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$ hg merge core-javascript.closure-exercises # done, at least for now
author jf3243
date Wed, 09 Nov 2011 17:00:38 +0000
parents cfb78eef6a57
children 4e6cfa066d1a
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1 Counters memory
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2 ===============
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3
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4 Memory
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5 ------
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6
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7 Understanding garbage collection and memory leaks is an advanced
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8 topic, but even beginners should know that there can be problems here.
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9 Correct use of a suitable framework is the simplest way to avoid
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10 memory leaks.
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11
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12 The code in the counters example indicates how to construct and use such
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13 a framework.
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14
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15 IE memory leaks
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16 ---------------
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17
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18 Internet Explorer, prior to IE8 (check) had **two memory heaps**, one
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19 for JavaScript and the other for the DOM. Each heap had its own
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20 independent garbage collector.
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21
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22 This means that if a DOM node *d* held a reference to a JavaScript
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23 object *j* and also that *j* held a reference to *d* then neither
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24 garbage collector could collect *j* or *d*. What's worse, even when
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25 the page was unloaded IE did not reclaim this memory.
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26
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27 Thus, prior to IE8, JavaScript could cause Internet Explorer to leak
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28 memory. This lost memory could be reclaimed only by closing the
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29 browser! Closing the page was not enough.
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30
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31 Garbage collection
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32 ------------------
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33 The JavaScript garbage collection deletes objects provide it can
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34 discover that they can never be used again. The simplest case is:
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35
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36 .. code-block:: javascript
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37
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38 x = [1, 2, 3, 4, 5, 6, 7];
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39 x = null;
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40
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41 The second assignment to *x* ensures that the original array (created
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42 by the array literal) can no longer be accessed, and so it can be
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43 garbage collected.
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44
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45 After the counters namespace anonyomous function has executed there
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46 are no global objects holding even indirect references to the
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47 function. Therefore it can and will be garbage collected. However, as
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48 we shall see, the execution context of the function continues.
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49
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50 The whole of the discussion here reduces to two things
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51
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52 * The execution of the context of the namespace anonymous function
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53 continues to exist after the function has completed execution.
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54
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55 * If the line
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57 .. code-block: javascript
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58 element.onclick = onclick_factory(models);
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59
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60 were replaced by
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61
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62 .. code-block: javascript
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63 element.onclick = onclick_factory(models);
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64
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65 then after the execution of the namespace anonymous function all the
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66 JavaScript objects it created would be garbage collected.
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67
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68 The trick is to allow garbage collection to take place by releasing,
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69 when the time comes, all DOM reference to JavaScript objects.
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70
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71 Residue
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72 -------
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73
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74 The *element* DOM node continues to exist, as it can be reached from
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75 the *document* node. It has an *onclick* attribute, which is the
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76 function created by the *onclick_factory*, with *models* as the
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77 argument. So that function is not garbage collected and neither is
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78 *models*.
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79
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80 This is just as it should be. Anything that can be reached from the
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81 rendered web page is not garbage collected. It has to be there so
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82 that click has the desired effect.
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83
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84 The function *element.onclick* retains a reference to *models* just as
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85 surely as executing
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86
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87 .. code-block:: javascript
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88
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89 var f = function(arg){ return [1, 2, 3, arg] };
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90 var y = f(x);
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91
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92 causes *y* to retain a reference to the value of *x*.
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93
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94 Finally, *models* contains references to the *Counter* instances and
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95 thus, by the hidden prototype reference, to the *counter* prototype
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96 object.
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97
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98 The *counter* prototype object lies in the execution context of the
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99 anonymous function, and as it happend that keeps alive the whole of
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100 the execution context.
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101
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102 Reclaiming memory
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103 -----------------
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104
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105 Notice that the JavaScript holds only two reference to DOM nodes,
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106 namely *document* and *example*. However, the line of code
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107
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108 .. code-block:: javascript
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109
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110 element = undefined; // Release reference to DOM node.
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111
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112 means that the JavaScript no longer holds a reference to *element*.
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113
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114 The DOM node *example* holds a reference, via its *onclick* function,
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115 to the JavaScript on the page, and in particular to the *Counter*
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116 instances. This cannot be avoided, because we want certain DOM nodes
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117 to change the state of the counters.
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118
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119 However, the converse is not true. We can write the JavaScript code
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120 so that it has but one reference to a DOM object, namely the
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121 *document* node held for example as a property of the *global* object.
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122
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123 Summary
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124 -------
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125
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126 To avoid the IE memory leaks the trick is to ensure that, when the
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127 page unloads, there is nothing that is keeping the garbage is alive.
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128 If we execute
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129
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130 .. code-block:: javascript
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131
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132 element.onclick = undefined
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133
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134 then there is nothing on the page that is keeping our JavaScript code
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135 alive. And once all the JavaScript is garbage collected then there's
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136 nothing to prevent all the DOM notde being garbage collected.
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137
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138 The key principle in this strategy is that the page unload event
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139 should unbind all JavaScript event handlers and other data attached to
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140 the page. This will ensure that the JavaScript is garbage collected,
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141 and hence there is nothing to obstruct garbage collection of the DOM
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142 nodes.
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143
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144 Clearly, this strategy requires us to keep track of what we add to the
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145 DOM. Delegation makes this much easier to do.