RxJava concat符操作处理多数据源

1 concat操作符概述

从concatMap操作我们知道，concat操作符肯定也是有序的，而concat操作符是接收若干个Observables，发射数据是有序的，不会交叉。

下面看看官方文档和流程图的说明：

Returns an Observable that emits the items emitted by three Observables, 
one after the other, without interleaving them.

2 使用示例

示例一

下面使用concat操作符 来实现个多数据源的例子，比如一个商品详情需要展示商品的信息、艺术家信息、与该商品类似的商品，可能需要访问三个接口。这个时候就可以使用concat操作符。

卖家信息

    private Observable<Object> sellerInfo = Observable.create(new Observable.OnSubscribe<Object>() {
        @Override
        public void call(Subscriber<? super Object> subscriber) {
            Seller d = goodsApi.getSeller();
            if (d != null) {
                subscriber.onNext(d);
            }
            subscriber.onCompleted();
        }
    }).subscribeOn(Schedulers.io());

商品信息

    private Observable<Object> goodsInfo = Observable.create(new Observable.OnSubscribe<Object>() {
        @Override
        public void call(Subscriber<? super Object> subscriber) {
            Goods d = goodsApi.getGoodInfo();
            if (d != null) {
                subscriber.onNext(d);
            }
            subscriber.onCompleted();
        }
    }).subscribeOn(Schedulers.io());

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    private Observable<Object> relateGoods = Observable.create(new Observable.OnSubscribe<Object>() {
        @Override
        public void call(Subscriber<? super Object> subscriber) {
            List<Goods> d = goodsApi.getRelateGoods();
            if (d != null) {
                subscriber.onNext(d);
            }
            subscriber.onCompleted();
        }
    }).subscribeOn(Schedulers.io());

Observable.concat(sellerInfo, goodsInfo, relateGoods)
                        .observeOn(AndroidSchedulers.mainThread())
                        .subscribe(new Action1<Object>() {
                            @Override
                            public void call(Object s) {
                                printLog(tvLogs, "Getting data from ", s);
                                //push data to ui
                            }
                        }, new Action1<Throwable>() {
                            @Override
                            public void call(Throwable throwable) {
                                throwable.printStackTrace();
                                printLog(tvLogs, "Error: ", throwable.getMessage());
                            }
                        });

为什么泛型去设置成Object呢，因为Observable.concat只接受相同泛型的参数。

也许会有人问后去商品信息的时候没办法传递参数啊(比如商品ID), 在外面包一层方法即可，方法参数为goodID。

示例二

使用 concat操作符 对缓存进行检查，如：内存缓存、本地缓存、网络，那一层有数据立即返回。

使用一个数组来表示三个数据源：

String data[] = {null, null, "network"};
//String data[] = {null, "disk","network"};
//String data[] = {"memory", null,"network"};
//String data[] = {"memory", "disk",null};
//String data[] = {"memory", "disk","network"};

内存缓存

  private Observable<String> memorySource = Observable.create(new Observable.OnSubscribe<String>() {
        @Override
        public void call(Subscriber<? super String> subscriber) {
            String d = data[0];
            printLog(tvLogs, "", "----start check memory data. value is null? " + (d == null));
            if (d != null) {
                subscriber.onNext(d);
            }
            subscriber.onCompleted();
        }
    });

本地缓存

    private Observable<String> diskSource = Observable.create(new Observable.OnSubscribe<String>() {
        @Override
        public void call(Subscriber<? super String> subscriber) {
            String d = data[1];
            printLog(tvLogs, "", "----start check disk data. value is null? " + (d == null));
            if (d != null) {
                subscriber.onNext(d);
            }
            subscriber.onCompleted();
        }
    }).subscribeOn(Schedulers.io());

网络

    private Observable<String> networkSource = Observable.create(new Observable.OnSubscribe<String>() {
        @Override
        public void call(Subscriber<? super String> subscriber) {
            String d = data[2];
            printLog(tvLogs, "", "----start check network data. value is null? " + (d == null));
            if (d != null) {
                subscriber.onNext(d);
            }
            subscriber.onCompleted();
        }
    }).subscribeOn(Schedulers.io());

从例子1我们知道，concat会三个数据源都会请求的。如何使得哪层有数据就用哪层的，之后就不走后面的逻辑了。
可以配合first()操作符来实现这样的效果。

Observable.concat(memorySource, diskSource, networkSource)
                        .first()
                        .observeOn(AndroidSchedulers.mainThread())
                        .subscribe(new Action1<String>() {
                            @Override
                            public void call(String s) {
                                printLog(tvLogs, "Getting data from ", s);
                            }
                        }, new Action1<Throwable>() {
                            @Override
                            public void call(Throwable throwable) {
                                throwable.printStackTrace();
                                printLog(tvLogs, "Error: ", throwable.getMessage());
                            }
                        });

需要注意的是如果memorySource, diskSource, networkSource返回的都null，那么会报一个异常：
java.util.NoSuchElementException: Sequence contains no elements……

可以使用takeFirst操作，即使都没有数据，也不会报异常。

Observable.concat(memorySource, diskSource, networkSource)
                        //first()-> if no data from observables will cause exception :
                        //java.util.NoSuchElementException: Sequence contains no elements
                        //takeFirst -> no exception
                        .takeFirst(new Func1<String, Boolean>() {
                            @Override
                            public Boolean call(String s) {
                                return s != null;
                            }
                        })
                        .observeOn(AndroidSchedulers.mainThread())
                        .subscribe(new Action1<String>() {
                            @Override
                            public void call(String s) {
                                printLog(tvLogs, "Getting data from ", s);
                            }
                        }, new Action1<Throwable>() {
                            @Override
                            public void call(Throwable throwable) {
                                throwable.printStackTrace();
                                printLog(tvLogs, "Error: ", throwable.getMessage());
                            }
                        });

github源码下载