UITableView
創(chuàng)建UITableView之后,會(huì)先numberOfRowsAtIndexPaht和heightForRowAtIndexPath 計(jì)算大概高度 (因?yàn)橹蓝嗌贄l 每條高度多少 可以算出UIScrollView的contentSize一樣的東西)
(如果有100條)heightForRow就會(huì)走100次
然后cellForRowAtIndexPath顯示每個(gè)cell
比如:
- (void)viewDidLoad {
[super viewDidLoad];
self.array = [NSMutableArray arrayWithObjects:@"000", @"111", @"222", @"333", @"444", @"555", @"666", @"777", @"888", @"999", @"101010", nil];
self.tblView = [[UITableView alloc] initWithFrame:self.view.bounds];
self.tblView.dataSource = self;
self.tblView.delegate = self;
[self.view addSubview:self.tblView];
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
NSLog(@"------------%s", __func__);
return self.array.count;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
NSLog(@"++++++++%s+++++++++%ld", __func__, indexPath.row);
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"reuseId"];
if (!cell) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"reuseId"];
}
cell.textLabel.text = [self.array objectAtIndex:indexPath.row];
return cell;
}
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
CGFloat height = 100;
NSLog(@"++++++++%s+++++++++%ld", __func__, indexPath.row);
return height;
}
輸出的結(jié)果如下:
-------------[ViewController tableView:numberOfRowsInSection:]
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++0
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++1
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++2
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++3
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++4
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++5
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++6
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++7
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++8
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++9
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++10
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++0
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++0
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++1
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++1
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++2
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++2
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++3
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++3
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++4
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++4
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++5
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++5
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++6
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++6
可以看出執(zhí)行的順序.即使畫(huà)面只能顯示到666,heightForRow還是會(huì)執(zhí)行到10
非等高cell布局思路:
- 在heightForRow里創(chuàng)建一個(gè)cell
(注意是創(chuàng)建,不是調(diào)用cellForRow方法,否則會(huì)死循環(huán),因?yàn)閏ellForRow時(shí)候會(huì)調(diào)用heightForRow).
布局一次(layoutIfNeeded) 得到高度之后 return 得到的高度.
這是最low的方法, 因?yàn)樵趆eightForRow里創(chuàng)建了一個(gè)cell只為了得到cell的高度. 得到之后就干掉這個(gè)cell. 上面也說(shuō)了 如果有100條 會(huì)調(diào)用100次 創(chuàng)建100個(gè)cell只為獲得他的高度. 很耗性能. - 上面也介紹了如果cellForRow那一定要先知道cell的高度.其實(shí)我們可以改變一下順序的 蘋(píng)果的API有提供一個(gè)方法
// Use the estimatedHeight methods to quickly calcuate guessed values which will allow for fast load times of the table.
// If these methods are implemented, the above -tableView:heightForXXX calls will be deferred until views are ready to be displayed, so more expensive logic can be placed there.
- (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(7_0);
這個(gè)方法會(huì)讓tableview估算一個(gè)cell的值(別太小的離譜,大概200 - 300),因?yàn)橛泄浪愕闹?所以會(huì)先走cellForRow,然后tableview覺(jué)得估算的height不靠譜,這時(shí)才會(huì)heightForRow.因?yàn)檫@時(shí)候cellForRow已經(jīng)走完了, 可以在cellForRow中拿到cell的高度(cellForRow只是賦值,不會(huì)布局,記得layoutIfNeeded,這樣才會(huì)走cell的layoutSubviews
),保存起來(lái)(用數(shù)組或者字典都可以),然后在heightForRow里根據(jù)保存的值顯示出來(lái).
而且因?yàn)橛辛斯浪愕腸ell高度,所以即使有100條,也不會(huì)再最開(kāi)始就走100次heightForRow了,也算是對(duì)性能稍微優(yōu)化了一些.
// 修改每個(gè)cell高度不相等
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
CGFloat height = 40 + 10 * indexPath.row;
NSLog(@"++++++++%s+++++++++%ld", __func__, indexPath.row);
return height;
}
// 增加估算的cell高度
- (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath {
return 240;
}
輸出結(jié)果:
-------------[ViewController tableView:numberOfRowsInSection:]
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++0
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++0
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++1
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++1
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++2
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++2
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++3
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++3
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++4
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++4
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++5
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++5
++++++++-[ViewController tableView:cellForRowAtIndexPath:]+++++++++6
++++++++-[ViewController tableView:heightForRowAtIndexPath:]+++++++++6
- 在上面2方法中 需要再建立個(gè)數(shù)組或者字典存放cell的高度,有些麻煩.
因?yàn)檎R粋€(gè)cell應(yīng)該是對(duì)應(yīng)一個(gè)數(shù)據(jù)模型,所以可以將cell的高度存放在模型中 然后在heightForRow里去獲取model.cellHeight.
另外:xib自動(dòng)布局中 UILabel比較特殊 設(shè)置寬度約束之后可以不設(shè)置高度約束,也可以自動(dòng)顯示出UILabel. 這種情況下可能cell里的label下面可能會(huì)有一個(gè)空行(多行的自動(dòng)換行時(shí)候可能會(huì)有).這時(shí)最好告訴label最大寬度(label.preferredMaxLayoutWidth屬性)
因?yàn)閘ayoutIfNeeded是強(qiáng)制布局,強(qiáng)制布局時(shí)候如果不準(zhǔn)確的告訴程序文字的最大寬度label.preferredMaxLayoutWidth 可能估算的最大高度可能會(huì)有偏差 所以可能會(huì)造成label下面可能會(huì)有一個(gè)空行的問(wèn)題
最后貼一下代碼:
模型:
@interface DataModel : NSObject
@property (nonatomic, copy)NSString *textValue;
@property (nonatomic, assign)CGFloat cellHeight;
@end
cell:
@interface MyTableViewCell : UITableViewCell
@property (nonatomic, strong)DataModel *model;
@end
@implementation MyTableViewCell
-(void)setModel:(DataModel *)model {
_model = model;
self.textLabel.text = _model.textValue;
// 這里偷懶了, 正常應(yīng)該是文字或者圖片的自適應(yīng)高度
CGFloat height = 40 + 10 * [_model.textValue integerValue] * 0.01;
// 布局之后獲取高度
[self layoutIfNeeded];
model.cellHeight = height;
}
@end
- (void)viewDidLoad {
[super viewDidLoad];
self.array = [NSMutableArray array];
for (NSInteger i = 0; i <= 10; i++) {
DataModel *data = [[DataModel alloc] init];
data.textValue = [NSString stringWithFormat:@"%ld%ld%ld", i, i, i];
[self.array addObject:data];
}
self.tblView = [[UITableView alloc] initWithFrame:self.view.bounds];
self.tblView.dataSource = self;
self.tblView.delegate = self;
[self.view addSubview:self.tblView];
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
NSLog(@"------------%s", __func__);
return self.array.count;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
NSLog(@"++++++++%s+++++++++%ld", __func__, indexPath.row);
MyTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"reuseId"];
if (!cell) {
cell = [[MyTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"reuseId"];
}
cell.model = [self.array objectAtIndex:indexPath.row];
return cell;
}
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
return [[self.array objectAtIndex:indexPath.row] cellHeight];
}
- (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath {
return 240;
}
結(jié)果圖: