Builder模式

前言

之前在研究RxJava的时候，就深深的迷上了这种代码调用方式，简洁明了，逻辑也清晰。后台在使用Glide的时候发现这种.调用方法真的很舒服。

附上一段代码：

Glide.with(this)
.load(imageUrl)
.placeholder(image)
.error(image)
.into(imageView);

但后来才发现，最好的例子就是Android自带的AlertDialog.

new AlertDialog.Builder(MainActivity.this)
               .setTitle("One Button")
               .setMessage("Thanks for visiting The Code of a Ninja - codeofaninja.com")
               .setPositiveButton("OK", new DialogInterface.OnClickListener() {
                   public void onClick(DialogInterface dialog, int id) {

                       dialog.cancel();
                   }
               }).show();

实现

现在看一段代码
AlertDialog源码

public Person(String name, int sex) {
        this(name, sex, 0);
}

public Person(String name, int sex, int age) {
        this(name,"",age,sex);
}

public Person(String firstName, String lastName, int age, int sex) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
        this.sex = sex;
}

person有若干个属性，有三个构造方法。倘若某天person增加了几个字段，然后你就啪啪啪的把这几个字段加在了后面

public Person(String firstName, String lastName, int age, int sex,String arg1,String arg2) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
        this.sex = sex;
        this.arg1 = arg1;
        this.arg2 = arg2;

这样看起来就很臃肿，如果参数再多呢，看起来就很难受了。调用的时候还需要去确认每一个位置的参数是什么。
两种方式可以解决：

1. Set/Get
2. Builder

Set/Get方法的确可以解决。

public class Person {
    String name;
    String lastName;
    ...
    public Person(Builder builder) {
        this.firstName = builder.name;
        this.lastName = builder.lastName;
        ...
    }

// 请注意这里返回的是一个Person类而不是void，因为可以链式调用
    public Person SetName(String name){
        this.name = name;
        return this;
    }
    
    public String getName(){
        return this.name;
    }
    
    public Person SetLastName(String lastName){
        this.name = lastName;
        return this;
    }
    
    public String getLastName(){
        return this.lastName;
    }
}

Builder

public class Person {
    String name;
    String lastName;
    ...
    
    public Person(Builder builder) {
        this.firstName = builder.name;
        this.lastName = builder.lastName;
        ...
    }
    
    // 请注意这里返回的是一个Person类而不是void，因为可以链式调用
    public Person SetName(String name){
        this.name = name;
        return this;
    }
    
    public String getName(){
        return this.name;
    }
    
    public Person SetLastName(String lastName){
        this.name = lastName;
        return this;
    }
    
    public String getLastName(){
        return this.lastName;
    }
    
    public static class Builder{
        String name;
        String lastName;
        ...
        public Builder SetName(String name){
                this.name = name;
                return this;
        }   
        
        public String getName(){
                return this.name;
        }
        
        public Builder SetLastName(String lastName){
                this.name = lastName;
                return this;
        }
            
        public String getLastName(){
                return this.lastName;
        }
        
        public static Builder createBuilder(){
            return new Builder();
        }
        
        public Person Build(){
            return new Person(this);
        }
    }
}

调用

Person p = Builder.createBuilder()
 .SetName("name")
 .SetLastName("lastname")
 .Build()

例子有很多，但不变的是思想。