我有一个用例，我使用多个数据源异步加载一些数据。因此，每个数据源都有一个完整的方法，我的 View Controller 实现了一个定义了该方法的协议(protocol)。例如，我的一个数据源是 fetchApples，它向我的 Controller 返回一个 Fruit 对象数组，另一个数据源是 fetchOranges 等。在我的 viewController 中，我想让苹果先出现，然后是橙子，然后是葡萄(我填充了一个 uiview带有用于渲染水果的自定义单元格)。如果没有苹果，橘子应该首先出现等等。当我的数据源被异步返回时，我如何映射这个顺序。 IE。当橘子回来时，我不知道我是否会有苹果，因此我还不能用它们填充 uiview？

最佳答案

使用 dispatch_group。每个服务器调用都会填充它自己的项目数组，您可以将它添加到一个主数组(数据源)或单独使用它们。

当所有服务器调用完成后，您将收到通知，然后您可以使用您想要的任何数据源以任何顺序重新加载您的表。

示例(使用 dispatch_group 和 UITableView):

//
//  ViewController.m
//  StackOverflowExample
//
//  Created by Brandon Anthony on 2016-07-16.
//  Copyright © 2016 XIO. All rights reserved.
//

#import "ViewController.h"


typedef NS_ENUM(NSInteger, Fruit) {
    Apples,
    Oranges,
    Grapes
};

#define kImageCellIdentifier @"kImageCellIdentifier"

@interface ViewController () <UITableViewDelegate, UITableViewDataSource>
@property (nonatomic, strong) UIButton *testButton;
@property (nonatomic, strong) UITableView *tableView;
@property (nonatomic, strong) NSMutableArray *apples;
@property (nonatomic, strong) NSMutableArray *oranges;
@property (nonatomic, strong) NSMutableArray *grapes;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    _apples = [[NSMutableArray alloc] init];
    _oranges = [[NSMutableArray alloc] init];
    _grapes = [[NSMutableArray alloc] init];

    [self initControls];
    [self setTheme];
    [self registerClasses];
    [self doLayout];

    [self loadData];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}

- (void)initControls {
    _testButton = [[UIButton alloc] init];
    _tableView = [[UITableView alloc] initWithFrame:CGRectZero style:UITableViewStyleGrouped];
}

- (void)setTheme {
    [_testButton setTitle:@"Test Again" forState:UIControlStateNormal];
    [_testButton setBackgroundColor:[UIColor lightGrayColor]];
    [_testButton setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
    [[_testButton layer] setCornerRadius:5.0f];
    [_testButton addTarget:self action:@selector(loadData) forControlEvents:UIControlEventTouchUpInside];

    [_tableView setDelegate:self];
    [_tableView setDataSource:self];
}

- (void)registerClasses {
    [_tableView registerClass:[UITableViewCell class] forCellReuseIdentifier:kImageCellIdentifier];
}

- (void)doLayout {
    [self.view addSubview:_testButton];
    [self.view addSubview:_tableView];

    NSDictionary *views = @{@"testButton":_testButton, @"tableView":_tableView};
    NSMutableArray *constraints = [[NSMutableArray alloc] init];

    [constraints addObject:[NSString stringWithFormat:@"H:[testButton(%d)]-%d-|", 150, 15]];
    [constraints addObject:[NSString stringWithFormat:@"H:|-%d-[tableView]-%d-|", 0, 0]];
    [constraints addObject:[NSString stringWithFormat:@"V:|-%d-[testButton(%d)]-%d-[tableView]-%d-|", 25, 44, 10, 0]];

    for (NSString *constraint in constraints) {
        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:constraint options:0 metrics:nil views:views]];
    }

    for (UIView *view in self.view.subviews) {
        [view setTranslatesAutoresizingMaskIntoConstraints:NO];
    }
}


- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    NSInteger sectionCount = _apples.count ? 1 : 0;
    sectionCount += _oranges.count ? 1 : 0;
    sectionCount += _grapes.count ? 1 : 0;
    return sectionCount ? sectionCount : 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    if (section == 0) {
        return _apples.count ?: _oranges.count ?: _grapes.count ?: 0;
    }

    if (section == 1) {
        return _oranges.count ?: _grapes.count ?: 0;
    }

    return _grapes.count;
}

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section {
    if (section == 0) {
        return _apples.count ? @"Apples" : (_oranges.count ? @"Oranges" : (_grapes.count ? @"Grapes" : @"No Fruits"));
    }

    if (section == 1) {
        if (_apples.count) {
            return _oranges.count ? @"Oranges" : (_grapes.count ? @"Grapes" : @"No Fruits");
        }
    }

    return @"Grapes";
}

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
    return 60.0f;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    if (indexPath.section == 0) {
        Fruit fruit = _apples.count ? Apples : (_oranges.count ? Oranges : (_grapes.count ? Grapes : 0));
        return [self cellForFruit:fruit tableView:tableView indexPath:indexPath];
    }

    if (indexPath.section == 1) {
        if (_apples.count) {
            Fruit fruit = _oranges.count ? Oranges : (_grapes.count ? Grapes : 0);
            return [self cellForFruit:fruit tableView:tableView indexPath:indexPath];
        }
    }

    return [self cellForFruit:Grapes tableView:tableView indexPath:indexPath];
}

- (UITableViewCell *)cellForFruit:(Fruit)kind tableView:(UITableView *)tableView indexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell = [_tableView dequeueReusableCellWithIdentifier:kImageCellIdentifier forIndexPath:indexPath];

    switch (kind) {
        case Apples: {
            NSString *kind = [_apples objectAtIndex:indexPath.row];

            [[cell imageView] setImage:nil]; //Some Image..
            [[cell textLabel] setText:kind];
        }
            break;

        case Oranges: {
            NSString *kind = [_oranges objectAtIndex:indexPath.row];

            [[cell imageView] setImage:nil]; //Some Image..
            [[cell textLabel] setText:kind];
        }
            break;

        case Grapes: {
            NSString *kind = [_grapes objectAtIndex:indexPath.row];

            [[cell imageView] setImage:nil]; //Some Image..
            [[cell textLabel] setText:kind];
        }
            break;

        default:
            break;
    }
    return cell;
}

- (void)loadData {
    [_apples removeAllObjects];
    [_oranges removeAllObjects];
    [_grapes removeAllObjects];

    dispatch_group_t group = dispatch_group_create();

    dispatch_group_enter(group);
    [self getApples:^(NSArray *apples) {

        if (apples.count) {
            @synchronized (self.apples) {
                [_apples addObjectsFromArray:apples];
            }
        }

        dispatch_group_leave(group);
    }];

    dispatch_group_enter(group);
    [self getOranges:^(NSArray *oranges) {

        if (oranges.count) {
            @synchronized (self.oranges) {
                [_oranges addObjectsFromArray:oranges];
            }
        }

        dispatch_group_leave(group);
    }];

    dispatch_group_enter(group);
    [self getGrapes:^(NSArray *grapes) {

        if (grapes.count) {
            @synchronized (self.grapes) {
                [_grapes addObjectsFromArray:grapes];
            }
        }

        dispatch_group_leave(group);
    }];

    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        [self.tableView reloadData];
    });
}






//Simulating server calls..

- (void)getApples:(void(^)(NSArray *apples))completion {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSArray *apples = @[@"Red Apple", @"Sweet Apple", @"Sour Apple"];

        bool error = arc4random_uniform(100) <= 50;
        if (error) {
            completion(nil);
        }
        else {
            completion(apples);
        }
    });
}

- (void)getOranges:(void(^)(NSArray *oranges))completion {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSArray *apples = @[@"Tiny Orange", @"Bruised Orange", @"Red Orange"];

        bool error = arc4random_uniform(100) <= 50;
        if (error) {
            completion(nil);
        }
        else {
            completion(apples);
        }
    });
}

- (void)getGrapes:(void(^)(NSArray *grapes))completion {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSArray *apples = @[@"Baby Grapes", @"Green Grapes", @"I ran out of ideas for names.. Grapes"];

        bool error = arc4random_uniform(100) <= 50;
        if (error) {
            completion(nil);
        }
        else {
            completion(apples);
        }
    });
}

@end

http://i.imgur.com/S1DiqGW.png

关于ios - 异步加载数据但按顺序填充 View ，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/38416479/