typescript 离子 3/打字稿承诺
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Ionic 3/Typescript Promise
提问by Wang
Im trying to use Promise to get data from web service
我正在尝试使用 Promise 从 Web 服务获取数据
login(){
var obj = {email:'WHATEVER', firstname:''};
var b = this.getUsers().then(data => {
this.films = data;
obj = this.films.customer_info;
console.log('inside:' + obj.firstname);
});
console.log('outside:' + obj.firstname);
}
}
My expected result:
我的预期结果：
outside:Nam Nam login.ts:51 inside:Nam Nam
login.ts:48
外部：Nam Nam login.ts:51 内部：Nam Nam
login.ts:48
My actual result :
我的实际结果：
outside: login.ts:51 inside:Nam Nam
login.ts:48
外部：login.ts:51 内部：Nam Nam
login.ts:48
Can someone tell me how to solve this problem? thanks
有人能告诉我如何解决这个问题吗？谢谢
回答by Fenton
Your expected result is not possible, because you are asserting that you want a value (that will be obtained asynchronously) to be available synchronously.
您的预期结果是不可能的，因为您断言您希望一个值（将异步获得）同步可用。
Let's look into it in more detail... here is a short, but fully working example of your problem:
让我们更详细地研究一下……这是您的问题的一个简短但完全有效的示例：
class Example {
private films: any;
login() {
var obj = { email: 'WHATEVER', firstname: '' };
var b = this.getUsers().then(data => {
this.films = data;
obj = this.films.customer_info;
console.log('inside:' + obj.firstname);
});
console.log('outside:' + obj.firstname);
}
getUsers() {
return new Promise((resolve, reject) => {
window.setTimeout(() => {
console.log('Result obtainined... for example from a service');
resolve({ customer_info: { firstname: 'Fenton' } });
}, 1000);
})
}
}
const example = new Example();
example.login();
The output from this program is:
这个程序的输出是：
outside:
Result obtainined... for example from a service
inside:Fenton
As you can see, the statement that is outside of the thenhandler executes before the promise resolves. If you are calling out to a service, this line of code will run before the server has responded.
如您所见，then处理程序之外的语句在承诺解析之前执行。如果您正在调用服务，这行代码将在服务器响应之前运行。
That's actually the whole point of promises - which is why it works insideyour thenhandler.
这实际上是 promises 的全部意义——这就是它在你的then处理程序中工作的原因。
If you have code that depends on the logincompleting, you could return a promise from the loginmethod. Based on the above example, you could use:
如果您有依赖于login完成的代码，您可以从该login方法返回一个承诺。基于上面的例子，你可以使用：
login(): Promise<{ firstname: string}> {
return new Promise((resolve, reject) => {
this.getUsers().then(data => {
this.films = data;
resolve(this.films.customer_info);
});
});
}
When the login completes, the customer info is available:
登录完成后，客户信息可用：
example.login()
.then((customer) => console.log(customer.firstname));
