On a field-by-field basis, the XForms processor checks the instance data against its constraints.
X Forms处理器根据实例数据的约束逐个字段地对其进行检查。
The XForms processor evaluates and enforces these constraints without the need for additional code.
X Forms处理器无需额外代码即可计算并强制这些约束。
When the functions attribute is present on the model, the XForms processor reads it before loading the form.
当 model 元素中含有 functions 属性时，XForms 处理器会在加载表单之前 读取该属性。
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