先贴一下该方法的源码：

  /*** Handles errors emitted by this [Future].** This is the asynchronous equivalent of a "catch" block.** Returns a new [Future] that will be completed with either the result of* this future or the result of calling the `onError` callback.** If this future completes with a value,* the returned future completes with the same value.** If this future completes with an error,* then [test] is first called with the error value.** If `test` returns false, the exception is not handled by this `catchError`,* and the returned future completes with the same error and stack trace* as this future.** If `test` returns `true`,* [onError] is called with the error and possibly stack trace,* and the returned future is completed with the result of this call* in exactly the same way as for [then]'s `onError`.** If `test` is omitted, it defaults to a function that always returns true.* The `test` function should not throw, but if it does, it is handled as* if the `onError` function had thrown.** Note that futures don't delay reporting of errors until listeners are* added. If the first `catchError` (or `then`) call happens after this future* has completed with an error then the error is reported as unhandled error.* See the description on [Future].*/// The `Function` below stands for one of two types:// - (dynamic) -> FutureOr<T>// - (dynamic, StackTrace) -> FutureOr<T>// Given that there is a `test` function that is usually used to do an// `isCheck` we should also expect functions that take a specific argument.// Note: making `catchError` return a `Future<T>` in non-strong mode could be// a breaking change.Future<T> catchError(Function onError, {bool test(Object error)});
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翻译一下主要的意思，就是这个catchError方法可以捕获其他Futrue的异常信息，如果重写了test方法，test返回true就可以在catchError的onError方法里捕获到异常，如果test返回false，就把该异常继续抛出而不会在catchError方法里被捕获，如果不写test默认实现一个返回true的test方法，注意catchError只能捕获Future的异常，而不能捕获同步代码的异常，测试代码如下：

import 'dart:async';Future testFutureError() {return new Future(() {throw "error";//1
//    return "abc";//2});
}main() {testFutureError().then((value) {print("then " + value);}).catchError((e) {print("catchError " + e);}, test: (Object o) {print("test " + o);return true;//3
//    return false;//4}).catchError((e) {print("catchError2 " + e);}, test: (_) => true);
}
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如果是上面的代码，会输出

test error
catchError error
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因为//3这行返回了true，所以会在第一个catchError里被捕获。 如果//4不注释了，把//3注释，那么第一个catchError不能捕获该异常，该异常会继续抛出，然后在第二个catchError里被捕获。具体的可自行测试。

还有就是用whenComplete方法的时候，不是所有then都执行完再执行whenComplete方法，而是then、catchError、whenComplete这些方法会按照顺序（除非中途有异常会进入下一个catchError）执行，这一点和rxjava+retrofit不一样，原生开发请知悉。