Use of Vue.watch function and implementation of data monitoring

Use of Vue.watch function and implementation of data monitoring

Vue.js is a front-end framework that provides many practical features to simplify the front-end development process. One of them is data monitoring. Vue provides a built-in functionwatchfor monitoring changes in Vue instance data. This article will introduce how to use thewatchfunction, and use code examples to show how to implement the data monitoring function.

1. Basic usage ofwatchfunction

watchThe function can be defined inside the Vue instance to monitor changes in instance data. It receives two parameters: the first parameter is the data to be monitored, which can be a string or a function; the second parameter is the callback function, which will be called when the monitored data changes.

The following is a simple example:

var vm = new Vue({ data: { message: 'Hello, Vue!' }, watch: { message: function(newVal, oldVal) { console.log('数据发生了变化：', newVal, oldVal); } } });

In the above code, we create a Vue instance and define amessagedata. In thewatchoption, we monitoredmessageand specified a callback function. Whenmessagedata changes, the callback function will be called.

2. Advanced usage of thewatchfunction

In addition to basic usage, thewatchfunction can also support more options. We can specify more options by passing an object to thewatchfunction.

Here is an example of using theimmediateoption:

var vm = new Vue({ data: { message: 'Hello, Vue!' }, watch: { message: { handler: function(newVal, oldVal) { console.log('数据发生了变化：', newVal, oldVal); }, immediate: true } } });

In the above code, we define the value ofwatchas an object , and pass in thehandlercallback function andimmediateoptions in the object.immediateThe optiontruemeans that the callback function will be executed immediately when the listener is created.

In addition to theimmediateoption, thewatchfunction also supports other options, such asdeep,deep:trueindicates depth Monitor the internal changes of the object. For more options, check out the Vue official documentation for details.

3. Implement the data monitoring function

In Vue, data monitoring is implemented through theObject.definePropertymethod. Vue internally implements data monitoring and updating by hijacking the getters and setters of data.

The following is a simplified version of the implementation example:

function watch(obj, key, callback) { var value = obj[key]; Object.defineProperty(obj, key, { get: function() { return value; }, set: function(newVal) { var oldVal = value; value = newVal; callback(newVal, oldVal); } }); } var data = { message: 'Hello, Vue!' }; watch(data, 'message', function(newVal, oldVal) { console.log('数据发生了变化：', newVal, oldVal); });

In the above code, we define awatchfunction, which receives an object and a key, and a callback function. TheObject.definePropertymethod is used internally to hijack thegetterandsetterof the object, and the callback function is called in thesetmethod.

Through the above example, we can see how to implement the data monitoring function through thewatchfunction.

This article introduces how to use Vue'swatchfunction, and shows how to implement data monitoring through code examples. Thewatchfunction can help us obtain changes in data in time and perform corresponding operations. In actual development, reasonable use of thewatchfunction can improve the robustness and maintainability of the code. I hope this article will help you understand the use of thewatchfunction and implement data monitoring.

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