How to use the watch function in Vue documentation

Vue is a popular JavaScript framework that provides many convenient features to help developers build applications more easily. One of the important features is data binding. Data binding in Vue is two-way. When the data changes, the view will automatically update. In Vue, we can use the watch function to monitor data changes.

The watch function is a property of the Vue instance. It allows us to perform some custom logic when the data properties of the Vue instance change. The watch function is useful when you want to perform some action when data changes.

How to use the watch function

The watch function can be used in two ways: object or function. When watch monitors an object, we need to use "key-value" pairs to define the listening function. For example:

watch: {
  name: function () {
    // code for handling name changes
  },
  age: function () {
    // code for handling age changes
  }
}

In this example, when the value of the name attribute or age attribute changes, Vue will call the corresponding listening function to execute the code.

Another way is to define a set of functions in watch, each function listening to a property. As shown below:

watch: {
  'name': {
    handler: function (val, oldVal) {
      // code for handling name changes
    },
    deep: true
  },
  'age': {
    handler: function (val, oldVal) {
      // code for handling age changes
    },
    immediate: true
  }
}

In this example, "name" and "age" are the properties to be monitored. Handler is a function that is called whenever the corresponding property changes. In the handler, the first parameter val is the new value of the attribute, and oldVal is the old value. These parameters can be used to check for property changes and execute the corresponding code. In addition, we can also set the "deep" and "immediate" options to control the behavior of the watch.

The "deep" option indicates whether to monitor deeply when the object properties change. If the properties of an object change, Vue by default only detects whether the reference to the object has changed, and will not detect whether the properties in the object have changed. If we use the "deep" option, Vue will examine the entire object tree to determine which properties have changed. This is useful for listening to object properties. For example:

watch: {
  user: {
    handler: function () {
      // code for handling user changes
    },
    deep: true
  }
}

In this example, when any property of the user object changes, Vue will automatically detect the change and call the handler function.

The "immediate" option indicates whether the watch function is executed immediately after the Vue instance is created. If set to true, Vue will call the handler function immediately when the watch function is bound. For example:

watch: {
  age: {
    handler: function () {
      // code for handling age changes
    },
    immediate: true
  }
}

In this example, even if the age attribute has not changed, Vue will immediately call the handler function when the watch function is bound.

Summary

watch is a property of the Vue instance that allows us to perform some custom logic when data properties change. The watch function can be used in two ways: object and function. In the object approach we need to use "key-value" pairs to define listening functions, while in the functional approach we need to manually listen to each property and define a listening function to handle property changes. At the same time, we can also set the "deep" and "immediate" options to control the behavior of the watch. Using Vue's watch function will help us write more powerful and flexible applications.

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