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1 /*******************************************************************************
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2 * Copyright (c) 2005, 2008 IBM Corporation and others.
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3 * All rights reserved. This program and the accompanying materials
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4 * are made available under the terms of the Eclipse Public License v1.0
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5 * which accompanies this distribution, and is available at
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6 * http://www.eclipse.org/legal/epl-v10.html
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7 *
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8 * Contributors:
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9 * IBM Corporation - initial API and implementation
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10 * Brad Reynolds - bug 116920
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11 * Brad Reynolds - bug 147515
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12 *******************************************************************************/
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13 module org.eclipse.core.databinding.observable.value.ComputedValue;
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81
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14 import org.eclipse.core.databinding.observable.value.ValueDiff;
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15 import org.eclipse.core.databinding.observable.value.AbstractObservableValue;
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16 import org.eclipse.core.databinding.observable.value.IValueChangeListener;
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78
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17
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18 import java.lang.all;
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19
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20 import org.eclipse.core.databinding.observable.ChangeEvent;
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21 import org.eclipse.core.databinding.observable.IChangeListener;
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22 import org.eclipse.core.databinding.observable.IObservable;
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23 import org.eclipse.core.databinding.observable.IStaleListener;
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24 import org.eclipse.core.databinding.observable.ObservableTracker;
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25 import org.eclipse.core.databinding.observable.Realm;
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26 import org.eclipse.core.databinding.observable.StaleEvent;
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27
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28 /**
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29 * A Lazily calculated value that automatically computes and registers listeners
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30 * on its dependencies as long as all of its dependencies are IObservable
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31 * objects
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32 * <p>
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33 * This class is thread safe. All state accessing methods must be invoked from
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34 * the {@link Realm#isCurrent() current realm}. Methods for adding and removing
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35 * listeners may be invoked from any thread.
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36 * </p>
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37 *
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38 * @since 1.0
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39 */
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40 public abstract class ComputedValue : AbstractObservableValue {
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41
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42 private bool dirty = true;
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43
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44 private bool stale = false;
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45
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46 private Object cachedValue = null;
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47
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48 /**
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49 * Array of observables this computed value depends on. This field has a
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50 * value of <code>null</code> if we are not currently listening.
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51 */
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52 private IObservable[] dependencies = null;
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53
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54 /**
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55 *
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56 */
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57 public this() {
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58 this(Realm.getDefault(), null);
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59 }
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60
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61 /**
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62 * @param valueType
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63 * can be <code>null</code>
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64 */
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65 public this(Object valueType) {
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66 this(Realm.getDefault(), valueType);
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67 }
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68
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69 /**
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70 * @param realm
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71 *
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72 */
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73 public this(Realm realm) {
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74 this(realm, null);
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75 }
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76
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77 /**
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78 * @param realm
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79 * @param valueType
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80 */
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81 public this(Realm realm, Object valueType) {
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82 super(realm);
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83 this.valueType = valueType;
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84 }
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85
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86 /**
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87 * Inner class that implements interfaces that we don't want to expose as
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88 * public API. Each interface could have been implemented using a separate
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89 * anonymous class, but we combine them here to reduce the memory overhead
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90 * and number of classes.
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91 *
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92 * <p>
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93 * The Runnable calls computeValue and stores the result in cachedValue.
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94 * </p>
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95 *
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96 * <p>
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97 * The IChangeListener stores each observable in the dependencies list. This
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98 * is registered as the listener when calling ObservableTracker, to detect
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99 * every observable that is used by computeValue.
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100 * </p>
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101 *
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102 * <p>
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103 * The IChangeListener is attached to every dependency.
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104 * </p>
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105 *
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106 */
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107 private class PrivateInterface : Runnable, IChangeListener,
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108 IStaleListener {
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109 public void run() {
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110 cachedValue = calculate();
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111 }
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112
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113 public void handleStale(StaleEvent event) {
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114 if (!dirty && !stale) {
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115 stale = true;
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116 fireStale();
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117 }
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118 }
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119
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120 public void handleChange(ChangeEvent event) {
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121 makeDirty();
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122 }
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123 }
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124
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125 private PrivateInterface privateInterface = new PrivateInterface();
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126
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127 private Object valueType;
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128
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129 protected final Object doGetValue() {
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130 if (dirty) {
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131 // This line will do the following:
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132 // - Run the calculate method
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133 // - While doing so, add any observable that is touched to the
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134 // dependencies list
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135 IObservable[] newDependencies = ObservableTracker.runAndMonitor(
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136 privateInterface, privateInterface, null);
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137
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138 stale = false;
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139 for (int i = 0; i < newDependencies.length; i++) {
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140 IObservable observable = newDependencies[i];
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141 // Add a change listener to the new dependency.
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142 if (observable.isStale()) {
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143 stale = true;
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144 } else {
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145 observable.addStaleListener(privateInterface);
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146 }
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147 }
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148
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149 dependencies = newDependencies;
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150
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151 dirty = false;
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152 }
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153
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154 return cachedValue;
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155 }
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156
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157 /**
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158 * Subclasses must override this method to provide the object's value.
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159 *
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160 * @return the object's value
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161 */
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162 protected abstract Object calculate();
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163
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164 protected final void makeDirty() {
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165 if (!dirty) {
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166 dirty = true;
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167
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168 stopListening();
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169
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170 // copy the old value
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171 final Object oldValue = cachedValue;
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172 // Fire the "dirty" event. This implementation recomputes the new
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173 // value lazily.
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174 fireValueChange(new class() ValueDiff {
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175
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176 public Object getOldValue() {
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177 return oldValue;
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178 }
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179
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180 public Object getNewValue() {
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181 return getValue();
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182 }
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183 });
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184 }
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185 }
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186
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187 /**
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188 *
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189 */
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190 private void stopListening() {
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191 // Stop listening for dependency changes.
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192 if (dependencies !is null) {
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193 for (int i = 0; i < dependencies.length; i++) {
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194 IObservable observable = dependencies[i];
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195
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196 observable.removeChangeListener(privateInterface);
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197 observable.removeStaleListener(privateInterface);
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198 }
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199 dependencies = null;
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200 }
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201 }
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202
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203 public bool isStale() {
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204 // we need to recompute, otherwise staleness wouldn't mean anything
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205 getValue();
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206 return stale;
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207 }
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208
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209 public Object getValueType() {
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210 return valueType;
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211 }
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212
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213 // this method exists here so that we can call it from the runnable below.
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214 /**
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215 * @since 1.1
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216 */
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217 protected bool hasListeners() {
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218 return super.hasListeners();
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219 }
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220
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221 public synchronized void addChangeListener(IChangeListener listener) {
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222 super.addChangeListener(listener);
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223 // If somebody is listening, we need to make sure we attach our own
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224 // listeners
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225 computeValueForListeners();
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226 }
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227
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228 /**
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229 * Some clients just add a listener and expect to get notified even if they
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230 * never called getValue(), so we have to call getValue() ourselves here to
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231 * be sure. Need to be careful about realms though, this method can be
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232 * called outside of our realm. See also bug 198211. If a client calls this
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233 * outside of our realm, they may receive change notifications before the
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234 * runnable below has been executed. It is their job to figure out what to
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235 * do with those notifications.
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236 */
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237 private void computeValueForListeners() {
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238 getRealm().exec(new class() Runnable {
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239 public void run() {
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240 if (dependencies is null) {
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241 // We are not currently listening.
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242 if (hasListeners()) {
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243 // But someone is listening for changes. Call getValue()
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244 // to make sure we start listening to the observables we
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245 // depend on.
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246 getValue();
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247 }
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248 }
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249 }
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250 });
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251 }
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252
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253 public synchronized void addValueChangeListener(
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254 IValueChangeListener listener) {
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255 super.addValueChangeListener(listener);
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256 // If somebody is listening, we need to make sure we attach our own
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257 // listeners
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258 computeValueForListeners();
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259 }
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260
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261 public synchronized void dispose() {
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262 super.dispose();
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263 stopListening();
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264 }
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265
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266 }
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