The ‘isolate’ scope takes an object hash which defines a set of local scope properties derived from the parent scope. These local properties are useful for aliasing values for templates. Locals definition is a hash of local scope property to its source:

= or =attr – set up bi-directional binding between a local scope
property and the parent scope property of name defined via the value
of the attr attribute. If no attr name is specified then the
attribute name is assumed to be the same as the local name. Given
<widget my-attr="parentModel"> and widget definition of scope: {
localModel:'=myAttr' },then widget scope property localModel will
reflect the value of parentModel on the parent scope. Any changes to
parentModel will be reflected in localModel and any changes in
localModel will reflect in parentModel. If the parent scope
property doesn’t exist,it will throw a
NON_ASSIGNABLE_MODEL_EXPRESSION exception. You can avoid this behavior
using =? or =?attr in order to flag the property as optional.

它应该在影响范围属性的每个摘要上触发预期的错误：

parentSet = parentGet.assign || function() {
// reset the change,or we will throw this exception on every $digest
lastValue = scope[scopeName] = parentGet(parentScope);
     throw Error(NON_ASSIGNABLE_MODEL_EXPRESSION + attrs[attrName] +
     ' (directive: ' + newScopeDirective.name + ')');
};

//...


if (parentValue !== scope[scopeName]) {
    // we are out of sync and need to copy
    if (parentValue !== lastValue) {
        // parent changed and it has precedence
        lastValue = scope[scopeName] = parentValue;
    } else {
        // if the parent can be assigned then do so
        parentSet(parentScope,lastValue = scope[scopeName]);
    }
}