *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
id<MASConstraintDelegate> strongDelegate = self.delegate;
MASConstraint *newConstraint = [strongDelegate constraint:self addConstraintWithLayoutAttribute:layoutAttribute];
newConstraint.delegate = self;
[self.childConstraints addObject:newConstraint];
return newConstraint;
}
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能夠發現,這裏又是經過 delegate 方式,調用 MASConstraintMaker
工廠類中的:
- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
MASViewAttribute *viewAttribute = [[MASViewAttribute alloc] initWithView:self.view layoutAttribute:layoutAttribute];
MASViewConstraint *newConstraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:viewAttribute];
if ([constraint isKindOfClass:MASViewConstraint.class]) {
//replace with composite constraint
NSArray *children = @[constraint, newConstraint];
MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];
compositeConstraint.delegate = self;
[self constraint:constraint shouldBeReplacedWithConstraint:compositeConstraint];
return compositeConstraint;
}
if (!constraint) {
newConstraint.delegate = self;
[self.constraints addObject:newConstraint];
}
return newConstraint;
}
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此時,注意到兩個 if
體都沒有走進去,既不像第一次,也不像第二次約束設置的時候。因此,此次僅僅是初始化了個 MASViewConstraint
對象就直接返回了,而後回到上個方法中添加到 MASCompositeConstraint
的私有數組 self.childConstraints
中返回備用。
關於三次 約束設置以後的 .equalTo(@20)
,由於執行完 .left
時,返回的是 MASCompositeConstraint
對象,到這一步的時候會有點變化,調用棧以下:
- (MASConstraint * (^)(id))equalTo {
return ^id(id attribute) {
return self.equalToWithRelation(attribute, NSLayoutRelationEqual);
};
}
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- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation {
return ^id(id attr, NSLayoutRelation relation) {
for (MASConstraint *constraint in self.childConstraints.copy) {
constraint.equalToWithRelation(attr, relation);
}
return self;
};
}
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能夠發現,這裏會循環以前準備好的私有數組 self.childConstraints
,調用 MASViewConstraint.m 的 equalToWithRelation
方法,和上面講的同樣了。
mas_makeConstraints
方法的最後會調用 [constraintMaker install]
方法來添加全部存儲在 self.constraints
數組中的全部約束。
- (NSArray *)install {
if (self.removeExisting) {
NSArray *installedConstraints = [MASViewConstraint installedConstraintsForView:self.view];
for (MASConstraint *constraint in installedConstraints) {
[constraint uninstall];
}
}
NSArray *constraints = self.constraints.copy;
for (MASConstraint *constraint in constraints) {
constraint.updateExisting = self.updateExisting;
[constraint install];
}
[self.constraints removeAllObjects];
return constraints;
}
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(1). 若是須要從新構建約束,也就是 調用 mas_remakeConstraints:
方法,會先取出視圖的全部約束,而後經過一個 for
循環,調用 uninstall
來清空全部約束:
(2). 若是不須要從新構建約束,會取出 self.constraints
數組中準備好的約束,經過 for
循環,調用 install
來把約束添加到視圖上。
關於 install
,是基類 MASConstraint
的抽象方法,方法體由MASViewConstraint
或 MASCompositeConstraint
實現。而 MASCompositeConstraint
的 install
方法體中其實也是調用的由MASViewConstraint
類實現的install
。
- (void)install { MASMethodNotImplemented(); }
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- (void)install {
for (MASConstraint *constraint in self.childConstraints) {
constraint.updateExisting = self.updateExisting;
[constraint install];
}
}
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這裏代碼較多,就不分開解析了,直接分爲7步寫到源碼的註釋中,以下所示:
- (void)install {
//【1】若是約束以及存在並是 active 會直接返回。
if (self.hasBeenInstalled) {
return;
}
//【2】若是 self.layoutConstraint 響應了 isActive 方法而且不爲空,會激活這條約束並添加到 mas_installedConstraints 數組中,最後返回。
if ([self supportsActiveProperty] && self.layoutConstraint) {
self.layoutConstraint.active = YES;
[self.firstViewAttribute.view.mas_installedConstraints addObject:self];
return;
}
//【3】這邊是獲取即將用於初始化 NSLayoutConstraint 的子類 MASLayoutConstraint 的幾個屬性。
MAS_VIEW *firstLayoutItem = self.firstViewAttribute.item;
NSLayoutAttribute firstLayoutAttribute = self.firstViewAttribute.layoutAttribute;
MAS_VIEW *secondLayoutItem = self.secondViewAttribute.item;
NSLayoutAttribute secondLayoutAttribute = self.secondViewAttribute.layoutAttribute;
// alignment attributes must have a secondViewAttribute
// therefore we assume that is refering to superview
// eg make.left.equalTo(@10)
//【4】這邊是判斷當前即將添加的約束是不是 size 類型的而且 self.secondViewAttribute 也就是約束的第二個參數是 nil,(eg make.left.equalTo(@10))會自動將約束添加到約束的第一個參數視圖的 superview 上。
if (!self.firstViewAttribute.isSizeAttribute && !self.secondViewAttribute) {
secondLayoutItem = self.firstViewAttribute.view.superview;
secondLayoutAttribute = firstLayoutAttribute;
}
//【5】而後就會初始化 NSLayoutConstraint 的子類 MASLayoutConstraint。
MASLayoutConstraint *layoutConstraint
= [MASLayoutConstraint constraintWithItem:firstLayoutItem
attribute:firstLayoutAttribute
relatedBy:self.layoutRelation
toItem:secondLayoutItem
attribute:secondLayoutAttribute
multiplier:self.layoutMultiplier
constant:self.layoutConstant];
layoutConstraint.priority = self.layoutPriority;
layoutConstraint.mas_key = self.mas_key;
//【6】這段代碼會先判斷是否有約束第二個參數的視圖,有的話會尋找約束第一個和第二參數視圖的公共 Superview,至關於求兩個數的最小公倍數;若是不知足第一個條件,會判斷約束第一個參數是不是 size 類型的,是的話直接取到它的視圖;最後都不知足會直接取到約束第一個參數視圖父視圖。
if (self.secondViewAttribute.view) {
MAS_VIEW *closestCommonSuperview = [self.firstViewAttribute.view mas_closestCommonSuperview:self.secondViewAttribute.view];
NSAssert(closestCommonSuperview,
@"couldn't find a common superview for %@ and %@",
self.firstViewAttribute.view, self.secondViewAttribute.view);
self.installedView = closestCommonSuperview;
} else if (self.firstViewAttribute.isSizeAttribute) {
self.installedView = self.firstViewAttribute.view;
} else {
self.installedView = self.firstViewAttribute.view.superview;
}
//【7】若是須要升級當前的約束就會獲取原有的約束,並替換爲新的約束,這樣就不須要再次爲 view 安裝約束。若是原來的 view 中不存在能夠升級的約束,那麼就會在上一步尋找到的 installedView 上面添加約束。
MASLayoutConstraint *existingConstraint = nil;
if (self.updateExisting) {
existingConstraint = [self layoutConstraintSimilarTo:layoutConstraint];
}
if (existingConstraint) {
// just update the constant
existingConstraint.constant = layoutConstraint.constant;
self.layoutConstraint = existingConstraint;
} else {
[self.installedView addConstraint:layoutConstraint];
self.layoutConstraint = layoutConstraint;
[firstLayoutItem.mas_installedConstraints addObject:self];
}
}
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其中第【6】步中的mas_closestCommonSuperview
方法,它會尋找 firstLayoutItem 和 secondLayoutItem 兩個視圖的公共 superview, 至關於求兩個數的最小公倍數.
- (instancetype)mas_closestCommonSuperview:(MAS_VIEW *)view {
MAS_VIEW *closestCommonSuperview = nil;
MAS_VIEW *secondViewSuperview = view;
while (!closestCommonSuperview && secondViewSuperview) {
MAS_VIEW *firstViewSuperview = self;
while (!closestCommonSuperview && firstViewSuperview) {
if (secondViewSuperview == firstViewSuperview) {
closestCommonSuperview = secondViewSuperview;
}
firstViewSuperview = firstViewSuperview.superview;
}
secondViewSuperview = secondViewSuperview.superview;
}
return closestCommonSuperview;
}
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make.edges.equalTo(view)
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咱們再來看看這種寫法,調用棧以下:
- (MASConstraint *)edges {
return [self addConstraintWithAttributes:MASAttributeTop | MASAttributeLeft | MASAttributeRight | MASAttributeBottom];
}
- (MASConstraint *)addConstraintWithAttributes:(MASAttribute)attrs {
__unused MASAttribute anyAttribute = (MASAttributeLeft | MASAttributeRight | MASAttributeTop | MASAttributeBottom | MASAttributeLeading
| MASAttributeTrailing | MASAttributeWidth | MASAttributeHeight | MASAttributeCenterX
| MASAttributeCenterY |
......
NSMutableArray *attributes = [NSMutableArray array];
if (attrs & MASAttributeLeft) [attributes addObject:self.view.mas_left];
if (attrs & MASAttributeRight) [attributes addObject:self.view.mas_right];
if (attrs & MASAttributeTop) [attributes addObject:self.view.mas_top];
......
NSMutableArray *children = [NSMutableArray arrayWithCapacity:attributes.count];
for (MASViewAttribute *a in attributes) {
[children addObject:[[MASViewConstraint alloc] initWithFirstViewAttribute:a]];
}
MASCompositeConstraint *constraint = [[MASCompositeConstraint alloc] initWithChildren:children];
constraint.delegate = self;
[self.constraints addObject:constraint];
return constraint;
}
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代碼太多省略了一部分,能夠發現這段代碼做用就是返回一個包含多條約束的 MASCompositeConstraint
對象,接着後面的操做也都是同樣的了。
上面3.1中例子的寫法還能夠改爲這樣:
make.edges.equalTo(UIEdgeInsetsMake(0.0f, 0.0f, 0.0f, 0.0f));
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這裏的 equalTo
須要注意下,它是一個宏,定義在 MASConstraint.h 中:
#define mas_equalTo(...) equalTo(MASBoxValue((__VA_ARGS__)))
#define mas_greaterThanOrEqualTo(...) greaterThanOrEqualTo(MASBoxValue((__VA_ARGS__)))
#define mas_lessThanOrEqualTo(...) lessThanOrEqualTo(MASBoxValue((__VA_ARGS__)))
#define mas_offset(...) valueOffset(MASBoxValue((__VA_ARGS__)))
#ifdef MAS_SHORTHAND_GLOBALS
#define equalTo(...) mas_equalTo(__VA_ARGS__)
#define greaterThanOrEqualTo(...) mas_greaterThanOrEqualTo(__VA_ARGS__)
#define lessThanOrEqualTo(...) mas_lessThanOrEqualTo(__VA_ARGS__)
#define offset(...) mas_offset(__VA_ARGS__)
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代入上述宏定義,前面的代碼等效成:
make.edges.equalTo(MASBoxValue(UIEdgeInsetsMake(0.0f, 0.0f, 0.0f, 0.0f)));
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能夠發現,其實裏面調用的是 MASBoxValue
這個宏,它將 C 和 Objective-C 語言中的一些基本數據結構好比說 double
CGPoint
CGSize
這些值用 NSValue
進行包裝。
這裏還支持直接調用 size、center 等,具體實現都差很少,就不熬述了:
make.center.equalTo(CGPointMake(0, 50));
make.size.equalTo(CGSizeMake(200, 100));
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make.height.equalTo(@[redView, blueView])
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再來看看這種傳數組的,在走到 .equalTo
時,最終會調用 MASViewConstraint.m 裏面的 equalToWithRelation
方法
- (MASConstraint * (^)(id))equalTo {
return ^id(id attribute) {
return self.equalToWithRelation(attribute, NSLayoutRelationEqual);
};
}
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- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation {
return ^id(id attribute, NSLayoutRelation relation) {
if ([attribute isKindOfClass:NSArray.class]) {
NSAssert(!self.hasLayoutRelation, @"Redefinition of constraint relation");
NSMutableArray *children = NSMutableArray.new;
for (id attr in attribute) {
MASViewConstraint *viewConstraint = [self copy];
viewConstraint.layoutRelation = relation;
viewConstraint.secondViewAttribute = attr;
[children addObject:viewConstraint];
}
MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];
compositeConstraint.delegate = self.delegate;
[self.delegate constraint:self shouldBeReplacedWithConstraint:compositeConstraint];
return compositeConstraint;
} else { .... }
};
}
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這邊仍是遍歷數組,而且 MASViewConstraint
實現 NSCopying 協議
,調用 [self copy]
會建立 MASViewConstraint
對象
- (id)copyWithZone:(NSZone __unused *)zone {
MASViewConstraint *constraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:self.firstViewAttribute];
constraint.layoutConstant = self.layoutConstant;
constraint.layoutRelation = self.layoutRelation;
constraint.layoutPriority = self.layoutPriority;
constraint.layoutMultiplier = self.layoutMultiplier;
constraint.delegate = self.delegate;
return constraint;
}
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而後會根據傳的數組裏面的 Value 類型來作不一樣的操做,前面講過就不熬述了:
- (void)setSecondViewAttribute:(id)secondViewAttribute {
if ([secondViewAttribute isKindOfClass:NSValue.class]) {
[self setLayoutConstantWithValue:secondViewAttribute];
} else if ([secondViewAttribute isKindOfClass:MAS_VIEW.class]) {
_secondViewAttribute = [[MASViewAttribute alloc] initWithView:secondViewAttribute layoutAttribute:self.firstViewAttribute.layoutAttribute];
} else if ([secondViewAttribute isKindOfClass:MASViewAttribute.class]) {
_secondViewAttribute = secondViewAttribute;
} else {
NSAssert(NO, @"attempting to add unsupported attribute: %@", secondViewAttribute);
}
}
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最後即是生成 MASCompositeConstraint
對象,並經過 delegate
方式,調用 MASConstraintMaker
的方法,替換 self.constraints
數組裏的約束:
- (void)constraint:(MASConstraint *)constraint shouldBeReplacedWithConstraint:(MASConstraint *)replacementConstraint {
NSUInteger index = [self.constraints indexOfObject:constraint];
NSAssert(index != NSNotFound, @"Could not find constraint %@", constraint);
[self.constraints replaceObjectAtIndex:index withObject:replacementConstraint];
}
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MASConstraintMaker
類就是一個工廠類,負責建立MASConstraint
類型的對象(依賴於MASConstraint
接口,而不依賴於具體實現)。在UIView的View+MASAdditions
分類中就是調用的MASConstraintMaker
類中的一些方法。上述咱們在使用Masonry給subView添加約束時,mas_makeConstraints
方法中的Block的參數就是MASConstraintMaker
的對象。用戶能夠經過該Block回調過來的MASConstraintMaker
對象給View指定要添加的約束以及該約束的值。該工廠中的constraints
屬性數組就記錄了該工廠建立的全部MASConstraint
對象。
MASConstraintMaker
之因此成爲約束工廠類,由於MASConstraintMaker
賦值建立NSLayoutConstraint
對象,由於Masonry將NSLayoutConstraint
類進一步封裝成了MASViewConstraint
,因此MASConstraintMaker
是負責建立MASViewConstraint
的對象,並調用MASViewConstraint
對象的Install
方法將該約束添加到相應的視圖中。
說了這麼多,總結一下,若是你調用maker.top
, maker.left
等等這些方法都會調用下方的工廠方法來建立相應的MASViewConstraint
對象,並記錄在工廠對象的約束數組中。之因此能鏈式調用,就是講當前的工廠對象(MASConstraintMaker
)指定爲MASViewConstraint
對象的代理,因此一個MASViewConstraint
對象就能夠經過代理來調用工廠方法來建立另外一個新的MASViewConstraint
對象了,此處用到了代理模式。
角色分析
Client:UIView
,經過分類View+MASAdditions
來扮演
工廠類:MASConstraintMaker
抽象產品:MASConstraint
具體產品:MASViewConstraint
, MASCompositeConstraint
換一種角度看,Masonry 並不是單純的工廠模式,而是採用了經典的 Composite 設計模式,中文可譯做組合模式。
UIView
,經過分類View+MASAdditions
來調用Masonry
MASConstraintMaker
MASConstraint
MASViewConstraint
MASCompositeConstraint
Objective-C是一門動態語言,它使用了一種動態的消息發送機制,即對象(object)或類(class)調用方法。而OC中的點語法則只能經過setter和getter方法做用於類的屬性,而不能做用於某個方法。想實現鏈式語法,只能經過相似block屬性的getter方法。
鏈式編程思想:核心思想爲將block做爲方法的返回值,且返回值的類型爲調用者自己,並將該方法以setter的形式返回,這樣就能夠實現了連續調用,即爲鏈式編程。
【舉例】簡單使用鏈式編程思想實現一個簡單計算器的功能:
// CaculateMaker.h
// ChainBlockTestApp
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
@interface CaculateMaker : NSObject
@property (nonatomic, assign) CGFloat result;
- (CaculateMaker *(^)(CGFloat num))add;
@end
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// CaculateMaker.m
// ChainBlockTestApp
#import "CaculateMaker.h"
@implementation CaculateMaker
- (CaculateMaker *(^)(CGFloat num))add;{
return ^CaculateMaker *(CGFloat num){
_result += num;
return self;
};
}
@end
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CaculateMaker *maker = [[CaculateMaker alloc] init];
maker.add(20).add(30);
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經過上面Masonry佈局能夠看出,它爲UIView寫了一個category,拓展了mas_makeConstraints
方法,並將MASConstraintMaker
對象做爲block的參數傳遞,在block的實現裏完成UIView的佈局,提現了函數式編程思想。
// NSObject+Caculate.h
// ChainBlockTestApp
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
#import "CaculateMaker.h"
@interface NSObject (Caculate)
- (CGFloat)caculate:(void (^)(CaculateMaker *make))block;
@end
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// NSObject+Caculate.m
// ChainBlockTestApp
#import "NSObject+Caculate.h"
@implementation NSObject (Caculate)
- (CGFloat)caculate:(void (^)(CaculateMaker *make))block;{
CaculateMaker *make = [[CaculateMaker alloc] init];
block(make);
return make.result;
}
@end
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CGFloat result = [NSObject caculate:^(CaculateMaker *maker) {
maker.add(10).add(20).add(30);
}];
NSLog(@"結果爲:%.2f",result);
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Masonry解析
工廠模式
組合模式
鏈式編程