Testing media queries programmatically

[yaoningvital]

https://developer.mozilla.org/en-US/docs/Web/CSS/Media_Queries/Testing_media_queries

The DOM provides features that can test the results of a media query programmatically, via the MediaQueryList interface and its methods and properties. Once you've created a MediaQueryList object, you can check the result of the query or receive notifications when the result changes.

1. Creating a media query list

Before you can evaluate the results of a media query, you need to create the MediaQueryList object
representing the query. To do this, use the window.matchMedia method.

For example, to set up a query list that determines if the device is in landscape or portrait orientation:

var mediaQueryList = window.matchMedia("(orientation: portrait)");

2. Checking the result of a query

Once you've created your media query list, you can check the result of the query by looking at the value of its matches property:

if (mediaQueryList.matches) {
  /* The viewport is currently in portrait orientation */
} else {
  /* The viewport is not currently in portrait orientation, therefore landscape */
}

3. Receiving query notifications

If you need to be aware of changes to the evaluated result of the query on an ongoing basis, it's more efficient to register a listener than to poll the query's result. To do this, call the addListener() method on the MediaQueryList object, with a callback function to invoke when the media query status changes (e.g., the media query test goes from true to false):

var mediaQueryList = window.matchMedia("(orientation: portrait)"); // Create the query list.
function handleOrientationChange(mql) { ... } // Define a callback function for the event listener.
mediaQueryList.addListener(handleOrientationChange); // Add the callback function as a listener to the query list.

handleOrientationChange(mediaQueryList); // Run the orientation change handler once.

This code creates the orientation-testing media query list, then adds an event listener to it. After adding the listener, we also call the listener directly. This makes our listener perform adjustments based on the current device orientation; otherwise, our code might assume the device is in portrait mode at startup, even if it's actually in landscape mode.

The handleOrientationChange() function would look at the result of the query and handle whatever we need to do on an orientation change:

function handleOrientationChange(evt) {
  if (evt.matches) {
    /* The viewport is currently in portrait orientation */
  } else {
    /* The viewport is currently in landscape orientation */
  }
}

Above, we define the parameter as evt — an event object. This makes sense because newer implementations of MediaQueryList handle event listeners in a standard way. They no longer use the unusual MediaQueryListListener mechanism, but a standard event listener setup, passing an event object of type MediaQueryListEvent as the argument to the callback function.

This event object also includes the media and matches properties, so you can query these features of the MediaQueryList by directly accessing it, or accessing the event object.

我创建了一个媒体查询列表（media query list），给它添加了一个监听器（listener），当媒体查询列表对象的matches状态发生变化时，我将event对象打印出来：

4. Ending query notifications

To stop receiving notifications about changes to the value of your media query, call removeListener() on the MediaQueryList, passing it the name of the previously-defined callback function:

mediaQueryList.removeListener(handleOrientationChange);