Proxies Design Principles for Robust OO Intercession APIs

Transcription

1 Proxies Design Principles for Robust OO Intercession APIs Tom Van Cutsem Mark S. Miller

2 Overview Context: a new -programming API for Javascript (d on ECMAScript 5th ed.) Focus on intercession, intercepting -level operations on proxies Paper: generalized design principles for messaged OO -programming APIs

3 Ecmascript 5 New reflection API exposes property attributes var point = { x: 5, get y() { return this.x; } }; Object.defineProperty(point, z, { value: 42, writable: false, enumerable: true, configurable: true });

4 Tamper-proof Objects var point = { x: 5, get y() { return this.x; } }; Object.preventExtensions(point); point.z = 0; // can t add new properties Object.seal(point); delete point.x; // can t delete properties Object.freeze(point); point.x = 7; // can t assign properties

5 Proxies

6 Intercession in Javascript today non-standard nosuchmethod hook in Firefox modelled after Smalltalk s doesnotunderstand: function makeproxy(target) { return { } } }; nosuchmethod : function(name, args) { return target[name].apply(target, args);

7 nosuchmethod not stratified (defined as a regular application-level method) limited intercession (intercepts only missing method calls) var p = makeproxy({foo: 42}); foo in p // false p. nosuchmethod // reveals the method for (var name in p) { } // reveals nosuchmethod but not foo

8 Dynamic Proxies Objects that look and feel like normal objects, but whose behavior is controlled by another Javascript object Some use cases: Generic wrappers around existing objects: access control, tracing, profiling, contracts,... Virtual objects: remote objects, mock objects,...

9 Tracing: dynamic proxies function maketracer(obj) { var = Proxy.create({ } get: function(rcvr, name) { trace( get, name, obj); return obj[name]; }, set: function(rcvr, name, val) { trace( set, name, obj, val); },... obj[name] = val; return true; }, Object.getPrototypeOf(obj)); return ;

10 Tracing: dynamic proxies function maketracer(obj) { var = Proxy.create({ } get: function(rcvr, name) { trace( get, name, obj); return obj[name]; }, set: function(rcvr, name, val) { trace( set, name, obj, val); },... obj[name] = val; return true; }, Object.getPrototypeOf(obj)); return ;

11 Stratified API var = Proxy.create(, proto);

12 Stratified API var = Proxy.create(, proto);.get(, foo ).foo

13 Stratified API var = Proxy.create(, proto);.get(, foo ).set(, foo, 42).foo.foo = 42

14 Stratified API var = Proxy.create(, proto);.get(, foo ).set(, foo, 42).get(, foo ).apply(,[1,2,3]).foo.foo = 42.foo(1,2,3)

15 Stratified API var = Proxy.create(, proto);.get(, foo ).set(, foo, 42).get(, foo ).apply(,[1,2,3]).get(, get ).foo.foo = 42.foo(1,2,3).get

16 Stratified API var = Proxy.create(, proto);.get(, foo ).set(, foo, 42).get(, foo ).apply(,[1,2,3]).get(, get ).foo.foo = 42.foo(1,2,3).get proto

17 Not just property access var = Proxy.create(, proto);

18 Not just property access var = Proxy.create(, proto);.has( foo ) foo in

19 Not just property access var = Proxy.create(, proto);.has( foo ).delete( foo ) foo in delete.foo

20 Not just property access var = Proxy.create(, proto);.has( foo ).delete( foo ) var props =.enumerate(); for (i=0;i<props.length;i++) { } var prop = props[i];... foo in delete.foo for (var prop in ) {... }

21 Not just property access var = Proxy.create(, proto);.has( foo ).delete( foo ) var props =.enumerate(); for (i=0;i<props.length;i++) { } var prop = props[i];....defineproperty( foo, pd) foo in delete.foo for (var prop in ) {... } Object.defineProperty(, foo, pd)

22 But not quite everything either var = Proxy.create(, proto);

23 But not quite everything either var = Proxy.create(, proto); === obj

24 But not quite everything either var = Proxy.create(, proto); === obj Object.getPrototypeOf() => proto proto

25 But not quite everything either var = Proxy.create(, proto); === obj Object.getPrototypeOf() => proto instanceof Fun proto

26 But not quite everything either var = Proxy.create(, proto); === obj Object.getPrototypeOf() => proto instanceof Fun typeof => object proto

27 Example: no-op forwarding function ForwardingHandler(obj) { this.target = obj; } target ForwardingHandler.prototype = { has: function(name) { return name in this.target; }, } get: function(rcvr,name) { return this.target[name]; }, set: function(rcvr,name,val) { this.target[name]=val;return true; }, delete: function(name) { return delete this.target[name]; }... var = Proxy.create(new ForwardingHandler(o), Object.getPrototypeOf(o));

28 Example: counting property access function makesimpleprofiler(target) { var forwarder = new ForwardingHandler(target); } var count = Object.create(null); forwarder.get = function(rcvr, name) { count[name] = (count[name] 0) + 1; return this.target[name]; }; return { } : Proxy.create(forwarder, Object.getPrototypeOf(target)), get stats() { return count; }

29 Example: counting property access function makesimpleprofiler(target) { var forwarder = new ForwardingHandler(target); } var count = Object.create(null); forwarder.get = function(rcvr, name) { count[name] = (count[name] 0) + 1; return this.target[name]; }; return { } : Proxy.create(forwarder, Object.getPrototypeOf(target)), get stats() { return count; } var subject = {... }; var profiler = makesimpleprofiler(subject); runapp(profiler.); display(profiler.stats);

30 Selective interception object

31 Selective interception VM territory (C++) object Javascript territory

32 Selective interception VM territory (C++) VM object Javascript territory

33 Selective interception VM territory (C++) Host VM host object object Javascript territory

34 Selective interception VM territory (C++) Host VM host object object Javascript territory

35 Selective interception VM territory (C++) Host VM host object object Javascript territory

36 Selective interception VM territory (C++) Host VM host object object Javascript territory

37 Fixing a Proxy (temporary intercession) var = Proxy.create(, proto);

38 Fixing a Proxy (temporary intercession) var = Proxy.create(, proto); Object.freeze() Object.seal() Object.preventExtensions()

39 Fixing a Proxy (temporary intercession) var = Proxy.create(, proto); var desc =.fix(); if (desc === undefined) throw TypeError(); become(, Object.freeze( Object.create(proto, desc))); Object.freeze() Object.seal() Object.preventExtensions()

40 Fixing a Proxy (temporary intercession) var = Proxy.create(, proto); var desc =.fix(); if (desc === undefined) throw TypeError(); become(, Object.freeze( Object.create(proto, desc))); Object.freeze() Object.seal() Object.preventExtensions() Trapping fix Fixed

41 Fixing a Proxy (temporary intercession) var = Proxy.create(, proto); var desc =.fix(); if (desc === undefined) throw TypeError(); become(, Object.freeze( Object.create(proto, desc))); Object.freeze() Object.seal() Object.preventExtensions() Trapping fix Fixed

42 Status Accepted proposal for ES-Harmony Draft specification at tinyurl.com/harmony-proxies Andreas Gal (Mozilla) prototyped an implementation in Tracemonkey

43 Status Available in Firefox 4 Betas today:

44 Summary Dynamic proxies enable: Generic wrappers: access control, profiling, adaptors,... Virtual objects: remote objects, mock objects, emulated host objects,... API: Robust (stratified, not all operations intercepted) Secure (can t trap non- or fixed objects) Performance: no overhead for non- objects es-lab.googlecode.com