Shallow comparer

CHANGELOG.md

@@ -1,3 +1,5 @@
+* Added `comparer.shallow` for shallow object/array comparisons [#1561](https://github.com/mobxjs/mobx/issues/1561).
+
 # 5.14.0 / 4.14.0
 
 * Added experimental `reactionRequiresObservable` & `observableRequiresReaction` config [#2079](https://github.com/mobxjs/mobx/pull/2079), [Docs](https://github.com/mobxjs/mobx/pull/2082)

docs/refguide/api.md

@@ -237,7 +237,7 @@ The expression will automatically be re-evaluated if any observables it uses cha
 
 There are various `options` that can be used to control the behavior of `computed`. These include:
 
--   **`equals: (value, value) => boolean`** Comparison method can be used to override the default detection on when something is changed. Built-in comparers are: `comparer.identity`, `comparer.default`, `comparer.structural`.
+-   **`equals: (value, value) => boolean`** Comparison method can be used to override the default detection on when something is changed. Built-in comparers are: `comparer.identity`, `comparer.default`, `comparer.structural`, `comparer.shallow`.
 -   **`name: string`** Provide a debug name to this computed property
 -   **`requiresReaction: boolean`** Wait for a change in value of the tracked observables, before recomputing the derived property
 -   **`get: () => value)`** Override the getter for the computed property.

docs/refguide/computed-decorator.md

@@ -159,7 +159,7 @@ When using `computed` as modifier or as box, it accepts a second options argumen
 -   `name`: String, the debug name used in spy and the MobX devtools
 -   `context`: The `this` that should be used in the provided expression
 -   `set`: The setter function to be used. Without setter it is not possible to assign new values to a computed value. If the second argument passed to `computed` is a function, this is assumed to be a setter.
--   `equals`: By default `comparer.default`. This acts as a comparison function for comparing the previous value with the next value. If this function considers the previous and next values to be equal, then observers will not be re-evaluated. This is useful when working with structural data, and types from other libraries. For example, a computed [moment](https://momentjs.com/) instance could use `(a, b) => a.isSame(b)`. `comparer.structural` comes in handy if you want to use structural comparison to determine whether the new value is different from the previous value (and as a result notify observers).
+-   `equals`: By default `comparer.default`. This acts as a comparison function for comparing the previous value with the next value. If this function considers the previous and next values to be equal, then observers will not be re-evaluated. This is useful when working with structural data, and types from other libraries. For example, a computed [moment](https://momentjs.com/) instance could use `(a, b) => a.isSame(b)`. `comparer.structural` and `comparer.shallow` come in handy if you want to use structural/shallow comparison to determine whether the new value is different from the previous value (and as a result notify observers).
 -   `requiresReaction`: It is recommended to set this one to `true` on very expensive computed values. If you try to read it's value, but the value is not being tracked by some observer (in which case MobX won't cache the value), it will cause the computed to throw, instead of doing an expensive re-evalution.
 -   `keepAlive`: don't suspend this computed value if it is not observed by anybody. _Be aware, this can easily lead to memory leaks as it will result in every observable used by this computed value, keeping the computed value in memory!_
 
@@ -174,6 +174,7 @@ MobX provides three built-in `comparer`s which should cover most needs:
 -   `comparer.identity`: Uses the identity (`===`) operator to determine if two values are the same.
 -   `comparer.default`: The same as `comparer.identity`, but also considers `NaN` to be equal to `NaN`.
 -   `comparer.structural`: Performs deep structural comparison to determine if two values are the same.
+-   `comparer.shallow`: Performs shallow structural comparison to determine if two values are the same.
 
 ## Computed values run more often than expected
 

src/utils/comparer.ts

@@ -12,12 +12,17 @@ function structuralComparer(a: any, b: any): boolean {
     return deepEqual(a, b)
 }
 
+function shallowComparer(a: any, b: any): boolean {
+    return deepEqual(a, b, 1)
+}
+
 function defaultComparer(a: any, b: any): boolean {
     return Object.is(a, b)
 }
 
 export const comparer = {
     identity: identityComparer,
     structural: structuralComparer,
-    default: defaultComparer
+    default: defaultComparer,
+    shallow: shallowComparer
 }

src/utils/eq.ts

@@ -9,13 +9,13 @@ import {
 declare const Symbol
 const toString = Object.prototype.toString
 
-export function deepEqual(a: any, b: any): boolean {
-    return eq(a, b)
+export function deepEqual(a: any, b: any, depth: number = -1): boolean {
+    return eq(a, b, depth)
 }
 
 // Copied from https://github.com/jashkenas/underscore/blob/5c237a7c682fb68fd5378203f0bf22dce1624854/underscore.js#L1186-L1289
 // Internal recursive comparison function for `isEqual`.
-function eq(a: any, b: any, aStack?: any[], bStack?: any[]) {
+function eq(a: any, b: any, depth: number, aStack?: any[], bStack?: any[]) {
     // Identical objects are equal. `0 === -0`, but they aren't identical.
     // See the [Harmony `egal` proposal](http://wiki.ecmascript.org/doku.php?id=harmony:egal).
     if (a === b) return a !== 0 || 1 / a === 1 / b
@@ -26,11 +26,7 @@ function eq(a: any, b: any, aStack?: any[], bStack?: any[]) {
     // Exhaust primitive checks
     const type = typeof a
     if (type !== "function" && type !== "object" && typeof b != "object") return false
-    return deepEq(a, b, aStack, bStack)
-}
 
-// Internal recursive comparison function for `isEqual`.
-function deepEq(a: any, b: any, aStack?: any[], bStack?: any[]) {
     // Unwrap any wrapped objects.
     a = unwrap(a)
     b = unwrap(b)
@@ -84,6 +80,13 @@ function deepEq(a: any, b: any, aStack?: any[], bStack?: any[]) {
             return false
         }
     }
+
+    if (depth === 0) {
+        return false
+    } else if (depth < 0) {
+        depth = -1
+    }
+
     // Assume equality for cyclic structures. The algorithm for detecting cyclic
     // structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`.
 
@@ -109,7 +112,7 @@ function deepEq(a: any, b: any, aStack?: any[], bStack?: any[]) {
         if (length !== b.length) return false
         // Deep compare the contents, ignoring non-numeric properties.
         while (length--) {
-            if (!eq(a[length], b[length], aStack, bStack)) return false
+            if (!eq(a[length], b[length], depth - 1, aStack, bStack)) return false
         }
     } else {
         // Deep compare objects.
@@ -121,7 +124,7 @@ function deepEq(a: any, b: any, aStack?: any[], bStack?: any[]) {
         while (length--) {
             // Deep compare each member
             key = keys[length]
-            if (!(has(b, key) && eq(a[key], b[key], aStack, bStack))) return false
+            if (!(has(b, key) && eq(a[key], b[key], depth - 1, aStack, bStack))) return false
         }
     }
     // Remove the first object from the stack of traversed objects.

test/base/extras.js

@@ -487,3 +487,29 @@ test("deepEquals should yield correct results for complex objects #1118 - 2", ()
     expect(mobx.comparer.structural(a1, a2)).toBe(true)
     expect(mobx.comparer.structural(a1, a4)).toBe(false)
 })
+
+test("comparer.shallow should work", () => {
+    const sh = mobx.comparer.shallow
+
+    expect(sh(1, 1)).toBe(true)
+
+    expect(sh(1, 2)).toBe(false)
+
+    expect(sh({}, {})).toBe(true)
+    expect(sh([], [])).toBe(true)
+
+    expect(sh({}, [])).toBe(false)
+    expect(sh([], {})).toBe(false)
+
+    expect(sh({ a: 1, b: 2, c: 3 }, { a: 1, b: 2, c: 3 })).toBe(true)
+
+    expect(sh({ a: 1, b: 2, c: 3, d: 4 }, { a: 1, b: 2, c: 3 })).toBe(false)
+    expect(sh({ a: 1, b: 2, c: 3 }, { a: 1, b: 2, c: 3, d: 4 })).toBe(false)
+    expect(sh({ a: {}, b: 2, c: 3 }, { a: {}, b: 2, c: 3 })).toBe(false)
+
+    expect(sh([1, 2, 3], [1, 2, 3])).toBe(true)
+
+    expect(sh([1, 2, 3, 4], [1, 2, 3])).toBe(false)
+    expect(sh([1, 2, 3], [1, 2, 3, 4])).toBe(false)
+    expect(sh([{}, 2, 3], [{}, 2, 3])).toBe(false)
+})