$q.when
$q.when() can be used to wrap an object or function if we are not sure whether it returns a promise or not. If we wrap a value object or a value returned function, the value will be passed as a result argument to the next function. When we wrap a function, the when() method will ensure that the function is executed first before going to the next one.
Below are some code examples:
$q.when('aaa').then(function(result) {
alert(result);
})
var vm = this;
vm.chars = '';
$q.when(_test()).then(function(){
vm.chars = vm.chars + 'END'
})
_test = function() {
for (i = 0; i <= 10000; i++) {
vm.chars = vm.chars + '.'
}
}
$q.all
$q.all() is used to combine promise returned functions and only continue to progress further after all of the functions complete successfully. If one of the functions fails, immediately as soon as $q.all() gets a rejected promise from one of the functions, it will end the execution and pass forward to an error callback if there is any.
vm.numbers = '';
_promiseFunction = function(x) {
var deferred = $q.defer();
if (x > 0) {
deferred.resolve(x * 100);
} else {
deferred.reject(x + " is smaller than 0")
}
return deferred.promise;
}
var first = _promiseFunction(1);
var second = _promiseFunction(5);
var third = _promiseFunction(10);
var four = _promiseFunction(-1);
var five = _promiseFunction(-10)
$q.all([first, second, third /*, four, five*/]).
then(function(results){
console.log(results.length);
for(i=0; i < results.length; i++) {
vm.numbers = vm.numbers + ' ' + results[i];
}
},
function(errors){
// if there is any error in the sequence, the success callback will not be called
console.log(errors);
})
$q.then
$q.then() is useful for executing asynchronous functions (or blocks of codes) sequentially in a chain. If the function inside a then() does not return a promise, it will be wrapped as a resolved promise. Otherwise the resolved or rejected promise will be passed forward.
_promiseFunction(5).then(
function() { alert('aaa')}
).then(
function() { alert('bbb');}
).then(
function() {alert('ccc');}
);
_promiseFunction(5).then(
function() { return $q.reject('failed')}
).then(
function(){ alert('result success')},
function(){ alert('result fail')}
)
Note also that if we return a non-promise value in the error callback inside then(), this will be treated as a resolved promise. So we need to be careful when handling error if our intention is to pass the error forward to the next function.
$q.reject().then(
// success
function() {},
// error
function(error) {
return 'i got the error'
}
).then(
//success
function() {
alert('success');
},
// error
function() {
alert('error')
}
)
$q.reject
Last thing, we have $q.reject() that is useful as a quick way to create then send a rejected promise (with a value optionally) to the next function in a chain. For example, in a chain of multiple then(). We can see this in the last two examples.
See the examples in Plunker.
