A pipe class must implement the PipeTransform interface. For example, if the name is "myPipe", use a template binding expression such as the following:
{{ exp | myPipe }}
{{ exp | myPipe }}
The result of the expression is passed to the pipe's transform() method.
A pipe must belong to an NgModule in order for it to be available to a template. To make it a member of an NgModule, list it in the declarations field of the NgModule metadata.
When true, the pipe is pure, meaning that the transform() method is invoked only when its input arguments change. Pipes are pure by default.
pure: boolean
pure: boolean
如果该管道具有内部状态（也就是说，其结果会依赖内部状态，而不仅仅依赖参数），就要把 pure 设置为 false。 这种情况下，该管道会在每个变更检测周期中都被调用一次 —— 即使其参数没有发生任何变化。
If the pipe has internal state (that is, the result depends on state other than its arguments), set pure to false. In this case, the pipe is invoked on each change-detection cycle, even if the arguments have not changed.
