Analyze and solve the problem that uniapp variable assignment does not work

uniapp is a cross-platform front-end development framework that can develop applications for multiple platforms at the same time. But in actual work, we sometimes encounter the problem that variable assignment does not work, which may cause great trouble to our development. This article will analyze and solve this problem.

Problem description:

When using uniapp for development, we often need to assign values ​​to variables for use in different scenarios. But sometimes it is found that the value of the variable has not changed after the assignment, which means that the variable assignment does not work.

For example, we have the following component:

<template>
  <div>{{ message }}</div>
</template>

<script>
export default {
  data() {
    return {
      message: 'Hello, uniapp!'
    }
  },
  methods: {
    changeMessage() {
      this.message = 'Hello, world!'
    }
  }
}
</script>

This component has a message variable and a changeMessage method. The changeMessage method is used to change the value of the message variable to 'Hello, world! '. However, after using the changeMessage method, it is found that the value of message has not changed and is still 'Hello, uniapp!'.

Analysis reasons:

When analyzing this problem, we need to clarify two concepts: responsive system and Vue instance.

Uniapp’s reactive system is implemented through the Object.defineProperty method, which is the core of uniapp data binding. When a piece of data is bound to a view, any modification to the data will trigger an update of the view.

The Vue instance is the entity of the uniapp program, which contains the data, methods, etc. in the application. When a Vue instance is created, it handles the data property reactively.

When we assign a variable to a property in data, it is actually equivalent to assigning the variable to a property of the Vue instance. This property will be processed by the reactive system. When the value of this property changes, the corresponding view will also change.

However, when we directly assign values ​​to the properties of the object, the reactive system does not work. Because the reactive system only handles the data attribute when the Vue instance is initialized, not its attributes.

Solution:

For the problem that variable assignment does not work, we can have some solutions:

1. Use the Vue.set method or this.$ set method.

The Vue.set method or this.$set method can effectively solve the problem of variable assignment not working. These two methods are provided by Vue and are specifically used to add properties to objects to make them responsive.

For example, we can modify the code in the changeMessage method to:

changeMessage() {
  this.$set(this, 'message', 'Hello, world!')
}

In this way, when the changeMessage method is called, the value of the message variable will be changed, and the corresponding view will also be changed. renew.

2. Use the object expansion symbol (...).

The object expansion symbol (...) can be used to expand the properties of one object into another object. This new object will have responsive functionality.

For example, we can modify the code in the changeMessage method to:

changeMessage() {
  this.message = { ...this.message, 'Hello, world!' }
}

In this way, when the changeMessage method is called, the value of the message variable will be changed, and the corresponding view will also be changed. renew.

3. Use the array update method.

For arrays, uniapp provides some update methods, such as push, pop, shift, unshift, splice, sort, reverse, etc. These methods can add, delete, and modify the array and trigger the update logic of the responsive system.

For example, we can modify the message variable to an array:

data() {
  return {
    message: ['Hello, uniapp!']
  }
},

Then, use the array modification method in the changeMessage method:

changeMessage() {
  this.message.splice(0, 1, 'Hello, world!')
}

In this way, when the changeMessage method is called , the value of the message variable will be changed, and the corresponding view will be updated.

Summary:

When using uniapp for development, we should pay attention to the problem that variable assignment does not work. If this problem occurs, it can be solved using the Vue.set method, this.$set method, the object expansion symbol (...) or the array update method. Mastering these methods can improve the efficiency of our development and make our programs more robust.

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