173 lines
No EOL
7.2 KiB
Markdown
173 lines
No EOL
7.2 KiB
Markdown
## m.deferred
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This is a low-level method in Mithril. It's a modified version of the Thenable API.
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A deferred is an asynchrony monad. It exposes a `promise` property which can *bind* callbacks to build a computation tree.
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The deferred object can then *apply* a value by calling either `resolve` or `reject`, which then dispatches the value to be processed to the computation tree.
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Each computation function takes a value as a parameter and is expected to return another value, which in turns is forwarded along to the next computation function (or functions) in the tree.
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The deferred object returned by `m.deferred` has two methods: `resolve` and `reject`, and one property called `promise`. The methods can be called to dispatch a value to the promise tree. The `promise` property is the root of the promise tree. It has a method `then` which takes a `successCallback` and a `errorCallback` callbacks. Calling the `then` method attaches the computations represented by `successCallback` and `errorCallback` to the promise, which will be called when either `resolve` or `reject` is called. The `then` method returns a child promise, which, itself, can have more child promises, recursively.
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The `promise` object is actually a function - specifically, it's an [`m.prop`](mithril.prop.md) getter-setter, which gets populated with the value returned by `successCallback` or `errorCallback` (depending on whether `resolve` or `reject` got called).
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Note that Mithril promises are not automatically integrated to its automatic redrawing system. If you wish to use third party asynchronous libraries (for example, `jQuery.ajax`), you should also consider using [`m.startComputation` / `m.endComputation`](mithril.computation.md) if you want views to redraw after requests complete.
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---
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### Usage
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```javascript
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//standalone usage
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var greetAsync = function() {
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var deferred = m.deferred();
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setTimeout(function() {
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deferred.resolve("hello");
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}, 1000);
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return deferred.promise;
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};
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greetAsync()
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.then(function(value) {return value + " world"})
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.then(function(value) {console.log(value)}); //logs "hello world" after 1 second
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```
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#### Retrieving a value via the getter-setter API
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```javascript
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//asynchronous service
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var greetAsync = function() {
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var deferred = m.deferred();
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setTimeout(function() {
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deferred.resolve("hello");
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}, 1000);
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return deferred.promise;
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};
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//asynchronous consumer
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var greeting = greetAsync()
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var processed = greeting.then(function(value) {return value + " world"})
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console.log(greeting()) // undefined - because `deferred.resolve` has not been called yet
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setTimeout(function() {
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//now `deferred.resolve` has been called
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console.log(greeting()) // "hello"
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console.log(processed()) // "hello world"
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}, 2000)
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```
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#### Integrating to the Mithril redrawing system
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```javascript
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//asynchronous service
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var greetAsync = function() {
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m.startComputation();
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var deferred = m.deferred();
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setTimeout(function() {
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deferred.resolve("hello");
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m.endComputation();
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}, 1000);
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return deferred.promise;
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};
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```
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---
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### Differences from Promises/A+
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For the most part, Mithril promises behave as you'd expect a [Promise/A+](http://promises-aplus.github.io/promises-spec/) promise to behave, but with a few key differences:
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Mithril promises forward a value downstream if a resolution callback returns `undefined`. This allows simpler debugging of promise chains:
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```javascript
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var data = m.request({method: "GET", url: "/data"})
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.then(console.log) //Mithril promises let us debug like this
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.then(doStuff)
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var data = m.request({method: "GET", url: "/data"})
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.then(function(value) { // Promises/A+ would require us to declare an anonymous function
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console.log(value) // here's the debugging snippet
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return value // and we need to remember to return the value as well
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})
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.then(doStuff) // or else `doStuff` will break
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```
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Another subtle difference is that the Promises/A+ require a callback to run in a different execution context than its respective `then` method. This requirement exists to support an obscure edge cases and incurs [a significant performance hit on each link of a promise chain](http://thanpol.as/javascript/promises-a-performance-hits-you-should-be-aware-of/). To be more specific, the performance hit can come either in the form of a 4ms minimum delay (if the implementation uses `setTimeout`), or from having to load a [bunch of hacky polyfill code](https://raw.githubusercontent.com/NobleJS/setImmediate/master/setImmediate.js) for a [feature that is not being considered for addition by some browser vendors](https://developer.mozilla.org/en-US/docs/Web/API/Window.setImmediate).
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To illustrate the difference between Mithril and A+ promises, consider the code below:
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```javascript
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var deferred = m.deferred()
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deferred.promise.then(function() {
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console.log(1)
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})
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deferred.resolve("value")
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console.log(2)
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```
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In the example above, A+ promises are required to log `2` before logging `1`, whereas Mithril logs `1` before `2`. Typically `resolve`/`reject` are called asynchronously after the `then` method is called, so normally this difference does not matter.
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---
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### Signature
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[How to read signatures](how-to-read-signatures.md)
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```clike
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Deferred deferred()
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where:
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Deferred :: Object { Promise promise, void resolve(any value), void reject(any value) }
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Promise :: GetterSetter { Promise then(any successCallback(any value), any errorCallback(any value)) }
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GetterSetter :: any getterSetter([any value])
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```
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- **GetterSetter { Promise then([any successCallback(any value) [, any errorCallback(any value)]]) } promise**
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A promise has a method called `then` which takes two computation callbacks as parameters.
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The `then` method returns another promise whose computations (if any) receive their inputs from the parent promise's computation.
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A promise is also a getter-setter (see [`m.prop`](mithril.prop)). After a call to either `resolve` or `reject`, it holds the result of the parent's computation (or the `resolve`/`reject` value, if the promise has no parent promises)
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- **Promise then([any successCallback(any value) [, any errorCallback(any value)]])**
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This method accepts two callbacks which process a value passed to the `resolve` and `reject` methods, respectively, and pass the processed value to the returned promise
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- **any successCallback(any value)** (optional)
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The `successCallback` is called if `resolve` is called in the root `deferred`.
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The default value (if this parameter is falsy) is the identity function `function(value) {return value}`
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If this function returns undefined, then it passes the `value` argument to the next step in the thennable queue, if any
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- **any errorCallback(any value)** (optional)
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The `errorCallback` is called if `reject` is called in the root `deferred`.
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The default value (if this parameter is falsy) is the identity function `function(value) {return value}`
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If this function returns undefined, then it passes the `value` argument to the next step in the thennable queue, if any
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- **returns Promise promise**
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- **void resolve(any value)**
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This method passes a value to the `successCallback` of the deferred object's child promise
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- **void reject(any value)**
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This method passes a value to the `errorCallback` of the deferred object's child promise
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